GB2633064A

GB2633064A - Light turning component

Info

Publication number: GB2633064A
Application number: GB2313259.0A
Authority: GB
Inventors: Cole Alexander
Original assignee: Envisics Ltd
Current assignee: Envisics Ltd
Priority date: 2023-08-31
Filing date: 2023-08-31
Publication date: 2025-03-05
Also published as: WO2025045629A1; GB202313259D0

Abstract

Light turning component comprising: first layer comprising a first lens array (1022, fig. 10), wherein each lenslet 1024 of the first lens array 1022 has an axis 1036 extending in a first dimension; a second layer (1032, fig. 10), arranged over the first layer, comprising a second lens array, wherein each lenslet 1034 of the second lens array has an axis 1026 extending in the first dimension, wherein the axis of each lenslet of the first lens array is offset from the axis of each adjacent lenslet of the second lens array, such that light propagating through the first and second layers is turned. The lenses of the first and second lens arrays may be elongate along the first dimension. The offset between the axis of each lens of the first lens array and the axis of each adjacent lens of the second lens array is in a second dimension, orthogonal to the first dimension. The first and second lens array may comprise a one-dimensional array of elongate lenslets extending in the second direction. the depth of each lenslet in the third direction may correspond to the focal distance of the lenslet.

Description

LIGHT TURNING COMPONENT

FIELD

The present disclosure relates to pupil expansion or replication, in particular, for a diffracted light field comprising diverging ray bundles and turning of the ray bundles. More specifically, the present disclosure relates a system comprising a pupil replicator and a light turning component. Some embodiments relate to two-dimensional pupil expansion, using first and second waveguide pupil expanders. Some embodiments relate to picture generating unit and a head-up display, for example an automotive head-up display (HUD).

BACKGROUND AND INTRODUCTION

Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or "hologram", comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object.

Computer-generated holography may numerically simulate the interference process. A computer-generated hologram may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel/Fourier transform holograms or simply Fresnel/Fourier holograms. A Fourier hologram may be considered a Fourier domain/plane representation of the object or a frequency domain/plane representation of the object. A computer-generated hologram may also be calculated by coherent ray tracing or a point cloud technique, for example.

A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and/or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micro-mirrors, for example.

A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device.

The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission.

A holographic projector may be provided using the system described herein. Such projectors have found application in head-up displays, "HUD".

SUMMARY

Aspects of the present disclosure are defined in the appended independent claims.

Broadly, the present disclosure relates to image projection. It relates to a method of image projection and an image projector which comprises a display device. The present disclosure also relates to a projection system comprising the image projector and a viewing system, in which the image projector projects or relays light from a display device to the viewing system. The present disclosure is equally applicable to a monocular and binocular viewing system. The viewing system may comprise a viewer's eye or eyes. The viewing system comprises an optical element having optical power (e.g., lens/es of the human eye) and a viewing plane (e.g., retina of the human eye/s). The projector may be referred to as a 'light engine'. The display device and the image formed (or perceived) using the display device are spatially separated from one another. The image is formed, or perceived by a viewer, on a display plane. In some embodiments, the image is a virtual image and the display plane may be referred to as a virtual image plane. In other embodiments, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. The image is formed by illuminating a diffractive pattern (e.g., hologram) displayed on the display device.

The display device comprises pixels. The pixels of the display may display a diffractive pattern or structure that diffracts light. The diffracted light may form an image at a plane spatially separated from the display device. In accordance with well-understood optics, the magnitude of the maximum diffraction angle is determined by the size of the pixels and other factors such as the wavelength of the light.

In embodiments, the display device is a spatial light modulator such as liquid crystal on silicon ("LCOS") spatial light modulator (SLM). Light propagates over a range of diffraction angles (for example, from zero to the maximum diffractive angle) from the LCOS, towards a viewing entity/system such as a camera or an eye. In some embodiments, magnification techniques may be used to increase the range of available diffraction angles beyond the conventional maximum diffraction angle of an LCOS.

In some examples, an image (formed from the displayed hologram) is propagated to the eyes. For example, spatially modulated light of an intermediate holographic reconstruction / image formed either in free space or on a screen or other light receiving surface between the display device arid the viewer, may be propagated to the viewer.

In some other examples, the (light of a) hologram itself is propagated to the eyes. For example, spatially modulated light of the hologram (that has not yet been fully transformed to a holographic reconstruction, i.e. image) -that may be informally said to be "encoded" with/by the hologram -is propagated directly to the viewer's eyes. A real or virtual image may be perceived by the viewer. In these embodiments, there is no intermediate holographic reconstruction / image formed between the display device and the viewer. It is sometimes said that, in these embodiments, the lens of the eye performs a hologram-to-image conversion or transform. The projection system, or light engine, may be configured so that the viewer effectively looks directly at the display device.

Reference is made herein to a "light field" which is a "complex light field". The term "light field" merely indicates a pattern of light having a finite size in at least two orthogonal spatial directions, e.g. x and y. The word "complex" is used herein merely to indicate that the light at each point in the light field may be defined by an amplitude value and a phase value, and may therefore be represented by a complex number or a pair of values. For the purpose of hologram calculation, the complex light field may be a two-dimensional array of complex numbers, wherein the complex numbers define the light intensity and phase at a plurality of

discrete locations within the light field.

In accordance with the principles of well-understood optics, the range of angles of light propagating from a display device that can be viewed, by an eye or other viewing entity/system, varies with the distance between the display device and the viewing entity. At a 1 meter viewing distance, for example, only a small range of angles from an LCOS can propagate through an eye's pupil to form an image at the retina for a given eye position. The range of angles of light rays that are propagated from the display device, which can successfully propagate through an eye's pupil to form an image at the retina for a given eye position, determines the portion of the image that is 'visible' to the viewer. In other words, not all parts of the image are visible from any one point on the viewing plane (e.g., any one eye position within a viewing window such as eye-motion box.) In some embodiments, the image perceived by a viewer is a virtual image that appears upstream of the display device -that is, the viewer perceives the image as being further away from them than the display device. Conceptually, it may therefore be considered that the viewer is looking at a virtual image through an 'display device-sized window' or 'eye box', which may be very small, for example 1 cm in diameter, at a relatively large distance, e.g., 1 meter. And the user will be viewing the display device-sized window via the pupil(s) of their eye(s), which can also be very small. Accordingly, the field of view becomes small and the specific angular range that can be seen depends heavily on the eye position, at any given time.

A pupil expander (also known as a replicator) addresses the problem of how to increase the range of angles of light rays that are propagated from the display device that can successfully propagate through an eye's pupil to form an image. The display device is generally (in relative terms) small and the projection distance is (in relative terms) large. In some embodiments, the projection distance is at least one -such as, at least two -orders of magnitude greater than the diameter, or width, of the entrance pupil and/or aperture of the display device (i.e., size of the array of pixels). Embodiments of the present disclosure relate to a configuration in which a hologram of an image is propagated to the human eye rather than the image itself. In other words, the light received by the viewer is modulated according to a hologram of the image. However, other embodiments of the present disclosure may relate to configurations in which the image is propagated to the human eye rather than the hologram -for example, by so called indirect view, in which light of a holographic reconstruction or "replay image" formed on a screen (or even in free space) is propagated to the human eye.

Use of a pupil expander increases the viewing area (i.e., user's eye-box) in at least one dimension, thus enabling some movement of the eye/s to occur, whilst still enabling the user to see the image. As the skilled person will appreciate, in an imaging system, the viewing area (user's eye box) is the area in which a viewer's eyes can perceive the image. The present disclosure relates to non-infinite virtual image distances -that is, near-field virtual images.

Conventionally, a two-dimensional pupil expander comprises one or more one-dimensional optical waveguides each formed using a pair of opposing reflective surfaces, in which the output light from a surface forms a viewing window -e.g., eye-box or eye motion box for viewing by the viewer. Light received from the display device (e.g., spatially modulated light from a LCOS) is replicated by the or each waveguide so as to increase the field of view (or viewing area) in at least one dimension. In particular, the waveguide enlarges the viewing window due to the generation of extra rays or "replicas" by division of amplitude of the incident wavefront.

The display device may have an active or display area having a first dimension that may be less than 10 cm such as less than 5 cm or less than 2 cm. The propagation distance between the display device and viewing system may be greater than 1 m such as greater than 1.5 m or greater than 2 m. The optical propagation distance within the waveguide may be up to 2 m such as up to 1.5 m or up to 1 m. The method may be capable of receiving an image and determining a corresponding hologram of sufficient quality in less than 20 ms such as less than 15 ms or less than 10 ms.

In some embodiments -described only by way of example of a diffracted or holographic light field in accordance with this disclosure -a hologram is configured to route light into a plurality of channels, each channel corresponding to a different part (i.e. sub-area) of an image. The hologram may be represented, such as displayed, on a display device such as a spatial light modulator. When displayed on an appropriate display device, the hologram may spatially modulate light transformable by a viewing system into the image. The channels formed by the diffractive structure are referred to herein as "hologram channels" merely to reflect that they are channels of light encoded by the hologram with image information. It may be said that the light of each channel is in the hologram domain rather than the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram and the hologram domain is therefore the Fourier or frequency domain. The hologram may equally be a Fresnel or Fresnel transform hologram. The hologram is described herein as routing light into a plurality of hologram channels merely to reflect that the image that can be reconstructed from the hologram has a finite size and can be arbitrarily divided into a plurality of image sub-areas, wherein each hologram channel would correspond to each image sub-area. Importantly, the hologram of this example is characterised by how it distributes the image content when illuminated. Specifically, the hologram divides the image content by angle. That is, each point on the image is associated with a unique light ray angle in the spatially modulated light formed by the hologram when illuminated -at least, a unique pair of angles because the hologram is two-dimensional. For the avoidance of doubt, this hologram behaviour is not conventional. The spatially modulated light formed by this special type of hologram, when illuminated, may be arbitrarily divided into a plurality of hologram channels, wherein each hologram channel is defined by a range of light ray angles (in two-dimensions). It will be understood from the foregoing that any hologram channel (i.e. sub-range of light ray angles) that may be considered in the spatially modulated light will be associated with a respective part or sub-area of the image. That is, all the information needed to reconstruct that part or sub-area of the image is contained within a sub-range of angles of the spatially modulated light formed from the hologram of the image. When the spatially modulated light is observed as a whole, there is not necessarily any evidence of a plurality of discrete light channels. However, in some arrangements, a plurality of spatially separated hologram channels is formed by intentionally leaving areas of the target image, from which the hologram is calculated, blank or empty (i.e., no image content is present).

Nevertheless, the hologram may still be identified. For example, if only a continuous part or sub-area of the spatially modulated light formed by the hologram is reconstructed, only a sub-area of the image should be visible. If a different, continuous part or sub-area of the spatially modulated light is reconstructed, a different sub-area of the image should be visible.

A further identifying feature of this type of hologram is that the shape of the cross-sectional area of any hologram channel substantially corresponds to (i.e. is substantially the same as) the shape of the entrance pupil although the size may be different -at least, at the correct plane for which the hologram was calculated. Each light / hologram channel propagates from the hologram at a different angle or range of angles. Whilst these are example ways of characterising or identifying this type of hologram, other ways may be used. In summary, the hologram disclosed herein is characterised and identifiable by how the image content is distributed within light encoded by the hologram. Again, for the avoidance of any doubt, reference herein to a hologram configured to direct light or angularly-divide an image into a plurality of hologram channels is made by way of example only and the present disclosure is equally applicable to pupil expansion of any type of holographic light field or even any type

of diffractive or diffracted light field.

Broadly, a system is disclosed herein that provides pupil expansion for an input light field, wherein the input light field is a diffracted or holographic light field comprising diverging ray bundles. As discussed above, pupil expansion (which may also be referred to as "image replication" or "replication" or "pupil replication") enables the size of the area at/from which a viewer can see an image (or, can receive light of a hologram, which the viewer's eye forms an image) to be increased, by creating one or more replicas of an input light ray (or ray bundle). The pupil expansion can be provided in one or more dimensions. For example, two-dimensional pupil expansion can be provided, with each dimension being substantially orthogonal to the respective other.

The system can be provided in a compact and streamlined physical form. This enables the system to be suitable for a broad range of real-world applications, including those for which space is limited and real-estate value is high. For example, it may be implemented in a head-up display (HUD) such as a vehicle or automotive HUD.

In accordance with the present disclosure, pupil expansion is provided for diffracted or diffractive light, which may comprise diverging ray bundles. The diffractive or diffracted light may be output by a display device such as a pixelated display device such as a spatial light modulator (SLM) arranged to display a diffractive structure such as a hologram. The diffracted light field may be defined by a "light cone". Thus, the size of the diffracted light field (as defined on a two-dimensional plane) increases with propagation distance from the corresponding diffractive structure (i.e. display device).

The spatial light modulator may be arranged to display a hologram. The diffracted or diverging light may comprise light encoded with/by the hologram, as opposed to being light of an image or of a holographic reconstruction. In such embodiments, it can therefore be said that the pupil expander replicates the hologram or forms at least one replica of the hologram, to convey that the light delivered to the viewer is spatially modulated in accordance with a hologram of an image, not the image itself. That is, a diffracted light field is propagated to the viewer.

In some embodiments, two one-dimensional waveguide pupil expanders are provided, each one-dimensional waveguide pupil expander being arranged to effectively increase the size of the exit pupil of the system by forming a plurality of replicas or copies of the exit pupil (or light of the exit pupil) of the spatial light modulator. The exit pupil may be understood to be the physical area from which light is output by the system. It may also be said that each waveguide pupil expander is arranged to expand the size of the exit pupil of the system. It may also be said that each waveguide pupil expander is arranged to expand/increase the size of the eye box within which a viewer's eye can be located, in order to see/receive light that is output by the system.

In some embodiments, an optical element comprising a light turning component (or light turning layer) is provided. The light turning component may be arranged to couple light emitted from the output region of a first pupil expander (or replicator) to a second pupil expander (or replicator). The light turning component changes the propagation direction of, or "turns", the output light such that the directions of expansion in the first and second expanders (or replicators) are aligned. Additionally or alternatively, a light turning component (or light turning layer) may be arranged to couple emitted light from a second pupil expander (or replicator) to an eye-box. The use of a light turning component may allow for a less bulky system in comparison to conventional optics to turn the light.

A light turning component may change the direction of the output light by means of diffraction of light, such as with a diffractive optical element. Alternatively, a light turning component may change the direction of the output light based on refraction of light, such as with an array of prism elements. A component that turns light by means of refraction may be conveniently applied to an output surface of an optical element, such as the output surface of a pupil expander (or replicator) as described herein.

Through simulation and experimentation, the inventors have found that, in some arrangements, the use of a light turning component in the form of layer comprising an array of prisms (or "prismatic layer") may introduce artefacts in the turned light. These artefacts could be due to stray or scattered light rays and/or breaks in the coherence of the turned light. For example, light rays may be scattered by the sloped prism surfaces, leading to blind spots and loss of light, which may contribute to dark bands and reduce optical efficiency. In embodiments where the system is arranged to expand an exit pupil and to propagate light to an eye-box from which a virtual image is viewable, these artefacts could adversely affect the quality of the virtual image viewed at the exit pupil. Having recognised that a prismatic layer may introduce artefacts in the turned light, the inventors have devised an improved light turning component for turning light, which may minimise the occurrence of artefacts.

There is provided a light turning component. The light turning component may be used with an optical component of a head-up display. The light turning component comprises a first layer comprising a first lens array. Each lenslet of the first lens array has an axis extending in a first dimension. The light turning component further comprises a second layer, arranged over the first layer, comprising a second lens array. Each lenslet of the second lens array has an axis extending in the first dimension. The axis of each lenslet of the first lens array is offset from the axis of each lenslet of the second lens array. The offset may be in a second dimension. The first and second dimensions may be orthogonal dimensions of the major surfaces of the first and second layers. Light propagating through the first and second layers is turned. For example, the light may be turned in a direction in the second dimension.

In some embodiments, the lenslets of the first and second lens arrays are elongate along the first dimension. The axis of each lenslet of the first and second lens arrays is a geometric (i.e., longitudinal) axis extending in the dimension of elongation. In some examples, each of the first and second lens arrays comprises a one dimensional array of elongate lenslets extending in the second dimension, orthogonal to the first dimension.

In some embodiments, light propagating through the first and second layers is turned in a direction of the second dimension.

In some embodiments, each of the first arid second lens arrays comprises an array of cylindrical lenslets arranged to provide optical power in the second dimension, and not in the first dimension. It may be said that the lenslets of the first and second lens arrays are arranged to provide optical power only in the second dimension. The geometric/cylinder axis of each lenslet of the first lens array is offset from the geometric/cylinder axis of each lens of the second lens array in the second dimension. It may be said that the optical axis/plane of each cylindrical lenslet of the first lens array is offset from the optical axis/plane of each cylindrical lenslet of the second lens array in the second dimension.

In some embodiments, the lenslets of the first and second lens arrays are arranged in abutment, that is without spatial separation between adjacent lenslets of the array in the second dimension. It may be said that each of the first and second lens arrays comprises an array of lenslets that forms continuous, optically transparent layer having a width and length in the first and second dimensions and a depth in a third dimension orthogonal to the first and second dimensions. Thus, the light turning component may comprise a two-layer optical structure, which may be conveniently applied to a surface of an optical component for the input or output of light to be turned, such as the output surface of a waveguide pupil expander (or replicator) as described herein.

Since the axes of the lenslets of the first and second lens arrays are offset, the direction of light propagating through the first and second layers of the light turning component is changed by virtue of the refractive effect of the offset lenslets of the first and second lens arrays. For example, light incident in a direction that is normal to the first and second layers (e.g., in the third dimension) is output in a direction that is off-normal (e.g., in the second and third dimensions). It may be said that the light turning layer has a turning effect on light. However, unlike an array of prisms that has inclined surfaces and sharp corners, the light turning component has smoothly curved lensing surfaces. This may lead to a number of advantages.

First, so-called "edge artefacts" are associated with the geometry of prisms, in particular straight edges (e.g. inclined prism surfaces) and sharp corners may have undesirable optical effects on the light. For instance, light may be scattered leading to stray light rays, blind spots and/or breaks in the coherence of the turned light. Such effects may also contribute to dark bands. The absence of straight edges and sharp corners in the first and second lens arrays may therefore lead to fewer edge artefacts in the turned light.

Secondly, gaps/dead areas may be created with a prism-based turning layer and optical efficiency may be reduced due to loss of light. However, since the lenslets of the first lens array focus light, and the lenslets of the second lens array have the opposite effect, rays of light incident on a particular lenslet are "contained" within that lenslet. Thus, there are fewer gaps/dead areas.

Finally, the arrangement is readily compatible, and may work synergistically with, techniques for the filtering out of stray light rays, particularly caused by sunlight reflections, and for shuttering arrangements, as described herein.

In some embodiments, the light turning component may comprise a first surface and a second surface. The first surface may be opposite the second surface. The first surface may form an input port of the light turning component. Accordingly, it may be said that the first surface is arranged to receive a light field or wavefront, such as a diffractive light field or holographic wavefront, at an angle of incidence. The second surface may form an output port of the light turning component. Accordingly, it may be said that the second surface is arranged to output (or transmit) a wavefront, such as a holographic wavefront, at an output angle (or transmission angle). The output angle (or transmission angle) of light output by the light turning component is different from the angle of incidence of input light.

In some embodiments, the lenslets of the first lens array have a depth in the third dimension. For instance, the lenslets may be mounted on, and/or integrated with, a parallelepiped-shaped optically transparent substrate to provide a depth in the third dimension. In some arrangement, the depth of each lenslets in the third dimension may substantially correspond to the focal distance thereof.

In some embodiments, the first and second layers formed by the first and second lens arrays each comprise an internal surface and an external surface. The internal surface of the first layer is adjacent to, or abuts, the internal surface of the second layer. In some examples, the lenslets of first/second lens array have opposed planar and curved/lensing surfaces. It may be said that the lenses are piano-cylindrical. In examples in which the lenslets comprise a parallelepiped substrate, the planar surface of each lenslet may be separated from its curved/lensing surface by a depth in the third dimension.

In some examples, the planar surfaces of the lenslets form the internal surface of the first/second layer and the curved/lensing surfaces form the external surface of the first/second layer. It may be said that the input and output ports of the light turning component, corresponding to the external surfaces of the first and second layers, comprise curved/lensing surfaces of an array of elongate (e.g., cylindrical) lenses.

The shape of the curved/lensing surface of the lenslets of the first lens array may be complementary (e.g., substantially the same), and the shape of the curved/lensing surface of the lenslets of the second array may be complementary (e.g., substantially the same). Thus, all the lenslets of the first lens array have the same optical power, and all the lenslets of the first lens array have the same optical power. In some embodiments, the shape of the curved/lensing surface of the lenslets of the first lens array is complementary to (e.g., substantially the same as) the shape of the curved/lensing surface of the lenslets of the second lens array. Thus all the lenslets of the first and second lens arrays have the same optical power. As the skilled person will appreciate, other arrangements, in which the optical power of the lenslets differs within each lens array, or between the first and second lens arrays, is possible and contemplated.

In embodiments comprising first/second arrays of cylindrical lenslets (e.g., convex cylindrical lenses), the curved/lensing surface of the lenslets is cylindrical (i.e., at least part of the surface of a cylinder). For example, the curved/lensing surface of the lenslets of the first/second lens array may comprises the whole of a surface of a cylindrical lens or a fraction of the surface of a cylindrical lens. In some embodiments, the fraction may be greater than or less than half the curved/lensing surface of a cylindrical lens. The curved/cylindrical surface may have an asymmetric geometry. It may be said that the curved/cylindrical surface of each lens is asymmetric in the second direction.

The optical power, geometry and pitch of lenslets of the first and second lens arrays may be chosen according to application requirements. The offset between the axis of each lenslet of the first lens array and the axis of the adjacent lenslet of the second lens array may be chosen according to the desired the turning angle of the light.

In some embodiments, the light turning component is arranged to turn substantially coherent light. For example, the coherent light may be light that has been emitted by a laser. In such embodiments, the first replicator may be arranged to replicate a substantially coherent diffractive light field. The light output by the first replicator may be substantially coherent light. It is desirable that the light remains substantially coherent after it has been received (and turned) by the turning component.

In some embodiments the light turning component may be manufactured separately from an optical element (e.g., pupil replicator) with which it is to be used, and attached (e.g., by bonding) to the optical element. In some embodiments the light turning component may be formed directly on the optical element.

In some embodiments, the light turning component comprises a two layer structure. Each layer comprises an array of microstructures (e.g., cylindrical lenses), as described previously. In some embodiments, the light turning component may be substantially planar and extend in the first and second dimensions. In some embodiments, the first dimension may be substantially horizontal and the second dimension may be substantially vertical. The lenses of the first and second layers may have a depth in a third dimension. Light propagates through the first and second layers of the light turning component. The first and second layers may advantageously be arranged to change the direction of light output from an exit surface/output port of a first replicator so that it propagates at the correct angle of incidence on and entrance surface/input port of a second replicator for replication. Such an arrangement may result in larger regions of coherent light which, as above, is advantageous.

A light turning component or layer may be described as an optical component that turns or rotates a propagation axis of light such that the propagation axis of light input to the turning component or layer and the propagation axis of light output from the turning component or later are not parallel. In this context, the propagation axis refers to the overall or group propagation direction of light, such as an axis at the centre of a light beam. For a diffracted light field the input light rays may have a range of values, and therefore the turning layer may turn each of the rays a different amount. An amount of turning of the turning component or layer may be defined based on the magnitude of the overall turn in the propagation axis/direction of the diverging light beam.

In some embodiments the amount of turning may be between 5° to 25°. However, the amount of turning is not limited to these values. The amount of turning may depend upon the specific arrangement of the optical system.

In some embodiments, the light turning component may be arranged on, or with, a first pupil expander (or replicator), a second pupil expander (or replicator) or both a first replicator and a second pupil expander (or replicator). This may reduce the size of the device, and also reduce the size of any gaps between elements of the display system. This in turn may reduce the amount of divergence of light in the gaps, and therefore lead to a more efficient system.

In some embodiments, the light turning component is arranged on the output port or surface of the first pupil expander (or replicator). This may reduce the size of the device, and also may reduce the requirement for alignment of elements of the device, reducing the complexity of fabrication.

In some embodiments, the light turning component is arranged on the output port or surface of the second pupil expander (or replicator). This may reduce the size of the device, and also may reduce the requirement for alignment of elements of the system, reducing the complexity of fabrication. In addition, the global tilt of the display system in situ, such as within a dashboard of a vehicle, may be optimised according to application requirements, such as available space.

In some embodiments, the light turning component is arranged on the input port or surface of the second pupil expander (or replicator). This reduces the gap between the elements, reducing the amount of divergence of the light and increasing efficiency of the system. Such an arrangement may be used in combination with a light component arranged on the output port or surface of the first and/or second pupil expander.

In some embodiments, the light turning component is arranged on, or with, an optical element comprising an aperture device (also called a switching device). The aperture device may be arranged to selectively block parts of the diffracted light beam to reduce cross-talk. In other words, the aperture device may comprise the turning component or layer. Applying the light turning component to the aperture device also reduces gaps between components, which reduces the amount of divergence of the light and increases the efficiency of the system. Such an aperture device may be arranged on, or with, the output port or surface of a first pupil expander/the input port or surface of a second pupil expander, and/or on, or with, the output port or surface of a second pupil expander, as described herein.

Accordingly, there is provided a replicator and a light turning component. The replicator is arranged to receive a light field and replicate the light field in one direction. The light turning component comprises a first layer and a second layer arranged over the first layer. It may be said that the first and second layers are provided in a stacked or layered arrangement. The first layer comprises a first lens array. Each lenslet of the first lens array has an axis extending in a first dimension. The second layer comprises a second lens array. Each lenslet of the second lens array has an axis extending in the first dimension. The axis of each lenslet of the first lens array is offset from the axis of each lenslet of the second lens array. The turning component is arranged to turn light output from the replicator. In some embodiments, the turned light is coupled into another replicator and replicate the light field in another direction. In other embodiments, the turned light is transmitted to an eye-box.

In some arrangements, a device is disclosed. The device may alternatively be known as a switching device or an aperture device. The device comprises a 1D array of cells, wherein each cell is independently switchable between a first state and a second state. The device further comprises a light turning component comprising a first layer and a second layer arranged over the first layer. The first layer comprises a first lenslet array. Each lens of the first lens array has an axis extending in a first dimension. The second layer, comprises a second lens array. Each lenslet of the second lens array has an axis extending in the first dimension. The axis of each lenslet of the first lens array is offset from the axis of each lenslet of the second lens array. The 1D array of cells is arranged between the first and second layers of the light turning component. The turning component is arranged to change the direction of the transmitted light.

In some embodiments, each cell of the 1D array of cells is positioned at a focus (e.g., focal line) of a respective lenslet of the first lens array. This arrangement allows the size and area of the cells to be reduced as well as providing space for the routing of wires. In addition, the arrangement mitigates concerns about the optical effects of inter-cell gaps.

Each cell of the 1D array of cells may be configured to receive diffracted light from an output region of a first pupil expander (or replicator) via the first lens array. Each cell, if in the first state, is configured to output the diffracted light via the second lens array towards an input region of a second pupil expander (or replicator). Each cell, if in the second state, is configured to interact with the diffracted light such that the diffracted light remains uncoupled into the second lens array/second pupil expander (or replicator).

In some embodiments, the device is a liquid crystal device (LCD). Each cell of the LCD may be switched between a substantially transparent state and a substantially opaque state. In the transparent light may be allowed to couple into the second pupil replicator. However, it is to be understood that alternative arrangements are possible, where the light is coupled into the second pupil replicator when the LCD cell is in a transparent state.

In some embodiments, the device comprises a first transparent substrate configured to receive diffracted light and a second transparent substrate configured to output light, and wherein the 1D array of cells is located in an optical path between the first transparent substrate and the second transparent substrate. The first arid second transparent substrates may act as containing layers for the array of cells such as an LCD. The first lens array may be arranged adjacent, or form a part of, the first transparent substrate and the second lens array may be arranged adjacent, or form part of, the second transparent substrate.

In some embodiments, the device is a microelectromechanical systems (MEMS) device, wherein each cell comprises a switchable mirror, the switchable mirror switchable between the first state and the second state.

In some embodiments, each switchable mirror is configured to direct the diffracted light towards a sensor for monitoring the diffracted light. Directing light to a sensor allows for the integrity of the image displayed to the user to be monitored, without requiring a sensor in the eye-line of the user. This may allow malfunctions or other issues to be detected, which is particularly useful when the system is part of a safety critical function, such as part of a HUD in a vehicle.

In some embodiments, switching between the first state and the second state is based on an output of an eye tracking sensor. This may allow for the switching device to be optimised such that the switching is dependent on where the user is looking.

There is also disclosed herein a display system comprising a first replicator, a second replicator and a light turning component. The first replicator is arranged to (directly or indirectly) receive a diffracted (e.g., holographic) light field (from a display device e.g., spatial light modulator) and replicate the diffracted light field in a first direction. The second replicator is arranged to receive the output from the first replicator and further replicate the diffracted light field in a second direction (perpendicular to the first direction). The light turning component comprises a first layer and a second layer arranged over the first layer.

The first layer comprises a first lens array. Each lenslet of the first lens array has an axis extending in a first dimension. The second layer, comprises a second lens array. Each lenslet of the second lens array has an axis extending in the first dimension. The axis of each lenslet of the first lens array is offset from the axis of each lenslet of the second lens array. The light turning component may be arranged to optically-couple output light from the first replicator to the input of the second replicator. Thus, the light turning component is arranged to change the angle of incidence of the light coupled from the first replicator to the second replicator (such that the first replicator and second replicator are substantially co-planar and/or a plane of the first replicator is substantially parallel to a plane of the second replicator). The light turning component may be arranged to turn the output light from the second replicator towards and eye-box.

In the present disclosure, the term "replica" is merely used to reflect that spatially modulated light is divided such that a complex light field is directed along a plurality of different optical paths. The word "replica" is used to refer to each occurrence or instance of the complex light field after a replication event -such as a partial reflection-transmission by a pupil expander. Each replica travels along a different optical path. Some embodiments of the present disclosure relate to propagation of light that is encoded with a hologram, not an image -i.e., light that is spatially modulated with a hologram of an image, not the image itself. It may therefore be said that a plurality of replicas of the hologram are formed. The person skilled in the art of holography will appreciate that the complex light field associated with propagation of light encoded with a hologram will change with propagation distance. Use herein of the term "replica" is independent of propagation distance and so the two branches or paths of light associated with a replication event are still referred to as "replicas" of each other even if the branches are a different length, such that the complex light field has evolved differently along each path. That is, two complex light fields are still considered "replicas" in accordance with this disclosure even if they are associated with different propagation distances -providing they have arisen from the same replication event or series of replication events.

A "diffracted light field" or "diffractive light field" in accordance with this disclosure is a light field formed by diffraction. A diffracted light field may be formed by illuminating a corresponding diffractive pattern. In accordance with this disclosure, an example of a diffractive pattern is a hologram and an example of a diffracted light field is a holographic light field or a light field forming a holographic reconstruction of an image. The holographic light field forms a (holographic) reconstruction of an image on a replay plane. The holographic light field that propagates from the hologram to the replay plane may be said to comprise light encoded with the hologram or light in the hologram domain. A diffracted light field is characterized by a diffraction angle determined by the smallest feature size of the diffractive structure and the wavelength of the light (of the diffracted light field). In accordance with this disclosure, it may also be said that a "diffracted light field" is a light field that forms a reconstruction on a plane spatially separated from the corresponding diffractive structure. An optical system is disclosed herein for propagating a diffracted light field from a diffractive structure to a viewer. The diffracted light field may form an image.

The term "hologram" is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, regarding the object. The term "holographic reconstruction" is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The system disclosed herein is described as a "holographic projector" because the holographic reconstruction is a real image and spatially-separated from the hologram. The term "replay field" is used to refer to the 2D area within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in the form of a plurality diffracted orders wherein each diffracted order is a replica of the zeroth-order replay field. The zeroth-order replay field generally corresponds to the preferred or primary replay field because it is the brightest replay field. Unless explicitly stated otherwise, the term "replay field" should be taken as referring to the zeroth-order replay field. The term "replay plane" is used to refer to the plane in space containing all the replay fields. The terms "image", "replay image" and "image region" refer to areas of the replay field illuminated by light of the holographic reconstruction. In some embodiments, the "image" may comprise discrete spots which may be referred to as "image spots" or, for convenience only, "image pixels".

The terms "encoding", "writing" or "addressing" are used to describe the process of providing the plurality of pixels of the SLM with a respective plurality of control values which respectively determine the modulation level of each pixel. It may be said that the pixels of the SLM are configured to "display" a light modulation distribution in response to receiving the plurality of control values. Thus, the SLM may be said to "display" a hologram and the hologram may be considered an array of light modulation values or levels.

It has been found that a holographic reconstruction of acceptable quality can be formed from a "hologram" containing only phase information related to the Fourier transform of the original object. Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram but the present disclosure is equally applicable to amplitude-only holography.

The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information related to the Fourier transform of the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram which contains both amplitude and phase information related to the original object. Such a hologram may be referred to as a fully-complex hologram because the value (grey level) assigned to each pixel of the hologram has an amplitude and phase component. The value (grey level) assigned to each pixel may be represented as a complex number having both amplitude and phase components. In some embodiments, a fully-complex computer-generated hologram is calculated.

Reference may be made to the phase value, phase component, phase information or, simply, phase of pixels of the computer-generated hologram or the spatial light modulator as shorthand for "phase-delay". That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2-rr) which represents the amount of phase retardation provided by that pixel.

For example, a pixel of the spatial light modulator described as having a phase value of 1r/2 will retard the phase of received light by Tr/2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term "grey level" may be used to refer to the plurality of available modulation levels. For example, the term "grey level" may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term "grey level" may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator.

The hologram therefore comprises an array of grey levels -that is, an array of light modulation values such as an array of phase-delay values or complex modulation values. The hologram is also considered a diffractive pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light having a wavelength comparable to, generally less than, the pixel pitch of the spatial light modulator.

Reference is made herein to combining the hologram with other diffractive patterns such as diffractive patterns functioning as a lens or grating. For example, a diffractive pattern functioning as a grating may be combined with a hologram to translate the replay field on the replay plane or a diffractive pattern functioning as a lens may be combined with a hologram to focus the holographic reconstruction on a replay plane in the near field.

Although different embodiments and groups of embodiments may be disclosed separately in the detailed description which follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments are described by way of example only with reference to the following figures: Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen; Figure 2 shows an image for projection comprising eight image areas/components, V1 to V8, and cross-sections of the corresponding hologram channels, H1-H8; Figure 3 shows a hologram displayed on an LCOS that directs light into a plurality of discrete 15 areas; Figure 4 shows a system, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3; Figure 5A shows a perspective view of a first example two-dimensional pupil expander comprising two replicators each comprising pairs of stacked surfaces; Figure 5B shows a perspective view of a first example two-dimensional pupil expander; Figure 6A shows a system comprising an optical wedge; Figure 6B shows the system in side view; Figure 7 shows a plan view of a system comprising a turning layer; Figure 8A shows a side view of a system comprising a switching element; Figure 8B shows a rear view of a system comprising a switching element, the switching element comprising a first and second turning layer; Figure 9 shows a side view of a system comprising a switching device comprising a turning layer formed as a single layer; Figure 10 is a schematic perspective view of a light turning component comprising first and second lens arrays in accordance with the present disclosure, and Figure 11 is a transverse cross section through a pair of lenslets of the light turning component of Figure 10.

The same reference numbers will be used throughout the drawings to refer to the same or like parts.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is not restricted to the embodiments described in the following but extends to the full scope of the appended claims. That is, the present invention may be embodied in different forms and should not be construed as limited to the described embodiments, which are set out for the purpose of illustration.

Terms of a singular form may include plural forms unless specified otherwise.

A structure described as being formed at an upper portion/lower portion of another structure or on/under the other structure should be construed as including a case where the structures contact each other and, moreover, a case where a third structure is disposed there between.

In describing a time relationship -for example, when the temporal order of events is described as "after", "subsequent", "next", "before" or suchlike-the present disclosure should be taken to include continuous and non-continuous events unless otherwise specified. For example, the description should be taken to include a case which is not continuous unless wording such as "just", "immediate" or "direct" is used.

Although the terms "first", "second", etc. may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the appended claims.

Features of different embodiments may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other. Some embodiments may be carried out independently from each other, or may be carried out together in codependent relationship.

In the present disclosure, the term "substantially" when applied to a structural units of an apparatus may be interpreted as the technical feature of the structural units being produced within the technical tolerance of the method used to manufacture it.

Conventional optical configuration for holographic proiection Figure 1 shows an embodiment in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object for reconstruction. It may therefore be said that the hologram is a Fourier domain or frequency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon, "LCOS", device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a replay field, for example, a light receiving surface such as a screen or diffuser.

A light source 110, for example a laser or laser diode, is disposed to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a generally planar wavefront of light to be incident on the SLM. In Figure 1, the direction of the wavefront is off-normal (e.g. two or three degrees away from being truly orthogonal to the plane of the transparent layer).

However, in other embodiments, the generally planar wavefront is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. In the embodiment shown in Figure 1, the arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a light-modulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to optics including a Fourier transform lens 120, having its focus at a screen 125. More specifically, the Fourier transform lens 120 receives a beam of modulated light from the SLM 140 and performs a frequency-space transformation to produce a holographic reconstruction at the screen 125.

Notably, in this type of holography, each pixel of the hologram contributes to the whole reconstruction. There is not a one-to-one correlation between specific points (or image pixels) on the replay field and specific light-modulating elements (or hologram pixels). In other words, modulated light exiting the light-modulating layer is distributed across the replay field.

In these embodiments, the position of the holographic reconstruction in space is determined by the dioptric (focusing) power of the Fourier transform lens. In the embodiment shown in Figure 1, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and the Fourier transform is performed optically. Any lens can act as a Fourier transform lens but the performance of the lens will limit the accuracy of the Fourier transform it performs. The skilled person understands how to use a lens to perform an optical Fourier transform In some embodiments of the present disclosure, the lens of the viewer's eye performs the hologram to image transformation.

Hologram calculation In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or Fourier-based hologram, in which an image is reconstructed in the far field by utilising the Fourier transforming properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field in the replay plane back to the lens plane. Computer-generated Fourier holograms may be calculated using Fourier transforms. Embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms which may be calculated by a similar method. In some embodiments, the hologram is a phase or phase-only hologram. However, the present disclosure is also applicable to holograms calculated by other techniques such as those based on point cloud methods.

In some embodiments, the hologram engine is arranged to exclude from the hologram calculation the contribution of light blocked by a limiting aperture of the display system.

British patent application 2101666.2, filed 5 February 2021 and incorporated herein by reference, discloses a first hologram calculation method in which eye-tracking and ray tracing are used to identify a sub-area of the display device for calculation of a point cloud hologram which eliminates ghost images. The sub-area of the display device corresponds with the aperture, of the present disclosure, and is used exclude light paths from the hologram calculation. British patent application 2112213.0, filed 26 August 2021 and incorporated herein by reference, discloses a second method based on a modified Gerchberg-Saxton type algorithm which includes steps of light field cropping in accordance with pupils of the optical system during hologram calculation. The cropping of the light field corresponds with the determination of a limiting aperture of the present disclosure. British patent application 2118911.3, filed 23 December 2021 and also incorporated herein by reference, discloses a third method of calculating a hologram which includes a step of determining a region of a so-called extended modulator formed by a hologram replicator. The region of the extended modulator is also an aperture in accordance with this disclosure.

In some embodiments, there is provided a real-time engine arranged to receive image data and calculate holograms in real-time using the algorithm. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the holograms are pre-calculated, stored in computer memory and recalled as needed for display on a SLM. That is, in some embodiments, there is provided a repository of predetermined holograms.

Large field of view using small display device

Broadly, the present disclosure relates to image projection. It relates to a method of image projection and an image projector which comprises a display device. The present disclosure also relates to a projection system comprising the image projector and a viewing system, in which the image projector projects or relays light from the display device to the viewing system. The present disclosure is equally applicable to a monocular and binocular viewing system. The viewing system may comprise a viewer's eye or eyes. The viewing system comprises an optical element having optical power (e.g., lens/es of the human eye) and a viewing plane (e.g., retina of the human eye/s). The projector may be referred to as a 'light engine'. The display device and the image formed (or perceived) using the display device are spatially separated from one another. The image is formed, or perceived by a viewer, on a display plane. In some embodiments, the image is a virtual image and the display plane may be referred to as a virtual image plane. In other examples, the image is a real image formed by holographic reconstruction and the image is projected or relayed to the viewing plane. In these other examples, spatially modulated light of an intermediate holographic reconstruction formed either in free space or on a screen or other light receiving surface between the display device and the viewer, is propagated to the viewer. In both cases, an image is formed by illuminating a diffractive pattern (e.g., hologram or kinoform) displayed on the display device.

In some embodiments, the (light of a) hologram itself is propagated to the eyes. For example, spatially modulated light of the hologram (that has not yet been fully transformed to a holographic reconstruction, i.e. image) -that may be informally said to be "encoded" with/by the hologram -is propagated directly to the viewer's eyes. A real or virtual image may be perceived by the viewer. In these embodiments, there is no intermediate holographic reconstruction / image formed between the display device and the viewer. It is sometimes said that, in these embodiments, the lens of the eye performs a hologram-to-image conversion or transform. The projection system, or light engine, may be configured so that the viewer effectively looks directly at the display device.

Reference is made herein to a "light field" which is a "complex light field". The term light field" merely indicates a pattern of light having a finite size in at least two orthogonal spatial directions, e.g. x and y. The word "complex" is used herein merely to indicate that the light at each point in the light field may be defined by an amplitude value and a phase value, and may therefore be represented by a complex number or a pair of values. For the purpose of hologram calculation, the complex light field may be a two-dimensional array of complex numbers, wherein the complex numbers define the light intensity and phase at a plurality of discrete locations within the light field.

In accordance with the principles of well-understood optics, the range of angles of light propagating from a display device that can be viewed, by an eye or other viewing entity/system, varies with the distance between the display device and the viewing entity. At a 1 metre viewing distance, for example, only a small range of angles from an LCOS can propagate through an eye's pupil to form an image at the retina for a given eye position. The range of angles of light rays that are propagated from the display device, which can successfully propagate through an eye's pupil to form an image at the retina for a given eye position, determines the portion of the image that is 'visible' to the viewer. In other words, not all parts of the image are visible from any one point on the viewing plane (e.g., any one eye position within a viewing window such as eye-box.) In some embodiments, the image perceived by a viewer is a virtual image that appears upstream of the display device -that is, the viewer perceives the image as being further away from them than the display device. Conceptually, it may therefore be considered that the viewer is looking at a virtual image through an 'display device-sized window', which may be very small, for example 1cm in diameter, at a relatively large distance, e.g., 1 metre. And the user will be viewing the display device-sized window via the pupil(s) of their eye(s), which can also be very small. Accordingly, the field of view becomes small and the specific angular range that can be seen depends heavily on the eye position, at any given time.

A pupil expander addresses the problem of how to increase the range of angles of light rays that are propagated from the display device that can successfully propagate through an eye's pupil to form an image. The display device is generally (in relative terms) small and the projection distance is (in relative terms) large. In some embodiments, the projection distance is at least one -such as, at least two -orders of magnitude greater than the diameter, or width, of the entrance pupil and/or aperture of the display device (i.e., size of the array of pixels).

Use of a pupil expander increases the viewing area (i.e., user's eye-box) laterally, thus enabling some movement of the eye/s to occur, whilst still enabling the user to see the image. As the skilled person will appreciate, in an imaging system, the viewing area (user's eye box) is the area in which a viewer's eyes can perceive the image. The present disclosure encompasses non-infinite virtual image distances -that is, near-field virtual images.

Conventionally, a two-dimensional pupil expander comprises one or more one-dimensional optical waveguides each formed using a pair of opposing reflective surfaces, in which the output light from a surface forms a viewing window or eye-box. Light received from the display device (e.g., spatially modulated light from a LCOS) is replicated by the or each waveguide so as to increase the field of view (or viewing area) in at least one dimension. In particular, the waveguide enlarges the viewing window due to the generation of extra rays or "replicas" by division of amplitude of the incident wavefront.

The display device may have an active or display area having a first dimension that may be less than 10 cms such as less than 5 cms or less than 2 cms. The propagation distance between the display device and viewing system may be greater than 1 m such as greater than 1.5 m or greater than 2 m. The optical propagation distance within the waveguide may be up to 2 m such as up to 1.5 m or up to 1 m. The method may be capable of receiving an image and determining a corresponding hologram of sufficient quality in less than 20 ms such as less than 15 ms or less than 10 ms.

In some embodiments -described only by way of example of a diffracted or holographic light field in accordance with this disclosure -a hologram is configured to route light into a plurality of channels, each channel corresponding to a different part (i.e. sub-area) of an image. The channels formed by the diffractive structure are referred to herein as "hologram channels" merely to reflect that they are channels of light encoded by the hologram with image information. It may be said that the light of each channel is in the hologram domain rather than the image or spatial domain. In some embodiments, the hologram is a Fourier or Fourier transform hologram and the hologram domain is therefore the Fourier or frequency domain. The hologram may equally be a Fresnel or Fresnel transform hologram. The hologram may also be a point cloud hologram. The hologram is described herein as routing light into a plurality of hologram channels to reflect that the image that can be reconstructed from the hologram has a finite size and can be arbitrarily divided into a plurality of image sub-areas, wherein each hologram channel would correspond to each image sub-area.

Importantly, the hologram of this example is characterised by how it distributes the image content when illuminated. Specifically and uniquely, the hologram divides the image content by angle. That is, each point on the image is associated with a unique light ray angle in the spatially modulated light formed by the hologram when illuminated -at least, a unique pair of angles because the hologram is two-dimensional. For the avoidance of doubt, this hologram behaviour is not conventional. The spatially modulated light formed by this special type of hologram, when illuminated, may be divided into a plurality of hologram channels, wherein each hologram channel is defined by a range of light ray angles (in two-dimensions). It will be understood from the foregoing that any hologram channel (i.e. sub-range of light ray angles) that may be considered in the spatially modulated light will be associated with a respective part or sub-area of the image. That is, all the information needed to reconstruct that part or sub-area of the image is contained within a sub-range of angles of the spatially modulated light formed from the hologram of the image. When the spatially modulated light is observed as a whole, there is not necessarily any evidence of a plurality of discrete light channels.

of diffractive or diffracted light field.

In accordance with the present disclosure, pupil expansion is provided for diffracted or diffractive light, which may comprise diverging ray bundles. The diffracted light field may be defined by a "light cone". Thus, the size of the diffracted light field (as defined on a two-dimensional plane) increases with propagation distance from the corresponding diffractive structure (i.e. display device). It can be said that the pupil expander/s replicate the hologram or form at least one replica of the hologram, to convey that the light delivered to the viewer is spatially modulated in accordance with a hologram.

Light channelling The hologram formed in accordance with some embodiments, angularly-divides the image content to provide a plurality of hologram channels which may have a cross-sectional shape defined by an aperture of the optical system. The hologram is calculated to provide this channelling of the diffracted light field. In some embodiments, this is achieved during hologram calculation by considering an aperture (virtual or real) of the optical system, as described above.

Figures 2 and 3 show an example of this type of hologram that may be used in conjunction with a pupil expander as disclosed herein. However, this example should not be regarded as limiting with respect to the present disclosure.

Figure 2 shows an image 252 for projection comprising eight image areas/components, V1 to V8. Figure 2 shows eight image components by way of example only and the image 252 may be divided into any number of components. Figure 2 also shows an encoded light pattern 254 (i.e., hologram) that can reconstruct the image 252 -e.g., when transformed by the lens of a suitable viewing system. The encoded light pattern 254 comprises first to eighth sub-holograms or components, H1 to H8, corresponding to the first to eighth image components/areas, V1 to V8. Figure 2 further shows how a hologram may decompose the image content by angle. The hologram may therefore be characterised by the channelling of light that it performs. This is illustrated in Figure 3. Specifically, the hologram in this example directs light into a plurality of discrete areas. The discrete areas are discs in the example shown but other shapes are envisaged. The size and shape of the optimum disc may, after propagation through the waveguide, be related to the size and shape of an aperture of the optical system such as the entrance pupil of the viewing system.

Figure 4 shows a system 400, including a display device that displays a hologram that has been calculated as illustrated in Figures 2 and 3.

The system 400 comprises a display device, which in this arrangement comprises an LCOS 402. The LOOS 402 is arranged to display a modulation pattern (or 'diffractive pattern') comprising the hologram and to project light that has been holographically encoded towards an eye 405 that comprises a pupil that acts as an aperture 404, a lens 409, and a retina (not shown) that acts as a viewing plane. There is a light source (not shown) arranged to illuminate the LCOS 402. The lens 409 of the eye 405 performs a hologram-to-image transformation. The light source may be of any suitable type. For example, it may comprise a laser light source.

The viewing system 400 further comprises a waveguide 408 positioned between the LCOS 402 and the eye 405. The presence of the waveguide 408 enables all angular content from the LCOS 402 to be received by the eye, even at the relatively large projection distance shown. This is because the waveguide 508 acts as a pupil expander, in a manner that is well known and so is described only briefly herein.

In brief, the waveguide 408 shown in Figure 4 comprises a substantially elongate formation. In this example, the waveguide 408 comprises an optical slab of refractive material, but other types of waveguide are also well known and may be used. The waveguide 408 is located so as to intersect the light cone (i.e., the diffracted light field) that is projected from the LCOS 402, for example at an oblique angle. In this example, the size, location, and position of the waveguide 408 are configured to ensure that light from each of the eight ray bundles, within the light cone, enters the waveguide 408. Light from the light cone enters the waveguide 408 via its first planar surface (located nearest the LCOS 402) and is guided at least partially along the length of the waveguide 408, before being emitted via its second planar surface, substantially opposite the first surface (located nearest the eye). As will be well understood, the second planar surface is partially reflective, partially transmissive. In other words, when each ray of light travels within the waveguide 408 from the first planar surface and hits the second planar surface, some of the light will be transmitted out of the waveguide 408 and some will be reflected by the second planar surface, back towards the first planar surface.

The first planar surface is reflective, such that all light that hits it, from within the waveguide 408, will be reflected back towards the second planar surface. Therefore, some of the light may simply be refracted between the two planar surfaces of the waveguide 408 before being transmitted, whilst other light may be reflected; and thus may undergo one or more reflections, (or 'bounces') between the planar surfaces of the waveguide 408, before being transmitted.

Figure 4 shows a total of nine "bounce" points, BO to B8, along the length of the waveguide 408. Although light relating to all points of the image (V1-V8) as shown in Figure 2 is transmitted out of the waveguide at each "bounce" from the second planar surface of the waveguide 408, only the light from one angular part of the image (e.g. light of one of V1 to V8) has a trajectory that enables it to reach the eye 405, from each respective "bounce" point, BO to B8. Moreover, light from a different angular part of the image, V1 to V8, reaches the eye 405 from each respective "bounce" point. Therefore, each angular channel of encoded light reaches the eye only once, from the waveguide 408, in the example of Figure 4.

The waveguide 408 forms a plurality of replicas of the hologram, at the respective "bounce" points B1 to B8 along its length, corresponding to the direction of pupil expansion. As shown in Figure 5, the plurality of replicas may be extrapolated back, in a straight line, to a corresponding plurality of replica or virtual display devices 402'. This process corresponds to the step of "unfolding" an optical path within the waveguide, so that a light ray of a replica is extrapolated back to a "virtual surface" without internal reflection within the waveguide. Thus, the light of the expanded exit pupil may be considered to originate from a virtual surface (also called an "extended modulator" herein) comprising the display device 402 and the replica display devices 402'.

Although virtual images, which require the eye to transform received modulated light in order to form a perceived image, have generally been discussed herein, the methods and arrangements described herein can be applied to real images.

Two-Dimensional Pupil Expansion Whilst the arrangement shown in Figure 4 includes a single waveguide that provides pupil expansion in one dimension, pupil expansion can be provided in more than one dimension, for example in two dimensions. Moreover, whilst the example in Figure 4 uses a hologram that has been calculated to create channels of light, each corresponding to a different portion of an image, the present disclosure and the systems that are described herebelow are not limited to such a hologram type.

Figure 5A shows a perspective view of a system 500 comprising two replicators, 504, 506 arranged for expanding a light beam 502 in two dimensions.

In the system 500 of Figure 5A, the first replicator 504 comprises a first pair of surfaces, stacked parallel to one another, and arranged to provide replication -or, pupil expansion in a similar manner to the waveguide 408 of Figure 4. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially elongate in one direction. The collimated light beam 502 is directed towards an input on the first replicator 504. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), which will be familiar to the skilled reader, light of the light beam 502 is replicated in a first direction, along the length of the first replicator 504.

Thus, a first plurality of replica light beams 508 is emitted from the first replicator 504, towards the second replicator 506.

The second replicator 506 comprises a second pair of surfaces stacked parallel to one another, arranged to receive each of the collimated light beams of the first plurality of light beams 508 and further arranged to provide replication -or, pupil expansion -by expanding each of those light beams in a second direction, substantially orthogonal to the first direction. The first pair of surfaces are similarly (in some cases, identically) sized and shaped to one another and are substantially rectangular. The rectangular shape is implemented for the second replicator in order for it to have length along the first direction, in order to receive the first plurality of light beams 508, and to have length along the second, orthogonal direction, in order to provide replication in that second direction. Due to a process of internal reflection between the two surfaces, and partial transmission of light from each of a plurality of output points on one of the surfaces (the upper surface, as shown in Figure 5A), light of each light beam within the first plurality of light beams 508 is replicated in the second direction. Thus, a second plurality of light beams 510 is emitted from the second replicator 506, wherein the second plurality of light beams 510 comprises replicas of the input light beam 502 along each of the first direction and the second direction. Thus, the second plurality of light beams 510 may be regarded as comprising a two-dimensional grid, or array, of replica light beams.

Thus, it can be said that the first and second replicators 504, 505 of Figure 5A combine to provide a two-dimensional replicator (or, "two-dimensional pupil expander"). Thus, the replica light beams 510 may be emitted along an optical path to an expanded eye-box of a display system, such as a head-up display.

In the system of Figure 5A, the first replicator 504 is a waveguide comprising a pair of elongate rectilinear reflective surfaces, stacked parallel to one another, and, similarly, the second replicator 504 is a waveguide comprising a pair of rectangular reflective surfaces, stacked parallel to one another. In other systems, the first replicator may be a solid elongate rectilinear waveguide and the second replicator may be a solid planar rectangular shaped waveguide, wherein each waveguide comprises an optically transparent solid material such as glass. In this case, the pair of parallel reflective surfaces are formed by a pair of opposed major sidewalls optionally comprising respective reflective and reflective-transmissive surface coatings, familiar to the skilled reader.

Figure 5B shows a perspective view of a system 500 comprising two replicators. 520, 540 arranged for replicating a light beam 522 in two dimensions, in which the first replicator is a solid elongated waveguide 520 and the second replicator is a solid planar waveguide 540.

In the system of Figure 5B, the first replicator/waveguide 520 is arranged so that its pair of elongate parallel reflective surfaces 524a, 524b are perpendicular to the plane of the second replicator/ waveguide 540. Accordingly, the system comprises an optical coupler arranged to couple light from an output port of first replicator 520 into an input port of the second replicator 540. In the illustrated arrangement, the optical coupler is a planar/fold mirror 530 arranged to fold or turn the optical path of light to achieve the required optical coupling from the first replicator to the second replicator. As shown in Figure 5B, the mirror 530 is arranged to receive light -comprising a one-dimensional array of replicas extending in the first dimension -from the output port! reflective-transmissive surface 524a of the first replicator/waveguide 520. The mirror 530 is tilted so as to redirect the received light onto an optical path to an input port in the (fully) reflective surface of second replicator 540 at an angle to provide waveguiding and replica formation, along its length in the second dimension. It will be appreciated that the mirror 530 is one example of an optical element that can redirect the light in the manner shown, and that one or more other elements may be used instead, to perform this task.

In the illustrated arrangement, the (partially) reflective-transmissive surface 524a of the first replicator 520 is adjacent the input port of the first replicator/waveguide 520 that receives input beam 522 at an angle to provide waveguiding and replica formation, along its length in the first dimension. Thus, the input port of first replicator/waveguide 520 is positioned at an input end thereof at the same surface as the reflective-transmissive surface 524a. The skilled reader will understand that the input port of the first replicator/waveguide 520 may be at any other suitable position.

Accordingly, the arrangement of Figure 5B enables the first replicator 520 and the mirror 530 to be provided as part of a first relatively thin layer in a plane in the first and third dimensions (illustrated as an x-z plane). In particular, the size or "height" of a first planar layer -in which the first replicator 520 is located -in the second dimension (illustrated as the y dimension) is reduced. The mirror 530 is configured to direct the light away from a first layer/plane, in which the first replicator 520 is located (i.e. the "first planar layer"), and direct it towards a second layer/plane, located above and substantially parallel to the first layer/plane, in which the second replicator 540 is located (i.e. a "second planar layer"). Thus, the overall size or "height" of the system -comprising the first and second replicators 520, 540 and the mirror 530 located in the stacked first and second planar layers in the first and third dimensions (illustrated as an x-z plane) -in the second dimension (illustrated as the y dimension) is compact. The skilled reader will understand that many variations of the arrangement of Figure 5B for implementing the present disclosure are possible and contemplated.

The image projector may be arranged to project a diverging or diffracted light field. In some embodiments, the light field is encoded with a hologram. In some embodiments, the diffracted light field comprises diverging ray bundles. In some embodiments, the image formed by the diffracted light field is a virtual image.

In some embodiments, the first pair of parallel / complementary surfaces are elongate or elongated surfaces, being relatively long along a first dimension and relatively short along a second dimension, for example being relatively short along each of two other dimensions, with each dimension being substantially orthogonal to each of the respective others. The process of reflection/transmission of the light between/from the first pair of parallel surfaces is arranged to cause the light to propagate within the first waveguide pupil expander, with the general direction of light propagation being in the direction along which the first waveguide pupil expander is relatively long (i.e., in its "elongate" direction).

There is disclosed herein a system that forms an image using diffracted light and provides an eye-box size and field of view suitable for real-world application -e.g. in the automotive industry by way of a head-up display. The diffracted light is light forming a holographic reconstruction of the image from a diffractive structure -e.g. hologram such as a Fourier or Fresnel hologram. The use diffraction and a diffractive structure necessitates a display device with a high density of very small pixels (e.g. 1 micrometer) -which, in practice, means a small display device (e.g. 1 cm). The inventors have addressed a problem of how to provide 2D pupil expansion with a diffracted light field e.g. diffracted light comprising diverging (not collimated) ray bundles.

In some embodiments, the display system comprises a display device -such as a pixelated display device, for example a spatial light modulator (SLM) or Liquid Crystal on Silicon (LCoS) SLM -which is arranged to provide or form the diffracted or diverging light. In such aspects, the aperture of the spatial light modulator (SLM) is a limiting aperture of the system. That is, the aperture of the spatial light modulator -more specifically, the size of the area delimiting the array of light modulating pixels comprised within the SLM -determines the size (e.g. spatial extent) of the light ray bundle that can exit the system. In accordance with this disclosure, it is stated that the exit pupil of the system is expanded to reflect that the exit pupil of the system (that is limited by the small display device having a pixel size for light diffraction) is made larger or bigger or greater in spatial extend by the use of at least one pupil expander.

The diffracted or diverging light field may be said to have "a light field size", defined in a direction substantially orthogonal to a propagation direction of the light field. Because the light is diffracted / diverging, the light field size increases with propagation distance.

In some embodiments, the diffracted light field is spatially-modulated in accordance with a hologram. In other words, in such aspects, the diffractive light field comprises a "holographic light field". The hologram may be displayed on a pixelated display device. The hologram may be a computer-generated hologram (CGH). It may be a Fourier hologram or a Fresnel hologram or a point-cloud hologram or any other suitable type of hologram. The hologram may, optionally, be calculated so as to form channels of hologram light, with each channel corresponding to a different respective portion of an image that is intended to be viewed (or perceived, if it is a virtual image) by the viewer. The pixelated display device may be configured to display a plurality of different holograms, in succession or in sequence. Each of the aspects and embodiments disclosed herein may be applied to the display of multiple holograms.

The output port of the first waveguide pupil expander may be coupled to an input port of a second waveguide pupil expander. The second waveguide pupil expander may be arranged to guide the diffracted light field -including some of, preferably most of, preferably all of, the replicas of the light field that are output by the first waveguide pupil expander -from its input port to a respective output port by internal reflection between a third pair of parallel surfaces of the second waveguide pupil expander.

The first waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a first direction and the second waveguide pupil expander may be arranged to provide pupil expansion, or replication, in a second, different direction. The second direction may be substantially orthogonal to the first direction. The second waveguide pupil expander may be arranged to preserve the pupil expansion that the first waveguide pupil expander has provided in the first direction and to expand (or, replicate) some of, preferably most of, preferably all of, the replicas that it receives from the first waveguide pupil expander in the second, different direction. The second waveguide pupil expander may be arranged to receive the light field directly or indirectly from the first waveguide pupil expander. One or more other elements may be provided along the propagation path of the light field between the first and second waveguide pupil expanders.

The first waveguide pupil expander may be substantially elongated and the second waveguide pupil expander may be substantially planar. The elongated shape of the first waveguide pupil expander may be defined by a length along a first dimension. The planar, or rectangular, shape of the second waveguide pupil expander may be defined by a length along a first dimension and a width, or breadth, along a second dimension substantially orthogonal to the first dimension. A size, or length, of the first waveguide pupil expander along its first dimension make correspond to the length or width of the second waveguide pupil expander along its first or second dimension, respectively. A first surface of the pair of parallel surfaces of the second waveguide pupil expander, which comprises its input port, may be shaped, sized, and/or located so as to correspond to an area defined by the output port on the first surface of the pair of parallel surfaces on the first waveguide pupil expander, such that the second waveguide pupil expander is arranged to receive each of the replicas output by the first waveguide pupil expander.

The first and second waveguide pupil expander may collectively provide pupil expansion in a first direction and in a second direction perpendicular to the first direction, optionally, wherein a plane containing the first and second directions is substantially parallel to a plane of the second waveguide pupil expander. In other words, the first and second dimensions that respectively define the length and breadth of the second waveguide pupil expander may be parallel to the first and second directions, respectively, (or to the second and first directions, respectively) in which the waveguide pupil expanders provide pupil expansion. The combination of the first waveguide pupil expander and the second waveguide pupil expander may be generally referred to as being a "pupil expander".

It may be said that the expansion/replication provided by the first and second waveguide expanders has the effect of expanding an exit pupil of the display system in each of two directions. An area defined by the expanded exit pupil may, in turn define an expanded eye-box area, from which the viewer can receive light of the input diffracted or diverging light field. The eye-box area may be said to be located on, or to define, a viewing plane.

The two directions in which the exit pupil is expanded may be coplanar with, or parallel to, the first and second directions in which the first and second waveguide pupil expanders provide replication/expansion. Alternatively, in arrangements that comprise other elements such as an optical combiner, for example the windscreen (or, windshield) of a vehicle, the exit pupil may be regarded as being an exit pupil from that other element, such as from the windscreen. In such arrangements, the exit pupil may be non-coplanar and non-parallel with the first and second directions in which the first and second waveguide pupil expanders provide replication/expansion. For example, the exit pupil may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication/expansion.

The viewing plane, and/or the eye-box area, may be non-coplanar or non-parallel to the first and second directions in which the first and second waveguide pupil expanders provide replication/expansion. For example, a viewing plane may be substantially perpendicular to the first and second directions in which the first and second waveguide pupil expanders provide replication/expansion.

In order to provide suitable launch conditions to achieve internal reflection within the first and second waveguide pupil expanders, an elongate dimension of the first waveguide pupil expander may be tilted relative to the first and second dimensions of the second waveguide pupil expander.

Combiner shape compensation An advantage of projecting a hologram to the eye-box is that optical compensation can be encoded in the hologram (see, for example, European patent 2936252 incorporated herein by herein). The present disclosure is compatible with holograms that compensate for the complex curvature of an optical combiner used as part of the projection system. In some embodiments, the optical combiner is the windscreen of a vehicle. Full details of this approach are provided in European patent 2936252 and are not repeated here because the detailed features of those systems and methods are not essential to the new teaching of this disclosure herein and are merely exemplary of configurations that benefit from the teachings of the present disclosure.

Control device The present disclosure is also compatible with optical configurations that include a control device (e.g. light shuttering device) to control the delivery of light from a light channelling hologram to the viewer. The holographic projector may further comprise a control device arranged to control the delivery of angular channels to the eye-box position. British patent application 2108456.1, filed 14 June 2021 and incorporated herein by reference, discloses the at least one waveguide pupil expander and control device. The reader will understand from at least this prior disclosure that the optical configuration of the control device is fundamentally based upon the eye-box position of the user and is compatible with any hologram calculation method that achieves the light channeling described herein. It may be said that the control device is a light shuttering or aperturing device. The light shuttering device may comprise a 1 D array of apertures or windows, wherein each aperture or window independently switchable between a light transmissive and a light non-transmissive state in order to control the deliver of hologram light channels, and their replicas, to the eye-box.

Each aperture or window may comprise a plurality of liquid crystal cells or pixels.

Turning laver Figures 6A and 6B illustrate a pupil replication system 600 in accordance with some embodiments. The pupil replication system 600 comprises a first replicator 604, a second replicator 606 (not shown in Figure 6A), a triangular wedge 612, and a turn mirror 614.

Diffracted input light 602 is coupled into the first replicator 604 at an acute angle of incidence in order to cause light to propagate along the first replicator 604 arid therefore allow for pupil expansion. The first replicator 604 and the second replicator 606 are waveguides, each waveguide configured to expand light in a single dimension. The first replicator 604 and the second replicator 606 act together to expand or replicate the exit pupil of the display in a horizontal and vertical dimension, such that the eye box or viewing area is increased.

Turn mirror 614 directs light towards the second replicator 606 (not shown in Figure 6A). The triangular wedge 612 is located in an optical path between the turn mirror 614 and the first replicator 602. The triangular wedge 612 imparts a change in angle of the rays exiting the first replicator 612 in order to enable stacking of the replicated pupils in the second replicator 606. The triangular wedge 612 is a prism comprising at least two optical surfaces. An optical surface is a surface is a surface of the prism that is intended to receive or transmit light.

Figure 6B illustrates a side view of the pupil replication system 600 as described in relation to Figure 6A. An aperture device 616 is shown located in front of the input region or surface of the second replicator 606. The aperture device 616 may be used to prevent cross-talk between channels, as described in GB2108456.1 filed on 14 June 2021, the contents of which are herein incorporated by reference. In other arrangements, the aperture device 616 may be located after the output region or surface of the second replicator 606. The aperture device 616 may also be referred to as a switching device.

As can be seen in Figures 6A and 6B and in general, propagation refers to a general or group direction of light propagation in the waveguide. The direction of propagation may also be referred to as the optical axis (or plane) of the waveguide.

The triangular wedge 612 adds mass to the pupil replication system 600, and may also increase the gap between the first replicator 604 and the second replicator 606. The inventors have recognised that it may be replaced by a light turning component comprising a "turning layer". The turning layer may be conveniently applied to the aperture device 616, or another component in the system.

A system comprising a light turning component is illustrated in Figure 7. The system is similar to the system shown in Figure 6A and 6B, where the triangular wedge 612 is replaced by a light turning component 718. The system comprises a first replicator 704, the first replicator 704 configured to replicate a pupil of light such that the eye box or viewing area is expanded in a first dimension. In order to achieve the replication or 'bounce', diffracted light 702 may be input at an acute angle relative to the optical axis of the first replicator 704.

The turning layer of the light turning component 718 turns the ray direction of light output from first replicator 704 such that light is correctly orientated to be input into fold mirror 714 and in turn a second replicator (not shown in Figure 7). The second replicator is configured to replicate light such that the eye box or viewing area is expanded in a second dimension orthogonal to the first dimension.

The use of a turning layer allows the turn mirror 714 and first replicator to be oriented substantially parallel, reducing the volume of the system. This may be particularly useful when the system is used in a part of HUD system in a vehicle, where space may be limited. The replacement of the triangular wedge 612 also allows for the first replicator 704 and the fold mirror 714 to be located closer together. Reducing the space between devices not only reduces the volume, but also improves coupling between devices. Light will diverge when passing through an air gap, so reducing the gap reduces the amount of dispersion and therefore improves the overall efficiency of the system.

A second replicator (not shown in Figure 7) receives light from turn mirror 714 and outputs light towards an output. The output may comprise a user, or may comprise a combiner or other type of display screen.

In some embodiments the turning layer may comprise a layer of microstructures. In some embodiments the turning layer may be applied to an optical element 718 to form a light turning component. In some embodiments the turning layer may be formed directly on the optical element 718. In some embodiments the layer of microstructures may be formed separate from the optical element 718 then bonded to the optical element 718.

Figure 8A illustrates an alternative system according to some embodiments. The alternative system is similar to the system shown in Figure 7, where that the optical element 718 comprises a switching device 820. The switching device 820 may be used to prevent cross-talk between channels, as described in GB2108456.1 filed on 14 June 2021, the contents of which are herein incorporated by reference.

Diffracted light 802 is input into first replicator 804 at an acute angle relative to the optical axis of the first replicator 804. Light in first replicator 804 propagates along the first replicator 804 and is replicated such that the eye box or viewing area is expanded in a first dimension.

Light is output by the first replicator 804 at an acute angle towards the switching device 820. The switching device 820 is used to prevent cross talk between channels and may be particularly useful when using holographic or diffraction-based systems as described in more detail in GB2108456.1.

A turning layer may be applied in two parts to the switching device 820, the first turning layer 818 may be applied to the part of the switching device 820 facing the output of the first replicator 804, and the first turning layer 818 may be configured to turn a ray direction of the light output from the first replicator 804 such that the angle is correct for input into the second replicator 806. As can be recognised from the description, the first turning layer performs substantially the same function as the triangular wedge 612 shown in Figures 6A and 6B.

A second turning layer 818' is applied to the part of the switching device 820 facing the input of the second replicator 806. Light in the second replicator 806 propagates along the second replicator 806 and is replicated such that the eye box or viewing area is expanded in a second dimension, orthogonal to the first dimension. The second turning layer 818' turns the rays exiting the switching device 820 towards the input area of the second replicator 806.

The turning direction of the second turning layer 818' may be orthogonal to the turning direction of the first turning layer 818. As can be recognised from the description, the second turning layer 818' performs substantially the same function as the fold mirror 714 as shown in Figure 7A.

The second replicator 806 outputs light towards an output 822. The output may comprise a user, or may comprise a combiner or other type of display screen.

The alternative system described with reference to Figures 8A and 8B replaces both the triangular wedge 612 and the turning mirror 614 shown in Figure 6B. This may result in a less bulky and lighter system in comparison to both the system of Figures 6A, 6B and 7.

Furthermore, the first replicator 604 and the second replicator 606 may be located closer together increasing the coupling efficiency and the quality of the image displayed to the user.

Figure 9 illustrates another alternative device, comprising a first replicator 904 to receive input diffracted light 902 and replicate the light such that the eye box or viewing area is expanded in a first dimension. Light is output towards a switching device 920 comprising a turning layer 918. Turning layer 918 performs the function of turning mirror 614 and triangular wedge 612. Turning layer 918 both sets the appropriate angle into the second replicator 906 and turns the light towards the second replicator 906. Second replicator replicates 906 light such that the eye box or viewing area is expanded in a second dimension orthogonal to the first dimension.

The second replicator 906 outputs light towards an output 922. The output may comprise a user, or may comprise a combiner or other type of display screen.

The device illustrated in Figure 9 is substantially similar to the alternative device described with relation to Figures SA and 8B, where the first turning layer and second turning layer are applied as the turning layer in a single layer 918. The turning layer 918 may comprise two layers formed on top of each other, forming a single layer. The single layer may be etched in one or more discrete processes.

Light Turning Component Figures 10 and 11 show a light turning component 1018 in accordance with the present

disclosure.

The light turning component 1018 comprises a two-layered optically transparent structure comprising a first layer 1020 and a second layer 1030, arranged (or stacked) over the first layer 1020.

Each of the first and second layers 1020, 1030 comprises an array of microstructures in the form of a one-dimensional array of cylindrical lenslets. In particular, the first layer 1020 comprises a first lens array 1022 comprising a plurality of first cylindrical lenslets 1024. The first cylindrical lenslets 1024 are arranged as a one dimensional array with their (cylinder/geometric) axes extending in a first dimension (illustrated as the x dimension) and the array extending in a second dimension (illustrated as the y dimension), wherein the second dimension is orthogonal to the first dimension. Thus, the first cylindrical lenslets 1024 are elongate in the first dimension and their (cylinder/geometric) axes are substantially parallel. Similarly, the second layer 1030 comprises a second lens array 1032 comprising a plurality of second cylindrical lenslets 1034. The second cylindrical lenslets 1034 are arranged as a one dimensional array with their axes 1036 extending in a first dimension (illustrated as the x dimension) and the array extending in a second dimension (illustrated as the y dimension). Thus, the second cylindrical lenslets 1034 are elongate in the first dimension and their axes 1026, are substantially parallel.

The first cylindrical lenslets 1024 are arranged with adjacent lenslets in abutment in their elongate direction (i.e., without spatial separation between adjacent lenses in the second dimension) so as to form a continuous first layer 1020. Similarly, and the second cylindrical lenslets 1034 are arranged with adjacent lenslets in abutment in their elongate direction so as to form a continuous second layer 1030. Accordingly, the first and second lens arrays 1022, 1032 forming respective first and second layers 1020, 1030, each having a width and length in the first and second dimensions and a depth in a third dimension (illustrated as the z dimension) orthogonal to the first and second dimensions.

The first lens array 1022 comprises an internal surface 1022A and an external surface 1022B, and the second lens array 1032 comprises an internal surface 1032A and an external surface 1032B. The internal surface 1022A of the first lens array 1022 is adjacent to, or abuts, the internal surface 1032A of the second lens array 1032. In the illustrated arrangement, the first and second lenslets 1024, 1034 of first and second lens arrays 1022, 1032 have opposed planar and curved/cylindrical surfaces. The first and second lens arrays 1022, 1032 are arranged so that the planar surfaces of the first and second lenslets 1024, 1034 form the internal surfaces 1022A, 1032A of the first and second lens arrays 1022, 1032. Accordingly, the curved/lensing surfaces of the first and second lenslets 1024, 1034 form the external surfaces 1022B, 1032B of the first and second lens arrays 1022, 1032.

As shown by the illustrative rays in Figures 10 and 11, light propagates through the light turning component from the external surface 1022B of the first layer 1020 to the external surface 1032B of the second layer 1030. It may be said that light propagates through the light turning component (mostly) in the third dimension. Accordingly, the external surface 1022B of the first layer 1020, comprising the curved/lensing surfaces of first cylindrical lenslets 1034, forms an input port of the light turning component. Similarly, the external surface 1032B of the second layer 1030, comprising the curved/lensing surfaces of the second cylindrical lenslets 1034, forms an output port of the light turning component.

In the illustrated arrangement, the shape of the curved/lensing surface of each of the first cylindrical lenslets 1024 is substantially the same, and the shape of the curved/lensing surface of each of the second cylindrical lenslets 1034 is substantially the same. Thus, the lenslets of each of the first and second lens arrays 1022, 1032 provide the same optical power. In addition, in the illustrated arrangement, the shape of the curved/lensing surface of each of the first cylindrical lenslets 1024 is substantially the same as the shape of the curved/lensing surface of each of the second cylindrical lenses 1034. For example, the first and second lens arrays may comprise identical first and second cylindrical lenses 1024, 1034. Thus, the optical power of the lenslets of the first and second lens arrays 1022, 1032 is matched. The skilled person will appreciate that other arrangements may comprises first and second arrays of lenslets with different optical power, and the lenslets in each of the first and second arrays may have different optical power, according to application requirements.

The first and second cylindrical lenslets 1024, 1034 are arranged to provide optical power in the second dimension, and not in the first dimension. It may be said that the first and second cylindrical lenslets 1024, 1034 are arranged to provide optical power only in the second dimension.

Figure 11 is a cross section of the light turning component of Figure 10, showing a pair of lenslets comprising a first cylindrical lenslet 1024 and a second cylindrical lenslet. The drawing is in a plane of the second and third dimensions (shown as the y and z dimensions in Figure 10). First cylindrical lenslet 102 has an optical plane 1026 and second cylindrical lenslet has an optical plane 1036. As the skilled person will appreciate, the "optical planes" 1026, 1036 of the cylindrical lenslets 1024, 1034 shown in Figure 11, which are analogous to an "optical axis" of a spherical lens, extend in the first, elongate dimension and orthogonally bisect the apex of the curved/lensing surface in the third dimension. Thus, the optical planes 1026, 1036 are geometric planes in the first and third dimensions that contain/align with the cylinder/geometric axes of the lenslets 1024, 1034, which extend in the first dimension. The optical plane 1026 of the first lenslet 1024 also contains/aligns with the focal line F formed at its planar surface 1022A. Each of the lenslets 1024, 1034 has a pair of parallel sidewalls defining a depth in the third dimension. As the skilled person will appreciate, individual lenslets may be formed with straight sidewalls in the shape illustrated in Figure 11. Alternatively, an array of individual lenslets may mounted on, and/or integrated with, a shared parallelepiped-shaped optically transparent substrate for the respective array, so that each lenslet may be associated with a part of the substrate as illustrated in Figure 11. In the illustrated arrangement, the depth of each lenslet in the third dimension (as measured from the apex of the curved/lensing surface to the planar surface) may substantially correspond to the focal distance thereof.

The axis of each of the first cylindrical lenslets 1024 is offset from the axis of each of the (adjacent/neighbouring) second cylindrical lenslets 1034 by a distance o in the second dimension. This is illustrated by the distance between the optical plane 1026 of the first cylindrical lenslet 1024 and the optical plane 1036 of the second cylindrical lenslet in Figure 11. It may be said that the first lens array 1022 is offset from the second lens array 1032 by a distance o in the second dimension or a fraction of the lens width in the second dimension. In the illustrated example, the offset o is about 50% of the dimension of the lenslets in the second dimension. By virtue of this offset between the first and second cylindrical lenslets 1024, 1034 in the second dimension (illustrated as the y dimension), light propagating through the first and second layers of the light turning component -in a direction corresponding to, or including a component in, the third dimension (illustrated as the z dimension) -may be turned in a direction in the second dimension (illustrated as the y dimension) as shown in Figure 11.

Since the axes of the second cylindrical lenslets 1034 are offset from the axes of the first cylindrical lenslets 1024, the direction of light propagating through light turning component is changed by virtue of the refractive effect of the offset lenslets 1024, 1034 of each pair of lenslets.

In the embodiment of Figures 10 and 11, the curved/lensing surface of the first and second cylindrical lenslets 1024, 1034 comprises a fraction (or quadrant) of a cylindrical lens, specifically just over half of a cylindrical lens. In particular, the cylindrical lenslets 1024, 1034 comprise about 60% of the curved/lensing surface. Fractions o cylindrical lenses greater than half in the range of 60-80% are contemplated. Thus, the curved/lensing surfaces of the first and second cylindrical lenslets 1024, 1034 have an asymmetric geometry. It may be said that (the curved/lensing surface of) each cylindrical lenslet is asymmetric in the second direction. In other embodiments, the fraction may be less than half the curved/lensing surface of a cylindrical lens. As the skilled person will appreciate, it is also possible to use lenslets comprising the complete curved/lensing surface or other fractions thereof according to design requirements.

The optical power of the first and second lenslets 1024, 1034 is chosen according to application requirements, for example the light turning effect and/or the geometry required.

Figure 11 shows light incident on the input surface 1022B of the light turning component in a direction that is normal to the first and second layers 1020, 1030 is output from the output surface 1032B in a direction that is off-normal. In particular, the light is turned in the positive direction of the second dimension (illustrated as the y dimension in Figure 10). It may be said that the light turning layer has a turning effect on light. However, unlike an array of prisms that has inclined surfaces and sharp corners, the light turning component has smoothly curved/lensing surfaces. This may lead to a number of advantages, in particular light fewer edge artefacts and gaps/dead areas in the turned light, and reduced light losses resulting in improved optical efficiency.

The light turning component thus comprises a two-layer optical structure, which may be conveniently applied to a surface of an optical component for the input or output of light to be turned, such as the output surface of a waveguide pupil expander (or replicator) as described herein. In addition, the light turning component is readily compatible, and works synergistically, with aperture or switching devices and/or filtering layers, as described below.

In some embodiments, the light turning component is arranged to turn substantially coherent light. For example, the coherent light may be light that has been emitted by a laser. In some embodiments, a replicator may be arranged to replicate a substantially coherent diffractive light field in one direction. The light output by the first replicator may be substantially coherent light. It is desirable that the light remains substantially coherent after it has been received (and turned) by the turning component.

However, the inventors consider that the arrangement comprising first and second arrays of cylindrical lenslets may help to maintain coherence of light propagated through adjacent/neighbouring lenslets of the light turning component.

Figure 11 shows how a collimated/parallel light beam propagates through the pair of lenslets comprising first and second lenslets 1024,1034 such that the direction of the beam is changed or turned. In particular, light is incident on the external, curved/lensing surface of a first lenslet 1024 of the first lens array 1022. The light propagates through the first lenslet 1026 and is focussed at a focal line F at its internal, planar surface 1022A which forms an interface with the planar surface 1032A of the second lenslet 1034. The light then diverges from the focal line F, propagates through the second lenslet 1034 and exits its external, curved/lensing surface. As a result of (i) the offset o between first and second lenslets 1024, 1034 in the second dimension and (ii) refraction at the curved/lensing surface of second lenslet 1034, the light is turned in a direction of the second dimension, as shown be the illustrative light rays. As the skilled person will appreciate, a similar turning effect could be achieved when the incident light rays are off-normal, for example light incident in a direction with a component in the second dimension as well as a component the third dimension.

In some embodiments, each of the above described pair of lenslets 1024, 1034 of the light turning component may receive and propagate a diffracted light field, such as a replica of a diffracted light field output by a pupil expander.

The light turning component may be used in conjunction with an aperture device, such as the aperture device 616 of Figure 6B or the switching devices 820 and 920 of Figures 8A and 8B and 9A and 9B, respectively. Alternatively, the aperture device may be located at the output port of surface of a second pupil replicator. In such an arrangement, an array of cells of an aperture device, comprising cells that are switchable between a first state and second state, may be arranged between the first and second layers 1020, 1030 of the light turning component, so that each cell is located at one of the focal lines F of the first lenslets 1024. Such an arrangement allows for a reduction in the size of the array of cells, including optimising the size of the individual cells, and allowing space for routing of wires. In addition, the arrangement mitigates problems associated with large "inter-pixel" gaps between the cells of the array because light is focussed onto the cells, and thus away from the adjacent inter-pixel areas. As the skilled person will appreciate, a gap will typically be required between the planar surfaces of the first and second lens arrays 1022, 1032 to accommodate the aperture device, and the first lenslets 1024 will focus light rays into a line at the centre of the gap, beyond its planar surface.

A similar arrangement is envisaged for a layer to filter stray light, such as sunlight, incident on the external surface 1032B of second lens array 1032,. In particular, a filter layer may be arranged between the first and second layers 1020, 1030 of the light turning component, as described above, to filter out light incident from the second lens array 1032 whilst allowing transmission of light from the first lens array 1022.

In some embodiments each layer of the light turning component may be manufactured as a sheet of cylindrical lenslets or similar microstructures, and assembled together to form the light turning component. Thus, the light turning component may be manufactured separately from an optical element (e.g., pupil replicator) and attached (e.g. by bonding) to the optical element. In some embodiments the light turning component may be formed directly on the optical element.

In summary, the light turning component comprising a two layer optical structure is provided.

Each layer comprises an array of microstructures comprising curved/lensing surfaces (e.g., cylindrical lenses). In some embodiments, the light turning component may be substantially planar and extend in the first and second dimensions. In some embodiments, the first dimension may be substantially horizontal and the second dimension may be substantially vertical. The lenses of the first and second layers may have a depth in a third dimension. Light propagates through the first and second layers of the light turning component. The light turning component may advantageously be arranged to change (or turn) the ray direction of the replicas of a light field (e.g. a diffracted/holographic light field) output from a pupil replicator of a display system, according to application requirements.

A light turning component may be described as an optical component that turns or rotates a propagation axis of light such that the propagation axis of light input to the turning layer and the propagation axis of light output from the turning layer are not parallel. In this context, the propagation axis refers to the overall or group propagation direction of light, such as an axis at the centre of a light beam. For a diffracted light field the input light rays may have a range of values, and therefore the turning layer may turn each of the rays a different amount. An amount of turning of the turning layer may be defined based on the magnitude of the overall turn in the propagation axis/direction of the diverging light beam.\ In some embodiments comprising a system including first and second pupil expanders as described herein, the light turning component may be arranged on the first pupil expander (or replicator), the second pupil expander (or replicator) or a combination of the first replicator and the second pupil expander (or replicator). This may reduce the size of the device, and also reduce the size of any gaps between elements of the display system. This in turn may reduce the amount of divergence of light in the gaps, and therefore lead to a more efficient system.

Display System There is disclosed herein a display system that forms an image using diffracted light and provides an eye-box size and field of view suitable for real-world application -e.g. in the automotive industry by way of a head-up display. The diffracted light is light forming a holographic reconstruction of the image from a diffractive structure -e.g. hologram such as a Fourier or Fresnel hologram or point cloud hologram. The use diffraction and a diffractive structure necessitates a display device with a high density of very small pixels (e.g. 1 micrometer) -which, in practice, means a small display device (e.g. 1 cm). The inventors have addressed a problem of how to provide 2D pupil expansion with a diffracted light field e.g. diffracted light comprising diverging (not collimated) ray bundles.

In some embodiments the light field is described as being a diffracted light field (or holographic), however it is to be understood that the systems described may be applicable to other types of light fields which are not diffracted light fields.

In some embodiments, the diffracted light field comprises diverging ray bundles. In some embodiments, the image formed by the diffracted light field is a virtual image.

In aspects, the display system comprises a display device -such as a pixelated display device, for example a spatial light modulator (SLM) or Liquid Crystal on Silicon (LCoS) SLM -which is arranged to provide or form the diffracted or diverging light. In such aspects, the aperture of the spatial light modulator (SLM) is a limiting aperture of the system. That is, the aperture of the spatial light modulator -more specifically, the size of the area delimiting the array of light modulating pixels comprised within the SLM -determines the size (e.g. spatial extent) of the light ray bundle that can exit the system. In accordance with this disclosure, it is stated that the exit pupil of the system is expanded to reflect that the exit pupil of the system (that is limited by the small display device having a pixel size for light diffraction) is made larger or bigger or greater in spatial extend by the use of at least one pupil expander.

In order to provide suitable launch conditions to achieve internal reflection within the first and second waveguide pupil expanders, an elongate dimension of the first waveguide pupil expander may be tilted relative to the first and second dimensions of the second waveguide pupil expander. This tilt between the first and second waveguide pupil expanders increases the volume occupied by the system, such as under the dashboard in an automotive applications. However, by providing a light turning component between the first and second waveguide pupil expanders as described herein, it is possible to reduce the required tilt, and thus provide a more compact system.

Additional features The methods and processes described herein may be embodied on a computer-readable medium. The term "computer-readable medium" includes a medium arranged to store data temporarily or permanently such as random-access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" shall also be taken to include any medium, or combination of multiple media, that is capable of storing instructions for execution by a machine such that the instructions, when executed by one or more processors, cause the machine to perform any one or more of the methodologies described herein, in whole or in part.

The term "computer-readable medium" also encompasses cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example form of a solid-state memory chip, an optical disc, a magnetic disc, or any suitable combination thereof. In some example embodiments, the instructions for execution may be communicated by a carrier medium. Examples of such a carrier medium include a transient medium (e.g., a propagating signal that communicates instructions).

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. The present disclosure covers all modifications and variations within the scope of the appended claims and their equivalents.

Claims

CLAIMS1. A light turning component, comprising: a first layer comprising a first lens array, wherein each lenslet of the first lens array has an axis extending in a first dimension; a second layer, arranged over the first layer, comprising a second lens array, wherein each lenslet of the second lens array has an axis extending in the first dimension, wherein the axis of each lenslet of the first lens array is offset from the axis of each adjacent lenslet of the second lens array, such that light propagating through the first and second layers is turned.
2. A light turning component as claimed in claim 1 wherein the lenslets of the first and second lens arrays are elongate along the first dimension.
3. A light turning component as claimed in claim 1 or 2 wherein the offset between the axis of each lens of the first lens array and the axis of each adjacent lens of the second lens array is in a second dimension, orthogonal to the first dimension, optionally wherein light propagating through the first and second layers is turned in a direction of the second dimension.
4. A light turning component as claimed in claim 3 wherein the first/second lens array comprises a one dimensional array of elongate lenslets extending in the second dimension.
5. A light turning component as claimed in any preceding claim wherein the lenslets of at least the first lens array have a depth in the third dimension, optionally wherein each lenslet is mounted on, and/or is integrated with, a parallelepiped substrate to provide a depth in the third dimension.
6. A light turning component as claim in claim 5 wherein the depth of each lenslet of at least the first lens array in the third dimension may substantially correspond to the focal distance of the lenslet.
7. A light turning component as claimed in any preceding claim wherein the lenslets of the first and second lens arrays are arranged in abutment.
8. A light turning component as claimed in any preceding claim wherein each of the first and second lens arrays comprises an array of cylindrical lenslets arranged to provide optical power in the second dimension, wherein the cylinder axis (or optical/plane) of each cylindrical lenslet of the first lens array is offset from the cylinder axis (or optical axis/plane) of each cylindrical lenslet of the second lens array in the second dimension.
9. A light turning component as claimed in claim 8 wherein: a curved/lensing surface of each lenslet of the first/second lens array comprises a fraction of surface of a cylindrical lens, optionally greater than half the surface of a cylindrical lens such as 60-80%, and/or a curved/lensing surface of each lenslet of the first/second lens array has an asymmetric geometry, optionally wherein each of the lenslets of the first/second lens array is asymmetric in the second direction.
10. A light turning component as claimed in any preceding claim wherein each lenslet of first/second lens array has opposed planar and curved/lensing surfaces.
11. A light turning component as claimed in claim 10 wherein the planar surfaces of the lenslets of the first and second lens arrays form the internal surface of the first and second layers and the curved/lensing surfaces of the lenslets of the first and second lens arrays form the external surface of the first and second layers, optionally wherein the planar surfaces of the lenslets of the first lens array are arranged in abutment with the planar surfaces of the lenslets of the second lens array.
12. A light turning component as claimed in any preceding claim wherein: a shape of a curved/lensing surface of the lenslets of the first lens array is substantially the same; a shape of a curved/lensing surface of the lenslets of the second array is substantially the same, and/or a shape of the curved/lensing surface of the lenslets of the first lens array is substantially the same as the shape of the curved/lensing surface of the lenslets of the second lens array.
13. A device comprising: a one dimensional array of array of cells, wherein each cell is independently switchable between a first state and a second state, and a light turning component as claimed in any preceding claim, wherein the one dimensional array of cells is arranged between the first and second layers of the light turning component.
14. A device as claimed in claim 13 wherein each cell of the one dimensional array of cells is arranged at a focus of a lenslets of the first lens array.
15. A device comprising: a light turning component as claimed in any preceding claim, and a filter layer, wherein the filter layer is arranged between the first and second layers of the light turning component.
16. A system comprising: a replicator arranged to receive a light field and replicate the light field in first dimension, and a light turning component as claimed in any one of claims 1 to 12 arranged to change the direction of output light from the replicator.
17. A system as claimed in claim 16 comprising: a first replicator arranged to receive a light field and replicate the light field in a first direction; a second replicator arranged to receive output light from the first replicator and replicate the light field in a second direction, the second direction substantially perpendicular to the first direction, wherein the light turning component is arranged to optically-couple output light from the first replicator to an input of the second replicator, wherein the turning layer is arranged to change the direction of output light from the first replicator.