WO2025099316A1 - High-resolution holocam by way of free-form exposure - Google Patents

Abstract

The invention relates to an imaging system comprising a diffraction-based incoupling element, a diffraction-based outcoupling element, the IE being configured to deflect light incident on the IE from a field of view of the IE to the OE at least in part, the OE being configured to outcouple the deflected light at least in part and the deflected light from different regions of the field of view of the IE traversing different optical path lengths through the imaging system, and at least one non-rotationally symmetric refractive element that is configured to reduce a path length-dependent aberration.

Description

High-resolution holocam through freeform exposure

1. Technical area

The present invention relates to imaging systems, e.g., for holocams, in particular RGB holocams. Examples include imaging systems based on a diffraction-based input coupling element (EE) for coupling light incident on the EE into the imaging system for redirection to a corresponding diffraction-based output coupling element (AE), e.g., via total internal reflection within a waveguide.

2. State of the art

Diffraction-based optical elements, such as gratings or holograms, are primarily used to redirect light incident on the optical elements. This allows, for example, customized imaging systems to be created.

However, the use of diffraction-based EE and AE, which may include, for example, holographic optical elements, also poses several challenges: The larger the field of view of the imaging systems, the greater the imaging errors that occur. Correcting such errors, such as aberrations, proves to be complicated: For example, an improvement in image quality at the edges of the field of view often leads to an (unacceptable) deterioration in image quality in the center of the field of view, and vice versa. This problem is particularly acute with large fields of view.

DE 10 2013 223 964 B3 relates to an imaging optics for a display device that can be placed on the head of a user and generates an image, comprising an optical element that has an entrance surface and a spectacle lens that has a coupling-out section, wherein the imaging optics is suitable for guiding the generated image, which is fed to the optical element via the entrance surface, in the optical element, coupling it from there into the spectacle lens, in which it is guided to the coupling-out section and via the coupling-out section to generate a virtual image is coupled out, characterized in that the optical element has, in addition to the entrance surface, at least one reflective surface on which the generated image is reflected for guidance in the optical element, and in that the optical element (8) and the spectacle lens are formed together as a one-piece optical part.

DE 102015 114833 Ai relates to a spectacle lens for imaging optics for generating a virtual image. The spectacle lens has an inner surface and an outer surface. Furthermore, the spectacle lens contains an input coupling section for coupling in an imaging beam path and an output coupling structure for outputting the imaging beam path. The input coupling section is arranged such that an imaging beam path is guided to the output coupling structure by reflections between the inner surface and the outer surface. An edge adjustment is present between the input coupling section and the output coupling structure, which is realized by a corresponding shaping of the inner surface. The curvature of the inner surface in the area through which the eye looks when viewing straight ahead is so closely approximated to the curvature of a conventional spectacle lens inner surface that it does not induce any perceptible optical aberrations when viewing straight ahead. In the area of the edge adjustment, the inner surface has a shape that deviates significantly from the curvature of the usual inner surface, allowing the imaging beam path coupled into the lens to be guided to the outcoupling structure via reflections. The entire inner surface is described by a single freeform surface.

DE 102019 102604 Ai relates to a functionalized waveguide for a detector system, wherein the waveguide has a transparent base body with a front side and a back side, wherein the base body has a partially transparent coupling-in region and a coupling-out region spaced therefrom in a first direction, wherein the coupling-in region comprises a diffractive structure which deflects only a part of the radiation coming from an object to be detected and striking the front side in such a way that the deflected part propagates as coupled-in radiation in the base body by reflections to the coupling-out region and strikes the coupling-out region, wherein the coupling-out region deflects at least a part of the coupled-in radiation striking it in such a way that the deflected part exits the base body via the front or back to strike the detector system, wherein the extent of the coupling-in region in a second direction transverse to the first direction is greater than the extent of the coupling-out region in the second direction.

US 2010/0214659 Ai relates to a diffractive beam expander comprising a substantially planar waveguide substrate, an input grating for generating an input beam propagating in the substrate, and an output grating for generating an output beam. The expander further comprises four or more further grating sections to increase the height of the input beam. A portion of the input light is diffracted by a first deflection grating section to generate a first deflected beam. A portion of the input light is refracted by a second deflection grating section to generate a second deflected beam. The first deflected beam propagates downwardly, and the second deflected beam propagates upwardly with respect to the input beam. The first deflected beam impinges on a first direction-restoring grating section, and the second deflected beam impinges on a second direction-restoring grating section. The first direction-restoring grating section provides a first restored beam, and the second direction-restoring grating section provides a second restored beam, both of which have the same direction as the input beam. The output coupling provides an output beam that is parallel to the input beam and has a larger vertical dimension than the input beam.

The present invention is therefore based on the object of at least partially improving corresponding imaging systems.

3. Summary of the invention

This task is at least partially solved by the aspects described herein.

An imaging system comprises a diffraction-based input coupling element (EE) and a diffraction-based output coupling element (AE). The EE is configured to at least partially redirect light incident on the EE from a field of view of the EE to the AE to redirect. The AE is configured to at least partially decouple the redirected light, with the redirected light from different areas of the EE's field of view traveling different optical path lengths through the imaging system. The imaging system also has at least one non-rotationally symmetric optical element configured to manipulate light non-rotationally symmetrically to reduce a path-length-dependent aberration error.

By reducing the path-length-dependent aberration error, the quality of the image can be improved and/or the size of the field of view can be increased with acceptable aberration error. The aberration error described herein can, in particular, include path-length effects (e.g., described herein). Reducing the aberration error can, for example, be understood as meaning that an aberration error is reduced (even) for rays of the same wavelength that traverse different optical path lengths through the imaging system, e.g., because they impinge on the EE at different angles or from different areas of the EE's field of view.

The solution according to the invention and the understanding of the underlying problem are explained below using Figs. 1a - 3c, which describe exemplary imaging systems with waveguides, but the concepts explained also apply to imaging systems without waveguides:

Fig. 1a schematically shows a view of the imaging system 10 with the EE 20 and an output coupling element, AE, 40 in the x-z plane. The imaging system 10 further comprises a waveguide 30 with a cross-section in the x-z plane in the shape of a rectangle elongated in the z direction and a detection system 50, which may include, for example, a sensor and/or a lens. The EE 20 is located at the upper end of the waveguide 30 on the left side of the waveguide 30, and the AE 40 is located at the lower end of the waveguide 30 on the right side of the waveguide 30.

The EE 20 and AE 40 may, in certain exemplary embodiments, comprise holographic optical elements. Such EE 20 and/or AE 40 are However, this is to be understood purely as an example: They can still differ in their (relative) positioning from the embodiment shown. Alternatively, they can be placed on the same side of the waveguide 30 (e.g. along the x-axis: to the left or right of the waveguide 30, e.g. along the y-axis in the middle or offset therefrom, e.g. at the edge of the waveguide 30 and e.g. along the z-axis: at the top, center or bottom of the waveguide 30). They can be configured to redirect light into reflection, as shown by way of example in Fig. 1a, or in another way as described below: The EE 20 and/or the AE 40 could, with the same, similar or different placement, be configured to redirect light into transmission and for this purpose have a reflective side, so that light can enter the EE 20 and/or the AE 40 from a front side and pass through it, be reflected at the back of the latter and be redirected into transmission when it exits the front side. Furthermore, the EE 20 and/or the AE 40 can be configured, e.g., in a geometry analogous to Fig. 1a, to redirect light in transmission. For this purpose, the EE 20 and/or the AE 40 can be mounted, e.g., on the respective opposite side of the waveguide 30 (in the example shown, the EE 20 is on the right and the AE 40 is on the left of the waveguide 30). If the EE 20 and/or the AE 40 is mounted on the opposite side (in the example shown, the EE 20 on the right and the AE 40 on the left) of the waveguide 30 (compared to the position shown in Fig. 1a), the EE 20 and/or the AE 40 can be configured to redirect light as follows: The EE 20 and/or the AE 40 can, for example, have a reflective side for this purpose, so that light can enter the EE 20 and/or the AE 40 from a front side and pass through it, be reflected at its rear side and be redirected in reflection upon impact on the front side. According to the invention, the EE 20 and the AE can redirect light in the same or a similar way (e.g., reflection or transmission) or in different ways.

The EE 20 has a vertical field of view, v-FOV. The light incident from the v-FOV is deflected by the EE 20 into the waveguide 30, as schematically shown by the black arrows. Within the waveguide, the deflected light reaches the AE 40 via total internal reflection. In other embodiments, the light can, for example, reach the AE 40 with more, fewer, or even no reflections within the waveguide 30. The reflection within the waveguide 30 shown in Fig. 1a is It is shown in a highly simplified manner and serves only to illustrate the basic concept of internal reflection in Fig. 1a. Figs. 2a - 2c show a more complete representation of the light path in waveguide 30.

The EE 20, the waveguide 30 and the AE 40 are coordinated in their respective shape, extent, relative position and/or relative orientation such that the deflected light from the EE 20 reaches the AE 40 as efficiently as possible, from where the deflected light is at least partially coupled out to the detection system 50, as schematically shown by the black arrows.

The v-FOV comprises an aperture angle ct _v , where ct _v = o in the example of Fig. 1a describes a light incidence perpendicular to the surface of the waveguide 30.

In principle, the deflection of light by the EE 20 and the AE 40 can depend on the angle of incidence of the incoming light, e.g. in that the deflection angle and/or the spectral distribution of the deflected light depends on the angle of incidence and/or only a part of the light reaches the AE 40.

Fig. 1b schematically shows a cross-sectional view of the imaging system 10 from Fig. 1a in the x-y plane, so that in particular the horizontal field of view, h-FOV, of the EE 10 can be represented. Fig. 1b shows the horizontal coupling to the EE 20 and the horizontal coupling from the AE 40 to the detection system 50, while Fig. 1a best illustrates the vertical components.

Fig. 1c schematically shows a cross-sectional view of the imaging system 10 from Figs. 1a and 1b in the yz-plane. It can be seen that the EE 20 is elongated in one direction, in this example along the y-axis, and has a shorter length in the direction perpendicular thereto, in this example along the z-axis, in the plane of the waveguide 30. The EE cross-section in the form of an elongated rectangle defines the region in which light incident on the EE 20 is at least partially deflected within the waveguide 30. Since the area of the AE 40 in the plane of the waveguide 30 is smaller than that of the EE 20, all paths along which the light is deflected from the EE 20 to the AE 40 run, in the projection in the yz-plane, within the light gray trapezoidal region between the EE 20 and the AE 40. Light that is at least partially outside this trapezoid does not come from the EE 20 and/or does not strike the AE 40 and therefore does not play a decisive role in the imaging of the imaging system 10.

Imaging systems as described in Figs. 1a to 1c are sometimes modified in the prior art by using multiple parallel holograms as EE and/or by using RGB holograms as EE and/or AE instead of simple monochromatic holograms. These exhibit the following problem, which is related to how light travels from the EE to the AE depending on its angle of incidence:

Figs. 2a - 2c show the same imaging system 10 as Figs. 1a - 1c (in the same view as Fig. 1a, in the xz plane). The imaging system 10 with an EE 20, an AE 40, and detection system 50, which may include, for example, a sensor and/or a lens, is shown for three different representative angles of incidence ct _v of the v-FOV (Fig. 2a: positive a _v ,i, Fig. 2b: a _v ,2 = o, and Fig. 2c: negative a _Vj3 ). The comparison of the three Figs. 2a - 2c exemplifies the problem existing in the prior art and at least partially solved by this invention:

In Fig. 2a, the light is incident on the EE 20 at a positive angle a _v ,i and is then redirected into the waveguide 30 such that it reaches the AE 40 via two reflections at the side surfaces of the waveguide 30. On the right side of the waveguide 30, further blocks the size of the waveguide 30 are shown, arranged in a row. In addition, the path of the redirected light in the direction of the redirected light after the first redirection by the EE 20 is extrapolated to show the entire distance traveled by the light within the waveguide 30. This distance corresponds to the optical path length through the imaging system for the corresponding region of the field of view of the EE or the associated direction of incidence and ends in the illustrative representation at the location of the equivalent AE 40a. This additional illustration does not represent an actual light path or actual elements, but merely serves to illustrate the overall path that the light travels within the waveguide 30. In Fig. 2b, the light is incident on the EE 20 at an angle a _v ,2 = o and is then redirected into the waveguide 30 such that it reaches the AE 40 via four reflections on the side surfaces of the waveguide 30. On the right side of the waveguide 30, further blocks the size of the waveguide 30 are shown, arranged in a row. In addition, the path of the redirected light in the direction of the redirected light after the first redirection by the EE 20 is extrapolated to show the entire distance traveled by the light within the waveguide 30. This distance corresponds to the optical path length through the imaging system for the corresponding region of the field of view of the EE or the associated direction of incidence and ends in the illustrative representation at the location of the equivalent AE 40b. This additional representation again does not represent an actual light path or actual elements, but merely serves to illustrate the overall path that the light travels within the waveguide 30.

In Fig. 2c, the light is incident on the EE 20 at a negative angle a _v , ₃ and is then redirected into the waveguide 30 so that it reaches the AE 40 through six reflections at the side surfaces of the waveguide 30. On the right side of the waveguide 30, further blocks the size of the waveguide 30 are shown, arranged in a row. In addition, the path of the redirected light in the direction of the redirected light after the first redirection by the EE 20 is extrapolated to show the entire distance traveled by the light within the waveguide 30. This distance corresponds to the optical path length through the imaging system for the corresponding region of the field of view of the EE or the associated direction of incidence and ends in the illustrative representation at the location of the equivalent AE 40c. This additional representation again does not represent an actual light path or actual elements, but merely serves to illustrate the overall path that the light travels within the waveguide 30.

The representations of Fig. 2a - 2c illustrate, in comparison to each other, that different angles of incidence of the light from the field of view of the EE or the imaging system 10 result in different equivalent distances of the AE 40 or different optical path lengths through the imaging system, as shown at the positions of the equivalent AE 40a, 40b, 40c. The resulting optical path length difference scales in the example of Fig. 2a - 2c with the thickness of the Waveguide 30 and can be applied analogously to imaging systems in which the waveguide 30 is in the form of a cavity delimited on at least two sides, e.g. by planar elements (not shown). The EE 20 and AE 40 of the examples in Fig. 2a - 2c are, for example, so-called deflection holograms that diffract or deflect light but have no refractive properties, i.e. they do not focus or scatter the light. When creating images, e.g. using an objective integrated in the detection system 50, path length-dependent aberration errors therefore inevitably occur: Such an exemplary objective (or any other suitable optics) can only correctly focus and/or image light from one angle. If such an objective is correctly adjusted for one angle of incidence, blurring and/or imaging errors occur at other angles.

A similar problem arises when the refractive properties for imaging are not (solely) implemented by a lens, but the EE and/or AE are formed with properties of a lens, as explained in Fig. 3a - 3c below:

Fig. 3a shows the imaging system 10 from Fig. 1a for the light that strikes the input coupling element 20 at a positive angle a _v ,i and the input coupling element 20 and the output coupling element 40 additionally have the function of spherical lenses. Fig. 3b shows the imaging system 10 from Fig. 3a for the light that strikes the input coupling element 20 at an angle a _v ,2 = 0. Fig. 3c shows the imaging system 10 from Fig. 3a for the light that strikes the input coupling element 20 at a negative angle a _v>3 .

Essentially, Figs. 3a - 3c represent the same imaging system as Figs. 2a - 2c, but with the difference that the EE 20 and the AE 40 comprise a spherical lens function, so that the image of a point of the object 60 to be imaged, depending on the angle of incidence a _v ,i, c _v ,2, a _v , _3, is collimated by the EE 20 and deflected within the waveguide as in Figs. 2a - 2c via internal reflection to the AE 40. The AE 40 then generates an image of the object 6o', which can be recorded by the detection system 50. However, it can be seen that aberration errors also occur in this constellation and the focal length is too short in Fig. 3a and too long in Fig. 3c. Therefore, path length-dependent Aberration errors and the image quality deteriorates increasingly towards the edges of the field of view of the imaging system 10.

The image of object 6o' is shown in Fig. 3a - 3c at the same "target" position. In an aberration-free image, the points in Fig. 3a and 3c at which the image is created would therefore lie in the "target plane." However, since aberration-free imaging is only possible for the angle of incidence shown in Fig. 3b, the distances Ax _a (in Fig. 3a) result, where the focus of the deflected light is closer to the AE 40 than the "target" plane, and Ax _c (in Fig. 3c) result, where the "target" plane is closer to the AE 40 than the focus of the deflected light. Therefore, it can be schematically seen from Fig. 3a - 3c that different degrees of aberration errors can occur in the various regions of the field of view.

The inventors of the present invention have come to the realization that path-length-dependent aberration errors are so difficult to compensate for, among other reasons, because, in contrast to conventional lenses, for example, the system described is a so-called "off-axis" system: light from different angles of incidence or different areas of the EE's field of view takes different optical path lengths through the system. Based on this new understanding, the inventors have recognized that it is generally necessary to use non-rotationally symmetric optical elements rather than simple lenses to reduce this path-length-dependent aberration error. This can, for example, improve the image quality and sharpness.

In order to manipulate the light non-rotationally symmetrically in order to reduce a path-length-dependent aberration error, the non-rotationally symmetrical optical element can have diffractive (e.g., if the non-rotationally symmetrical optical element comprises a hologram) and/or refractive (e.g., if the non-rotationally symmetrical optical element comprises a freeform lens) properties. The manipulation of the light across its entire beam cross-section can include redirecting and/or focusing and/or scattering, wherein the focusing and/or scattering occurs non-rotationally symmetrically. The imaging system can, for example, comprise exactly one (e.g., spatially coherent) non-rotationally symmetrical optical element. Exactly one non-rotationally symmetric optical element or the plurality of non-rotationally symmetric optical elements can be configured to manipulate the light deflected to the AE (essentially) completely, e.g. over its entire beam cross-section, in a non-rotationally symmetric manner.

Typical approaches to compensating, reducing, and/or avoiding aberrations, particularly for holographic imaging systems, can offer solutions that address dispersion-related aberrations rather than path-length-dependent aberrations. As understood herein, these can involve different problems or effects: The path-length-dependent aberration is due to the fact that light from different areas of the EE's field of view travels different optical path lengths through the imaging system. Dispersion effects, on the other hand, result from the fact that different wavelengths from the same area of the EE's field of view are deflected differently by the EE.

The reduction of the aberration error by the non-rotationally symmetric optical element (compared to an imaging system without a non-rotationally symmetric optical element) can, for example, comprise a first adjustment of a first focal length for a first outcoupling direction associated with a first region of the field of view of the EE (with a first optical path length through the imaging system). It can also comprise a second adjustment of a second focal length for a second outcoupling direction associated with a second region of the field of view of the EE (with a second optical path length). The first adjustment and the second adjustment and/or the first focal length and the second focal length can be different and/or adjusted to the corresponding first and/or second optical path length through the imaging system.

In an exemplary embodiment, the EE and/or the AE may comprise the at least one non-rotationally symmetric optical element.

In principle, this represents an advantageous, space- and material-saving possibility to carry out not only the diffraction of light but also the refraction of light by the EE and/or AE This can reduce the space requirements, costs and/or labor required to manufacture the imaging system, etc.

A fundamental distinction is made here between diffraction or redirection of light and refraction: Pure diffraction or redirection of light involves a change in the direction of light, as can be achieved, for example, by holograms illuminated with two plane waves. Non-rotationally symmetric optical manipulation of light can be essentially independent of this and involve focusing and/or scattering light. This manipulation can be performed by the same or a separate hologram.

This may be particularly advantageous if, in certain embodiments, the EE and/or the AE comprises a holographic optical element:

In the example of holographic EE and/or AE, the non-rotationally symmetric optical properties (for non-rotationally symmetric manipulation of light in order to reduce a path-length-dependent aberration error) in addition to the diffraction properties can in principle be inscribed into the EE and/or the AE by exposure to a plane wave and a point wave using conventional methods for exposing holograms. For concrete application in the present invention, for example, to achieve the non-rotationally symmetric optical property of the EE and/or the AE, the plane and/or the point wave can comprise a freeform exposure, e.g. as described herein. In other words, the (e.g. holographic) EE and/or the (e.g. holographic) AE can comprise a holographic element and the non-rotationally symmetric optical element. For example, the diffraction properties of the diffraction-based EE and/or AE and the non-rotationally symmetric properties of the non-rotationally symmetric optical element can be equally inscribed in the (e.g., holographic) EE and/or the (e.g., holographic) AE, e.g., in an identical hologram of the holographic EE and/or the holographic AE. The EE and/or the AE itself can be transmissive or reflective. This means that a non-rotationally symmetric optical function can be configured to be transmissive or reflective. For example, one can write a freeform lens function (transmission) and/or a freeform mirror function (reflection) into the EE and/or the AE.

The statements herein regarding the fabrication and properties of the EE and/or AE can also be transferred to the non-rotationally symmetric optical element, which can be manufactured analogously (e.g., as a hologram).

In some embodiments, the imaging system may be configured such that the EE and/or the AE comprises a three- or multi-color holographic optical element, preferably an RGB holographic optical element.

Basically, light of different wavelengths is redirected in different directions by holograms or holographic optical elements. In specific applications, this can mean that only light of one wavelength reaches its destination. If two- or multi-color holograms are used, images with a greater variety of colors can be created. RGB holograms, in particular, have proven advantageous for this purpose, as they cover the visible spectral range effectively and can redirect light of different wavelengths in the desired direction.

For example, the EE and/or the AE can be produced by means of at least one freeform exposure.

Freeform exposure can be controlled as described herein, e.g., using diffractive optically transparent components, refractive and/or diffractive hybrid optics and lenses, and/or spatial light modulators. Additionally or alternatively, optical elements such as aspherical lenses, integrated lens systems, diffractive optics, Fresnel lenses, off-axis parabolic mirrors, cylindrical lenses, gradient index lenses (GRISM), total internal reflection lenses (TIR), prisms, reflective optics, mirrors, cube optics, lens arrays, moth-eye optics, optical combiners, freeform optics, collimating lenses, and/or grating structures can be used to adjust the freeform exposure as desired and thus to control the non-rotationally symmetric to determine the optical properties of the non-rotationally symmetric optical element.

For example, the non-rotationally symmetric optical element may be configured to at least partially focus and/or disperse light incident on the non-rotationally symmetric optical element.

This has the advantage that the desired imaging can be achieved with reduced aberration error and, in particular, the refractive properties can be adapted to the geometry of the imaging system.

In some examples, the imaging system may comprise at least two non-rotationally symmetric optical, e.g., reflective, elements, wherein the at least two non-rotationally symmetric optical elements may be coordinated with one another and configured to create an image of the field of view.

This can be implemented, for example, in such a way that the EE and the AE, as described herein, each comprise a non-rotationally symmetric optical element. In this and other embodiments, the matching can be advantageous in that a first non-rotationally symmetric optical element, for example, to improve the aberration error at the edges of the field of view can lead to a deterioration of the aberration error in the center of the field of view, which, in this example, can be compensated for by a second non-rotationally symmetric optical element. Thus, the aberration error, as described herein, can be kept below an acceptable threshold over the largest possible field of view.

In exemplary embodiments, the imaging system may have a field of view of at least 50° by 25 ^° , preferably at least 60° by 50°.

This allows larger objects and/or larger fields of view to be imaged in general, enabling the application of the imaging system in new areas, which set minimum requirements for field-of-view sizes. For a field of view of 50° by 25 ^°, non-rotationally symmetric optical elements were able to improve the aberration in initial experiments to such an extent that the resolution was improved by an average of over 50%, and for fields of view of 60° by 50°, by an average of over 35%.

In some examples, the at least one non-rotationally symmetric optical element can be configured to reduce the path-length-dependent aberration error such that it is below a predefined value of 10 times the wavelength of the deflected light in the field of view on average (e.g., RMS (root mean square) error), preferably below a predefined value of 6 times the wavelength of the deflected light. The aberration here can be defined, for example, as the difference between the actual wavefront and the ideal wavefront. Typically, it is expressed in units of wavelength, and the root mean square over the field of view of the imaging system can representatively express the aberration of the imaging system in one value. Typically, an aberration expressed in this way by the solution according to the invention lies in a range of 0.3 to 6 times the wavelength of the deflected light. This corresponds to a significant reduction compared to an identical system that does not require a non-rotationally symmetric optical element, by a factor of at least 5, at least 10, or even at least 20.

The specified maximum values for aberration errors allow for high image quality imaging that meets a user's standard, thus improving user satisfaction and the applicability of conventional image post-processing techniques that may require a minimum level of image quality.

In further examples, the imaging system may further comprise a waveguide.

The waveguide can be arranged in relation to the EE and AE, for example, as shown in Figs. 1a - 3c, and can represent an advantageous way to fix the EE and AE in their relative position in a reliable, space-saving, and cost-effective manner. Waveguides can, for example, be made at least partially of glass and/or Plastics, etc. and/or can be tailored to the specific application in terms of their shape, size, surface quality, material composition and/or (spectrally dependent) transparency/light transmittance. This application can be, for example, the integration of the imaging system into a screen (where the waveguide can be, for example, part of the screen of a computer, a clock face/screen and/or a mobile phone), in windows (where the waveguide can be, for example, part of the windshield and/or another pane of a vehicle and/or a window of a building and/or a display) and/or in glasses (where the waveguide can be, for example, part of the lens of the glasses).

In some examples, the EE may be configured to at least partially couple light incident on the EE, preferably of at least two wavelengths, wherein the incident light of the two wavelengths preferably strikes the EE from the same direction, from a field of view of the EE into the waveguide.

Coupling into the waveguide offers, among other advantages, that the same element—the waveguide—which defines the stability and basic geometry of the system can be used for light transmission, providing an efficient, space-saving, and cost-saving solution. The coupled light can then be redirected, for example, as explained below.

Specifically, in exemplary embodiments, the EE may be configured to redirect the EE incident light from the field of view to the AE at least partially via internal reflection in the waveguide.

The angle between the light deflected by the EE and the surfaces of the waveguide can be adjusted, e.g. by illuminating the EE and/or the relative attachment or positioning on the waveguide, so that the deflected light reaches the AE via total internal reflection, which can lead to no, negligible and/or only small losses of light intensity within the waveguide. In some examples, the AE may be configured to at least partially couple deflected light of at least two wavelengths out of the waveguide.

For this purpose, the AE, if it comprises, for example, a holographic optical element, can be illuminated at two wavelengths, preferably from the same direction, to couple out both the light of a first wavelength and the light of a second wavelength in a desired direction. This can enable images with a greater color diversity.

If, for example, the EE and/or the AE is exposed to two wavelengths, the light incidence can be selected such that a portion of the light with the first wavelength and a portion of the light with the second wavelength strike the EE and/or AE to be exposed from the same direction. In typical examples, three wavelengths (red, green and blue (RGB)) are often selected in order to write an RGB hologram. Imaging systems with such EE and/or AE are then configured, as described herein, to couple light of at least two wavelengths incident on the EE, with the incident light of the two wavelengths preferably striking the EE from the same direction, from a field of view of the EE at least partially into the waveguide and to couple out the deflected light of the first and second wavelengths accordingly in a desired direction.

The same applies to RGB EE and/or AE for three-color light or to multi-color EE and/or AE for four- or multi-color light.

In exemplary embodiments, the AE may have a smaller surface area than the EE.

Such a relative size ratio is advantageous from the following perspectives, among others: The size of the EE affects the size of the field of view. This means that the larger the EE, the larger the field of view can be, which is generally preferred. Likewise, the size of the AE determines the beam cross-section of the outcoupled light. Therefore, the smaller the AE, the smaller the aperture of the lens and/or a detection system can be selected for image acquisition, which can generally have a positive effect on image quality. Generally, it is advantageous to find a compromise between aperture size, To find waveguide thickness, EE and/or AE size and/or field of view size in order to optimize the imaging system, e.g. with regard to the planned images, installation space requirements, etc.

In exemplary embodiments, the waveguide may have a first and a second surface opposite the first, which may preferably be separated from each other by a substantially constant layer thickness.

Such waveguides can therefore be particularly well suited to guide the deflected light from the EE to the AE in a controlled manner by total internal reflection and with low losses, which can have a positive effect on the efficiency and image quality of the imaging system.

For example, EE may be arranged substantially at the first surface of the waveguide and/or AE may be arranged substantially at the second surface of the waveguide.

If the EE and AE are arranged on opposite sides of the waveguide, the redirection of light from the EE to the AE is possible without reflection or by reflection within the waveguide.

In such exemplary embodiments, the EE and AE can be positioned and oriented relative to each other in the waveguide plane to optimize the deflection from the EE to the AE. The EE and AE can, for example, be attached to an outer surface of the waveguide, integrated into the waveguide, etc. Any holograms in examples where the EE and/or the AE comprise holograms can be a transmission or reflection hologram and can also be embedded in the pane or between two panes (e.g., laminated glass).

In one example, the imaging system may further comprise an objective lens, wherein the objective lens may be configured to at least partially influence light coupled out from the AE. A lens can compensate for the aberration of the image and thus improve the image quality.

The function of the lens can also be at least partially integrated into the AE, e.g. by the AE not only redirecting the light in that the direction of the redirected light is changed so that it is coupled out to the sensor, but that the AE also has a focusing and/or dispersing function, so that the AE alone or in combination with a lens applies the light appropriately to the sensor.

In one example, the objective lens may comprise the at least one and/or another non-rotationally symmetric optical element, which may be configured to reduce the path length-dependent aberration error.

Another non-rotationally symmetric optical element offers additional valuable degrees of freedom to reduce the path length-dependent aberration error and can thus further improve image quality.

Another aspect relates to a system that may include an imaging system as described herein and a sensor, wherein the AE may be configured to at least partially couple the deflected light to the sensor.

The sensor can be a suitable component directly integrated with the imaging system, enabling digital recording of the image created by the imaging system. This can enable digital post-processing and/or merging of the light contributions collected and redirected by the various EEs.

The superposition of identical and/or similar points in the images based on the light coming from different EEs on the sensor can be digitally processed and, if necessary, used to create the final image with post-image corrections. For example, shifts, distortions, different scaling, etc., can be compensated for using this digital processing, creating a coherent combined image. 4- Description of the characters

Fig. 1a shows a schematic view of an imaging system with an input coupling element and an output coupling element in the x-z plane.

Fig. ib shows a schematic view of an imaging system with an input coupling element and an output coupling element in the x-y plane.

Fig. ic shows a schematic view of an imaging system with an input coupling element and an output coupling element in the y-z plane.

Fig. 2a shows the imaging system of Fig. 1a for the light that hits the coupling element at a positive angle a _v ,i.

Fig. 2b shows the imaging system from Fig. 1a for the light that hits the coupling element at an angle a _v ,2 = o.

Fig. 2c shows the imaging system from Fig. 1a for the light that hits the coupling element at a negative angle a _v>3 .

Fig. 3a shows the imaging system of Fig. 1a for the light which strikes the input coupling element at a positive angle a _v ,i and the input coupling element and the output coupling element comprise a rotationally symmetric refractive element.

Fig. 3b shows the imaging system of Fig. 1a for the light which strikes the input coupling element at an angle a _v ,2 = o and the input coupling element and the output coupling element comprise a rotationally symmetric refractive element.

Fig. 3c shows the imaging system of Fig. 1a for the light which strikes the input coupling element at a negative angle a _v>3 and the input coupling element and the output coupling element comprise a rotationally symmetric refractive element. Fig. 4a shows a schematic view of an imaging system with an input coupling element, an output coupling element and a waveguide with a wedge-shaped cross-section in the xz-plane.

Fig. 4b shows a schematic view of an imaging system with an input coupling element, an output coupling element and a waveguide curved in the x-z plane.

Fig. 5 shows three possibilities for freeform exposure of holographic optical elements.

Fig. 6a shows a schematic view of an imaging system with an EE and an AE in the x-z plane, where the EE and the AE each comprise a non-rotationally symmetric optical element so that the aberration error is reduced for four exemplary angles of incidence on the EE.

Fig. 6b shows the aberration error as a function of position in a large field of view for an imaging system without a non-rotationally symmetric optical element.

Fig. 6c shows the aberration error as a function of position in a small field of view for an imaging system with a non-rotationally symmetric optical element.

Fig. 6d shows the aberration error as a function of position in a large field of view for an imaging system with a non-rotationally symmetric optical element.

Fig. 6e shows the aberration error as a function of position in a large field of view for an imaging system with two non-rotationally symmetric optical elements.

Fig. 7a shows an example of diffraction or redirection of light in contrast to the refraction in Fig. 7b. Fig. 7b shows an example of refraction of light in contrast to the diffraction or deflection in Fig. 7a.

Fig. 8a shows an exemplary non-rotationally symmetric optical element, which can be comprised, for example, by a coupling element.

Fig. 8b shows an exemplary non-rotationally symmetric optical element, which can be comprised, for example, by an outcoupling element.

5. Detailed description of preferred embodiments

The concept described herein can be applied not only to the imaging systems shown in Fig. 1a - 3c but also to those with waveguides of other shapes, such as those shown in Fig. 4a and 4b and described below:

For example, Fig. 4a shows a view of an imaging system 10 with an EE 20, an AE 40, and a waveguide 30 with a wedge-shaped cross-section in the x-z plane. The two side surfaces of the waveguide 30, to which either the EE 20 or the AE 40 is attached, are not parallel to each other. The AE 40 couples the deflected light, as described herein, to the detection system 50, which may include, for example, a sensor and/or a lens.

Fig. 4b schematically shows a view of an imaging system 10 with an EE 20, an AE 40, and a waveguide 30 curved in the xz-plane. The exemplary waveguide 30 has a constant thickness and a curvature in the xz-plane. Additionally or alternatively, the waveguide 30 could also have a curvature in the xy- and/or yz-plane. The respective curvature could be uniform or non-uniform on different sections of the waveguide 30, up to and including completely irregular curvatures. The AE 40 couples the deflected light, as described herein, to the detection system 50, which may comprise, for example, a sensor and/or a lens. Neither the relative inclination of the two side surfaces of the waveguide 30 from Fig. 4a nor the curvature of the waveguide from Fig. 4b represent an obstacle to the functionality of the imaging system 10. The functionalities described herein can be transferred to the imaging system 10 from Figs. 4a and 4b and the coupling to the respective detection system 50 takes place as described herein.

Fig. 5 shows three possibilities for freeform exposure of holographic optical elements. This can be applied, for example, in the exposure of an EE 20, an AE 40, and/or a non-rotationally symmetric optical element optionally included in the lens. The following exemplary explanation is directed to holographic optical EE 20, AE 40, and/or non-rotationally symmetric optical elements included in the lens. Holographic optical elements can comprise, for example, thin layers. The hologram can be written, for example, into such a thin layer, e.g., a photoemulsion coated on the surface of a photographic plate, and/or photopolymers (photosensitive polymers containing one or more light-sensitive molecular groups that react to exposure in such a way that a photo-induced rearrangement of functional groups leads, for example, to a change in the optical and/or mechanical properties). The thin layer can comprise, for example, a carrier medium such as gelatin with embedded light-sensitive halides such as silver chloride, silver bromide, or silver iodide. The impact of photons can lead to a chemical reaction of the halides, forming metallic silver. This reaction may be facilitated by coating the halides with dye molecules. Thus, exposure of such a film can lead to permanent inscription of the hologram using exposure as described herein.

These holograms can be inscribed into suitable media, e.g., photographic plates, using monochromatic light, as described herein. If, instead of a monochromatic first and second exposure (as described herein), multicolored (coherent) light is used, e.g., a red, a green, and a blue exposure can be superimposed in the first and second exposures to write so-called RGB holograms. For example, using dichroic mirrors, two, three, or more beams of different wavelengths be spatially superimposed. Wavelengths designated as red can, for example, cover the spectral range from 650 nm to 750 nm. Wavelengths designated as green can, for example, cover the spectral range from 490 nm to 575 nm. Wavelengths designated as blue can, for example, cover the spectral range from 420 nm to 490 nm. An exemplary combination can, for example, comprise a first and second exposure at 450 nm, 540 nm and 650 nm respectively. Alternatively or additionally, exposures in other wavelength ranges, e.g. ultraviolet (less than 380 nm), violet (380 nm to 420 nm), yellow (575 nm to 585 nm), orange (585 nm to 650 nm) or infrared (more than 750 nm) can also be used. Instead of multi-color exposure, two or more single-color exposed holograms can be combined to form hologram stacks, e.g., with suitable exposure, an RGB stack.

In addition to the color of the exposure, the direction of the exposure plays a crucial role in the diffraction and refractive properties of the holograms.

The three exemplary exposures from Fig. 5 include (i) a quasi-planar freeform exposure 601 and a point exposure 602 (left), (ii) a quasi-planar freeform exposure 603 and a quasi-point freeform exposure 604 (center), and (iii) a planar exposure 605 and a quasi-point freeform exposure 606 (right). Since at least one freeform exposure is used in all cases, non-rotationally symmetric optical elements 20, 40 can result from the process.

Fig. 6a schematically shows a view of an imaging system 10 with a waveguide 30, an EE 20, and an AE 40 in the xz-plane, wherein the EE 20 and the AE 40 each comprise a non-rotationally symmetric optical element, so that the aberration error for four exemplary angles of incidence a _v ,i, a _v>2 , a _v>3 , and a _Vj4 is reduced to the EE 20. However, such a reduction of the aberration error is not self-evident; Figs. 6b - 6e explain how the EE 20 and the AE 40 cooperate in the example of Fig. 6a to achieve this:

Fig. 6b shows the aberration error as a function of position in a large field of view for an imaging system without non-rotationally symmetric optical Element. Different areas of the field of view can correspond to different directions of incidence. The corresponding imaging system instead only has rotationally symmetric refractive elements. This means that no aberration error occurs only along one direction of incidence of the field of view (x = y = o). From there, the aberration error increases along the x and y axes with increasing distance from (x,y) = (o,o) and exceeds an acceptable aberration (AAT) at the edges of the field of view (FOV). The aberration here can be defined, for example, as the difference between the actual wavefront and the ideal wavefront. Typically, it is expressed in units of wavelength, and according to the so-called Marechal criterion, the root mean square of the aberration should ideally be below 0.07 wavelengths of the redirected light, but acceptable images can also be created for higher values. For the imaging system with a single sensor, the ATT depends on the sensor pixel size. Normally, it does not have to be at the diffraction limit. In the example of Fig. 6b, when light is incident at the exemplary angles of incidence a _v ,i, a _v ,3, and a _v>4 , aberrations above the AAT occur, which must be reduced, e.g., using the inventive solution. Without the inventive solution, one is typically limited to FOVs with aperture angles of approximately 20° in order not to exceed acceptable aberration values across the entire FOV.

If the FOV is so small that, for example, a _v>4 is not in the FOV of the imaging system, it is possible to keep the aberration below the AAT over the entire FOV with only one non-rotationally symmetric optical element in the imaging system, as shown, for example, in Fig. 6c, where the aberration error is shown as a function of position in a small field of view for an imaging system with one non-rotationally symmetric optical element.

In contrast, Fig. 6d shows that this approach with only one non-rotationally symmetric optical element is not sufficient to sufficiently reduce the aberration error as a function of position in a large field of view. In the example of Fig. 6d, the one non-rotationally symmetric optical element can indeed reduce the aberration error at the edge of the FOV so much that for the exemplary angle of incidence a _v>4 the aberration error is below the AAT. However, this does not reduce the aberration error at the other edge of the FOV as well as in the center of the FOV, e.g., for civ, 2, is increased so much that no acceptable result is achieved. To remedy this, a second non-rotationally symmetric optical element can be used, as shown schematically in Fig. 6e:

Fig. 6e shows the aberration error as a function of position in a large field of view for an imaging system with two non-rotationally symmetric optical elements. Compared to Fig. 6d, it can be seen that the aberration error can be kept below the AAT across the entire FOV by the second non-rotationally symmetric optical element, especially for a _v ,2, which was not the case in Fig. 6d.

Fig. 7a and 7b illustrate how diffraction (see Fig. 7a) and refraction (see Fig. 7b) are distinguished here:

For example, the diffraction-based EE and the diffraction-based AE diffract light by redirecting it. Two exemplary diffractions are shown in Fig. 7a: In a first example, the diffraction-based element 703 redirects the incident light 700a at an angle CIB,1 relative to the direction of the incident light 700a, so that the redirected light 701a is redirected downward. Alternatively, a second example is shown in the same figure: In the second example, the diffraction-based element 703 redirects the incident light 700a at an angle CIB,2 relative to the direction of the incident light 700a, so that the redirected light 702a is redirected upward. Thus, the two examples 701a and 702a differ in that the diffraction-based element 703a must have different diffraction properties in order to redirect or diffract the incident light 700a either in the direction 701a or 702a. In this sense, for example, the EE and the AE of the imaging system from Figs. 2a-2c are purely diffraction-based and not refractive.

In contrast to diffraction or deflection, refraction involves the process shown in Fig. 7b: Collimated light 700b, which strikes a refractive element 703b, is focused or scattered by the refractive element 703b. Fig. 7b shows two examples of focusing: once the light 701b is focused with a focal length fR,i and once the light 701b is focused with a focal length fR, ₂ . The focal length in the example of Fig. 7b depends on the refractive properties of the refractive element 703b. This also applies to refractive elements that scatter light (not shown).

Figs. 8a and 8b show exemplary phase patterns of non-rotationally symmetric optical elements, which can be comprised, for example, by EE 20 and AE 40, respectively. The gray-scale coded phase pattern represents the phase shift as a multiple of the wavelength (in Fig. 8a from -0.006 to 0.290 times the wavelength and in Fig. 8b from -0.0028 to 0.173 times the wavelength). In principle, the EE 20 is significantly longer in the x-direction than the AE 40, which can be the case, for example, in arrangements as best illustrated in Fig. 1c. Phase patterns other than those illustrated in Figs. 8a and 8b can also serve the inventive solution. The phase patterns are the result of an optimization of a system as illustrated in Figs. 6a-c, which minimizes the error in the imaging system 50 due to run-length-related aberration. As described herein, initial experiments by the inventors have shown that without non-rotationally symmetric optical elements, aberration values between 1.6 and 160 times the wavelength can be achieved; with non-rotationally symmetric optical elements, this can be greatly reduced to values between 0.3 - 6 times the wavelength of the deflected light.

The exemplary EE 20 from Fig. 8a has a substantially rectangular shape, extending a few hundred mm along the x-axis and slightly more than 10 mm along the y-axis. On the surface of the EE 20, the black and white color coding according to the legend shown in Fig. 8a shows the phase shift that light experiences when passing through and/or reflecting at the respective point of the EE 20. The EE 20 is configured so that light striking the ends along the x-axis experiences the greatest phase shift, while this is comparatively small in the center (at and around (x,y) = (0,0)) of the EE 20. The phase profile on the surface of the EE 20 can, for example, vary along both the x- and y-axis, as shown. In the example of Fig. 8a, the amplitude of the phase profile decreases along both the x- and y-axis from the center to the edges of the EE 20 (e.g., with varying degrees of slope and/or curvature). The exemplary AE 40 from Fig. 8b has a substantially rectangular shape, extending a few tens of mm along the x-axis and slightly more than 10 mm along the y-axis. On the surface of the AE 40, the black and white color coding according to the legend shown in Fig. 8b shows the phase shift that light experiences when passing through and/or reflecting at the respective point of the AE 20. The AE 20 is configured so that light striking the ends along the x-axis experiences the greatest phase shift, while this is comparatively small in the center (at and around (x,y) = (0,0)) of the AE 40. The phase profile on the surface of the AE 40 can, for example, vary along both the x- and y-axis, as shown. In the example of Fig. 8b, the amplitude of the phase profile increases along the x-axis from the center to the edges of the AE 40 and decreases along the y-axis from the center to the edges of the AE 40. In addition to the sign, the slope and/or curvature can also differ in amplitude for the x- and x-directions.

Claims

Claims 1. An imaging system comprising: a diffraction-based input coupling element, EE; a diffraction-based output coupling element, AE; wherein the EE is configured to at least partially redirect light incident on the EE from a field of view of the EE to the AE; wherein the AE is configured to at least partially outcouple the redirected light, wherein the redirected light from different regions of the field of view of the EE travels different optical path lengths through the imaging system; and at least one non-rotationally symmetric optical element configured to manipulate light non-rotationally symmetrically to reduce a path-length-dependent aberration error. 2. Imaging system according to claim i, wherein the EE and/or the AE comprises the at least one non-rotationally symmetric optical element. 3. Imaging system according to claim 1 or 2, wherein the EE and/or the AE comprises a holographic optical element. 4. Imaging system according to claim 3, wherein the EE and/or the AE comprises a three- or multi-color holographic optical element, preferably an RGB holographic optical element. 5. Imaging system according to claim 3 or 4, wherein the EE and/or the AE is produced by means of at least one freeform exposure. 6. Imaging system according to one of claims 1 - 5, wherein the non-rotationally symmetric optical element is configured to at least partially focus and/or disperse light incident on the non-rotationally symmetric optical element. 7- Imaging system according to one of claims 1-6, comprising at least two non-rotationally symmetrical optical elements, wherein the at least two non-rotationally symmetrical optical elements are coordinated with one another and are configured to create an image of the field of view. 8. Imaging system according to one of claims 1-7, wherein the imaging system has a field of view of at least 50° by 25 ^° , preferably of at least 60° by 50°. 9. Imaging system according to one of claims 1-8, wherein the at least one non-rotationally symmetric optical element is configured to reduce the path length-dependent aberration error such that it is below a predefined value of 10 times the wavelength of the deflected light, preferably below a predefined value of 6 times the wavelength of the deflected light, in the field of view. 10. Imaging system according to one of claims 1-9, further comprising a waveguide. 11. Imaging system according to claim 10, wherein the EE is configured to couple light incident on the EE, preferably of at least two wavelengths, from a field of view of the EE at least partially into the waveguide. 12. The imaging system of claim 11, wherein the EE is configured to redirect the EE incident light from the field of view to the AE at least partially via internal reflection in the waveguide. 13. Imaging system according to one of claims 1 - 12, wherein the AE is configured to at least partially couple deflected light of at least two wavelengths out of the waveguide. 14. Imaging system according to one of claims 1-13, wherein the AE has a smaller surface area than the EE. 15- Imaging system according to one of claims 1-14, wherein the waveguide has a first and a second surface opposite the first, which are preferably separated from each other by a substantially constant layer thickness. 16. The imaging system of claim 15, wherein the EE is disposed substantially on the first surface of the waveguide; and/or the AE is disposed substantially on the second surface of the waveguide. 17. Imaging system according to one of claims 1-16, further comprising an objective lens, wherein the objective lens is configured to at least partially influence light coupled out by the AE. 18. Imaging system according to claim 17, wherein the objective comprises the at least one and/or a further non-rotationally symmetric optical element configured to reduce the path length-dependent aberration error. 19. A system comprising: an imaging system according to any one of claims 1-18; and a sensor; wherein the AE is configured to at least partially couple the deflected light to the sensor.