A device for projecting an image formed by a screen

Abstract

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. – figure 2.;

Description

Title of the invention: Device for projecting an image formed by a screen. Technical field

[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. EARLIER ART

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

[0003] US patent 9632317, as well as the Martinez publication "See-through holography retinal projection display concept," Optica, Vol. 5 No. 10, Oct. 2018, describe a device for projecting onto the retina of an eye without a screen or optical system. The device comprises a transparent integrated optical circuit consisting of an array of nanometric light guides, an electrode array, and a holographic film. Such a device is compact and provides a wide field of view. Furthermore, it eliminates the need for a bulky and complex optical system.

[0004] Light guides allow a set of emission points to be defined 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 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 point of light on 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 large number of pixels.

[0005] Such a configuration makes it possible to form a very compact device. However, this assumes the use of a large number of different laser sources.

[0006] Other technologies have been described that allow an image to be projected onto an eye using a compact device. US patent 10254547, for example, describes a pair of glasses incorporating a virtual image projection device. The operating principle is shown schematically in [Fig. 1]. A light emitter E is arranged on the frame M of a pair of glasses. The light emitter E generates light beams F that propagate 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 a user's eye O. The light emitter consists of a light source coupled to a movable mirror. The movable mirror is moved so as to successively form light beams that sweep across the holographic reflector. Thus, the user perceives reflected light beams from different angular directions.When the intensity of each beam is modulated during the scan, the user perceives an image.

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

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

[0009] Another drawback is related to the use of a mechanical system for performing the scanning. The use of a mechanical scanning system and moving components increases the complexity and cost of the device.

[0010] The inventors propose an alternative configuration to the previously mentioned scanning projection devices. The objective is to offer a solution without moving components, while improving user comfort by increasing the size of the eye box. Description of the invention

[0011] 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;

[0012] the device being characterized in that: - the light emitter includes 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 include 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 dependent on the position of the screen pixel.

[0014] According to a preferred embodiment: - the screen is a directional screen, each pixel being configured to emit a diverging 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 modulated, 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 different networks fraction respectively coupled to different light guides, each electrode being configured to modulate each diffraction grating to 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;

[0016] so 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.

[0017] The screen may include 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, along the emission axis and the divergence angle of the pixel, under the effect of light extracted by the diffraction grating to which it is associated.

[0018] 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, feeding each light guide.

[0019] The device may include several light sources, each light source being optically connected to several light guides.

[0020] Different light sources can be configured to emit light respectively according to different wavelengths.

[0021] The pixels can be arranged according to: - lines, each line being defined by a light guide, the light guide extending along different pixels of the line; - columns, each column being defined by an electrode, the electrode extending along different pixels along the column.

[0022] The device may include: - several converging lenses, aligned parallel to the directional screen, each lens extending around a center and presenting the same object focal plane;

[0023] the device being such that - the screen is positioned 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 to which it is associated.

[0024] The device may be such that: - two adjacent pixels are separated by a spatial step; - the screen has a central part, surrounded by a peripheral part; - the spatial step between two adjacent pixels in the central part is less than the spatial step between two adjacent pixels in the peripheral part.

[0025] Preferably, the combiner is a holographic combiner.

[0026] The device may be such that: - the screen emits light according to at least one spectral emission band; - the holographic combiner is transparent outside of the emission spectral band(s); - the holographic combiner forms a converging lens in the spectral emission band(s).

[0027] The device may be such that the holographic combiner forms a reflector in the or each spectral emission band.

[0028] 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.

[0029] A collimated light wave is defined as a light wave whose divergence or convergence is sufficiently small that the wave is considered to be formed of beams propagating parallel to each other. Small divergence or convergence is defined as forming a divergence (or convergence) angle of less than 2° or 1°.

[0030] 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 emitting respectively divergent light waves propagating respectively along different emission axes; - each pixel being configured to emit a diverging light wave along a predefined emission axis, background light propagating along a predefined divergence angle relative to the emission axis.

[0031] 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 modulated, 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 to which she is associated; - each pixel of the screen corresponding to an association between an electrode and a diffraction grating coupled to a light guide;

[0032] so 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.

[0033] The screen may include 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, along the emission axis and the divergence angle of the pixel, under the effect of light extracted by the diffraction grating to which it is associated.

[0034] 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, feeding each light guide.

[0035] The screen may include several light sources, each light source being optically connected to several light guides. Different light sources may be configured to emit light at different wavelengths.

[0036] The pixels can be arranged according to: - lines, each line being defined by a light guide, the light guide extending along different pixels of the line; - columns, each column being defined by an electrode, the electrode extending along different pixels along the column.

[0037] The invention will be better understood upon reading the description of the exemplary embodiments presented later in this description, in connection with the figures listed below. FIGURES

[0038] Fig. 1 represents a prior art configuration.

[0039] Fig. 2 is an optical diagram of a device according to the invention.

[0040] Fig. 3A schematically illustrates the angular emission characteristics of pixels of a directional screen.

[0041] Fig. 3B shows the structure of a directional screen.

[0042] Figure 4 represents a recording of a hologram of a directional screen.

[0043] Fig. 5A represents a layer of a directional screen, in which light guides are formed.

[0044] Fig. 5B shows a layer of a directional screen, in which electrically activatable diffraction gratings coupled to electrodes are formed.

[0045] Fig. 5C shows a layer of a directional screen containing previously recorded holograms.

[0046] Fig. 5D represents a variant of a directional screen allowing the use of different light sources, potentially emitting in different spectral bands.

[0047] Figures 6A to 6F schematically illustrate the different layers forming a directional screen.

[0048] The [Fig.7A] is an optical diagram of a converging lens.

[0049] Fig. 7B shows a recording phase of part of the holographic lens.

[0050] Fig. 7C represents an application of the holographic lens.

[0051] Figures 8A and 8B schematically illustrate a first example of an embodiment.

[0052] Figures 8C and 8D schematically illustrate a second embodiment.

[0053] Figure 9 illustrates the collection, by a lens, of a beam emitted by a pixel of a directional screen.

[0054] Fig. 1OA shows the yield (ordinate axis) as a function of the position of the pixels relative to the optical axis (abscissa axis), and this for three angular values ​​of divergence.

[0055] Fig. 10B shows the size of the eye-box formed by the device (ordinate axis) as a function of the divergence angle.

[0056] Figure 11 shows a variant of a device, allowing the size of the Eye-Box to be increased

[0057] Fig. 12 shows a variant of a directional screen, comprising a high-resolution central part. PRESENTATION OF SPECIFIC IMPLEMENTATION METHODS

[0058] Figure 2 schematically illustrates the main elements of a device 1 according to the invention. The device comprises a screen 10, having pixels 10. Each pixel 10 emits a diverging light wave, in a spectral emission band, propagating along an emission axis A. The emission axis A is inclined at an emission angle θ with respect 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 simplicity, Figure 2 describes the operating principle in a plane, but the concept applies to a 3-dimensional device with pixels distributed according to a matrix and directions to be considered along two angles such as longitude and latitude.

[0059] The device includes a combiner 30. The term combiner refers to a component that combines both an optical transparency function, in a spectral band The component consists of transparency and an optical reflection function within a spectral band of interest, preferably narrow, and optionally, a beam-shaping function for an optical beam generated off-axis by the screen. The axis of vision corresponds to an axis centered and perpendicular to the exit pupil. The combiner can combine, within 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, and optical beams pass through it without significant disturbance.

[0060] Figure 2 schematically illustrates an "unfolded" optical scheme in which the combiner 30 is shown operating in transmission. From a diverging 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., a weakly convergent wave. Weakly diverging means a wave whose divergence angle is less than 1°. This allows the formation, at the eye, of an image "at a great distance," i.e., a distance greater than 2 m. Optical schemes in which the combiner operates in reflection are described subsequently, in connection with Figures 8B and 8D.

[0061] 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.

[0062] The use of a holographic combiner is known to those skilled in the art. A holographic combiner has the advantage of being compact, as it consists of a thin holographic film deposited on a substrate, such as a spectacle lens. A holographic combiner is very wavelength-selective. The hologram is transparent to most of the visible spectrum, except for a specific wavelength to which it is sensitive. The combiner's convergence function, as well as its angular deviation function, are encoded in the hologram, as described below.

[0063] The device includes 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 positioned at the level of 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 with respect to an optical axis Ao of the device. The use of the lens 20 allows the screen to be positioned further from the combiner. This facilitates the integration of the device into equipment such as virtual reality glasses or headsets.

[0064] In Figure 2, the notations MA2, ".'4. HA6 respectively denote the beam sizes of the light wave emitted by pixel 10i, at points B1, B1, B2, B3, C3, and B4 are located along the wave propagation axis. The screen extends along an object plane passing through point A1, with a normal vector coinciding with or close to the optical axis A0; by "close," we mean within an angular tolerance of ±5°. Lens 20 forms an image of the screen in an image plane with normal vector adjacent to the optical axis Ao; - point Cl corresponds to the center of lens 20; - point C2 corresponds to the center of combiner 30; Point C3 corresponds to a position, called the central position, which is the center of the pupil P of the user's eye O. When the user operates the device, they are assumed to center their pupil on this central position. Point C3 also corresponds to the center of the eye box formed by the device. The eye box corresponds to the movement of the user's pupil around point C3, allowing them to see the image displayed on the screen. The combiner is positioned to conjugate point Cl (the center of the lens, the entrance pupil) with point C3 (the center of the exit pupil). - point B1 corresponds to the position of pixel 10; 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 A; and the combiner. - point B4 corresponds to a point on the retina R conjugate to point B1 by the system formed by lens 20 and combiner 30. HA6 corresponds to the size of the beam at the level of a pixel of the image formed on the retina by the device. Points C1, C2, and C3 are aligned along the optical axis defined by lens 20 and combiner 30. The screen is in a plane with a normal vector close to the optical axis Ao. Preferably, point A1, which corresponds to the center of the screen, is also on the optical axis Ao, as shown in [Fig. 2]. The distance between points C2 and C3 corresponds to eye relief, commonly referred to as "eye relief." - In a reflection configuration, points C1, C2 and C3 are not aligned due to the beam folding performed by the combiner. See figures 8B and 8D.

[0065] The screen 10 extends in a plane with a normal vector close to the optical axis Ao, passing through AL. As previously stated, 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 Ao and passing through point A2. The image plane is located, or is sufficiently close to be considered as being located, on the object focal plane of the holographic combiner 30.

[0066] The holographic combiner 30 is arranged such that the image of point Cl, by the holographic combiner, corresponds to point C3, that is, a position at which the user places the pupil P of their eye O. The collimated light wave, resulting from combiner 30, propagates towards the pupil at an angle α, called the apparent angle. The apparent angle α depends on the position of point B1, that is, pixel 10i.

[0067] Thus, at each pixel 10 of the screen 10, the optical system formed by the lens 20 and the combiner 30 defines an angle α at which the light wave emitted by the pixel, and collimated by the combiner 30, reaches the user's pupil. The apparent angles α determined for two different pixels are different.

[0068] 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, the latter being different for each pixel 10. It is understood that the device allows the formation of the image formed by the screen 10 on the retina R.

[0069] The combiner allows the generation of a collimated wave at eye level so that when the user looks at a distant object (eye / object distance large compared to the size of the eye), for example, the summit of a mountain, they can also perceive the image on the screen, which generates augmented reality-type information, such as the name of the summit and its altitude. The concept of collimation is therefore relative. Although theoretically associated with an image placed at infinity, it can also be applied to an image placed at a great distance (typically more than 100 times the size of the eye, approximately 2 meters from the observer). The device described in [Fig. 2] can be fixed, its position determined 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 it were at infinity.Advantageously, the device can be made dynamic by moving the screen within the object plane of the lens or by using a 20x variable focal length lens. This allows the accommodation distance to be adjusted.

[0070] Preferably, the screen 10 is a directional screen. The term directional screen refers to a screen in which each pixel is configured to emit a light wave diverging along an emission axis, forming an emission angle with respect to a direction normal to the screen, the light wave propagating at an angle of divergence with respect 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 with respect to the normal direction on the screen.

[0071] The emission axis of pixel 10; is oriented towards the center of lens 20. Thus, each pixel 10; of the screen 10 is configured to emit a light wave, around an emission axis A; whose emission angle y; is such that the emission axis A; passes through the center Cl of lens 20. It follows that the respective emission angles of different pixels 10; are different from each other and converge towards the center Cl of lens 20.

[0072] Figures 3A and 3B show the operation of the directional screen 10. The directional screen is formed by several pixels 10, preferably arranged in rows and columns. Each pixel is configured to emit a light wave at an emission angle, with respect to a direction D normal to the plane along which the screen extends. In [Fig. 3A], the following are shown: - a pixel 10i, emitting a divergent light wave propagating along a propagation axis forming an emission angle yi with respect to the direction D, and forming a cone whose half-angle at the apex, called the divergence angle, is noted [3i; - a pixel 102, emitting a divergent light wave propagating along a propagation axis forming an emission angle y2 with respect to the direction D, and forming a divergence angle [32.

[0073] Figure 3B schematically illustrates a structure of the directional screen. The directional screen comprises light guides 11. Each light guide 11 is connected to a light source 1. Unlike the configuration described in US9632317, the light source 11 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 made of silicon nitride (SIN) deposited in a layer of SiO2.

[0074] 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 1 lin - 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, which can be electrically modulated. The diffraction gratings 12 coupled to the same light guide 11 are spaced apart along the light guide and are considered point-like. Each diffraction grating 12 can be formed of inclusions, defining a A periodic pattern is formed in silicon dioxide (SiO2), with each inclusion made of a material whose refractive index can be electrically modulated, for example, a liquid crystal. When the wavelength of the 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 along 10 periodic patterns, and thus extend over a length of 2 or 5 pm. A third layer contains transparent electrodes 13, the electrodes being configured to electrically modulate the refractive index of a material forming the diffraction gratings. The transparent electrodes can be made of a transparent conductive material, for example, ITO (indium tin oxide). Each electrode can thus activate a diffraction grating through 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, we mean a photosensitive substrate capable of recording a hologram. The holographic film is assumed to be sufficiently thin to be considered the emitting surface. The holographic film can be a photopolymer such as a photoresin or a suspension of light-sensitive compounds such as a silver halide.

[0075] The layers are formed on a transparent substrate. This can, for example, be a glass or polycarbonate substrate.

[0076] Under the effect of polarization by an electrode 13, each point diffraction grating 12 is activated, in the sense that it allows the 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 of the holographic film 14. Under the effect of illumination, the elementary zone of the holographic film emits a light wave with predefined angular characteristics. By angular characteristics, we mean an emission angle θ and a divergence angle [3].

[0077] Thus, each pixel 10 of the screen corresponds to a superposition of a point diffraction grating 12 coupled to a light guide 11, and an electrode 13, facing an elementary area 14 of the holographic film 14. The association between each electrode 13 and each diffraction grating 12 forms a structure for extracting part of the light propagating in a light guide 11.

[0078] 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 "intersects" several light guides, so as to define Several intersections, each intersection corresponding to a pixel on the screen. The term "intersect" should be interpreted as referring to the 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, illuminated by an extraction of light propagating in the light guide.

[0079] Figure 3B shows pixels 102 and 104. Each elementary area of ​​the hologram facing these pixels is configured to emit a light wave according to predefined angular characteristics encoded in the hologram. Figure 3B shows the emission angles Y2ety4 defined for pixels 102 and 104. The angular emission characteristics can be defined for each pixel independently of the other pixels.

[0080] Dividers 11' can be arranged, for example Y-junctions, so as to distribute the light emitted by a single light source 11 to different light guides 11. In order to modulate the intensity of the light propagating in a light guide, each light guide 11 can be coupled to a modulator. In [Fig. 3B], four modulators M1, M2, M3, and M4 are shown. Each modulator includes an extractor 16, configured to be electrically switchable, so as to extract all or part of the light propagating in a light guide. Each extractor can 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 allows the extracted light to dissipate, in order to avoid the propagation of stray light through the screen 10.The use of modulators allows adjustment of the intensity of simultaneously activated pixels on the screen.

[0081] The angular emission characteristics of each pixel 10 are defined during a preliminary recording phase of the holographic film 14. As is known, a hologram is formed by the interference of two light waves emitted by the same light source: an object light wave and a reference light wave. The generated interference fringes are stored physically or chemically in the holographic film 14. Figure 4 shows an assembly allowing the recording of an elementary area 14 of the holographic film 14.

[0082] A light source is coupled to two fibers using a splitter, so as to obtain one fiber forming an object beam 41 and one fiber forming a reference beam 42. The light source has a wavelength close to that of the light 1 lin to which the screen 10 is connected during its use. This source is typically a long coherence laser (greater than one meter).

[0083] The fiber forming the reference beam 42 reproduces the illumination conditions similar to those obtained by extracting light from a light guide 11 by activating a diffraction grating 12. Illumination conditions refer to the beam incidence, size, and divergence. The reference beam 42 is fixed and is formed by a reference optical shaping system 43.

[0084] The object beam is generated by an object-shaping optical system 44 coupled to a converging focusing optic 45. This allows for the adjustment of an angle of incidence θ and a divergence angle [3] of the object beam. The recording of a hologram in an elementary zone 14 is performed by simultaneously exposing said elementary zone to the object beam and the reference beam. The different holograms at each elementary zone are produced by moving the holographic film 14 and optionally modifying the characteristics of the object beam, in particular the angle of incidence θ and the divergence angle [3]. Thus, each elementary zone 14 is assigned an angle of incidence θ and a divergence angle [3], which correspond to the angles of incidence and divergence of the object beam during the recording of the hologram.

[0085] Figures 5A to 5D schematically illustrate the various layers mentioned above. Figure 5A shows a structured layer, defining the light guides 11, formed on a glass substrate 15. Figure 5B shows an extraction layer, formed by the diffraction gratings 12, 16 described above. Figure 5C shows a holographic layer, comprising the holographic elemental zones 14, as well as the absorber 17.

[0086] One advantage of the holographic screen is that it reduces the number of light sources compared to the configuration described in US9632317. The directional screen can be formed from a single light source. In this case, it is monochrome. The directional screen can also be formed from several light sources emitting in different spectral bands. Such a configuration is shown in [Fig. 5D]. Several light guides can be formed on the same layer to create independent light guide arrays. Each light guide array is intended to be optically coupled to a light source emitting in a specific spectral band. In [Fig. 5D], two light guide arrays intended to be optically coupled to two light sources, l11 and l112, are shown. The different light guides can be fabricated on the same substrate 15.The arrangement of the light guides prevents interference (cross-talk) between the light guides at each intersection.

[0087] Figures 6A to 6F illustrate the manufacturing steps of a directional screen 10. In [Fig. 6A], strips of reflective material (for example, a metal such as aluminum Al), intended to act as reflectors, are deposited on a substrate 15, forming lines. The aluminum strips, 1 µm wide, are spaced 5 pm from each other.

[0088] In [Fig. 6B], a layer of addressing electrodes 13 is shown, in the form of a structured layer of ITO (indium tin oxide), 40 nm thick. The ITO layer can be structured to form electrodes extending in columns, perpendicular to the rows.

[0089] Figure 6C shows the deposition of a SiO2 layer in which SiN light guides 11 are formed. The guides have a width of approximately 400 nm and a thickness between 100 nm and 400 nm. SiN was chosen because of its transparency properties in the visible range.

[0090] In [Fig. 00D], a deposit 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.

[0091] 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 the extraction of light propagating through the light guide. A transparent counter electrode 13', for example made of ITO deposited on a transparent film 13s (of glass or transparent plastic material), is arranged on the liquid crystal layer. See [Fig. 0E].

[0092] Figure 6F shows the deposition of a holographic layer 14, supported by a transparent substrate 14s, against the film 13s that encapsulates the liquid crystal. The holographic layer 14 can be made of a photopolymer, 15 µm thick, while the substrate 14s can be made of glass, 700 µm thick. The holographic layer 14 has been previously registered, as described in relation to Figure 4.

[0093] Figure 6F shows all the layers forming the directional screen 10. The total thickness is approximately 1.5 mm. The surface area of ​​each pixel can be 5 µm x 5 µm. A screen with a resolution of 1920 x 1080 can thus be formed with a surface area of ​​10 mm x 5 mm.

[0094] The configuration described in [Fig. 2] works regardless of the type of screen. However, optimal performance is obtained using a directional screen as described above. 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 lens 20. Thus, the emission angles assigned to each pixel are adjusted so as to converge towards the center of lens 20. This increases the amount of light forming the image on the retina.

[0095] Figure 7A details the operation of a converging lens. In Figure 7A, the lens conjugates point A' with point A. Points F and M belong to the plane object focal plane. The light waves emitted by points F and M are reflected to infinity by the lens. Point F corresponds to the focal point of the lens. Point M is offset relative to point F in the object focal plane. Upon exiting the lens, the beams passing through points F and M respectively are collimated and deviated angularly relative to each other.

[0096] Figure 7B illustrates a hologram recording phase on the holographic combiner. An elementary area of ​​the holographic material, forming the combiner, is exposed to a divergent reference beam Fl, emitted from a point F, and to a collimated object beam F2, both beams being emitted from 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 Fl and F2 beams is stored in the holographic material. 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 focal point 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'.

[0097] In the example shown in Figures 7B and 7C, the holographic combiner forms a holographic lens reflecting light at the wavelengths of the beams Fl and F2. This allows the formation of a holographic reflector, suitable for the reflection configurations shown in Figures 8B and 8D. The holographic combiner operates only within a narrow spectral band, which corresponds to the recording spectral band. Outside this spectral band, the holographic combiner transmits light. The holographic combiner can be mounted on the lens of a pair of glasses or on the visor of a virtual reality headset.

[0098] Figures 8A and 8B show a first example of an embodiment, respectively in 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 screen 10 and lens 20: = 16 mm - focal length of lens 20: / } = 10 mm; - focal length of the combiner 30: / 2 = 18 mm; - Screen size: 5 mm x 5 mm; - field of observation: FOV = 26°;

[0099] Defining the parameters allows the distance DL between lens 20 and the combiner 30:

[0101] We find a distance DL = 44.667 mm.

[0102] The field of view (FOV) is determined by the following relationship: [0i03] FOV = 2a^ = 2xtan-i(_A_ x

[0104] Where Umax is the apparent angle of the screen pixel furthest from the optical axis.

[0105] We find FOV = 26°.

[0106] If it is desired to modify the accommodation plane, this can be done by actively modifying the optical parameters of the system. The accommodation distance Zac is determined and given by the formula:

[0107] _ ...

[0108] The accommodation distance Zac can be varied by changing ze (screen-lens distance 20) or by changing the focal length using a variable focal length lens 20.

[0109] To move from an accommodation distance of infinity to Zac = 2 m, the screen must be moved from a distance of 16 mm to 15.94 mm, which corresponds to a displacement of 60 pm. Such a displacement can be obtained using a translation support, for example motorized or piezoelectric, allowing 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.

[0110] Figure 8B shows a reflection configuration in which the combiner acts as a reflector. Such a configuration, with significant eye relief (30 mm), is better suited for mounting on a mixed reality headset. Since the reflector is transparent, except at wavelengths sensitive to holograms, it allows the transmission of an image Is of a scene propagating towards the user's eye. The combiner can also be opaque for virtual reality applications. In this case, the combiner forms an optical reflector, which can be a holographic component as described above or a freeform mirror whose curvature provides the desired convergence effect. Angle θ corresponds to a folding angle. In this example, θ = 32°. In this reflection embodiment, the use of a holographic combiner is particularly advantageous.

[0111] Figures 8C and 8D show a second example of an embodiment, respectively in transmission and reflection, with the following parameters: - ER (Eye Relief): 15 mm; Distance between screen 10 and lens 20: ze = 20 mm

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[0122] - focal length of lens 20: fl = 10 mm; - focal length of the combiner 30: f2 = 10 mm; - Screen size: 5 mm x 5 mm; - field of observation: FOV = 28° For this configuration, we set DL = 30 mm. To go from accommodation from infinity to Zac = 2 m, the screen-lens distance must be changed from 20 mm to 19.95 mm. This configuration is more compact than the one described in connection with figures 8A and 8B. The eye relief is 15 mm, which is suitable for integration into a spectacle-type device. Figure 8D shows a reflected configuration with a folding angle of 0 = 45°. The configuration shown schematically in [Fig. 2], examples of which are given in connection with Figures 8A to 8D, can be used for any type of screen. Using a directional screen is advantageous because it increases the device's light output. As previously described, the emission of each pixel 10 is parameterized by angular characteristics θ and θ. It was previously established that it is preferable for the emission angle θ to be such that the emission axis of each pixel passes through the center of the lens 20. The divergence angle θ can be optimized to further increase the device's light output. Figure 9 shows a homogeneous beam F emitted by a pixel 10, whose emission angle is oriented perpendicular to the screen surface. The beam is distributed, in the plane formed by lens 20, according to a disk of surface area S and radius (see Figure 2). S2 corresponds to the surface of lens 20, with radius R20. S3 corresponds to the intersection of S and S2. S3 must be as high as possible to optimize the collection, by lens 20, of the beam emitted by the pixel. We can define an efficiency P such that: o _ A A3 The yield P can be calculated geometrically, by setting: wy^dy--R^ 2dy d -dy-d (3) Let Sa and Sb be the parts of the surfaces S2 and S; respectively delimited by the dotted line shown on [Fig.9]. Sa = wy22 x cos-i (7^) - d^wy^-d2 (5) sb = ^20 x cos- ' ) - d.'^R22 -d 6

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[0134] And : wj2 = Zetan(^.) (8) Z„ is the distance between lens 20 and screen 10. The efficiency P was calculated according to equation (7), considering three values ​​of θ: 2°, 10°, or 30°. Figure 1OA represents the efficiency value (y-axis) as a function of the distance of the pixels from the center of the screen. It can be observed that for a divergence of 10°, the efficiency is optimal only for pixels at the center of the screen. Since the emission angle is not controlled, the efficiency decreases as the pixels move away from the center of the screen. When the divergence angle is 2°, 100% efficiency is achieved for pixels located around the center of the screen. This corresponds to a configuration where the surface S2 is small and contained within the surface SL. The smaller the divergence angle, the better the efficiency. This confirms the advantage of a directional, low-divergence screen in the implementation of the invention. Low divergence is understood to mean that the divergence angle of each pixel is less than 20°, and preferably less than 15° or 10°. Recall that the divergence angle is half the angle at the apex of the pixel's emission cone. Besides yield, another performance indicator is the size of the eye-box, which also depends on the divergence angle. Indeed, the EB side of the eye-box is such that: EB = 2w5 = 2^-^ tan( / 1 ) where / 1 and / 2 are the respective focal lengths of lens 20 and combiner 30. Figure 10B shows the dimension EB of one side of the Eye-Box (ordinate axis - unit mm) as a function of the angle (abscissa axis - unit degrees). It can be observed that the greater the divergence, the larger the Eye-Box, thus improving the user comfort of the device. When the angle of each pixel is 11°, the Eye-Box measures 4 mm on each side, which is acceptable. In [Fig. 1OA], it can be seen that if the emission directivity of the pixels is not properly adjusted, a divergence angle of 11° leads to a 40% loss of efficiency for the pixels located at the periphery of the screen 10. According to our invention, with directivity control targeting the center of the lens 20, the efficiency is close to 100% for all pixels while maintaining a reasonable Eye-Box size. Figures 11 and 12 illustrate alternative embodiments. In [Fig. 11], a configuration is shown in which lens 20 is formed by several lenses. elementary converging lenses 20a, 20b, and 20c are coplanar and aligned perpendicularly to the optical axis Ao. Each lens is optically coupled to a group of pixels on 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. In [Fig. 11]: - the pixels located between points Mal and Ma2 are associated with elementary lens 20a: they form a first sub-screen; - the pixels located between points Mbl and Mb2 are associated with the elementary lens 20b: they form a second sub-screen; - the pixels located between the points Mcl and Mc2 are associated with the elementary lens 20c: they form a third sub-screen;

[0135] According to this configuration: - the pixel placed at point Mal is imaged, by lens 20a, at point B1; - the pixel placed at point Ma2 is imaged, by lens 20a, at point B2; - the pixel placed at point Mbl is imaged, by lens 20b, at point B1; - the pixel placed at point Mb2 is imaged, by lens 20b, at point B2; - the pixel placed at point Mc 1 is imaged, by lens 20c, at point B1; - the pixel placed at point Mc2 is imaged, by lens 20c, at point B2;

[0136] This configuration allows the formation, at the pupil, of three images Ia, Ib, and L, corresponding respectively to pixels Mal to Ma2, Mbl to Mb2, and Mcl 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 Ia, Ib, and Ic are identical. Such a configuration makes it possible to duplicate the Eye-Box, so as to increase the spatial movement of the eye.

[0137] Figure 12 illustrates a variant in which the spatial pitch, separating two adjacent pixels, is variable. More precisely, the screen is segmented into a central portion, comprising 8x8 pixels, surrounded by a peripheral portion. The spatial pitch is reduced in the central portion of the screen, compared to the spatial pitch separating adjacent pixels in the peripheral portion. This improves the spatial resolution of the screen at the fovea, which is a central area of ​​the retina with an angular radius extending 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 in the eye's spatial resolution.

Claims

Demands

1. Device (1) for projecting an image onto an eye (O), the device comprising: - a light emitter, configured to emit light waves respectively along different 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 propagating towards the pupil (P) of the eye; the device being characterized in that: - the light emitter includes a directional screen (10), comprising different pixels (10;), 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 at a predefined divergence angle (|3;) relative to their respective emission axis; - 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 (C3), corresponding to the center of the pupil of the eye; and in that The device comprises a converging lens (20) interposed between the screen (10) and the optical combiner (30), the converging lens extending around a center (Cl), the center of the lens forming, with a center of the combiner (C2), an optical axis (Ao) of the device, the lens being arranged such that: - the image of the screen, by the converging lens (20), is formed in an object focal plane of the combiner; - the image of the center of the converging lens, by the combiner (30), is formed at the central position (C3); - the respective emission axes of pixels converge towards the center of the converging lens; - so that the collimated light wave, resulting from the combiner, reaches the central position by forming an angle (a;) dependent on the position of the pixel (10i) of the screen.

2. A device according to claim 1 in which the screen comprises a stack including: - light guides (11), each light guide being coupled to a plurality of diffraction gratings (12), distributed along the light guide, each diffraction grating being electrically tunable, each diffraction grating (12) being configured to be electrically modulated so as to extract light propagating in the light guide; - electrodes (13), each electrode being associated with several diffraction gratings (12) 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;so that under the effect of illumination by light extracted from the light guide, each pixel is configured to emit a diverging 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.

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

4. A device according to any one of claims 2 or 3, wherein: - 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, powering each light guide.

5. Device according to any one of claims 2 to 4, comprising several light sources, each light source being optically connected to several light guides.

6. Device according to any one of claims 2 to 5, wherein different light sources are configured to emit light respectively at different wavelengths.

7. A device according to any one of claims 2 to 6, wherein the pixels are arranged in: - 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.

8. A device according to any one of the preceding claims, comprising: - several converging lenses (20a, 20b, 20c), aligned parallel to the directional screen, each lens extending around a center and presenting 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 to which it is associated.

9. A device according to any one of the preceding claims, wherein: - two adjacent pixels are separated by one spatial step; - the screen has a central part, surrounded by a peripheral part; - the spatial step between two adjacent pixels of the central part is less than the spatial step between two adjacent pixels of the peripheral part.

10. Device according to any one of the preceding claims, wherein the combiner is a holographic combiner.

11. Device according to claim 10, wherein: - the screen emits light in at least one spectral emission band; - the holographic combiner is transparent outside the or each spectral emission band; - the holographic combiner forms a converging lens in the or each spectral emission band.

12. Device according to claim 11, wherein the holographic combiner forms a reflector in the or each spectral emission band.

13. Device according to any one of the preceding claims, wherein the converging lens is movable in translation relative to the screen, along the optical axis.

14. Device according to any one of the preceding claims, wherein the converging lens has a variable focal length.

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