Bending light guides, imaging optical units, and HMDs

Abstract

A curved light guide (1) is disclosed which can be used as an imaging optical unit of an HMD or as part of an imaging optical unit. When used as intended, the light guide (1) is arranged in front of at least one eye and has a concave curvature as seen from the eye. The light guide comprises: a rear surface (6) facing the eye and a front surface (4) facing away from the eye, an input coupling structure (3) for coupling light into the light guide (1) in such a way that the light is guided through the light guide (1) by reflection at the front surface (4) and / or the rear surface (6), and an output coupling structure (13) for coupling out the light guided through the light guide (1). The light guide (1) has a diverging structure (11) whose focal length has an absolute value less than or equal to the radius of curvature of the light guide (1).

Description

Technical Field

[0001] The invention relates to a bent light guide for an HMD (Head Mounted Display), ie for a display device to be worn on the head. Furthermore, the invention relates to an imaging optical unit for an HMD and to the HMD itself. Background Art

[0002] An HMD is a display system in which an output image is imaged into the field of view of the wearer of the HMD by means of an imaging optical unit. In this case, it is usually desirable to allow an undisturbed view of the surroundings in addition to the provided image. This arrangement prevents a solution in which a non-transparent image generator and an associated imaging optical unit are arranged directly in front of the eye. Therefore, in a class of HMDs (sometimes also referred to as smart glasses), an output image outside the field of view of the eye is usually generated, and the generated output image is reflected in front of the eye along the desired viewing direction by means of an imaging optical unit. To this end, the HMD includes a light guide, for example, via which an imaging beam path for generating a virtual image is guided by internal reflection from an input coupling structure located outside the field of view to an at least partially transparent output coupling structure located within the field of view of the eye. In this case, the input coupling and the output coupling can be achieved by a refractive structure, by a reflective structure, or by a diffractive structure. Diffractive input and output coupling structures are described, for example, in DE 10 2015 122 055 A1, US 2006 / 012181 A1 and US 2010 / 0220295 A1. Examples of reflective or refractive input or output coupling structures are described, for example, in US 2012 / 00002294 A1, DE 10 2011 007 812 A1, WO 2015 / 075206 A1 and WO 2015 / 075207 A1.

[0003] In order to obtain a large output pupil, US 8,320,032 B2 proposes multiple output coupling. Although US 8,320,032 B2 describes a planar light guide, in US 8,830,584 B2, the principle of multiple output coupling is applicable to a spherical light guide. In order to achieve ideal multiple output coupling, in the case of a spherical concentric light guide, the output coupling structure must be parallel to the latitude circle on the sphere. Light can therefore be transferred from the input coupling structure to the output coupling structure and coupled out multiple times therefrom. In this regard, ideal point-to-point imaging can be achieved in the center of the sphere. In addition, multiple output coupling is also described in DE 10 2015 122 055 A1.

[0004] In US 2013 / 0088780 A1, different focal planes are achieved by multiple light guides using the point-to-point imaging principle. However, in this case, the light guide must be curved away from the eye, so that the light guide has a convex curvature as seen from the eye. However, for aesthetic reasons, the light guide should have a concave curvature as seen from the eye, so that the head-mounted display can take the form of conventional glasses.

[0005] Furthermore, in US 8,842,368 B2 a head mounted display with diffractive input and output coupling is described, in which a curved diffractive structure is used to generate pupil imaging. However, such pupil imaging would be incompatible with multiple output couplings.

[0006] US 2012 / 0300311 A1 describes a head-mounted display comprising a cylindrically curved light guide and having a diffractive input and output coupling. In order to achieve the effect that a collimated image coupled into the light guide can be coupled out as a collimated image with an enlarged exit pupil, the input coupling grating and the output coupling grating have a variable grating spacing in the curvature direction of the light guide.

[0007] DE 10 2012 213 685 A1 describes a display device in which a light beam coupled into a curved light guide is converted into a collimated light beam by an additional optical unit arranged between an image generator and the light guide before incoupling and is coupled out of the light guide as a collimated light beam. Instead of by means of an additional optical unit 9, the collimation can be performed by means of an incoupling grating for incoupling.

[0008] DE 10 2015 122 055 A1 describes a display device comprising a correction arrangement for correcting imaging aberrations.

[0009] DE 10 2008 009 095 A1 describes a light transmission display system in which an image is visible through a panel into which an intermediate image generated by an image generator on a diffuser panel is coupled via an imaging lens. A corrective lens is arranged between the diffuser panel and the imaging lens, with the aim of optimizing the image quality.

[0010] DE 602 05 722 T2 describes a relay lens system which projects an image generated by an image source onto a curved goggle to be worn in front of the eye, the light from the relay lens system being reflected from the goggle in the direction of the eye. The relay lens system comprises an optical element which performs chromatic correction.

[0011] WO 2008 / 023367 A1 describes an input coupling module for coupling a beam path into a light guide. The input coupling module comprises a polarization beam splitter and a retardation plate. Summary of the invention

[0012] Against the background of the cited prior art, a first object of the present invention is to provide an advantageous light guide for an HMD, which, when the light guide is concavely curved as seen from the eye, enables the generation of a virtual image at infinity or at a defined distance in front of the eye. A further object of the present invention is to provide an advantageous imaging optical unit for an HMD, which enables the generation of a virtual image at infinity or in a defined image plane in front of the eye by using a light guide that is concavely curved as seen from the eye, in particular a light guide that is concavely curved on the terrestrial surface. A third object of the present invention is to provide an HMD comprising a light guide that is concavely curved as seen from the eye, in particular a light guide that is concavely curved on the terrestrial surface, and which enables the generation of a virtual image at infinity or in a defined image plane in front of the eye.

[0013] The first object is achieved by a bent light guide as claimed in claim 1, the second object is achieved by an imaging optical unit as claimed in claim 12, and the third object is achieved by a HMD as claimed in claim 17. The dependent claims contain advantageous configurations of the invention.

[0014] When used as intended, the curved light guide for an HMD according to the invention is arranged in front of at least one eye and has a concave curvature, in particular a concave curvature of the earth's surface, as seen from the eye. The light guide comprises a rear surface facing the eye and a front surface facing away from the eye. Furthermore, the light guide comprises an input coupling structure for coupling light into the light guide in such a way that the light is guided through the light guide by reflection at the front surface and / or the rear surface; and an output coupling structure for coupling out the light guided through the light guide. Such a light guide has a converging effect on the light coupled in. Therefore, according to the invention, the light guide has a diverging structure, the absolute value of the focal length of which is less than or equal to the radius of curvature of the light guide. The diverging structure can be superimposed on the output coupling structure or the input coupling structure, whereby no additional optical elements need to be integrated into the light guide. However, it is also possible to subdivide the focal length of the diverging structure between two substructures, and to superimpose one substructure on the input coupling structure and the other substructure on the output coupling structure.

[0015] Integrating a diverging structure into a light guide enables the use of the light guide to generate a virtual image that is located beyond the light guide as seen from the eye. In this case, if the absolute value of the focal length of the diverging structure is equal to the radius of curvature of the light guide, the image is at infinity; if the absolute value of the focal length of the diverging structure is less than the radius of curvature of the light guide, i.e. the diverging effect of the diverging structure is greater than the converging effect of the light guide, the virtual image is located at a distance from the eye that is determined according to the strength of the diverging effect, i.e. according to the focal length of the diverging structure.

[0016] The divergent structure may specifically be a diffractive lens, for example in the form of a volume hologram or a blazed grating. The divergent effect of the diffractive lens is determined by its focal length. In order to generate a virtual image at infinity, a diffractive lens is used here, the absolute value of the focal length of which is equal to the radius of curvature of the light guide. This is particularly true when the centers of curvature of the front and rear surfaces coincide in the light guide. This relationship only approximately holds true if the centers of curvature of the front and rear surfaces do not coincide, or if the curvature of the light guide deviates from the spherical curvature, for example because in the context of smart glasses the light guide is intended to be used simultaneously for correcting defective vision. Thus, the precise focal length of the diffractive lens used to generate the virtual image depends on the specific curvature of the light guide and can be determined approximately or experimentally when appropriate. For example, if the centers of curvature of the front and rear surfaces of the light guide do not coincide, the focal length of the diffractive lens may be less than or equal to the smaller of the two radii of curvature of the front and rear surfaces. In this case, the virtual image may not be at infinity, but is so far away from the light guide that it can be considered to be approximately at infinity. In the case determined experimentally, it can be determined that a divergent light beam from a point light source is imaged to a point at infinity by a light guide without a divergent structure, and the distance between the point and the rear surface of the light guide can be considered as its radius of curvature.

[0017] A diffractive lens focuses light to a focal point by interference, wherein there is constructive interference at the focal point and destructive interference outside the focal point. The focal point is typically formed by the first diffraction peak that occurs during diffraction. In order to induce corresponding diffraction, the diffractive lens may include alternating transparent and non-transparent areas, or alternating areas with shorter and longer optical path lengths for the light to pass through the diffractive lens. In this case, the spacing and width of these areas of the diffractive lens depend on the wavelength of the light and the desired focal length of the diffractive lens. In the case of alternating transparent and non-transparent areas, diffraction is generated by virtue of the fact that only specific parts of the wavefront can pass through the diffractive lens, wherein these passing parts interfere constructively with each other in specific directions and destructively with each other in other directions. In the case of areas with different optical path lengths, when the wavefront passes through different areas of the diffractive lens, a phase shift occurs between the wavefront parts passing through different areas. The phase shift has the following effect: different parts of the wavefront interfere constructively with each other in specific directions and destructively interfere with each other in other directions. The configuration of the diffractive lens with transparent and non-transparent areas provides the possibility of producing the diffractive lens using a relatively simple method. In contrast, producing a diffractive lens comprising zones with different optical path lengths offers the advantage that in augmented reality applications it has no effect on ambient light penetrating the light guide from the outside and thus does not impair the perception of the surroundings.

[0018] However, diverging structures lead to axial chromatic aberration and lateral chromatic aberration. Axial chromatic aberration refers to the fact that the focal points of light of different colors (wavelengths) are at different distances from the diverging structure, so that only one color of light is focused at a specific focal point, while the other colors of light are slightly defocused at this focal point. In contrast, lateral chromatic aberration is the result of the diverging structure having different imaging proportions for different colors (wavelengths) of light guided through the light guide, which leads to color fringes in the imaging. Although axial chromatic aberration is usually small enough to be able to dispense with correction, lateral chromatic aberration at the field edges of the image field may become so large that it is considered to be disturbing. In order to minimize the lateral chromatic aberration caused by the diverging structure, it is therefore advantageous if the diverging effect of the diverging structure is centered in the center of the image field.

[0019] In each case, only a specific angular spectrum of the beam path can be transmitted through the light guide. The smaller the angular spectrum, the smaller the image field that can be transmitted through the light guide. In order to be able to transmit a large angular spectrum in order to obtain a large image field, it is possible that the input coupling structure and the output coupling structure each comprise a plurality of substructures, which respectively couple different parts of the angular spectrum of the light into the light guide and couple them out of the light guide. These different parts of the angular spectrum can then be designed such that they each correspond to a maximum value of the angular spectrum that can be transmitted by the light guide. The total angular spectrum of the transmitted light then results from the combination of these two parts, so that overall a large angular spectrum can be transmitted, thereby enabling a large image field to be generated.

[0020] In addition, the diffractive input and output coupling structures are optimized for waves of a specific length. As the deviation from this optimized wavelength increases, interference that can be used for output coupling no longer occurs. Therefore, the wavelength spectrum of the light to be coupled in and coupled out by the input and output coupling structures is limited. If the aim is to generate an image with a wide wavelength spectrum, ideally a color spectrum, it is therefore advantageous that the output coupling structure comprises a plurality of substructures in each case, each of which couples a different part of the spectral range of the light to be coupled into the light guide into the light guide and couples it out from the light guide. Therefore, in order to be able to generate color images, in a specific configuration of the light guide according to the invention, there are input coupling structures and output coupling structures, each of which comprises three substructures, for example, one of which is optimized for blue light, one for green light, and another for red light. Then, if the angular spectrum is also subdivided into a plurality of parts, the substructures must each exist multiple times, i.e. each exists once for each part of the angular spectrum. Therefore, in general, this will result in six substructures in the case where the angular spectrum is subdivided into two parts and three colors. If the intention is to subdivide the angular spectrum into n parts, this correspondingly results in 3n substructures for the input coupling structure and 3n substructures for the output coupling structure. In this case, it is possible to guide different spectral ranges and / or different parts of the angular spectrum through a common light guide, or to subdivide the light guide into a plurality of sub-light waveguides, which guide different spectral wavelength ranges and / or different parts of the angular spectrum. In this case, the sub-light guides can be regarded as separate transmission channels. The presence of different transmission channels makes it possible to reliably avoid crosstalk of the transmitted signals. In comparison, transmitting multiple spectral ranges and / or multiple parts of the angular spectrum through a common light guide reduces the complexity of the light guide and thereby simplifies its production.

[0021] Light can be coupled into the light guide specifically in the following manner: the coupled-in light extends along a great circle extending through those points whose spherical coordinates are given by polar angles 0° and 180° on the spherical front surface and / or spherical rear surface of the light guide. In general, these points themselves are not part of the spherical front surface and / or spherical rear surface of the light guide, because they only represent a part of the surface of an imaginary sphere containing these points. On the surface of the earth, such a great circle corresponds to a longitude circle. Therefore, the great circle will be referred to as the longitude circle of the light guide hereinafter. In this way, for all longitude circles along which the coupled-in light passes, there is a great circle that intersects these longitude circles at right angles, that is, the great circle that the light passes through at right angles, or will pass through this great circle at right angles if the light is coupled out before actually reaching this great circle. Therefore, the great circle intersecting the longitude circle at right angles extends substantially parallel to the structural elements of the input coupling structure and the output coupling structure, that is, in the case of diffractive input and output coupling, it extends parallel to the grating structure, or in the case of reflective or refractive input and output coupling, it extends parallel to the boundary line between the Fresnel structures. On the surface of the earth, this intersecting great circle corresponds to the equator. Therefore, the great circle intersecting the longitude circle at right angles is referred to as the equator of the light guide below. In this case, it is particularly advantageous to integrate the divergent structure into the output coupling structure, because the route of the light along the longitude circle in the light guide is not affected by the divergent effect of the divergent structure. The route of the light along the longitude circle in the light guide provides the possibility of arranging the output coupling structure closer to the equator of the light guide than the input coupling structure. In this case, it may be advantageous to arrange the output coupling structure to be no more than 5 degrees, preferably no more than 3 degrees, from the equator of the light guide. In particular, here, the output coupling structure can be arranged so that the equator of the light guide also intersects with the output coupling structure, specifically so that the equator also intersects symmetrically with the output coupling structure. Since the output coupling structure is close to the equator of the light guide, the circle of longitude along which the light rays propagate intersects the structural elements of the output coupling structure almost at right angles, so that the light rays hit the structural elements of the output coupling structure almost at right angles. Therefore, the difference in pupil shape due to different numbers of reflections within the light guide can be minimized. Different numbers of reflections within the light guide may result in the use of diffractive lenses at different positions, which may produce pupil distortion that may need to be corrected. By arranging the output coupling structure close to the equator, this pupil distortion can be reduced and possibly minimized.

[0022] The present invention also provides an imaging optical unit, which comprises an imaging beam path, a light guide, and at least one image forming optical element. The imaging optical unit according to the present invention is distinguished in that it comprises a light guide according to the present invention, wherein the light guide serves as the at least one image forming optical element. In this case, the imaging optical unit can in particular also consist only of the light guide according to the present invention. The simultaneous use of the light guide as an image forming optical element enables the production of a compact imaging optical unit. In this case, taking into account the concave curvature of the light guide as seen from the eye, in particular the spherical concave curvature of the light guide, the light guide according to the present invention enables the provision of a virtual image as seen from the eye, which is located at a finite or infinite distance beyond the light guide.

[0023] In a modification of the imaging optical unit according to the invention, a correcting element for pre-correcting the trapezoidal pupil distortion guided by the light guide is arranged upstream of the input coupling structure of the light guide along the imaging beam path. The correcting element produces an inverse pupil distortion relative to the pupil distortion caused by the light guide. Then, the two pupil distortions compensate each other. Additionally or alternatively, a compensating optical unit that compensates for static optical lateral chromatic aberrations can be arranged upstream of the output coupling structure of the light guide along the imaging beam path. The static lateral chromatic aberration is caused by the light guide comprising a divergent structure and is independent of eye movement. Therefore, the aberration can be directly compensated by the optical compensating optical unit. The compensating optical unit causes an inverse lateral chromatic aberration relative to the lateral chromatic aberration caused by the light guide, as a result of which the two lateral chromatic aberrations cancel each other out. If a manipulator is arranged upstream of the input coupling structure of the light guide along the imaging beam path, wherein the manipulator comprises a diffractive optical element and a refractive optical element, which are arranged such that they are displaceable relative to each other perpendicularly to the optical axis of the imaging beam path, and an eye tracker for detecting the viewing direction of the eye is also present, dynamic, field-constant lateral chromatic aberrations caused by the light guide can also be corrected in advance. Dynamic lateral chromatic aberrations arise due to eye movements, at least in the case that the chromatic bandwidth of the transmitted light exceeds a certain threshold.

[0024] Furthermore, a polarizing beam splitter and a retardation plate may be arranged upstream of the input coupling structure of the light guide along the imaging beam path. The polarizing beam splitter in combination with the retardation plate enables the imaging beam path to be coupled in through the input coupling structure while achieving a compact size of the imaging optical unit.

[0025] Another aspect of the present invention provides an HMD comprising a display for displaying an output image and an imaging optical unit according to the present invention for generating a virtual image based on the output image. Since the imaging optical unit according to the present invention can be compactly implemented due to the use of a light guide as an image forming optical element, it is also possible to provide a compact HMD by using the imaging optical unit according to the present invention, which enables generation of a virtual image located at infinity beyond the light guide or at a certain distance from the light guide as seen from the eye, in the case where the light guide is curved in a concave shape as seen from the eye. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Further features, characteristics, and advantages of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

[0027] Figure 1 It shows ideal point-to-point imaging, which can be achieved, for example, with a spherical light guide.

[0028] Figure 2 A bent light guide imaged to infinity is shown.

[0029] Figure 3 An example of a diffractive out-coupling structure for coupling out a bundle of light rays transmitted through a light guide is shown.

[0030] Figure 4 A diffractive lens is shown.

[0031] Figure 5 Shown by Figure 3 The output coupling structure and Figure 4 The output coupling structure is obtained by superposition of diffractive lenses.

[0032] Figure 6 is a diagram used to illustrate the field of view angle of the display under consideration.

[0033] Figure 7 Shows how to Figure 6 The field of view is subdivided between two different light guides.

[0034] Figure 8 Shows Figure 7 Output coupling of a light bundle transmitted through the light guide.

[0035] Fig. 9 A light guide comprising three substructures is shown, each substructure being adapted to couple out a different spectral range.

[0036] Fig.10 An embodiment variant of an imaging optical unit is shown, which comprises a wavefront manipulator for correcting lateral chromatic aberrations caused by the light guide.

[0037] Fig.11 An embodiment variant of an imaging optical unit is shown, which comprises a corrective element for correcting pupil distortion caused by the light guide, said corrective element being arranged upstream of an incoupling structure of the light guide.

[0038] Fig.12 An example of a compact imaging optical unit is shown. DETAILED DESCRIPTION

[0039] See below Figure 2 Before discussing the principles behind the present invention, see Figure 1 To explain the effect of spherical light guide.

[0040] Figure 1 A spherical light guide 1 is shown, comprising a diffractive in-coupling structure 3 at the edge of the light guide and a diffractive out-coupling structure 5 in the center of the light guide. A line between the center of the in-coupling structure 3 and the center of the out-coupling structure 5 extends substantially along a longitude circle 7 of the sphere of the light guide 1, i.e. along a great circle extending through the sphere (of which the light guide forms a part) and the points whose spherical coordinates are given by the polar angles 0° and 180°. In the present exemplary embodiment, the out-coupling structure 5 is located on the equator 9 of the sphere in a manner symmetrically intersecting the equator 9.

[0041] The diffractive in-coupling structure 3 and the diffractive out-coupling structure 5 are each formed by a diffraction grating with a specific grating vector, the grating lines of which extend parallel to the circle of latitude, i.e. parallel to the equator 9. By forming the diffractive in-coupling structure 3 by a diffraction grating, the divergent light bundle 2 coming out of the center of the sphere of the light guide 1 is deflected in such a way that the individual rays of the light bundle in the light guide 1 strike the front surface 4 of the light guide 1, which faces away from the eye when the light guide is used as intended in an HMD, so that the rays are reflected by total internal reflection in the direction of the rear surface 6 of the light guide 1, which faces the eye when the light guide is used as intended in an HMD, where the rays are in turn reflected by total internal reflection in the direction of the front surface 4 of the light guide 1. In this way, the rays of the light bundle are guided by multiple total internal reflections at the front surface 4 and the rear surface 6 of the light guide 1 to the out-coupling structure 5, where they are deflected by the diffraction grating to come out of the rear surface 6 of the light guide 1. In this case, the bundle of rays emerging from the rear surface 6 of the light guide 1 is a converging bundle of rays 8 which converges in such a way that its rays are again focused at the center of the sphere of the light guide 1. Thus, Figure 1 The shown light guide 1 comprising a diffractive in-coupling structure 3 and a diffractive out-coupling structure 5 ideally images a point located at the center of the sphere of the light guide 1 onto itself.

[0042] However, if Figure 1As shown, the light guide 1 is used only to a limited extent as part of the imaging optical unit of the HMD. In an HMD, it is usually desirable to generate an image at infinity (which the user of the HMD can view with relaxed eyes) or an image located beyond the light guide at a defined distance from the eye, so that the user has the impression that the image is floating at a certain distance in front of the user's eye. In other words, the imaging optical unit comprising the light guide is intended to be able to generate a virtual image beyond the light guide. Indeed, in principle, there is the possibility of generating a virtual imaging light beam bundle coupled out by the output coupling structure 5 by means of a lens arranged between the output coupling structure 5 and the eye. However, it is not desirable to arrange a lens between the light guide and the eye, because firstly there is only very little space between the light guide and the eye for such a lens, and secondly, damage to the eye by the optical element located directly in front of the eye should be avoided in the event that the HMD is blown. In addition, the lens also affects the image of the surrounding environment.

[0043] Therefore, the present invention proposes to integrate a diverging structure into the light guide 1 itself, wherein the absolute value of the focal length of the diverging structure is less than or equal to the curvature radius of the light guide 1. By means of the diverging structure, the light beam coupled out of the light guide 1 through the outcoupling structure 5 (e.g. Figure 2 The light guide 1 is a light guide having a plurality of optical components, each of which is coupled to the optical element 10 and is coupled to the optical element 11. The light guide 1 is a light guide having a plurality of optical components, each of which is coupled to the optical element 11 and is coupled to the optical element 11. The light guide 1 is a light guide having a plurality of optical components, each of which is coupled to the optical element 11. The light guide 1 is a light guide having a plurality of optical components, each of which is coupled to the optical element 11. The light guide 1 is a light guide having a plurality of optical components, each of which is coupled to the optical element 11. The light guide 1 is a light guide having a plurality of optical components, each of which is coupled to the optical element 11. The light guide 1 is a light guide having a plurality of optical components, each of which is coupled to the optical element 11.

[0044] The diverging structure can be a refractive, reflective or diffractive structure integrated into the light guide 1. In the current exemplary embodiment, a diffractive structure in the form of a diffractive lens is used. The use of a diffractive structure is advantageous here because it can be made very thin, so that the thickness of the light guide 1 does not need to be increased or does not need to be increased significantly. Furthermore, the diffractive diverging structure can be integrated into the diffractive output coupling structure 5 and / or the diffractive input coupling structure 3. In the case of multiple output couplings, it is particularly advantageous to integrate the diffractive diverging structure into the diffractive output coupling structure 5, because as a result, the light path in the glasses is not affected. See below for more information. Figures 3 to 5 This is explained based on the example of integration into the output coupling structure 5 .

[0045] Figure 3 An exemplary diffractive outcoupling structure 5 in the form of a diffraction grating is shown, with the grey scale indicating the phase deviation to be produced in the range 0-1λ. In the present exemplary embodiment, the outcoupling structure comprises a parallel grating structure extending along the latitudinal circle of the sphere of the light guide 1. Figure 3 The diffractive output coupling structure 5 shown produces Figure 1 The converging light beam 8 shown and thus produces Figure 1 Point-to-point imaging.

[0046] Figure 4 A diffractive lens 11 is shown. In the present exemplary embodiment, the focal length of the diffractive lens 11 is equal to the radius of curvature of the sphere of the light guide 1. The diffractive lens 11 is a concentric diffraction grating with an annular grating structure. In this case, the annular grating structure can be formed by alternating annular transparent and non-transparent areas, or by alternating annular areas with different optical path lengths, so that the light passing through adjacent areas undergoes different phase shifts. In both cases, interference occurs after passing through the diffractive lens, which interference enhances the light in a specific spatial area by constructive interference and weakens the light in other spatial areas by destructive interference. The deflection angle at which constructive and destructive interference occurs depends here on the distance between two adjacent annular structures. The closer this distance is, the larger the angle at which the first-order diffraction peak (i.e., the first constructive interference) appears. By reducing the distance towards the edge of the diffractive lens, these deflection angles can be adapted to each other so that the deflected light interferes constructively at a specific distance from the lens. This distance is then the focal length of the diffractive lens 11. In this case, for example, +1st order diffraction may be used to converge light incident on the diffraction lens 11, and -1st order diffraction may be used to diverge light incident on the diffraction lens 11.

[0047] If the +1-order diffraction is used, the parallel light beam will thus become a converging light beam focused at the focal point of the diffraction lens 11. In contrast, when the -1-order diffraction is used, the parallel light beam will become a diverging light beam after passing through the diffraction lens 11. In the current exemplary embodiment, the diffraction lens 11 is implemented to use the -1-order diffraction.

[0048] Will Figure 4 The diffractive lens 11 shown is superimposed on Figure 3 The output coupling structure 5 is shown on the output coupling grating. Figure 5 The results of superposition are shown. This type of output coupling structure 13 is replaced Figure 5 The output coupling structure 5 is used for Figure 2 The lens light guide 1 is shown.

[0049] As already mentioned, in the present exemplary embodiment, the diverging effect in the form of the diffractive lens 11 is completely integrated into the outcoupling structure 13. In principle, however, there is also the possibility of integrating a diverging structure, for example in the form of the described diffractive lens, into the incoupling structure 3. Furthermore, there is also the possibility of integrating corresponding diverging effects both into the incoupling structure 3 and into the outcoupling structure 13, the sum of which corresponds to Figure 4 The divergent effect of the diffractive lens of the embodiment of the present invention is achieved. The corresponding subdivided diffractive lens can be designed by a person skilled in the art in a straightforward manner. However, the divergent structure integrated only into the outcoupling structure 13 offers the following advantages over other variants: the divergent effect does not affect the guidance of the light through the light guide.

[0050] It can be produced, for example, by holographic exposure on a curved surface of a light guide as shown in FIG. Figures 3 to 5 Grating structures with different optical path lengths are described. Indeed, in principle, there is also the possibility of producing the grating structure by transparent and non-transparent areas of a light guide, wherein these non-transparent areas can, for example, be printed thereon, but this leads to light losses of ambient light passing through the light guide. However, especially in the context of the HMD being implemented as an augmented reality display, in which the image to be displayed is intended to be perceived as having been inserted into the surrounding environment, grating structures with different optical path lengths offer the advantage that the ambient light is not attenuated.

[0051] Furthermore, it should be pointed out that the position of the input coupling structure 3 on the light guide and the position of the output coupling structure 13 on the light guide can in principle be freely chosen. However, it should be noted that the light rays associated with the field points traverse along non-parallel great circles within the light guide, which, depending on the position of the input coupling structure and the output coupling structure, produces a more or less trapezoidal pupil distortion and thus astigmatism in the field. Therefore, ideally, the output coupling structure is located near the equator 7 of the sphere of the light guide 1 in order to minimize the pupil distortion and thus the astigmatism. Typically, considering a pupil diameter of 3 mm, a geometrical spot size of 1 to 2 arc minutes can thus be achieved at the field edge, which is generally considered no longer to be disturbing.

[0052] In the present invention, see Figure 2In the described exemplary embodiment, the light guide 1 comprises a single light guiding structure for guiding light from the input coupling structure 3 to the output coupling structure 13. Through such a light guide, the angular range of the field of view to be transmitted is relatively limited and is not sufficient to transmit an image in a 16:9 format with a 70° image angle. Nevertheless, in order to enable the transmission of such an image, the angular range of the field of view extending over the field of view angle α is subdivided into a plurality of local angular ranges of the field of view, which are then transmitted through the light guide separately from one another. This is described below based on the example of subdividing the angular range of the field of view extending over the field of view angle α into two angular ranges of the field of view extending over the field of view angle α1, α2. Figure 6 The subdivision of the viewing angle range into two viewing angle ranges is schematically shown in FIG. This figure shows the display 15 and the viewing angle α used for viewing the display 15. The viewing angle is subdivided into two viewing angles α1 and α2, which do not overlap each other in the current example. However, in principle, there are also possible local viewing angles α1 and α2. 1 , α 2 overlap. In this case, the local field of view angle α 1 , α 2 can be chosen so that each coverage can be Figure 2 The maximum viewing angle range of the light guide shown is transmitted. In order to transmit the entire viewing angle range given by the viewing angle α, two light guides are arranged one after the other, for example, as described in Figure 2 The sub-light guide 1 described and referred to below as light guide 1 1 , 1 2 . Figure 7 and Figure 8 This arrangement is shown schematically. Figure 7 Here, a section along a longitude circle is shown with the input coupling structure 3 1 and 3 2 The two sub-light guides 1 1 , 1 2 , the sub-light guides are arranged front and back. Input coupling structure 3 1 and 3 2 Each is assigned a dedicated display 15 1 , 15 2 , each display displays an image area that will be perceived with a corresponding viewing angle range α1, α2. As already mentioned, the viewing angle α 1 , α 2 is selected so that the viewing angle range covered by it can be 1 , 1 2 For example, if the field of view angle α is 40°, and the associated field of view angle range extends from -20° to +20°, then the local field of view angle α 1 , α 2It can be 20°, for example, wherein the local viewing angle α1 covers the viewing angle range of -20° to 0°, and the local viewing angle α 2 Covering a viewing angle range of 0° to +20°. 1 , 1 2 Together the complete image is transmitted, so together they form the light guide 1 .

[0053] Figure 8 The schematic diagram of the sub-light guide 1 1 , 1 2 The output coupling of the light guide 1 is formed. 1 , 1 2 Each includes an output coupling structure 13 1 , 13 2 , which basically corresponds to Figure 2 However, the structure of the basic grating 5 on which the diffractive lens is superimposed is selected so that the light guided through the sub-light guide 1 1 , 1 2 The light is coupled out at the right angle. Input coupling structure 3 1 , 3 2 Correspondingly, with the output coupling structure 13 1 , 13 2 Therefore, the local field angle α 1 , α 2 The limited field of view angle ranges are combined again to form a field of view angle range with an aperture angle α. This process is also called field stitching and is described in detail in WO 2017 / 102795 A1. For further details on field stitching, see the document.

[0054] A sufficiently large field of view can be achieved by using a suitable number of sub-light guides in the light guide 1. Since the diffractive incoupling structures and the diffractive outcoupling structures can be made very flat, a larger number of sub-light guides can also be arranged one behind the other.

[0055] In addition, in the case of a single input coupling structure and a single output coupling structure, it is generally not possible to transmit the entire wavelength spectrum of visible light through a single light guide. However, there is the possibility of providing the light guide with sub-light guides, each sub-light guide being designed to transmit a different wavelength spectrum of visible light. In this regard, for example, one sub-light guide having an input coupling structure and an output coupling structure may be designed to transmit red light, another sub-light guide having an input coupling structure and an output coupling structure may be designed to transmit green light, and a third sub-light guide having an input coupling structure and an output coupling structure may be designed to transmit blue light. In this way, a complete color image may be transmitted through the three sub-light guides. This is in Fig. 9Schematically shown in the figure, the figure shows a light guide comprising three sub-light guides 1 1 , 1 2 , 1 3 and the associated output coupling structure 13 1 , 13 2 , 13 3 , light guide 1. Each has an associated input coupling structure (not shown) and an output coupling structure 13 1 , 13 2 , 13 3 , the sub-light guide 1 1 , 1 2 , 1 3 Designed to transmit one color, for example, the first sub-light guide 1 1 Designed to transmit red light, the second sub-light guide 1 2 is designed to transmit green light, and the third sub-light guide 1 3 It is designed to transmit blue light. Therefore, in general, the three sub-light guides 1 1 , 1 2 , 1 3 To transmit color images.

[0056] See Figures 6 to 8 , if it is intended to transmit multiple field angle ranges and each field angle range is intended to represent a color image, then in each case three substructures 1 for red, green and blue light are required for each field angle defined by the local field angles α1, α2 1 , 1 2 , 1 3 . Overall, in the above example, there are then six sub-lightguides, three for each viewing angle range defined by the local viewing angle α1, and three for each viewing angle range defined by the local viewing angle α2. As already mentioned, these sub-lightguides with diffractive input coupling and diffractive output coupling can be made quite thin, so that a lightguide 1 comprising six sub-structures can be produced without problems with an acceptable total thickness.

[0057] Figure 4 The diffractive lens 11 shown is not chromatically corrected. This is associated with the fact that the angle at which constructive interference occurs depends on the wavelength of the light. The lack of chromatic correction results in axial and lateral chromatic aberrations.

[0058] In this case, the axial chromatic aberration is caused by the fact that the angles of the first diffraction peaks are different from each other, so that the lens has different focal lengths for different wavelengths. If the diffractive lens is designed, for example, so that its focal length for green light corresponds to the radius of curvature of the light guide, the focal length for red light is slightly smaller than the radius of curvature of the light guide 1, and the focal length for blue light is slightly larger than the radius of curvature of the light guide, the result is that a certain image unsharpness occurs in the red and blue parts of the image while the green part of the image is sharp. In the case of a 50 nm bandwidth at a wavelength of 500 nm (green light), assuming that the radius of curvature of the light guide 1 and the focal length of the diffractive lens 11 are each 150 mm, the loss of clarity due to the unsharpness of the red and blue parts of the image is approximately 0.6 dpt. A loss of clarity of 0.6 dpt can still be regarded as acceptable.

[0059] Lateral chromatic aberration is based on the different magnifications produced by different focal lengths and therefore on the different imaging proportions of the individual colors in the corresponding focal planes. Lateral chromatic aberration does not occur on the optical axis, it increases with the distance from the optical axis. If the exit pupil of the diffractive lens is 20 mm behind the output coupling structure, lateral chromatic aberration occurs at the field edge of the ±12° image field, where it has an order of magnitude of about 10 arc minutes. Such lateral chromatic aberration is no longer acceptable and must be corrected.

[0060] One possibility for correcting lateral chromatic aberration is that, in an imaging device comprising a light guide 1, a corrective element is arranged upstream of the input coupling structure 3 of the light guide 1, which corrective element produces an inverse lateral chromatic aberration relative to the lateral chromatic aberration caused by the diffractive lens 11, so that the lateral chromatic aberration of the diffractive lens 11 and the lateral chromatic aberration of the corrective element exactly cancel each other out. Fig.10An example of such a corrective element is shown. The figure shows a schematic diagram of an HMD comprising an image generator 15 on which an output image is displayed, which output image is intended to be converted into a virtual image by an imaging optical unit. This imaging optical unit comprises, in addition to the light guide 1 according to the invention, a corrective element 17 for correcting lateral chromatic aberration. In the current exemplary embodiment, the corrective element 17 comprises conventionally a refractive optical element 19 and a diffractive optical element 21. The two optical elements are coordinated in such a way that together they induce a lateral chromatic aberration which compensates for the lateral chromatic aberration of the diffractive lens 11 of the output coupling structure 13. In addition, the refractive optical element 19 and the diffractive optical element 21 can be displaced relative to each other, whereby dynamic lateral chromatic aberrations can be compensated. Dynamic lateral chromatic aberrations are generated due to the eye movements of the user, the effect of which is that the degree of lateral chromatic aberration occurring along the edge of the field of view changes. By appropriately displacing the two optical elements 19, 21 relative to each other, changes in lateral chromatic aberration can be compensated to a certain extent. To this end, the current exemplary embodiment comprises an eye tracker 23 for determining the viewing direction. Based on the determined viewing direction, the control unit 25 controls the relative displacement between the refractive optical element 19 and the diffractive element 21 of the corrective element 17. If it is not intended to compensate for dynamic lateral chromatic aberrations, there is the possibility of arranging the refractive optical element 19 and the diffractive optical element 21 fixedly relative to one another and of omitting the eye tracker 23 and the control unit 25.

[0061] The corrective element 17 also allows, by suitable configuration of the refractive optical element 19 and / or the diffractive optical element 21 , compensation of the pupil distortion already mentioned above, which occurs as a result of the light rays being guided along a circle of longitude.

[0062] However, if the aim is only to compensate for pupil distortion, this can be achieved using simpler means. Fig.11 An imaging optical unit for an HMD is shown, which comprises a light guide 1 according to the invention and a corrective element for compensating pupil distortion only. In this case, a parabolic reflector 27 with a radius of curvature of 480 mm is used as the corrective element. In this case, the radius of curvature of the light guide 1 and the focal length of the diffractive lens of the outcoupling structure 13 are again assumed to be 150 mm. From Fig.10 The starting point of the bundle of rays which strikes the parabolic reflector 27 on the left side is outside the image at the focus of the parabolic reflector. Linear astigmatism can also be reduced by a similarly generated secondary pupil distortion in the longitudinal direction.

[0063] Instead of using the described corrective element, the pupil distortion can also be corrected by other off-axis elements.By correcting the pupil distortion, in particular even longer transmission lengths in the light guide are possible.

[0064] In the imaging device according to the invention, if a polarization beam splitter 29 and a retardation plate 32 in the form of a λ / 4 plate are arranged upstream of the input coupling structure, the display 15 can be arranged closer to the input coupling structure, such as Fig.12 For example, see WO 2008 / 023367 A1. Figure 3 Input coupling by means of a polarization beam splitter and a retardation plate is described. Reference is therefore made to WO 2008 / 023367 A1 for its disclosure in relation to input coupling by means of a polarization beam splitter and a retardation plate.

[0065] The invention has been described in detail based on exemplary embodiments for illustrative purposes. However, it is clear to the person skilled in the art that deviations from these exemplary embodiments are possible within the scope of the invention. In this respect, for example, the number of partial field angles can be greater than two, and the number of color channels to be transmitted can be less than or greater than three. In addition, in principle, there is the possibility of providing a refractive lens, such as a Fresnel lens, instead of a diffractive lens to produce a divergent effect. As already mentioned, the divergent effect can also be superimposed on the input coupling structure instead of on the output coupling structure. In addition, there is the possibility of subdividing the divergent effect and superimposing one part on the input coupling structure and another part on the output coupling structure. In addition, other input coupling structures and output coupling structures can be used instead of the diffractive structure, such as reflective structures, such as partially transmitting mirrors, or the described diffractive structures can be replaced by blazed gratings. In the latter case, the diffractive structure can be optimized to a specific diffraction order by appropriately selecting the blaze angle. The diffractive lens can also be implemented as a blazed grating. In order to couple out a specific wavelength range, for example, a dichroic partially transmitting mirror can be used. Although in the described exemplary embodiments the outcoupling structures are not located in the rear surface of the light guide, these outcoupling structures may also be arranged in the front surface, so that the light first passes through the rear surface approximately vertically, for example, and then only reaches the deflection / diffraction structures on the front surface. Accordingly, the light, after passing through the light guide, may be deflected by the deflection / diffraction structures on the front surface in the direction of the rear surface, and then the light passes through the rear surface, for example approximately vertically, out of the light guide. Therefore, the present invention is not intended to be limited to the described exemplary embodiments, but only to the appended claims.

[0066] List of Reference Numerals

[0067] 1 Spherical light guide

[0068] 2 Diverging light beams

[0069] 3 Input coupling structure

[0070] 4 Front surface

[0071] 5 Output coupling structure

[0072] 6 Rear surface

[0073] 7 Longitude Circle

[0074] 8 Converging light beams

[0075] 9. Equator

[0076] 10 Parallel light beams

[0077] 11 Diffractive lens

[0078] 13 Superimposed output coupling structure

[0079] 15 Display

[0080] 17 Correction elements

[0081] 19 Refractive optical elements

[0082] 21 Diffractive optical elements

[0083] 23 Eye Tracker

[0084] 25 Control Unit

[0085] 27 Correction elements

[0086] 29 Polarization beam splitter

[0087] 31 Hysteresis film

Claims

1. A spherically curved light guide (1) for an HMD, wherein: When used as intended, the light guide (1) is arranged in front of at least one eye and has a concave curvature as seen from the eye, and comprises: - a rear surface (6) facing the eye and a front surface (4) facing away from the eye, - an input coupling structure (3) for coupling light into the light guide (1) in such a way that the light is guided through the light guide (1) by reflection at the front surface (4) and / or the rear surface (6), and - an output coupling structure (13) for coupling out the light guided through the light guide (1), It is characterized in that The light guide (1) has a diverging structure (11), the absolute value of the focal length of the diverging structure is less than or equal to the radius of curvature of the light guide (1), The input coupling structure (3) is configured to couple light into the light guide (1) so that the coupled light travels in the light guide (1) along a longitude circle (7) of the light guide (1). The output coupling structure (13) is closer to the equator (9) of the light guide (1) than the input coupling structure (3).

2. The light guide (1) according to claim 1, characterized in that The radiating structure (11) is superimposed on the output coupling structure (13) and / or the input coupling structure (3).

3. The light guide (1) according to claim 1 or 2, characterized in that The diverging structure is a diffractive lens.

4. The light guide (1) according to claim 3, characterized in that The diffractive lens comprises a plurality of zones which differ from one another in terms of their optical path lengths for the light guided through the light guide (1).

5. The light guide (1) according to claim 3, characterized in that The diffractive lens includes a transparent area and a non-transparent area.

6. The light guide (1) according to claim 1 or 2, characterized in that The diverging effect of the diverging structure (11) is centered on the center of the image field.

7. The light guide (1) according to claim 1 or 2, characterized in that The light to be coupled into the light guide (1) has an angular spectrum and / or a spectral range, and the input coupling structure (3) and the output coupling structure (13) each comprise a plurality of substructures (31, 32, 131, 132, 13 1 , 13 2 , 13 3 ), the substructures respectively couple different parts of the spectral range and / or different parts of the angular spectrum into and out of the light guide (1).

8. The light guide (1) according to claim 7, characterized in that including a plurality of sub-light guides (11, 12, 1 1 , 1 2 , 1 3 ), each of the sub-light guides respectively guides the different parts of the spectral range and / or a different part of the different parts of the angular spectrum from the input coupling structure (3) to the output coupling structure (13).

9. The light guide (1) according to claim 1 or 2, characterized in that The incoupling structure (3) is arranged in the light guide (1) in such a way that incoupling can occur through the rear surface (6) of the light guide (1).

10. An imaging optical unit comprising an imaging beam path, a light guide (1) and at least one image forming optical element, characterized in that: The imaging optical unit comprises a light guide (1) as claimed in any one of claims 1 to 9, wherein the light guide (1) serves as the at least one image forming optical element.

11. The imaging optical unit according to claim 10, characterized in that: A correcting element (17, 27) for pre-correcting pupil distortion caused by the light guide is arranged upstream of the input coupling structure (3) of the light guide (1) along the imaging beam path.

12. The imaging optical unit according to claim 10 or 11, characterized in that: A compensating optical unit is arranged upstream of the input coupling structure (3) of the light guide (1) along the imaging beam path, for compensating for the lateral chromatic aberration caused by the light guide (1).

13. The imaging optical unit according to claim 12, characterized in that: The compensating optical unit comprises a diffractive optical element (21) and a refractive optical element (19), which are arranged to be displaceable relative to each other perpendicularly to the optical axis of the imaging beam path, and an eye tracker (23).

14. The imaging optical unit according to claim 10 or 11, characterized in that: A polarization beam splitter (29) and a retardation plate (31) are arranged upstream of the input coupling structure (3) of the light guide (1) along the imaging beam path.

15. An HMD comprising a display (15) for displaying an output image and an imaging optical unit for generating a virtual image from the output image, characterized in that: The imaging optical unit is the imaging optical unit according to any one of claims 10 to 14.

Images