Optical system for augmented reality applications

Abstract

Various examples of the disclosure pertain to an optical system for augmented reality applications, e.g., glasses or head-up display. A Fresnel lens (111, 112) is used in combination with an optical waveguide (122). Various designs of the Fresnel lens are disclosed.

Description

[0001] D E S C R I P T I O N

[0002] OPTICAL SYSTEM FOR AUGMENTED REALITY APPLICATIONS

[0003] TECHNICAL FIELD

[0004] Various aspects of the disclosure pertain to optical systems for augmented reality applications that include an optical waveguide configured to form images as well as a Fresnel lens arranged along the optical waveguide. Various aspects specifically relate to design aspects of the Fresnel lens and techniques of attaching the Fresnel lens to the optical waveguide.

[0005] BACKGROUND

[0006] Augmented reality applications create images in a line of sight of a user observing the environment. Thus, the user has a field of view that includes, both, digitally created information depicted by the image as well as real-world surroundings, i.e. , environmental objects.

[0007] FIG. 1 illustrates an example optical system 100 for augmented reality applications. For instance, the optical system 100 can be integrated in a spectacle lens or a head- up display. The optical system 100 includes a see-through optical waveguide 122. The see-through optical waveguide is configured to form images in a display region 125 thereof. For instance, holographic images can be formed.

[0008] A lens 111 is arranged on an eye-side 182 of the optical system 100 adjacent to the optical waveguide 122. The lens 111 forms light exiting the optical waveguide 122.

[0009] To reduce or compensate any impact of the user's perception of the surrounding by the lens 111 , the optical system 100 also includes a further lens 112 located at the real-world side 181 adjacent to the optical waveguide 122. The lenses 111 , 112 form a conjugate pair of lenses and are sometimes referred to as push-pull lenses or push-pull system of lenses. To implement the optical system 100 having compact dimensions along a thickness direction 191 , it has been proposed to use Fresnel lenses for the lens 111 and / or the lens 112. See WO 2022 / 245447 A1 . However, it has been observed that the structural robustness of the optical system 100 can suffer from using Fresnel lenses. Also, optical properties of the optical system 100 can suffer from using Fresnel lenses.

[0010] SUMMARY

[0011] Accordingly, a need exists for advanced designs and manufacturing techniques for optical systems for augmented reality applications that combine an optical waveguide to form images with one or more Fresnel lenses.

[0012] This need is met by the features of the independent claims. The features of the dependent claims define embodiments.

[0013] An optical system for augmented reality applications is disclosed.

[0014] The optical system includes a Fresnel lens. It some examples, the optical system includes multiple Fresnel lenses, e.g., a negative and a positive Fresnel lens in a conjugated arrangement. More generally, the optical system includes a conjugate pair of Fresnel lenses. Two Fresnel lenses can sandwich an optical waveguide.

[0015] The one or more Fresnel lenses may have a circular design. Prism facets can form circles or ellipses around a center of the respective Fresnel lens.

[0016] Each one of one or more Fresnel lens can have a flat circular design. One side of each one of the one or more Fresnel lens can be flat. The total thickness of each one of the one or more Fresnel lens can be smaller than its clear aperture.

[0017] The prism facets typically include a slope part and a draft part. The prism facets can have a curved surface, to resemble an aspheric lens. In some cases, the outer prism facets can be linearized.

[0018] The one or more Fresnel lenses each include prism facets. These can be circular prism facets. A constant heigh design can be employed; all relevant prism facets have the same heigh. Alternatively, a constant width design is used; here the prism facets all have the same width.

[0019] In examples, some prism facets have a larger height than other prism facets. This enables to bring the respective Fresnel lens into contact with other optical objects, specifically an optical waveguide, only at some sections thereof, formed by those prism facets having the larger height.

[0020] It is to be understood that the features mentioned above and those yet to be explained below may be used not only in the respective combinations indicated, but also in other combinations or in isolation without departing from the scope of the invention.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 schematically illustrates an optical system for augmented reality applications including an optical waveguide sandwiched by a push-pull lens system according to various examples.

[0023] FIG. 2 schematically illustrates a reference implementation of a Fresnel lens arranged adjacent to and along an optical waveguide according to various examples.

[0024] FIG. 3 schematically illustrates a reference implementation of a Fresnel lens arranged adjacent to and along an optical waveguide according to various examples.

[0025] FIG. 4 schematically illustrates an example implementation of a relative arrangement of a Fresnel lens arranged adjacent to and along an optical waveguide according to various examples.

[0026] FIG. 5 schematically illustrates an example implementation of a relative arrangement of a Fresnel lens arranged adjacent to and along an optical waveguide according to various examples.

[0027] FIG. 6 is a top view of an example Fresnel lens.

[0028] FIG. 7 is a top view of an example Fresnel lens.

[0029] FIG. 8 is a top view of an example Fresnel lens.

[0030] FIG. 9 is a top view of an example Fresnel lens. FIG. 10 schematically illustrates an example implementation of a relative arrangement of a Fresnel lens arranged adjacent to and along an optical waveguide according to various examples.

[0031] DETAILED DESCRIPTION

[0032] Hereinafter, techniques for enabling augmented reality applications are disclosed. Images are created in the field of view of a user using see-through optics so that the user, both, perceives the digitally-created image as well as the environment. The images can, depending on the particular use case, provide additional information to the user, e.g., text messages, information regarding objects in the environment that are visible to the user, etc. Other use cases include depicting objects that seamlessly integrate into the environment, e.g., for games or route guidance for navigation use cases. The particular type of augmented reality application is not germane to the techniques disclosed herein.

[0033] Hereinafter, designs and manufacturing techniques of optical systems that are able to form and shape images in the field of view of the user for augmented reality applications are disclosed.

[0034] Optical systems are described that include a see-through optical waveguide that is configured to form images in a display region of the optical waveguide. For example, the optical waveguide can form holographic images. In examples, the optical waveguide includes a substrate region and a holographic optical element arranged adjacent to the substrate region. The holographic optical element includes a periodic modulation of the refractive index. This enables to couple out light from the optical waveguide, to thereby form the image in the display region.

[0035] An image forming unit is configured to create the light and provide the light towards an entry region of the optical waveguide. The image forming unit implements a display.

[0036] Internal reflections can guide the light within the optical waveguide from the entry region to the display region where the holographic optical element is arranged. Then, light can be coupled out from the waveguide by means of the holographic optical element at the display region.

[0037] Because the optical waveguide is see through, the user can perceive objects in the environment through the optical waveguide.

[0038] An example optical system 100 that can be employed in the various techniques disclosed herein as illustrated in FIG. 1 and has already been described above. Illustrated is the image forming unit 121 that generates the light that couples into the optical waveguide 122 in the input region. Then the light is guided from the input region to the display region 125 using multiple internal reflections between a real- world surface 132 and an eye-side surface 131 of the optical waveguide 122. This is achieved by total internal reflection due to the refractive index used by the optical waveguide 122. The optical waveguide is made, e.g., from glass or silica material.

[0039] Also illustrated is a pair of Fresnel lenses 111 , 112. The Fresnel lenses 111 , 112 — e.g., flat, circular lenses - are offset along a thickness direction 191 (or z-direction). The optical waveguide 122 is sandwiched in-between the Fresnel lenses 111 , 112.

[0040] The optical waveguide 122 and the Fresnel lenses 111 , 112 extend laterally, i.e., along lateral directions 192 (x and y directions).

[0041] The Fresnel lens 111 is arranged along the optical waveguide 122 towards the eyeside 182 of the optical system 100. Images created by the optical waveguide 122 in the display region 125 are depicted / imaged by the Fresnel lens 111. The Fresnel lens 111 shapes light exiting the optical waveguide 122. The optical effect of the Fresnel lens 111 - here implemented as a negative lens - also acts upon light rays entering the optical system 100 from its surrounding at the real-world side 181.

[0042] Also illustrated is the Fresnel lens 112 that compensates the optical effect of the Fresnel lens 111. The Fresnel lens 112 is arranged along the optical waveguide 122 towards the real-world side 181 of the optical system 100. The clear aperture 116 of the Fresnel lens 112 corresponds to the clear aperture 115 of the Fresnel lens 111 , but may differ in other scenarios. The clear apertures 115, 116 can be as large as the field of view of the user. Considering first an observer without myopia or hyperopia, the Fresnel lens 111 is a negative lens that creates diverging rays from light rays that exit the optical waveguide 122 to form the image in the display region. In other words, a focal position of the negative lens 111 is arranged at a real-world side 181 of the optical system 100 (opposite to the eye side 182). The optical effect of the negative lens 111 also acts upon light rays entering the optical system 100 from its surrounding at the real-world side 181 . I.e. , divergent rays of light are created from the image formed by the optical waveguide 122; this creates the perception of objects depicted in the image being located further away from the eye of the user in the environment of the optical system 100 than the actual distance between the eye and the optical waveguide 122. Objects are apparently “pushed” away from the observer. This is why the Fresnel lens 111 is referred to as “push lens”.

[0043] This optical functionality of the Fresnel lens 111 can be superimposed with corrective functionality to compensate for myopia or hyperopia of the observer. A concrete example: The Fresnel lens may natively (without correcting for myopia or hyperopia) have a dioptrie of -0.53 to project the augmented-reality image at 2 m at the real- world side. The push lens is +0.58 dioptrie to compensate that. If eye correction is +18, then this value is added to the dioptrie referred above. Such addition can be achieved by appropriately shaping the Fresnel lens geometry; or using classic convex or concave lens geometries as a baseline for the Fresnel geometry. Alternatively or additionally, astigmatism correction may be provided by the Fresnel lens 111.

[0044] The clear aperture 115 of the Fresnel lens 111 is larger than the display region 125.

[0045] The Fresnel lens 111 and the Fresnel lens 112 can be made of polymer or plastics.

[0046] As a general rule, Fresnel lenses offer the possibility of thin designs, i.e., small dimensions between an outer surface of the optical system 100 at the real-world side 181 and an outer surface of the optical system 100 at the eye side 182. This enables flexible system integration, e.g., into spectacle lenses. Volume and weight are reduced.

[0047] As illustrated in FIG. 1 , a thickness 861 , 862 of the Fresnel lenses 111 , 112 is smaller than a thickness 869 of the optical waveguide, 122. Various disclosed techniques are based on the finding that the use of Fresnel lenses in an optical system for augmented reality applications according to reference implementations can cause optical imperfections due to stray light generated at discontinuities of the side profile of the Fresnel lens. Furthermore, it has been found that the use of Fresnel lenses can degrade the performance of the optical waveguide 122, e.g., reduce the internal reflection used for guiding the light towards the display region 125. Techniques are disclosed to mitigate such drawbacks.

[0048] Various disclosed techniques are based on the further finding that, to provide the optical waveguide 122 with the ability to guide the light with low losses towards the display region 125, the critical angle 0cfor total internal reflection associated with the optical waveguide 122 should be small. The critical angle is given by: wheretis the refractive index of the surrounding of the optical waveguide and n2is the refractive index of the material of the optical waveguide. Thus, a design goal is: nx« n2

[0049] The refractive index of the waveguide is typically n2= 1.5 - 1.7, due to manufacturing constraints. For the optical waveguide being surrounded by air, this would yield: nl =nair=1; #c(air)=36° — 42°

[0050] In a reference design illustrated in FIG. 2, the optical waveguide 122 is directly glued to the Fresnel lens 111 , at the surface 131. In this scenario the Fresnel lens 111 employs a so-called constant-height design. Here, the prism facets of the Fresnel lens 111 all have the same height (constant height design). In such a scenario, the refractive index in the surrounding of the optical waveguide 122 is that of the glue.

[0051] 0c(glue) = 52° - 64' The comparatively large critical angle degrades the light -guiding performance of the optical waveguide. This degrades the image quality for images formed in the display region.

[0052] Furthermore, it has been found that - beyond the degraded light-guiding capability of the optical waveguide 122 as explained above - local leakage of light from the optical waveguide 122 can be generated at sites along the profile of the optical waveguide 122 where the facet prims of the Fresnel lens 111 are adjacent to or in close contact with the surface 131 of the optical waveguide 122.

[0053] To mitigate such drawbacks of the design of the optical system 100 in the scenario of FIG. 2, a further reference design as illustrated in FIG. 3 can be used. The reference design of the optical system 100 as illustrated in FIG. 3 also employs a constantheight Fresnel lens 111 (as in FIG. 2). However, none of the prism facets of the Fresnel lens 111 is in contact with the optical waveguide 122 or specifically the surface 131. This is because the Fresnel lens 111 is spaced apart by a gap 211 from the optical waveguide 122. This gap 211 is filled with glue. The gap 211 avoids light leakage due to direct contact between the prism facets of the Fresnel lens 111 with the optical waveguide 122 as in the scenario of FIG. 2. However, also in the reference design of FIG. 3, the light-guiding performance of the optical waveguide 122 is degraded due to the surrounding of the optical waveguide 122 having the comparatively large refractive index of glue.

[0054] Hereinafter, techniques are disclosed that mitigate disadvantages of the reference designs according to FIG. 2 and FIG. 3 as described above.

[0055] Techniques are disclosed that enable to obtain and optimize light-guiding performance of the see-through optical waveguide 122. Designs are provided that reduce a loss of light due to reduced total reflection. Large parts of the surrounding of the optical waveguide can be filled with a low-index material having a refractive index smaller than the refractive index of glue, as in the reference designs of FIG. 2 and FIG. 3. Furthermore, light leakage due to contact between the prism facets of the Fresnel lens and the optical waveguide can be reduced.

[0056] According to examples, only a fraction of the prisms facets of the Fresnel lens is in contact with the eye-side surface 131 of the optical waveguide 122. In other words, the Fresnel lens includes contact prism facets that are in contact with the eye-side surface 131 of the optical waveguide 122, but also includes non-contact prism facets that are not in contact with the eye-side surface 131 of the optical waveguide 122. In other words, the non-contact prism facets are spaced apart from the optical waveguide 122. The contact prism facets can have a constant height; they can be in contact with the eye side surface 131 along their entire length. The contact prism facets alternatively have varying height along their length; and are, accordingly, in contact with the eye-side surface 131 only at certain sites or sections.

[0057] One example implementation as illustrated in FIG. 4.

[0058] FIG. 4 illustrates an example design of the optical system 100, specifically of the Fresnel lens 111. FIG. 4 schematically illustrates a profile of an example implementation of the Fresnel lens 111. The Fresnel lens can be a flat circular Fresnel lens. This means that the prism facets can form circles or ellipses at a certain radius with respect to the center 595 of the Fresnel lens 111 (not shown in the side view of FIG. 1 ). The Fresnel lens 111 in the scenario FIG. 4 includes contact prism facets 511 and non-contact prism facets 512. As illustrated in the scenario FIG. 4, the count of the contact prism facets 511 is significantly smaller than the count of the non-contact prism facets. In the specific illustrated example, there are a total of four contact prism facets 511 ; and 25 non-contact prism facets 512. More generally, the ratio of the count of the contact prism facets to the count of the non-contact prism facets is not larger than 1 :4.

[0059] By using a combination of contact prism facets and non-contact prism facets, the following effects can be achieved: Firstly, by using the contact prism facets, the Fresnel lens 111 can be attached to the optical waveguide 122 in a reliable manner. Specifically, an offset as in the scenario of FIG. 3 is not required across the entire clear aperture 115 of the Fresnel lens 111. Thereby, the relative arrangement of the Fresnel lens 111 to the optical waveguide 122 is well-designed and stable. Drifts due to expanding or contracting adhesive are avoided. Furthermore, light leakage due to an excessive number of prism facets of the Fresnel lens being in contact with the optical waveguide 122 can be avoided. The number of contact points can be minimized. Thereby, the optical properties of the optical waveguide, in particular, the light-guiding performance, is optimized. FIG. 4 includes an inset (dashed circle) that illustrates details with respect to one example implementation of the contact between the contact prism facets 511 and the eye-side surface 131 of the optical waveguide 122 (but the scenario of FIG. 4 is not tied to such implementation). As illustrated, the contact prism facets 511 include a flat contact surface 555 (e.g., chopped / cut) that is arranged in-between and tilted with respect to both a draft part 505 and a slope part 506 of the contact prism facets 511. The flat contact surface 555 engages the eye-side surface 131 , thereby creating a stable and robust contact.

[0060] Next, aspects with respect to the geometrical design of the prism facets 511 , 512 are discussed.

[0061] Illustrated in FIG. 4 is the facet height 551 for a number of prism facets 511 , 512. As is apparent from FIG. 4, the facet height 551 of the non-contact prism facets 512 increases from the center 595 of the Fresnel lens 111 towards the edge of the Fresnel lens 111 , more specifically towards the edge of the clear aperture 115 of the Fresnel lens 111. This is in contrast to the constant-height reference designs of FIG.

[0062] 2 and FIG. 3.

[0063] Also illustrated in FIG. 4 is the facet width 552 for a number of prism facets 511 , 512. In the scenario of FIG. 4, the non-contact prism facets 512 rather have a constant width 552. A constant-width design is used. As illustrated in FIG. 4, it is possible that also the contact prism facets 511 have the same width 552 as the non-contact prism facets 512.

[0064] In FIG. 4 a scenario is illustrated in which the two outmost prism facets are contact prism facets 511 in contact with the optical waveguide 122. As a general rule, it would be possible to use only a single contact prism facet or more than two contact prism facets.

[0065] In the scenario of FIG. 4, the contact prism facets 511 are all arranged at the edge of the clear aperture 115 of the Fresnel lens 111. In general, at least one of the contact prism facets may be arranged at the edge of the clear aperture 115. Such arrangement creates a cavity 605 in-between the eye-side surface 131 of the optical waveguide 122 and the Fresnel lens 111 . A low-index material can be arranged in the cavity 605. For example, the low-index material can be selected from the group including air, nitrogen, aerogel, liquid crystal, etc. The aerogel can provide mechanical support. As a general rule, the low-index material can have a refractive index that is smaller than 1 .35. For instance, the low-index material can have a refractive index in the range of 1 -1 .2. There are also low-index glues or optical cements available that can be used to fill the cavity 605. Thereby, the critical angle for total reflection in the waveguide is small. The light-guiding properties are good.

[0066] In the scenario of FIG. 4, the contact prism facets 511 surround the non-contact prism facets 512. In other scenarios, the contact prism facets 511 are arranged interleaved with the non-contact prism facets 512 in a radial direction (XY direction 192). Such scenarios illustrated in FIG. 5.

[0067] FIG. 5 schematically illustrates a profile of an example implementation of the Fresnel lens 111. While FIG. 5 illustrates a constant-height design of the non-contact prism facets 512, also a constant-width design of the non-contact prism facets 512 (e.g., as illustrated in FIG. 4) would be possible.

[0068] In the scenario of FIG. 5, the contact prism facets 511 and the non-contact prism facets 512 are arranged interleaved with each other, along the lateral directions 192. Thereby, the stability of the optical waveguide and generally the optical system 100 can be increased. Bending of the optical waveguide 122 is prevented, due to the multiple contact positions provided by the interleaved contact prism facets 511. Also, bending of the Fresnel lens is prevented.

[0069] This enables the Fresnel lens to be thinner than the optical waveguide. The Fresnel lens can be made from polymer or plastic material at greater flexibility than the optical waveguide, e.g., made of silica or other glass material having a high optical index.

[0070] FIG. 6 is a top view of the Fresnel lens 111. The design of the Fresnel lens 111 illustrated in FIG. 6 corresponds to the design illustrated in FIG. 4. In particular, the outmost two prism facets implement contact prism facets 511 ; while the inner prism facets are non-contact prism facets 512 (in FIG. 6 the contact prism facets, and more specifically sites at which the contact prism facets 511 are in contact with the side surface 131 of the optical waveguide 122, e.g., the contact regions 555 as discussed in the inset of FIG. 4 are illustrated with thick lines). In the scenario of FIG. 5, in other words, the height 551 of the contact prism facets 511 does not vary as a function of the perimeter position, i.e. , does not vary in the circumferential direction of the contact prism facets 511 . The contact regions 555 cover the entire 360° around the center 595. This is also the case for the scenario of the design / arrangement of the contact prism facets 511 and the non-contact prism in FIG. 7 (also a top view of the Fresnel lens 111 ). Here, the contact prism facets 511 and the non-contact prism facets 512 are arranged in an interleaved manner along the lateral directions 192 (cf. FIG. 5).

[0071] Different scenarios are illustrated in FIG. 8 in FIG. 9 (also top views of the Fresnel lens 111 ). Here, the height of some of the contact prism facets varies along its length, i.e., along the circumferential direction as a function of the perimeter position. I.e., the respective contact prism facets 511 include, both, contact regions 555 (bold lines) and non-contact regions 556 (narrow lines). Only the contact regions 555 are in contact with the eye-side surface 131 . Such interleaved arrangement of the contact regions 555 and the non-contact regions 556 further reduces the total area where the optical waveguide 122 is in contact with the Fresnel lens 111. Thereby, light leakage can be further reduced.

[0072] As a general rule, while two example scenarios of relative arrangement of the contact regions 555 with respect to the non-contact regions 556 of the contact prism facets 511 are illustrated in FIG. 8 and FIG. 9, other arrangements are possible. In particular, to provide strong mechanical stability, it is possible that the contact regions 555 are arranged at different circumferential positions of the Fresnel lens 111.

[0073] Thereby, mechanical stability in both orthogonal Cartesian directions in the lens plane (lateral x and y directions 192) can be provided. Further, the positions of the contact regions 555 can form a defined pattern or can be randomly placed.

[0074] Above, scenarios have been disclosed in which the clear aperture 115 of the Fresnel lenses 111 includes, both, contact prism facets 511 , as well as non-contact prism facets 512. Another scenario is illustrated in FIG. 10.

[0075] FIG. 10 schematically illustrates a profile of an example implementation of the Fresnel lens 111. In FIG. 10, the clear aperture 115 of the Fresnel lens 111 includes only non-contact prism facets 512. A contact element 711 is provided outside of the clear aperture 115 of the Fresnel lens 111. The contact element 711 supports contact between the Fresnel lens 111 and the optical waveguide 122.

[0076] Next, details with respect to the contact element 711 will be explained. The contact element 711 may also be combined with any other design of the Fresnel lens, e.g., as discussed above.

[0077] The contact element 711 extends at the perimeter of the flat, circular Fresnel lens 111. The contact element 711 and the Fresnel lens 111 are integrally formed. For instance, they could be formed from a single extrusion process or could be 3-D printed in a single printing process. The contact element 711 forms a circumferential edge of the cavity 605 filled with the low-index material. In that sense, the scenario of FIG. 10 is different than the scenario of FIG. 3. While the scenarios of FIG. 10 and FIG. 3 have some similarities (constant-height design of the Fresnel lens 111 and no contact between the prism facets 512 and the eye-side surface 131 of the optical waveguide 122), the optical waveguide 122 in the display region is not in contact with the adhesive, but rather with the low-index material in the cavity 605. In the scenario of FIG. 10, this is enabled by the contact element 711 that includes a region 724 that can be configured to seal the cavity 605 with respect to the environment. The region 724, for this purpose, engages and extends away from the eye-side surface 131 of the optical waveguide 122. The region 724 separates a notch 723 from the Fresnel lens 111 and the cavity 605. For a flat circular design of the Fresnel lens 111 , the notch (generally the contact element 711 ) extends around the full circumferential perimeter of the Fresnel lens 111. The notch 723 is filled with adhesive that fixes the Fresnel lens 111 and the contact element 711 to the optical waveguide 122. Also provided as an exit channel 721 that is designed to evacuate excess glue / adhesive.

[0078] The exit channel 721 connects the notch 723 with the surrounding / environment of the optical system 100. Thereby, excess adhesive in the notch can be discharged.

[0079] The channel 721 can also extend along the entire circumferential perimeter of the Fresnel lens 111. Alternatively, it would be possible that multiple distinct channels are formed at different circumferential positions.

[0080] While FIG. 10 illustrates a scenario in which the contact element 711 is combined with a constant-height Fresnel lens 111 , as a general rule, the contact element 711 could also be employed with other designs of the Fresnel lens such as those designs illustrated in connection with FIG. 4, FIG. 5, FIG. 6, FIG. 7, FIG. 8 or FIG. 9.

[0081] Summarizing, above, techniques have been disclosed that enable to maintain good performance of an optical waveguide by provisioning low-index material in its surrounding. Thus, the critical angle for total internal reflection in the optical waveguide is small. For instance, a cavity can be provided between the optical waveguide and a Fresnel lens, the cavity including the low-index material such as air, aerogel, etc. Techniques have been disclosed that facilitate a simplified assembly process, e.g., by providing notches of the Fresnel lenses that are filled with adhesive.

[0082] Various Fresnel lens designs have been disclosed such as constant height or constant width or a mixture thereof. Techniques have been disclosed to use contact and non-contact prism facets. This enables to optimize the width of the prism facets and / or the height of the prism facets. Also, an angle of the sidewalls of the prism facets can be optimized. The 2-D or 3-D profile of the Fresnel lens can be optimized. This enables to improve the performance of the Fresnel lens. The optical quality of imaging can be increased. Certain mechanical properties can be enforced. A compact push-pull system of Fresnel lenses can be used.

[0083] Although the invention has been shown and described with respect to certain preferred embodiments, equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications and is limited only by the scope of the appended claims.

[0084] For illustration, above, various aspects have been disclosed above in context of a Fresnel lens (lens 111) arranged adjacent to the optical waveguide at an eye-side of the optical system. Similar techniques can be readily applied alternatively or additionally to a positive Fresnel lens such as the Fresnel lens 112 of the push-pull lens system discussed in connection with FIG. 1 , arranged at the real-world side.

[0085] For further illustration, various aspects have been disclosed in the context of a Fresnel lens design. Similar techniques can also be used for diffraction lenses.

Claims

C L A I M S1 . An optical system (100) for augmented reality applications, comprising:- a see-through optical waveguide (122) configured to form images in a display region (125) of the see-through optical waveguide (122), and- a Fresnel lens (111 ) arranged along the see-through optical waveguide (122) towards an eye side (182) of the optical system (100) to image the images formed in the display region (125) using a focal position arranged at a real-world side (181 ) of the see-through optical waveguide (122), wherein the Fresnel lens (111 ) comprises a plurality of prism facets (511 , 512), the plurality of prism facets (511 , 512) comprising one or more contact prism facets (511 ) attached to an eye-side surface (131 ) of the see-through optical waveguide (122), the plurality of prism facets (511 , 512) further comprising non-contact prism facets (512) that are not in contact with the eye-side surface (131 ) of the see-through optical waveguide (122).

2. The optical system of claim 1 , wherein a ratio of a count of the one or more contact prism facets (511 ) to a count of the non-contact prism facets (512) is not larger than 1 :4.

3. The optical system of claim 1 or 2, wherein a facet height (551) of the non-contact prism facets (512) increases from a center (595) of the Fresnel lens (111 ) towards an edge of a clear aperture (115) of the Fresnel lens (111 ).

4. The optical system of any one of the preceding claims, wherein at least the non-contact prism facets (512) have a single fixed facet width (552).

5. The optical system of any one of the preceding claims, wherein at least one of the one or more contact prism facets (511 ) is arranged at an edge of the clear aperture (151 ) of the Fresnel lens (111 ).

6. The optical system of any one of the preceding claims,wherein the one or more contact prism facets (511 ) comprise multiple contact prism facets (511), wherein the multiple contact prism facets (511) are arranged interleaved with the non-contact prism facets (512) in a lateral direction (592) from a center (595) of the Fresnel lens (111 ) towards an edge of the clear aperture (115) of the Fresnel lens (111 ).

7. The optical system of any one of the preceding claims, wherein a height (551) of at least one of the one or more contact prism facets (511 ) varies along a length of the respective at least one of the one or more contact prism facets (511 ).

8. The optical system of claim 7, wherein each of the at least one of the one or more contact prism facets (511 ) comprises contact regions (555) and non-contact regions (556), wherein the contact regions (555) are in contact with the eye-side surface (131 ) of the see-through optical waveguide (122), wherein the non-contact regions (566) are not in contact with the eye-side surface (131 ) of the see-through optical waveguide (122).

9. The optical system of claim 8, wherein the at least one of the one or more contact prism facets (511 ) comprises multiple contact prism facets (511), wherein the contact regions of the multiple contact prism facets (511 ) are arranged at different circumferential positions of the Fresnel lens (111 ).

10. The optical system of any one of the preceding claims, wherein at least one of the one or more contact prism facets comprises a flat contact surface (555) arranged in between and being tilted with respect to both of a draft part (505) and a slope part (506) of the at least one of the one or more contact prism facets (511 ), the flat contact surface (555) engaging the eye-side surface (131 ) of the see-through optical waveguide (122).11 . The optical system (100) of any one of the preceding claims, further comprising:- a cavity (605) formed in-between the eye-side surface (131 ) of the see- through optical waveguide (122) and the Fresnel lens (111), and- a low-index material arranged in the cavity (605), the low-index material being that is selected from the group comprising: air; nitrogen; aerogel; liquid crystal.

12. An optical system (100) for augmented reality applications, comprising:- a see-through optical waveguide (122) configured to form images in a display region (125) of the see-through optical waveguide (122),- a Fresnel lens (111 ) arranged on an eye-side (182) of the see-through optical waveguide (122) to image the images using a focal position arranged at a real-world side (181 ) of the see-through optical waveguide (122), and- a contact element (711 ) extending at a perimeter of the Fresnel lens (111 ) and integrally formed with the Fresnel lens (111 ), wherein the contact element (711 ) comprises a notch (723) filled with an adhesive that fixes the contact element (711 ) and the Fresnel lens (111 ) to the see- through optical waveguide (122).

13. The optical system of claim 12, wherein the notch (723) is separated from the Fresnel lens (111 ) by a region (724) of the contact element (711 ) that engages and extends away from an eye-side surface (131 ) of the see-through optical waveguide (122).

14. The optical system of claim 12 or 13, wherein the contact element (711 ) comprises one or more channels (721 ) that connect the notch (723) to a surrounding of the optical system (100) and that are configured for discharging excess adhesive in the notch (723).

Images