WO2014009717A1

WO2014009717A1 - Head up displays

Info

Publication number: WO2014009717A1
Application number: PCT/GB2013/051816
Authority: WO
Inventors: Lilian Lacoste; Euan Christopher Smith
Original assignee: Light Blue Optics Ltd
Current assignee: Light Blue Optics Ltd
Priority date: 2012-07-10
Filing date: 2013-07-10
Publication date: 2014-01-16
Anticipated expiration: 2015-01-10
Also published as: GB201212270D0; GB2518315A

Description

Head Up Displays

FIELD OF THE INVENTION

This invention relates to apparatus and methods for expanding an eye box or exit pupil in a virtual image display system for application in, for example, a head up display (HUD).

BACKGROUND TO THE INVENTION

Some known techniques for HUD image expansion employ dual combiners but these requires good alignment and matching of the HUD combiners to avoid artefacts in the virtual images such as brightness overshoot or gaps. An example is shown in Figure 3a (the combiner is the surface combining the user's field of view and the projected image).

We have previously described in WO2010/092409 (hereby incorporated by reference in its entirety) optical techniques for replicating an image in a head-up display. We will now describe some related techniques which may be used with, but are not restricted to, such systems. We are particularly concerned with head-up displays but in principle applications of the techniques we describe are not limited to HUDs. Background prior art relating to light-guiding display systems can be found in WO2009/009268; US6,580,529; and WO2004/109349.

SUMMARY OF THE INVENTION

In broad terms the invention provides an eye box expander for a head-up display (HUD) based upon one or more Fresnel mirrors, that is repeated small structures extending over an surface, in embodiments wedge-like structures. In one approach a waveguide-based technique is employed; in another grazing angle beam expansion based on "sheering" a beam is used; these approaches may be combined. Optionally graded Fresnel reflection is employed to extract progressively more from a beam within the eye box expander as the beam progresses through the expander.

Thus in one aspect the invention provides an eye box expander for a head-up display (HUD), the eye box expander comprising: a pair of optical surfaces defining a region therebetween; and an optical input to receive into said region an input beam for expansion; wherein a first of said optical surfaces defines an output surface for an output beam of said eye box expander; and wherein a second of said optical surfaces comprises a structured surface having a plurality of reflecting facets, each configured to reflect a part of said input beam to turn the respective part of said input beam through an angle, such that in combination said reflecting facets direct an expanded version of said input beam through said output surface.

In embodiments the optical surfaces comprise planar parallel optical surfaces, in a waveguide-type approach reflecting surfaces. In embodiments these define substantially parallel planes spaced apart in a direction perpendicular to the parallel planes. In embodiments the facets are configured to turn the input beam through an acute angle, to direct an output beam substantially perpendicularly through the output surface. The magnification or expansion of the eye box may be determined by the degree of sheering applied to the input beam: a collimated input beam may be directed at an angle onto the structured surface to thereby define a region which, when viewed perpendicularly to the structured surface, is expanded, the degree of expansion being determined by the acute angle between the input beam and the structured surface. For a head-up display it is preferable that the spacing between the Fresnel reflections is smaller than the pupil size of the observer and thus in preferred embodiments each facet has a dimension of less than 2mm, 2mm or 1 mm.

In embodiments, particularly those employing a waveguide-type approach the reflecting facets may be disposed on the structured surface to define a variable reflectivity such that the reflectivity o the structured surface increases with increasing distance away from the optical input, for example by varying a fill factor of the Fresnel elements directing light towards the output surface. Such a waveguide-type embodiment may comprise a pair of substantially planar mirrors (1 bearing the structured, Fresnel surface) defining the outer optical surfaces of a waveguide configured such that light escapes from the waveguide through the output surface and reflected to provide a replicated version of the input image on reflection. In such an approach the front, output optical surface (mirror) may be configured to transmit a proportion of light (for example >0.1 %, >1 %, >10%, >50%) when reflecting light such that light for the replicated images escape through the front optical surface; the rear optical mirror surface may then be the structured, Fresnel surface. The partially reflective output surface may have a reflectivity of >35%, >50%, >90%, >95%, >99% or 99.9%.

In embodiments a further pair of optical services, again one having a structured, faceted surface, may be provided to provide two-stage beam expansion. In embodiments a first stage of beam expansion may be performed in a first direction, and a second subsequent stage of beam expansion in a second, perpendicular direction. In embodiments of this approach the optical surfaces of the first and second stages may be aligned so that they define substantially a pair of common planes. In such an arrangement the first stage of beam expansion may direct the beam expanded in one dimension into the second stage without the beam needing to leave an output optical surface of the first stage. The facets of a first stage of the beam expansion may be arranged to turn the expanded beam through 90° to provide an input to the region between the optical surfaces of the second stage of expansion.

The space between a pair of optical surfaces may be free space such as air or gas, or the beam expander may be fabricated as a solid element, the optical surfaces comprising outer surfaces of the solid element. In embodiments the beam expander may comprise a single, moulded hollow or solid block or formation of plastic.

The invention also provides a head-up display comprising an eye box expander as described above. Such a head-up display may comprise an image projector, and an optional light collimation system (if needed) to provide a beam of substantially collimated light bearing an image for expansion, to provide an input beam for the eye box expander.

Thus the invention also provides a method of expanding the eye box of a head-up display (HUD), the method comprising: providing a pair of optical surfaces defining a region therebetween, one of said optical surfaces comprising a structured surface having a plurality of reflecting facets; receiving a collimated input beam bearing a virtual image into said region; reflecting a part of said input beam from each said facet to turn the respective part of said input beam through an angle such that, in combination, said reflecting facets output an expanded version of said input beam. In a related aspect the invention provides a head up display comprising an eye box expander, said eye box expander comprising: a pair of optical surfaces defining a region therebetween, one of said optical surfaces comprising a structured surface having a plurality of reflecting facets; an optical input to receive a collimated input beam bearing a virtual image into said region; and configured to reflect a part of said input beam from each said facet to turn the respective part of said input beam through an angle such that, in combination, said reflecting facets output an expanded version of said input beam.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures, in which:

Figures 1 a to 1 d show, respectively, first and second example optical image replication systems with which embodiments of the invention may be used, a perspective view of the systems, and a perspective view of a pair of stacked pupil expanders; Figures 2a to 2b show, respectively, an example of a head-up display (HUD) incorporating an optical image replicator, and a vehicle rear-view mirror incorporating the HUD;

Figures 3a to 3c show, respectively, an HUD according to the prior art, a conceptual illustration of the operation of an embodiment of the invention, and a Fresnel reflector based HUD according to an embodiment of one aspect of the invention;

Figure 4 illustrates magnification in an eye box expansion system according to an embodiment of an aspect of the invention; Figure 5 shows an architecture of a HUD employing an eye box expander according to an embodiment of the invention; Figures 6a and 6b show a waveguide-type pupil expander employing Fresnel pupil expansion (FPE), and 6b a waveguide-type expander employing FPE together with optical axis correction, according to an embodiments of the invention;

Figures 7a and 7b show, respectively, eye box expanders according to embodiments of respective first and second aspects of the invention;

Figures 8a to 8c show a first example design of an eye box expander according to an embodiment of the invention, employing a two-stage waveguide-type expander; Figures 9a to 9c show a second example design of an eye box expander according to an embodiment of the invention, employing a first-stage grazing angle expander and a second stage waveguide-type expander;

Figures 10a to 10c show a third example design of an eye box expander according to an embodiment of the invention, employing a first-stage grazing angle expander and a second stage waveguide-type expander;

Figures 1 1 a to 1 1 c show a fourth example design of an eye box expander according to an embodiment of the invention, employing a two-stage grazing angle expander; and

Figure 12 shows a head-up display system incorporating a virtual image display system according to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It is helpful for understanding the invention to first describe examples of optical image replicators/head-up displays with which embodiments of the invention may be used: Image replicators and head-up displays

We have previously described an HUD image display system comprising: a laser or other light source; image generating optics coupled to the light source to provide a substantially collimated beam bearing an image; and image replication optics to replicate an image carried by the substantially collimated beam. The image replication optics comprises a pair of substantially planar reflecting optical surfaces defining substantially parallel planes spaced apart in a direction perpendicular to said parallel planes, the surfaces comprising a first, front optical surface and a second, rear optical surface. The system is configured to launch the collimated beam into a region between the parallel planes such that said reflecting optical surfaces waveguide the collimated beam between the optical surfaces in a plurality of successive reflections at the front and rear surfaces. The front optical surface is configured to transmit a proportion of the collimated beam when reflecting the beam such that at each reflection at the front optical surface a replica of the image is output from the image replication optics. This provides a simpler and cheaper manufacturing process for the image replication optics, colour compatibility, and improved optical efficiency.

In embodiments the rear optical surface is a mirrored surface and the front optical surface is a partially transmitting mirrored surface. The front surface may selectively transmit one polarisation and reflect an orthogonal polarisation, or it may transmit a proportion of the incident light substantially irrespective of polarisation. Thus the image replication optics may be fabricated as a bulk optical component or the front and rear optical surfaces have an air or gas-filled gap between them. A wide range of launch angles of the collimated beam into the region between the parallel planes may be employed, including angles close to a normal to the parallel planes, for example less than 30° to the normal. The distance between the parallel planes may be varied between wide limits, for example from less than 1 mm to greater than 3cm or 5cm. The image replication optics are not see-through and thus where the system is incorporated into a HUD the image replication optics may be followed by a combiner, such as a glass plate, to combine a view through the combiner with displayed symbology.

In one implementation the input beam to the image replication optics is an at least partially polarised beam and the front optical surface is configured to preferentially reflect light of a first (preferably linear) polarisation and to transmit light of a second polarisation orthogonal to the first. The image replication optics may then include a (directional) phase retarding layer to rotate polarisation of light passing through the layer. In some configurations light propagating within the waveguide has only has the first (reflected) polarisation (except adjacent the rear reflecting optical surface if this is where the polarisation rotating layer is located). Then, at each reflection from the rear surface (two passes through the polarisation changing region) a component of light at the second, orthogonal polarisation is introduced, which is transmitted through the front optical surface. Preferably the front optical surface reflects substantially no light at the second, orthogonal polarisation. Each beam of light transmitted through the front optical surface provides a replica of the image carried by the input beam and the intensity of a replica may be adjusted by adjusting the degree of polarisation change (rotation) introduced by the phase retarding layer., for example so that the output image replica beams have substantially the same brightness. In preferred embodiments the image display system comprises a multicolour image display system in which the different colours are displayed in a time multiplexed fashion. However it is not essential to tune the phase retardation for each colour and the system is still able to perform image replication for a multicolour display. In an alternative implementation rather than employ polarisation control the front surface, is configured as a partially transmissive mirror so that, at each internal reflection down the waveguide, a proportion of the internally reflected beam is transmitted to provide a replicated output image. In general the preferred range of percentage transmission will depend on the number of replicas. For example for less than 10 replicas, say N=4 replicas, a suitable transmission range is 10% to 50% transmission (to give a good compromise) whereas above 10 replicas a range of 0.1 % to 10% transmission is preferred. An embodiment may have 5-8 replicas (bounces) and a front mirror with of order 90% reflectivity. Two sets of image replication optics may be stacked one above another so that a replicated beam from a first set of image replication optics provides an input beam to a second set of image replication optics. This technique may be used to replicate beams in one dimension, in which case the or each output beam from one set of image replication optics may provide an input beam to a subsequent set of image replication optics (for example, via an aperture in the subsequent set of image reproduction optics) in which subsequent set of optics the light is waveguided in substantially the same direction as the first set of image replication optics. In this case the second set of image replication optics preferably has a smaller spacing between the planar reflectors than the first set of image replication optics, so that a plurality of output beams is provided from the second set of optics in the physical space between the output beams from the first set of image replication optics (which are the input beams to the second set).

Additionally or alternatively two (or more) sets of image replication optics (pupil expanders) may be stacked such that the direction of light propagation in a first of the expanders is substantially perpendicular to the direction of light propagation in the second expander. In such an arrangement the first expander may provide one- dimensional image replication and the second, following expander may provide two- dimensional image (pupil) replication, replicating each of the images from the first expander along an orthogonal direction to the direction of image replication by the first expander. To achieve this a plane defined by the parallel, planar surfaces of the first expander is non-parallel with a plane defined by the parallel, planar surfaces of the second expander. However the light exiting the first expander defines a direction or axis which is substantially aligned with the direction or axis of the light exiting the second expander. In this case the spacing of the planes may be the same and in the second set of optics the light may be propagating in a substantially orthogonal direction to that in which the light is propagating in the first or lower set of image replication optics.

The techniques we describe later are not restricted to any particular type or image projection system.

Example image replicator and HUD systems Figure 1 a shows an example of image replication optics 300, i.e. a pupil expander, configured to extract light based on polarisation. The optics comprises a first substantially planar mirror 302 and a substantially planar reflective polariser 304 substantially parallel to mirror 302 and spaced away from the mirror to form a waveguide. A directional phase retarder 306 is located adjacent to mirror 302. An input beam l₀ launched into the waveguide propagates along the waveguide in a direction parallel to the planes of reflectors 302, 304, alternately reflecting off surfaces 302, 304. The beams reflected off mirror 302 are labelled R₀, Ri and so forth and the beams reflected off polariser 304 are labelled \ l₂ and so forth since these can effectively be considered in the same way as beam l₀. At each reflection at the surface of reflective polariser 304 a proportion of the beam is transmitted to form an output beam O₀, and so forth. The reflective polariser reflects light of one polarisation and transmits light of a second, orthogonal polarisation. Beam l₀ bearing a collimated image is injected at an angle Θ to the normal to the plane of the device and at each reflection a small portion of the beam is extracted, at its original injection angle. The directional phase retarding layer 306 located inside the waveguide rotates the polarisation of the beam each time the beam passes through it.

Figure 1 b shows an alternative arrangement of image replication (pupil expander) optics 350, not relying on polarisation. Again the two mirrors in combination form a waveguide. A proportion of light is transmitted by mirror 304 at each reflection. The proportion depends on the number of replicas desired - for example for 20 replicas along one axis it is between 0.3 and 5% for good optical efficiency and uniformity of the replicated images (in general, the lower the number of replicas, the higher the gradient of transmission and therefore, the higher the final transmission). The pupil expander 350 operates in free space, the light transmitted through mirror 304 generating a replica image, the remainder of the light (losses apart) continuing to propagate within the cavity.

Figure 1 c shows a perspective view of image replication (pupil expander) optics of the type shown in Figures 1 a and 1 b. Figure 1 d shows a perspective view of a pair of stacked pupil expanders (image replicators) for expanding a beam in two dimensions. Each output beam from the first image replicator is itself replicated by a second image replicator. Apertures may be provided in the second image replicator(s) for the output beam(s) from the first. In the illustrated example there is substantially the same spacing between the parallel planes of the two sets of image replicators but this is not a requirement.

Figure 2a shows an example of a head-up display (HUD) 1000 comprising an image projector 1010 which provides a collimated beam to image replication optics 1050 of the type previously described, to generate a plurality of replicated images HUD images for viewing. A final, semi-transmissive optical element, combiner 1052, is used to combine the replicated images with an external view to provide a cockpit display to a user 1054. In some preferred embodiments the combiner comprises a flat glass plate although it may alternatively comprise, for example, a chromatic mirror. Although not shown in Figure 2a, there will be reflections off front and rear surfaces of combiner 1052, but these will comprise parallel beams, and because the virtual image is at infinity the viewer will not see a double-reflection. An example image projector 1010 comprises a holographic image projection system providing a polarised collimated beam to the image replication. The system comprises red R, green G, and blue B lasers and the following additional elements:

• SLM is the hologram SLM (spatial light modulator), for example a liquid crystal device or a pixellated MEMS-based piston-actuator type device.

• L1 , L2 and L3 are collimation lenses for the R, G and B lasers respectively (optional, depending upon the laser output).

• M1 , M2 and M3 are corresponding dichroic mirrors.

• PBS (Polarising Beam Splitter) transmits the incident illumination to the SLM.

Diffracted light produced by the SLM - naturally rotated (with a liquid crystal

SLM) in polarisation by 90 degrees - is then reflected by the PBS towards L4.

• Mirror M4 folds the optical path.

• Lenses L4 and L5 form an output telescope (demagnifying optics); output lens L5 may be a group of projection lenses.

· D may be a weak diffuser. It may comprise, for example, a microlens array

(MLA) or any other microstructured diffusers. The density of microlenses may be varied to vary the diffusion angle - for example for tiled replicas of 10° angular extent a diffusion angle of 1 °-2° is appropriate for the diffused intensity at the edge of a tile to be -80% of the intensity at the centre.

· System controller 1012 performs signal processing in dedicated hardware

and/or in software. Thus controller 1012 inputs image data (and optionally touch sensing data) and provides hologram data 1014 to the SLM. The controller also provides laser light intensity control data to each of the three lasers to control the overall laser power in the image. An alternative technique for coupling the output beam from the image projection system into the image replication optics employs a waveguide 1056, shown dashed in Figure 2a. This captures the light from the image projection system and has an angled end within the image replication optics waveguide to facilitate release of the captured light into the image replication optics waveguide. Use of an image injection element 1056 of this type facilitates capture of input light to the image replication optics over a range of angles, and hence facilitates matching the image projection optics to the image replication optics. Within element 1056 the light propagates by means of total internal reflection.

The arrangement of Figure 2a illustrates a system in which symbology from the head- up display is combined with an external view, and this is one approach which may be employed to provide a head-up display within a vehicle. Another approach is that schematically illustrated in Figure 2b. In this example the image replication optics 1050, more particularly the front optical surface of these optics, provides the function of the vehicle rear view mirror-that is the image display is incorporated into a vehicle rear- view mirror. The front optical surface of the image replication optics 1050 typically has a very high reflectivity, for example better than 95%, and thus provides a particularly high quality rear-view mirror whilst the symbology is displayed to the user at an effective image distance of 2m or greater, so that the accommodation of the users' eye need not change substantially when viewing the reflected image in the rear-view mirror and the head-up display symbology. The curved surface 1060 in Figure 2b schematically illustrates the front windscreen or windshield of a vehicle. The skilled person will appreciate that purely by way of example is image projector 1010 shown as a holographic image projector - any type of image projector, for example a DLP (Digital Light Processor), may be employed.

Fresnel eye box expanders

Broadly, we will describe optical devices implementing an expansion of the exit pupil of a collimated system. The general aim of such devices in a HUD is to allow a reduction in the collimation optics and generally a reduction in the size of the equipment. The techniques we describe reflect the same rays from two different reflecting surfaces to form two exit pupils separated in space, ideally the images tiling.

A Head-Up Display (HUD) eye-box is the area in which the virtual image of a HUD can be viewed - for a practical device this should be large enough to accommodate head movement while keeping both eyes in the eye-box. For a true far-field HUD where the image is at infinity this means that the final optic should be the dimensions of this eye- box. Using traditional optics this results in a very bulky optical assembly. The compact eye-box expander we described in WO2010/092409 uses the fact that far-field images may overlap substantially exactly when an eye-box is replicated with parallel copies, using a more compact optical design to produce the virtual image and then a separate optical assembly to split this view into multiple copies to form an expanded eye-box.

Now we describe using arrays of discrete mirrors (Fresnel mirrors) to chop-up and spread out the eye-box. This works best when the spacing between the sections is smaller than the pupil size of the observer. This is conceptually illustrated in Figure 3b. An advantage of these methods is that they can result in a lower cost more compact eye-box expander Referring now to Figure 3c, in embodiments the expansion of the exit pupil is done using a multitude of very small reflecting surfaces having the same orientation. With the Fresnel Pupil Expansion (later FPE), of Figure 3b the size of a reflector can be below 1 mm smoothing the tiling of the exit pupil. Further, choosing the expansion factor is straightforward since it only depends on the angle of the prism shaped grooves. However the nature of the FPE makes it inherently less preferable for see- through applications and thus a separate combiner is employed to show the virtual image.

One major advantage of this pupil expansion technique compared to the technique we previously described in WO2010/092409 is its very high efficiency. Indeed, the pupil magnification factor does not influence the efficiency of the element which is very close to the one of a conventional mirror. In embodiments each ray bounces exactly once off the Fresnel element. Practically speaking, the nature of the magnification is related to the angle of the Fresnel surface and the angle of the profile on top of it, as shown in Figure 4. Nevertheless, it is advisable to limit the magnification factor in order to avoid extra sensitivity to the tilt angle of the Fresnel surface: When the magnification increases, the surface is viewed by the projection optics at a very shallow angle and angular sensitivity to positioning tolerances increases rapidly. Thus it can be preferable to limit to magnification below a factor 5.

Since the implementation of this expansion works well for quite large surfaces, it is best used as the last element before the combiner in a HUD architecture, as shown in Figure 5. Since the Fresnel element can be employed to provide another optical function, combined with that of eye box expansion, it is also possible to use the FPE to reorient the topical axis. Thus this architecture can advantageously be combined with, for example, a waveguide-type pupil expander (as described in WO2010/092409), to perform a 2D pupil expansion in a very low volume. Figure 6a shows an example of a waveguide-type pupil expander employing FPE, and 6b shows an example of a waveguide-type pupil expander employing FPE together with optical axis correction. The architecture of Figure 6b is preferred to implement a very low volume HUD. The Fresnel reflector may be manufactured on a plastic substrate using a roll to roll process. This may optionally then be later laminated onto a more rigid material to assist in providing rigidity and achieving tolerances. The coating may be a single reflecting coating or may comprise reflecting and black stripes (the latter for non- reflecting parts).

Figure 7a shows an eye box expander 700 according to an embodiment of a first aspect of the invention and Figure 7b shows an eye box expander 750 according to an embodiment of a first aspect of the invention; these implement embodiments of the above described concepts.

In Figure 7a the light is guided within the moulding. The reflectivity of the Fresnel turning mirrors varied by changing the fill factor of the mirrors. Working back from the final section the fill factor steps down from 100% to 1 /2, 1/3, .... 1/n where n is the number of steps. In practice this is a continuous function rather than a stepped function, however the principal is the same. As the virtual images are in the far field the replicated images will overlap. In Figure 7b the input image is expanded by virtue of its angle of incidence and the distribution of the turning mirrors on the Fresnel reflector.

Figures 8a to 8c show a first example design of an eye box expander according to an embodiment of the invention, employing a two-stage waveguide-type expander (where FF denotes "fill factor").

The maximum spacing between Fresnel mirror elements is preferably <1 mm, so in embodiments the maximum Fresnel mirror size is determined by this plus the minimum aperture ratio (AR). It should be noted that this is an area AR and the mirrors do not need to be stripes. In embodiments the maximum size is of order like 300χ300μηι. In embodiments a spacing of order 100μηι may be employed.

Continuous stripe Fresnel mirrors may be undesirable as these may cause Moire effects in the output image which could become visible. A pseudo-random layout may thus be preferred. The stepped reflections may not be ideal and a continuous graduation of the density of the Fresnel elements may be preferred. A suitable design may be computer generated/modelled to determine an optimum layout of the Fresnel elements.

In embodiments the top surface is metal coated (the entrance window may to be masked during this process); the bottom surface may rely on TIR (total internal reflection) for guiding. An example design is based on a 20mm entrance projected beam size. Assuming a 50° propagation angle with respect to the normal of the TIR surface with a 5°x10 ° replay field (the design should preferably cope with up to ±7°), this gives an eye box of approximately 70mmx130mm. This can be adjusted by modifying the launch angle, number of repetitions and the like.

Fresnel wedge mirrors are potentially vulnerable to manufacturing distortions and artefacts related to the optical flatness of each reflector. Since these become more important the more grazing the angle of incidence, while a 2:1 or greater expansion with a grazing angle expander is practical, the expansion ratio is preferably kept below 10:1 to reduce these effects. By contrast, a lower expansion ratios such as a 2:1 expansion may be less preferable with a waveguide approach as the join between the replicated fields may become more visible. Thus overall a combination of grazing angle expansion, for example for one stage/in one direction (Y), and waveguide-type expansion, for example for a second stage/in a perpendicular direction (X) may be advantageous.

Thus Figures 9a to 9c show a second example design of an eye box expander according to an embodiment of the invention, employing a first-stage grazing angle expander and a second stage waveguide-type expander, with parallel Fresnel mirror surfaces. Figures 10a to 10c show a third example design of an eye box expander according to an embodiment of the invention, employing a first-stage grazing angle expander and a second stage waveguide-type expander, with arbitrary Fresnel mirror surfaces. Figures 1 1 a to 1 1 c show a fourth example design of an eye box expander according to an embodiment of the invention, employing a two-stage grazing angle expander with arbitrary Fresnel mirror surfaces. (The designs of Figures 9 to 1 1 do the Y expansion first, but there is no constraint requiring this to be the case).

Figure 12 shows an example of a head-up display system 800 incorporating an eye box pupil expander 700/750 as described above. The system comprises a projector 1012 providing an output beam 802 folded by mirrors 804, 806 and provided via an intermediate diffuser 708 to a collimation lens 710 and thence to the pupil expander 700/750. The exit pupil of the pupil expander is matched to the entrance pupil of the image replication optics 1050 which provide an expanded virtual image to a head-up display combiner (not shown in Figure 8).

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS:

1 . An eye box expander for a head-up display (HUD), the eye box expander comprising: a pair of optical surfaces defining a region therebetween; and an optical input to receive into said region an input beam for expansion; wherein a first of said optical surfaces defines an output surface for an output beam of said eye box expander; and wherein a second of said optical surfaces comprises a structured surface having a plurality of reflecting facets, each configured to reflect a part of said input beam to turn the respective part of said input beam through an angle, such that in combination said reflecting facets direct an expanded version of said input beam through said output surface.

2. An eye box expander as claimed in claim 1 wherein said optical surfaces comprise planar, parallel optical surfaces, and wherein said facets are configured to turn said input beams through an acute angle to direct said output beam substantially perpendicularly through said output surface.

3. An eye box expander as claimed in claim 1 or 2 wherein a said facet has a dimension of less than 3mm, 2mm or 1 mm.

4. An eye box expander as claimed in any preceding claim wherein pair of optical surfaces defines a waveguide, wherein said output surface is partially reflective, and wherein said part of said input beam reflected by said facets comprises a part of said input beam reflected into said region between said pair of optical surfaces from said output surface.

5. An eye box expander as claimed in any preceding claim wherein said reflecting facets are disposed on said structured surface to define a variable reflectivity such that a reflectivity of said structured surface increases with increasing distance from said optical input.

6. An eye box expander as claimed in claim 5 wherein said reflecting facets define a variable fill factor of said reflecting surface to define said variable reflectivity.

7. An eye box expander as claimed in any preceding claim comprising a second pair of optical surfaces, one with a faceted surface, said second pair of optical surfaces having a second optical input, and having a second optical output to provide said input beam to said optical input, to provide two-stage beam expansion.

8. An eye box expander as claimed in claim 7 wherein said pair of optical surfaces is configured to expand said beam in a first direction, and wherein said second pair of optical surfaces is configured to expand said beam in a second, perpendicular direction.

9. An eye box expander as claimed in claim 8 wherein said pair of optical surfaces is aligned with said second pair of optical surfaces such that they define a common pair of planes.

10. An eye box expander as claimed in claim 8 or 9 wherein said facets of said second pair of optical surfaces are configured to direct a beam at said second optical input into said optical input without leaving a region between said second optical surfaces.

1 1 . An eye box expander as claimed in claim 7, 8, 9 or 10 wherein the facets of a first stage of said beam expansion turn an output beam from said first stage to direct said output beam into a second stage of said beam expansion.

12. A HUD comprising the eye box expander of any preceding claim.

13. A HUD as claimed in claim 12 wherein said facets are configured to re-orient an optical axis of the HUD, in particular a topical axis.

14. A method of expanding the eye box of a head-up display (HUD), the method comprising: providing a pair of optical surfaces defining a region therebetween, one of said optical surfaces comprising a structured surface having a plurality of reflecting facets; receiving a collimated input beam bearing a virtual image into said region; reflecting a part of said input beam from each said facet to turn the respective part of said input beam through an angle such that, in combination, said reflecting facets output an expanded version of said input beam.

15. A head up display comprising an eye box expander, said eye box expander comprising: a pair of optical surfaces defining a region therebetween, one of said optical surfaces comprising a structured surface having a plurality of reflecting facets; an optical input to receive a collimated input beam bearing a virtual image into said region; and configured to reflect a part of said input beam from each said facet to turn the respective part of said input beam through an angle such that, in combination, said reflecting facets output an expanded version of said input beam.