DEVICE FOR PROJECTING AN IMAGE FORMED BY A SCREEN

Description

DOMAINE TECHNIQUE

[0001] The technical field of the invention relates to a directional screen and its use in a device for projecting an image onto an eye, for example in augmented reality type applications. ART ANTERIEUR

[0002] Wearable augmented reality devices, such as glasses, allow users to observe a real scene while viewing additional information. This type of device is often based on micro-screens, which allow an image to be formed in close proximity to a user's eye. Such micro-screens can, for example, be integrated into glasses. An optical system, comprising a set of lenses, allows the eye to perceive a clear image.

[0003] US Patent 9632317, as well as the Martinez publication "See-trough holographic retinal projection display concept", Optica, Vol. 5 No. 10, Oct. 2018, describe a device allowing projection onto the retina of an eye, without a screen or optical system. The device comprises a transparent integrated optical circuit composed of a network of nanometric light guides, an array of electrodes and a holographic film. Such a device is compact, and allows for a wide field of vision. In addition, it eliminates the need for a bulky optical system with a complex design.

[0004] The light guides make it possible to define a set of emission points on the holographic film, each point being capable of being illuminated by light extracted from a light guide. The set of emission points is subdivided into different subsets, each subset comprising emission points distributed, as randomly as possible, on the holographic film. The emission points of the same subset can be simultaneously illuminated by the different light guides. Under the effect of the illumination, each emission point of the same subset emits a light wave propagating in the same direction to the pupil of the eye, so as to form a single luminous point at the level of the retina. In this way, each subset of emission points allows the formation of a pixel of the image perceived by the user.An image can be formed by successively illuminating different subsets of points, so as to form an image with a high number of pixels.

[0005] This configuration allows for a very compact device. However, it requires the use of a large number of different laser sources.

[0006] Other technologies have been described, allowing an image to be projected onto an eye using a compact device. Patent US10254547, for example, describes glasses comprising a device for projecting a virtual image. The operating principle is shown diagrammatically in the figure 1 . A light emitter E is arranged on the frame M of a pair of glasses. The light emitter E generates light beams F propagating towards a holographic reflector H. The holographic reflector H is formed on the lens of the glasses. It is configured to reflect each light beam towards the pupil P of an eye O of a user. The light emitter is formed by a light source coupled to a movable mirror. The movable mirror is moved so as to successively form light beams scanning the holographic reflector. Thus, the user perceives reflected light beams from different angular directions. When the intensity of each beam is modulated during scanning, the user perceives an image.

[0007] Other documents describe configurations in which a light beam scans a holographic reflector. Examples include US2019 / 0285897 or US20180299680.

[0008] A disadvantage of scanning configurations is that the eye box is small. An eye box, commonly referred to as an "eye box," is a volume in which the eye can be moved to perceive a sharp image. Eye movement can be dynamic for a user, as the eye rotates to scan the field of view. It can also vary from one user to another due to differences in interpupillary distances. With a small eye box, a device may be suitable for one user but not for another, for example if the two users have different interpupillary distances.

[0009] Another disadvantage is the use of a mechanical system to perform scanning. The use of a mechanical scanning system and moving components increases the complexity and cost of the device.

[0010] US20060228073 describes devices for projecting an image into an eye comprising a screen and a waveguide as well as holographic patterns which are projected towards an eye.

[0011] The inventors propose an alternative configuration to the previously mentioned scanning projection devices. The objective is to provide a solution without moving components, while improving user comfort by increasing the size of the eye box. EXPOSE DE L'INVENTION

[0012] A first object of the invention is a device for projecting an image onto an eye, the device comprising: a light emitter, configured to emit light waves respectively along different emission axes; an optical combiner, optically coupled to the light emitter, and configured to form, from each light wave emitted by the light emitter, a collimated light wave propagating towards the pupil of the eye; the device being characterized in that: the light emitter comprises a screen, comprising different pixels, each pixel being configured to emit a divergent light wave propagating around an emission axis, the different pixels respectively emitting divergent light waves propagating respectively along different emission axes; the optical combiner is configured to receive each light wave emitted by a pixel and form a collimated light wave propagating towards a central position, corresponding to the center of the pupil of the eye.

[0013] The device may comprise a converging lens interposed between the screen and the optical combiner, the converging lens extending around a center, the center of the lens forming, with a center of the combiner, an optical axis of the device, the lens being arranged such that: the image of the screen, by the converging lens, is formed in an object focal plane of the combiner; the image of the center of the converging lens, by the combiner, is formed at the central position; so that the collimated light wave, resulting from the combiner, reaches the central position by forming an angle depending on the position of the pixel of the screen.

[0014] According to a preferred embodiment: the screen is a directional screen, each pixel being configured to emit a divergent light wave along a predefined emission axis, the light wave propagating at a predefined divergence angle relative to the emission axis; the respective emission axes of pixels converge towards the center of the converging lens.

[0015] The screen may include a stack comprising: light guides, each light guide being coupled to a plurality of diffraction gratings, distributed along the light guide, each diffraction grating being electrically modulatable, each diffraction grating being configured to be electrically modulated so as to extract light propagating in the light guide; electrodes, each electrode being associated with several diffraction gratings respectively coupled to different light guides, each electrode being configured to modulate each diffraction grating with which it is associated; each pixel of the screen corresponding to an association between an electrode and a diffraction grating coupled to a light guide; such that under the effect of illumination by light extracted from the light guide, each pixel is configured to emit a divergent light wave, propagating around an emission axis of the pixel, forming an emission cone, defined by a divergence angle around the emission axis of the pixel.

[0016] The screen may comprise a holographic film subdivided into different elementary zones, each elementary zone being associated with the diffraction grating of a pixel, and configured to emit the divergent light wave, according to the emission axis and the divergence angle of the pixel, under the effect of light extracted by the diffraction grating with which it is associated.

[0017] According to one possibility: several light guides are connected to the same light source; a light modulator extends between the light source and each light guide, so as to modulate an intensity of the light, emitted by the light source, supplying each light guide.

[0018] The device may comprise several light sources, each light source being optically connected to several light guides.

[0019] Different light sources can be configured to emit light at different wavelengths respectively.

[0020] Pixels can be arranged according to: rows, each row being defined by a light guide, the light guide extending along different pixels of the row; columns, each column being defined by an electrode, the electrode extending along different pixels along the column.

[0021] The device may include: several converging lenses, aligned parallel to the directional screen, each lens extending around a center and having the same object focal plane; the device being such that the screen is arranged parallel to each converging lens; each converging lens is associated with pixels of the directional screen; the emission axis of each pixel associated with the lens converges towards the center of the converging lens with which it is associated.

[0022] The device can be such that: two adjacent pixels are separated by a spatial pitch; the screen has a central part, surrounded by a peripheral part; the spatial pitch between two adjacent pixels of the central part is less than the spatial pitch between two adjacent pixels of the peripheral part.

[0023] Preferably, the combiner is a holographic combiner.

[0024] The device can be such that: the screen emits light according to at least one emission spectral band; the holographic combiner is transparent outside the or each emission spectral band; the holographic combiner forms a converging lens in the or each emission spectral band.

[0025] The device may be such that the holographic combiner forms a reflector in the or each emission spectral band.

[0026] The device may be such that the converging lens is movable in translation relative to the screen, along the optical axis. The device may be such that the converging lens has a variable focal length.

[0027] A collimated light wave is a light wave whose divergence or convergence is sufficiently low that the wave is considered to be formed of beams propagating parallel to each other. A low divergence or convergence is defined as forming an angle of divergence (or convergence) of less than 2° or 1°.

[0028] Another object of the invention is a directional screen, comprising different pixels: each pixel being configured to emit a divergent light wave propagating around an emission axis, the different pixels respectively emitting divergent light waves propagating respectively along different emission axes; each pixel being configured to emit a divergent light wave along a predefined emission axis, the light wave propagating at a predefined divergence angle relative to the emission axis.

[0029] The screen may include a stack comprising: light guides, each light guide being coupled to a plurality of diffraction gratings, distributed along the light guide, each diffraction grating being electrically modulatable, each diffraction grating being configured to be electrically modulated so as to extract light propagating in the light guide; electrodes, each electrode being associated with several diffraction gratings respectively coupled to different light guides, each electrode being configured to modulate each diffraction grating with which it is associated; each pixel of the screen corresponding to an association between an electrode and a diffraction grating coupled to a light guide; such that under the effect of illumination by light extracted from the light guide, each pixel is configured to emit a divergent light wave, propagating around an emission axis of the pixel, forming an emission cone, defined by a divergence angle around the emission axis of the pixel.

[0030] The screen may comprise a holographic film subdivided into different elementary zones, each elementary zone being associated with the diffraction grating of a pixel, and configured to emit the divergent light wave, according to the emission axis and the divergence angle of the pixel, under the effect of light extracted by the diffraction grating with which it is associated.

[0031] According to one possibility: several light guides are connected to the same light source; a light modulator extends between the light source and each light guide, so as to modulate an intensity of the light, emitted by the light source, supplying each light guide.

[0032] The display may have multiple light sources, each light source being optically connected to multiple light guides. Different light sources may be configured to emit light at different wavelengths respectively.

[0033] Pixels can be arranged according to: rows, each row being defined by a light guide, the light guide extending along different pixels of the row; columns, each column being defined by an electrode, the electrode extending along different pixels along the column.

[0034] The invention will be better understood by reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0035] There figure 1 represents a configuration of the prior art. The figure 2 is an optical diagram of a device according to the invention. The figure 3A schematizes the angular characteristics of pixel emission from a directional screen. The figure 3B shows the structure of a directional screen. The figure 4 represents a recording of a hologram of a directional screen. The figure 5A represents a layer of a directional screen, in which light guides are formed. The figure 5B shows a layer of a directional screen, in which electrically activatable diffraction gratings are formed, coupled to electrodes. The figure 5C shows a layer of a directional screen with previously recorded holograms. The figure 5D represents a variant of a directional screen allowing the use of different light sources, potentially emitting in different spectral bands. figures 6A à 6F schematize the different layers forming a directional screen. The figure 7A is an optical diagram of a converging lens. The figure 7B shows a recording phase of a part of the holographic lens. The figure 7C represents a use of the holographic lens. The figures 8A et 8B schematize a first example of realization. The figures 8C et 8D schematize a second example of realization. The figure 9 illustrates the collection, by a lens, of a beam emitted by a pixel of a directional screen. The figure 10A shows the efficiency (y-axis) as a function of the position of the pixels relative to the optical axis (x-axis), and this for three angular values ​​of divergence. The figure 10B shows the size of the eye-box formed by the device (y-axis) as a function of the divergence angle. The figure 11 shows a variant of a device, allowing to increase the size of the Eye-Box The figure 12 shows a variant of a directional screen, featuring a high-resolution central portion. EXPOSE DE MODES DE REALISATION PARTICULIERS

[0036] There figure 2 schematizes the main elements of a device 1 according to the invention. The device comprises a screen 10, comprising pixels 10 i . Each pixel 10 i emits a divergent light wave, in a spectral emission band, propagating along an emission axis Δ i . The emission axis Δ i is inclined by an emission angle γ i relative to a direction perpendicular to the screen. Advantageously, the screen 10 is a directional screen, as described below. The respective spectral emission bands of each pixel may be identical or different from each other. For simplification, the figure 2 describes the principle of operation in a plane but the concept applies to a 3-dimensional device with pixels distributed according to a matrix and directions to be considered according to two angles such as longitude and latitude.

[0037] The device comprises a combiner 30. The term combiner designates a component which combines both an optical transparency function, in a transparency spectral band, and an optical return function, in a spectral band of interest, preferably narrow, and possibly shaping of an optical beam, generated off the axis of vision by the screen. The axis of vision corresponds to an axis centered and perpendicular to the exit pupil. The combiner can combine in one or each spectral band of interest, the mirror-type optical function and the converging lens-type shaping function. Outside these spectral bands the component is transparent, the optical beams pass through it without notable disturbance.

[0038] There figure 2 schematizes an “unfolded” optical diagram, in which the combiner 30 is shown operating in transmission. From a divergent incident light wave, the combiner 30 takes the form of a lens forming a collimated light wave, or one that can be considered as such, i.e. weakly divergent. By weakly divergent, we mean a wave whose divergence angle is less than 1°. This makes it possible to form, at eye level, a “long-distance” image, i.e. a distance greater than 2 m. Optical diagrams according to which the combiner operates in reflection are described below, in connection with the figures 8B And 8D .

[0039] The combiner is configured to form, from each light wave emitted by a pixel, a collimated light wave propagating towards the pupil of the user's eye. Preferably, the combiner is a holographic combiner, the lens function and the mirror function being encoded in a hologram formed along the combiner.

[0040] The use of a holographic combiner is known to those skilled in the art. A holographic combiner has the advantage of being compact, since it is formed by a thin holographic layer deposited on a support, such as a spectacle lens. A holographic combiner is very wavelength selective. The hologram is transparent to the majority of the visible spectrum, except for a specific wavelength to which it is sensitive. The convergence function of the combiner, as well as the angular deflection function, are encoded in the hologram, as described below.

[0041] The device comprises a converging lens 20, defining an object focal plane and an image focal plane. The converging lens 20 is interposed between the screen 10 and the combiner 30. The screen 10 is arranged at an object plane of the converging lens 20. The converging lens forms an image of the screen in an image plane, the latter corresponding to the object focal plane of the combiner. The lens 20 and the combiner 30 are centered relative to an optical axis Δ 0 of the device. The use of the lens 20 makes it possible to move the screen away from the combiner. This facilitates the integration of the device into equipment such as virtual reality glasses or headsets.

[0042] On the figure 2 , the ratings wy 1 , wy 2 , wy 3 , wy 4 , wy 5 , wy 6 respectively designate the sizes of the beam of the light wave emitted by the pixel 10 i , at points B1, C1, B2, B3, C3 and B4 along the wave propagation axis. The screen extends along an object plane passing through point A1, with a normal vector coincident with or close to the optical axis Δ 0; By close, we mean according to an angular tolerance of ± 5°. the lens 20 forms an image of the screen in an image plane, with a normal vector close to the optical axis Δ 0; the point C1 corresponds to the center of the lens 20; the point C2 corresponds to the center of the combiner 30; the point C3 corresponds to a position, called the central position, corresponding to the center of the pupil P of an eye O of the user. When the user uses the device, he is assumed to center his pupil on the central position. The point C3 also corresponds to the center of the image box (Eye Box) formed by the device. The image box corresponds to the movement of the user's pupil, around the point C3, making it possible to observe the image displayed on the screen. The combiner is arranged so as to conjugate the point C1 (center of the lens, entrance pupil) with the point C3 (center of the exit pupil).point B1 corresponds to the position of pixel 10 i of screen 10; point B2 corresponds to the image of point B1 by lens 20; point B3 corresponds to an intersection between the emission axis Δ i and the combiner. point B4 corresponds to a point of the retina R conjugated to point B1 by the system formed by lens 20 and combiner 30. wy 6 corresponds to the size of the beam at a pixel of the image formed, on the retina, by the device. The points C1, C2 and C3 are aligned along the optical axis defined by the lens 20 and the combiner 30. The screen is in a plane with a normal vector close to the optical axis Δ 0 . Preferably, the point A1, which corresponds to the center of the screen, is also on the optical axis Δ 0 , as shown in the figure 2 The distance between points C2 and C3 corresponds to an eye relief, usually referred to as “EyeRelief”. In a reflection configuration, points C1, C2 and C3 are not aligned due to the beam folding carried out by the combiner. Cf. figures 8B And 8D .

[0043] The screen 10 extends in a plane with a normal vector close to the optical axis Δ 0 , passing through A1. As previously indicated, an image of the screen 10 is formed, by the lens 20, in an image plane with a normal vector close to the optical axis Δ 0 and passing through the point A2. The image plane is arranged, or is close enough to be considered as being arranged, on the object focal plane of the holographic combiner 30.

[0044] The holographic combiner 30 is arranged such that the image of the point C1, by the holographic combiner, corresponds to the point C3, that is to say a position at which the user places the pupil P of his eye O. The collimated light wave, resulting from the combiner 30, propagates towards the pupil forming an angle α i , called the apparent angle. The apparent angle α i depends on the position of the point B1, that is to say of the pixel 10 i .

[0045] Thus, at each pixel 10 i of the screen 10, the optical system formed by the lens 20 and the combiner 30, defines an angle α i according to which the light wave emitted by the pixel, and collimated by the combiner 30, reaches the user's pupil. The apparent angles α i , determined for two different pixels are different.

[0046] The collimated light wave resulting from the combiner is focused by the eye so as to form a pixel of an image of the screen, formed on the retina R of the eye. The pixel of the image formed on the retina corresponds to point B4. The position of B4 is defined by the apparent angle α i , the latter being different for each pixel 10 i . It is understood that the device allows formation of the image formed by the screen 10, on the retina R.

[0047] The combiner allows the generation of a collimated wave at eye level so that when the user looks at an object in the distance (eye / object distance large compared to the size of the eye), for example the summit of a mountain, he can also perceive the image of the screen, the latter generating augmented reality type information, for example the name of the summit and its altitude. The notion of collimation is therefore relative. Although theoretically associated with an image placed at infinity, it can also apply to an image placed at a great distance (typically beyond 100 times the size of the eye, approximately 2 meters from the observer). The device described in figure 2 can be fixed, frozen by the distances between the screen and the optics (lens 20 and combiner 30). The image projected into the eye represents the screen as if the latter were at infinity. Advantageously, it is possible to make the device dynamic by moving the screen in the object plane of the lens or by using a lens 20 with variable focal length. This makes it possible to adjust the accommodation distance.

[0048] Preferably, the screen 10 is a directional screen. The term directional screen designates a screen in which each pixel is configured to emit a divergent light wave along an emission axis, forming an emission angle relative to a direction normal to the screen, the light wave propagating at an angle of divergence relative to the emission axis, the screen being such that: the divergence angle is predetermined, preferably less than 45° or less than 30°; and / or the emission axes of two different pixels are different; and / or the divergence angles of two different pixels are different; and / or at least one emission axis of a pixel is inclined relative to the direction normal to the screen.

[0049] The emission axis of the pixel 10 i is oriented towards the center of the lens 20. Thus, each pixel 10 i of the screen 10 is configured to emit a light wave, around an emission axis Δ i whose emission angle γ i is such that the emission axis Δ i passes through the center C1 of the lens 20. As a result, the respective emission angles of different pixels 10 i are different from each other and converge towards the center C1 of the lens 20.

[0050] THE figures 3A et 3B show the operation of the directional screen 10. The directional screen is formed by several pixels 10 i , preferably arranged in rows and columns. Each pixel is configured to emit a light wave at an emission angle, relative to a direction D normal to the plane along which the screen extends. On the figure 3A , we have represented: a pixel 10 1 , emitting a divergent light wave propagating along a propagation axis forming an emission angle γ 1 relative to the direction D, and forming a cone whose half-angle at the apex, called the divergence angle, is noted β 1 ; a pixel 10 2 , emitting a divergent light wave propagating along a propagation axis forming an emission angle γ 2 relative to the direction D, and forming a divergence angle β 2 .

[0051] There figure 3B schematizes a structure of the directional screen. The directional screen comprises light guides 11. Each light guide 11 is connected to a light source 11 in . Unlike the configuration described in US9632317, the light source 11 in can be a laser source, but also a non-coherent source, for example a light-emitting diode. In the example shown, each light guide extends along a line, and more precisely along different pixels of the line. The light guides can for example be formed of SIN (silicon nitride), deposited in a layer of SiO 2 .

[0052] The screen includes: a first layer, in which light guides 11 are formed. The light guides are configured to receive the light emitted by the light source 11 in a second layer, in which diffraction gratings 12 are formed, such that each diffraction grating 12 is coupled with a light guide 11. The diffraction gratings 12 are electrically modulated. Each diffraction grating 12 corresponds to a periodic variation in refractive index, capable of being electrically modulated. The diffraction gratings 12 coupled to the same light guide 11 are spaced from each other along the light guide, and are considered to be point-like. Each diffraction grating 12 may be formed of inclusions, defining a periodic pattern, in silicon oxide (SiO 2 ), each inclusion being formed of a material whose refractive index is electrically modulatable, for example a liquid crystal.When the wavelength of light is 532 nm, the period of the diffraction grating pattern 12 can be between 200 nm and 500 nm. A diffraction grating can extend in 10 periodic patterns, and thus extend over a length of 2 or 5 µm. a third layer, in which transparent electrodes 13 are formed, the electrodes being configured to electrically modulate the refractive index of a material forming the diffraction gratings. The transparent electrodes may be formed from a transparent conductive material, for example ITO (indium tin oxide). Each electrode may thus activate a diffraction grating under the effect of electrical modulation. In the example shown, the transparent electrodes extend parallel to columns. a fourth layer, called the holographic layer, corresponding to a holographic film 14. By holographic film, is meant a photosensitive support capable of recording a hologram. The holographic film is assumed to be sufficiently thin to be assimilated to the emission surface. The holographic film may be a photopolymer of the photoresist type or a suspension of light-sensitive compounds such as a silver halide.

[0053] The layers are formed on a transparent support. This can be, for example, a glass or polycarbonate support.

[0054] Under the effect of polarization by an electrode 13, each point diffraction grating 12 is activated, in the sense that it allows extraction of a portion of the light propagating in a light guide 11 to which the diffraction grating 12 is coupled. The extracted light propagates towards the holographic film 14, and more precisely towards an elementary zone 14 i of the holographic film 14 i . Under the effect of illumination, the elementary zone of the holographic film emits a light wave according to predefined angular characteristics. By angular characteristics, we mean an emission angle γ i and a divergence angle β i .

[0055] Thus, each pixel 10 i of the screen corresponds to a superposition of a point diffraction grating 12 coupled to a light guide 11, and of an electrode 13, facing an elementary zone 14 i of the holographic film 14. The association between each electrode 13 and each diffraction grating 12 forms a structure for extracting a portion of the light propagating in a light guide 11.

[0056] In the example shown, the light guides 11 are coplanar. The same is true of the electrodes 13. Thus, the electrodes 13 are superimposed on the light guides 11. Each electrode “crosses” several light guides, so as to define several intersections, each intersection corresponding to a pixel of the screen. The term “cross” is to be interpreted as designating a superposition of an electrode and a light guide. The position of each pixel is defined by the positioning of the light guides and the electrodes. The angular emission characteristics are defined by the hologram forming the elementary zone 14 i , illuminated by an extraction of the light propagating in the light guide.

[0057] On the figure 3B , a 10 2 pixel and a 10 4 pixel are shown. Each elementary zone of the hologram facing these pixels is respectively configured to emit a light wave according to predefined angular characteristics, coded in the hologram. On the figure 3B , the emission angles γ 2 and γ 4 defined for the pixels 10 2 and 10 4 have been represented. The angular emission characteristics can be defined, for each pixel, independently of the other pixels.

[0058] Splitters 11' may be arranged, for example Y-junctions, so as to distribute the light, emitted by the same light source 11 in , to different light guides 11. In order to modulate the intensity of the light propagating in a light guide, each light guide 11 may be coupled to a modulator. On the figure 3B , four modulators M1, M2, M3 and M4 are shown. Each modulator comprises an extractor 16, configured to be electrically activatable, so as to extract all or part of the light propagating in a light guide. Each extractor may be similar to a diffraction grating 12 as previously described. When an extractor is activated, the light propagating through the light guide 11 is extracted, preferably towards an absorber 17. The presence of the absorber makes it possible to dissipate the extracted light, in order to avoid propagation of stray light through the screen 10. The use of the modulators makes it possible to adjust the intensity of simultaneously activated pixels of the screen.

[0059] The angular emission characteristics of each pixel 10 i are defined during a preliminary phase of recording the holographic film 14. In a known manner, a hologram is formed by interference between two light waves emitted by the same light source: an object light wave and a reference light wave. The interference fringes generated are stored physically or chemically in the holographic film 14. On the figure 4 , an assembly is shown allowing the recording of an elementary zone 14 i of the holographic film 14.

[0060] A light source is coupled to two fibers using a splitter, so as to obtain a fiber forming an object beam 42 and a fiber forming a reference beam 41. The light source has a wavelength close to that of the light 11 in to which the screen 10 is connected during its use. This source is typically a laser with a long coherence length (greater than one meter).

[0061] The fiber forming the reference beam 41 reproduces illumination conditions similar to those obtained by extracting light from a light guide 11 by activating a diffraction grating 12. Illumination conditions are understood to mean the incidence of the beam, its size and its divergence. The reference beam 41 is fixed and is formed by a reference shaping optical system 43.

[0062] The object beam is generated by an object shaping optical system 44 coupled to a converging focusing optic 45. This makes it possible to adjust an angle of incidence γ i and an angle of divergence β i of the object beam. The recording of a hologram, in an elementary zone 14 i , is carried out by simultaneously exposing said elementary zone to the object beam and to the reference beam. The different holograms, at the level of each elementary zone, are produced by moving the holographic film 14 and possibly modifying the characteristics of the object beam, in particular the angle of incidence γ i and the angle of divergence β i . Thus, each elementary zone 14 i is assigned an angle of incidence γ i and an angle of divergence β i , which correspond to the angles of incidence and divergence of the object beam during the recording of the hologram.

[0063] THE figures 5A à 5D schematize different layers previously mentioned. On the figure 5A , a structured layer is shown, defining the light guides 11, formed on a glass substrate 15. The figure 5B shows an extraction layer, formed by the diffraction gratings 12, 16 previously described. The figure 5C shows a holographic layer, comprising the elementary holographic zones 14 i as well as the absorber 17.

[0064] An advantage of the holographic screen is to reduce the number of light sources compared to the configuration described in US9632317. The directional screen can be formed from a single light source. It is then monochrome. The directional screen can be formed from several light sources emitting according to different spectral bands. Such a configuration is shown in the figure 5D . Several light guides can be formed, on the same layer, so as to form independent light guide networks. Each light guide network is intended to be optically coupled to a light source emitting in a given spectral band. On the figure 5D , two light guide networks are shown intended to be optically coupled to two light sources 11 in1 and 11 in2. The different light guides can be produced on the same substrate 15. The arrangement of the light guides makes it possible to avoid interference (cross-talk) between the light guides at each intersection.

[0065] THE figures 6A à 6F illustrate the manufacturing steps of a directional screen 10. On the figure 6A , Strips of reflective material (for example a metal such as aluminum Al), intended to act as a reflector, are deposited on a substrate 15, forming lines. The aluminum strips, 1 µm wide, are spaced 5 µm apart from each other.

[0066] On the figure 6B , a layer of addressing electrodes 13 is shown, in the form of a structured layer of ITO (Indium Tin Oxide), 40nm thick. The ITO layer can be structured, so as to form electrodes extending along columns, perpendicular to the lines.

[0067] On the figure 6C , the deposition of a layer of SlO 2 is shown, in which light guides 11 made of SiN are formed. The guides have a width of approximately 400 nm and a thickness between 100 nm and 400 nm. The choice of SiN is justified by its transparency properties in the visible range.

[0068] On the figure 6D , a deposition of a structured layer intended to form a diffraction grating 12 is shown. The diffraction grating is preferably formed in a material that is simple to structure, for example a sol-gel type material.

[0069] The diffraction grating is encapsulated in a liquid crystal layer LC, the refractive index of which can switch between two values ​​depending on the voltage applied by the electrodes. Depending on the value of the refractive index, the diffraction grating 12 allows extraction of the light propagating through the light guide. A transparent counter-electrode 13', for example made of ITO deposited on a transparent film 13s (made of glass or transparent plastic material), is arranged on the liquid crystal layer. Cf. figure 6E .

[0070] On the figure 6F , the deposition of a holographic layer 14, supported by a transparent support 14s, against the film 13s allowing the encapsulation of the liquid crystal is shown. The holographic layer 14 can be formed from a photopolymer, 15 µm thick, while the support 14s can be formed from glass, 700 µm thick. The holographic layer 14 has previously been recorded, as described in connection with the figure 4 .

[0071] On the figure 6F , we have represented all the layers forming the directional screen 10. The total thickness is of the order of 1.5 mm. The surface of each pixel can be 5 µm x 5 µm. We can thus form a screen with a resolution of 1920 x 1080 whose surface is 10 mm x 5 mm.

[0072] The configuration described on the figure 2 works regardless of the type of screen. However, optimal performance is achieved by using a directional screen as previously described. More specifically, it is advantageous for each pixel of the screen, contributing to the image formed on the retina, to emit a light wave along an emission axis passing through the center of the lens 20. Thus, the emission angles assigned to each pixel are adjusted so as to converge towards the center of the lens 20. This makes it possible to increase the quantity of light forming the image on the retina.

[0073] There figure 7A details the operation of a converging lens. On the figure 7A , the lens conjugates point A' with point A. Points F and M belong to the object focal plane. The light waves emitted by points F and M are returned to infinity by the lens. Point F corresponds to the focus of the lens. Point M is offset from point F in the object focal plane. At the exit of the lens, the beams passing respectively through points F and M are collimated, and angularly deviated relative to each other.

[0074] There figure 7B illustrates a phase of recording a hologram on the holographic combiner. An elementary zone of the holographic material, forming the combiner, is exposed to a divergent reference beam F1, emitted from a point F, and to a collimated object beam F2, the two beams being emitted by the same light source. The light source used is preferably coherent, for example a laser source, emitting in a recording spectral band. The hologram resulting from the interference between the beams F1 and F2 is stored in the holographic material. The figure 7C shows the use of the holographic combiner: under the effect of exposure to a divergent light beam F3, emitted from point F, which corresponds to the focus of the lens, the previously stored hologram reflects a collimated light beam F4. If the beam F3 is emitted from a point A, distant from the focus F, the combiner reflects a convergent beam towards a point A'.

[0075] In the example described on the figures 7B et 7C , the holographic combiner forms a holographic lens reflecting light in the wavelength of beams F1 and F2. This makes it possible to form a holographic reflector, suitable for reflection configurations described on the figures 8B And 8D The holographic combiner only operates in a narrow spectral band, which corresponds to the recording spectral band. Outside of this spectral band, the holographic combiner transmits light. The holographic combiner can be placed on a lens of a goggle or on a visor of a virtual reality headset.

[0076] THE figures 8A et 8B show a first example of realization, respectively according to an unfolded and folded representation. The design parameters are: ER (Eye Relief), corresponding to the distance between the diopter closest to the eye, in this case the combiner 30, and the eye: 30 mm; distance between the screen 10 and the lens 20: z e = 16 mm focal length of the lens 20: f 1 = 10 mm; combiner focal length 30: f 2 = 18 mm; Screen size: 5 mm x 5 mm; Field of view: FOV = 26 °;

[0077] Setting the parameters allows you to set the distance DL between lens 20 and combiner 30: DL = z e × f 1 z e − f 1 + f 2

[0078] We find a distance DL = 44.667 mm.

[0079] The field of observation ( FOV ) is determined by the following relation: FOV = 2 α max = 2 × tan − 1 d e 2 × f 2 × z e × DL − f 2

[0080] Or α max is the apparent angle of the screen pixel furthest from the optical axis.

[0081] We find FOV = 26°.

[0082] If we want to change the accommodation plane, we can do so by actively changing the optical parameters of the system. We determine Z ac the accommodation distance, it is given by the formula: Z ac = f 1 − z e × D L + z e × f 1 × f 2 z e × f 1 + D L − f 2 × f 1 − z e + ER

[0083] The accommodation distance Z ac can be varied by modifying z e (screen-lens distance 20) or by changing the focal length f 1, using a 20 lens with variable focal length.

[0084] To move from an accommodation distance of infinity to Z ac = 2 m it is necessary to move the screen from a distance of 16 mm to 15.94 mm, which corresponds to a displacement of 60 µm. Such a displacement can be obtained by using a translation support, for example motorized, or piezoelectric, allowing a translation of the screen relative to the lens, along the optical axis. The same effect is obtained with a lens 20 whose focal length varies from 10 mm to 10.022 mm.

[0085] On the figure 8B , a reflection configuration is shown, in which the combiner acts as a reflector. Such a configuration, with a significant eye relief (30 mm), is more suitable for mounting on a mixed reality headset. The reflector being transparent, except in wavelengths to which holograms are sensitive, it allows an image I s of a scene propagating towards the user's eye to be transmitted. The combiner can also be opaque to address virtual reality applications. The combiner then forms an optical reflector which can be constituted by a holographic component as described above or by a "free form" mirror whose curvature allows the desired convergence effect to be obtained. The angle θ corresponds to a folding angle. In this example, θ = 32°. In this embodiment, in reflection, the use of a holographic combiner is particularly advantageous.

[0086] On the figures 8C et 8D , we have schematized a second example of realization, respectively in transmission and in reflection, with the following parameters: ER (Eye Relief): 15 mm; distance between screen 10 and lens 20: z e = 20 mm focal length of lens 20: f1 = 10 mm; focal length of combiner 30: f2 = 10 mm; Screen size: 5 mm x 5 mm; field of view: FOV = 28°

[0087] For this configuration, DL is set to 30 mm. To move from infinity accommodation to Zac = 2 m, the screen-lens distance must be changed from 20 mm to 19.95 mm.

[0088] This configuration is more compact than the one described in connection with the figures 8A et 8B . The eye relief is 15 mm, which is suitable for integration into scope-type equipment. On the figure 8D , we have represented a configuration in reflection, with a folding angle of θ = 45°.

[0089] The configuration shown on the figure 2 , and examples of which have been given in connection with the figures 8A à 8D , can work for any type of screen. The use of a directional screen is advantageous, because it allows to increase the luminous efficiency of the device. As previously described, the emission of each pixel 10 j is parameterized by angular characteristics γ i and β i . It was previously established that it was preferable for the emission angle γ i to be such that the emission axis of each pixel passes through the center of the lens 20. The divergence angle β i can be optimized, so as to increase the luminous efficiency of the device.

[0090] On the figure 9 , a homogeneous beam F i emitted by a pixel 10 i is shown, the emission angle of which is oriented in a direction perpendicular to the surface of the screen. The beam is distributed, in the plane formed by the lens 20, according to a disk of surface S 1 and radius wy 2 (cf. figure 2 ). S 2 corresponds to the surface of the lens 20, of radius R 20 . S 3 corresponds to the intersection of S 1 and S2. S 3 must be as high as possible in order to optimize the collection, by the lens 20, of the beam emitted by the pixel. We can define an efficiency ρ , such as : ρ = s 3 s 1

[0091] The yield ρ can be calculated geometrically, by posing: d = wy 2 + dy 2 − R 20 2 2 dy d ′ = dy − d

[0092] Let S a And S b , the parts of the surfaces S 2 and S 1 respectively delimited by the dotted line shown on the figure 9 . S a + S b = S 1 + S 2 − S 3 S a = wy 2 2 × cos − 1 d wy 2 − d wy 2 2 − d 2 S b = R 20 × cos − 1 d ′ R 20 − d ′ R 20 2 − d ′ 2 ρ = S 3 πwy 2 2 And : wy 2 = Z e tan β i Z e is the distance between the lens 20 and the screen 10.

[0093] The yield was calculated ρ according to expression (7), considering three values ​​of β i : 2°, 10° or 30°. The figure 10A represents the efficiency value (y-axis) as a function of pixel distance from the center of the screen. We observe that for a divergence of 10° the efficiency is only optimal for pixels in the center of the screen. Since the emission angle is not controlled, the efficiency drops as the pixels move away from the center of the screen.

[0094] When the divergence angle is 2°, 100% efficiency is obtained for pixels located around the center of the screen. This corresponds to a configuration in which the surface S2 is small and contained within the surface S1.

[0095] The smaller the divergence angle, the better the efficiency. This confirms the interest of a directional screen, weakly divergent, in the implementation of the invention. By weakly divergent, it is understood that the divergence angle of each pixel is less than 20°, and preferably less than 15° or 10°. It is recalled that the divergence angle is the half-angle at the apex of the emission cone of the pixel.

[0096] Besides efficiency, another performance indicator is the size of the eyebox, which also depends on the divergence angle. Indeed, the EB side of the eyebox is such that: EB = 2 wy 5 = 2 f 2 Z e − f 1 f 1 tan β i Or f 1 and f 2 are the respective focal lengths of the lens 20 and the combiner 30.

[0097] On the figure 10B , we represented the dimension EB on one side of the Eye-Box (y-axis - unit mm) as a function of the angle β i (x-axis - unit degree). We observe that the higher the divergence, the larger the size of the Eye-Box, which improves the comfort of use of the device. When the angle β i of each pixel is equal to 10°, the Eye-Box measures 4 mm on each side, which is acceptable. On the figure 10A , we observe that if the emission directivity of the pixels is not adapted, a divergence angle of 10° leads to a loss of efficiency of 40% for the pixels arranged at the periphery of the screen 10. According to our invention, with a control of the directivity which targets the center of the lens 20, the efficiency is close to 100% for all the pixels while maintaining a reasonable eye-box.

[0098] THE figures 11 And 12 illustrate variants of realization. On the figure 11 , a configuration is shown in which the lens 20 is formed by several converging elementary lenses 20 a , 20 b and 20 c coplanar and aligned perpendicular to the optical axis Δ 0 . Each lens is optically coupled to a group of pixels of the screen, these pixels being associated with said lens. The pixels associated with a lens have an emission axis converging towards the center of said lens. On the figure 11 : the pixels located between the points Ma1 and Ma2 are associated with the elementary lens 20 a: they form a first sub-screen; the pixels located between the points Mb1 and Mb2 are associated with the elementary lens 20 b: they form a second sub-screen; the pixels located between the points Mc1 and Mc2 are associated with the elementary lens 20 c: they form a third sub-screen;

[0099] According to this configuration: the pixel placed at point Ma1 is imaged, by lens 20 a , at point B1; the pixel placed at point Ma2 is imaged, by lens 20 a , at point B2; the pixel placed at point Mb1 is imaged, by lens 20 b , at point B1; the pixel placed at point Mb2 is imaged, by lens 20 b , at point B2; the pixel placed at point Mc1 is imaged, by lens 20 c , at point B1; the pixel placed at point Mc2 is imaged, by lens 20 c , at point B2;

[0100] This configuration allows three images I a , I b , and I c to be formed at the pupil, corresponding respectively to the pixels Ma1 to Ma2, Mb1 to Mb2, Mc1 to Mc2. The screen is then segmented into three sub-screens, each sub-screen being associated with a lens. The pixels of the same sub-screen allow the formation of three images. If the three groups of pixels contain the same content, the images I a , I b , and I c are identical. Such a configuration allows the Eye-Box to be duplicated, so as to increase the spatial movement of the eye.

[0101] There figure 12illustrates a variant in which the spatial pitch separating two adjacent pixels is variable. More precisely, the screen is segmented into a central part, comprising 8 x 8 pixels, surrounded by a peripheral part. The spatial pitch is reduced at a central part of the screen, compared to the spatial pitch separating adjacent pixels in the peripheral part. This improves the spatial resolution of the screen at the fovea, which is a central area of ​​the retina whose angular radius extends up to 3° or 5°. At the fovea, the spatial resolution of the eye is particularly high. Such a screen, referred to as a foveated screen, is adapted to the variation of the spatial resolution of the eye.

Claims

1. A device (1) for projecting an image onto an eye (O), the device comprising: - a light emitter, configured to emit light waves along various respective emission axes; - an optical combiner (30), optically coupled to the light emitter, and configured to form, from each light wave emitted by the light emitter, a collimated light wave that propagates towards a pupil (P) of the eye; wherein: - the light emitter comprises a directional screen (10), comprising various pixels (10i), each pixel being configured to emit a divergent light wave that propagates around an emission axis, the various pixels emitting respective divergent light waves propagating at a predetermined divergence angle (βi) relative to their respective emission axis; - the optical combiner is configured to receive each light wave emitted by a pixel and to form a collimated light wave that propagates towards a central position (C3) corresponding to the centre of the pupil of the eye ; the device comprising a convergent lens (20) interposed between the screen (10) and the optical combiner (30), the convergent lens (20) extending around a centre, the centre of the lens forming, with a centre of the combiner (C2), an optical axis (Δ0) of the device, the lens being arranged so that - the image of the screen, as generated by the convergent lens (20), is formed in an object focal plane of the combiner; - the image of the centre of the convergent lens, as generated by the combiner (30), is formed at the central position (C3); - the respective emission axes of pixels converge to the centre of the convergent lens. - so that the collimated light wave, resulting from the combiner, reaches the central position at an angle (αi) dependent on the position of the pixel (10i) of the screen;2. The device according to Claim 1, wherein the screen comprises a stack comprising: - light guides (11), each light guide being coupled to a plurality of diffraction gratings (12), which are distributed over the length of the light guide, each diffraction grating being electrically modulatable, each diffraction grating being configured to be electrically modulated so as to extract light propagating through the light guide; - electrodes (13), each electrode being associated with a plurality of diffraction gratings (12) coupled to various light guides, respectively, each electrode being configured to modulate each diffraction grating with which it is associated; each pixel of the screen corresponding to an association between an electrode and a diffraction grating coupled to a light guide; so that when illuminated by light extracted from the light guide, each pixel is configured to emit a divergent light wave that propagates around an emission axis of the pixel, thereby forming an emission cone, defined by a divergence angle around the emission axis of the pixel.

3. The device according to Claim 2, wherein the screen comprises a holographic film (14), which is subdivided into various elementary zones, each elementary zone being associated with the diffraction grating (12) of one pixel, and configured to emit the divergent light wave, along the emission axis and at the divergence angle of the pixel, under the effect of light extracted by the diffraction grating with which it is associated.

4. The device according to any one of Claims 2 or 3, wherein: - a plurality of light guides are connected to the same light source; - a light modulator lies between the light source and each light guide, so as to modulate an intensity of the light emitted by the light source and fed to each light guide.

5. The device according to any one of Claims 2 to 4, comprising a plurality of light sources, each light source being optically connected to a plurality of light guides.

6. The device according to any one of Claims 2 to 5, wherein various light sources are configured to emit light at various respective wavelengths.

7. The device according to any one of Claims 2 to 6, wherein the pixels are arranged in: - rows, each row being defined by one light guide, the light guide extending over the length of various pixels in the row; - columns, each column being defined by one electrode, the electrode extending over the length of various pixels over the length of the column.

8. The device according to any one of the preceding Claims, comprising: - a plurality of convergent lenses (20a, 20b, 20c) aligned parallel to the directional screen, each convergent lens extending around a centre and having the same object focal plane; the device being such that - the screen is placed parallel to each convergent lens; - each convergent lens is associated with pixels of the directional screen; - the emission axis of each pixel associated with the lens converges to the centre of the convergent lens with which said pixels is associated.

9. The device according to any one of the preceding Claims, wherein: - two adjacent pixels are separated by a spatial pitch; - the screen comprises a central part, encircled by a peripheral part; - the spatial pitch between two adjacent pixels of the central part is smaller than the spatial pitch between two adjacent pixels of the peripheral part.

10. The device according to any one of the preceding Claims, wherein the combiner is a holographic combiner.

11. The device according to Claim 10, wherein: - the screen emits light in at least one emission spectral band; - the holographic combiner is transparent outside of the or each emission spectral band; - the holographic combiner forms a convergent lens in the or each emission spectral band.

12. The device according to Claim 11, wherein the holographic combiner forms a reflector in the or each emission spectral band.

13. The device according to any one of the preceding Claims, wherein the convergent lens is movable translationally with respect to the screen, along the optical axis.

14. The device according to any one of the preceding Claims, wherein the convergent lens has a variable focal length.