A device for projecting an image formed by a screen

Abstract

A device (1) for projecting an image onto an eye (O), comprising: a light emitter with different emission axes; an optical combiner (30) configured to form a collimated light wave from each light wave; the device being such that: the light emitter comprises a directional screen (10) with different pixels (10i) configured to emit a diverging light wave along a predefined emission axis; the combiner has an object focal plane; the directional screen is disposed in the object focal plane of the combiner; 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); the respective emission axes of different pixels of the directional screen converge towards the same viewing point, downstream of the combiner; Figure 2A

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 one 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. The eye movement can be dynamic, occurring during the rotation of the eye as it scans 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] An 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 combiner presents an object focal plane; - the light emitter includes a directional screen, comprising different pixels, 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 directional screen is positioned in the object focal plane of the combiner; - the optical combiner is configured to receive each light wave emitted by a pixel and form a collimated light wave propagating towards a position likely to be occupied by the pupil of the eye; - the respective emission axes of different pixels of the directional screen converge towards the same aiming point, downstream of the combiner; - the image of the point of view, by the combiner, corresponds to the position likely to be occupied by the pupil of the eye.

[0013] According to one possibility, the screen comprises a stack including: - 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 it is associated;

[0014] each pixel of the screen corresponding to an association between an electrode and a diffraction grating coupled to a light guide;

[0015] 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.

[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 diverging light wave, along the emission axis and the divergence angle of the pixel, under the effect of light extracted by the dif-

[0017] fraction to which it is associated. According to one embodiment: 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 the intensity of the light emitted by the light source, feeding the light guide.

[0018] 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.

[0019] 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.

[0020] According to one embodiment, the combiner extends around an optical axis; the pixels of the screen are segmented into groups of pixels; the emission axes of the pixels of the same group of pixels converge towards the same aiming point associated with the group of pixels; two different pixel groups are associated with two different aiming points, at least one aiming point associated with a pixel group being distant from the optical axis.

[0021] The optical axis may pass through the center of the combiner. The optical axis may extend between the center of the combiner and the position likely to be occupied by the pupil of the eye.

[0022] According to one embodiment, the screen has a first group of pixels, whose emission axes converge towards a first point of aim, the first group of pixels being configured to form a first part of an image when the pupil of the eye occupies a first position; The screen has a second group of pixels, whose emission axes converge towards a second sighting point, different from the first sighting point, the second group of pixels being configured to form a second part of the image when the pupil of the eye occupies a second position, angularly offset from the first position.

[0023] According to one embodiment, The screen comprises a first group of pixels, whose emission axes converge towards a first aiming point, the first group of pixels being configured to form an image when the pupil of the eye occupies a first position; - the screen has a second group of pixels, whose emission axes converge towards a second sighting point, different from the first sighting point, the second group of pixels being configured to form the image when the pupil of the eye occupies a second position, different from the first position.

[0024] According to one embodiment, - the screen pixels are segmented into macropixels, with the pixels of the same macropixel being configured to display the same content; - the emission axes of the pixels of the same macropixel aim at different aiming points; - the respective emission axes of the pixels of different macropixels converge towards the same point of aiming.

[0025] The combiner is advantageously a holographic combiner.

[0026] According to one possibility: - 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 holographic combiner can form a reflector in the or each spectral emission band.

[0028] 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°.

[0029] 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

[0030] Fig. 1 represents a prior art configuration.

[0031] Fig. 2A is an optical diagram of a device according to the invention.

[0032] The [Fig.2B] is a ray tracing carried out taking into account the device shown in the [Fig.2A].

[0033] Fig. 2C shows an example of an arrangement on the lens of a spectacle.

[0034] Fig. 3A schematically illustrates the angular emission characteristics of pixels of a directional screen.

[0035] Fig. 3B shows the structure of a directional screen.

[0036] Fig. 4 represents a recording of a hologram of a directional screen.

[0037] Fig. 5A represents a layer of a directional screen, in which light guides are formed.

[0038] Fig. 5B shows a layer of a directional screen, in which electrically activatable diffraction gratings coupled to electrodes are formed.

[0039] Fig. 5C shows a layer of a directional screen containing previously recorded holograms.

[0040] Fig. 5D represents a variant of a directional screen allowing the use of different light sources, potentially emitting in different spectral bands.

[0041] Figures 6A to 6F schematically illustrate the different layers forming a directional screen.

[0042] The [Fig.7A] is an optical diagram of a converging lens.

[0043] Fig. 7B shows a recording phase of part of the holographic lens.

[0044] Figure [Fig. 7C] represents an application of the holographic lens.

[0045] Figures 8A to 8G show a variant allowing duplication of an Eye-Box of the device.

[0046] Fig. 8A and Fig. 8C show configurations in which the eye is respectively positioned facing two angular directions.

[0047] Fig. 8B and 8D illustrate images perceived by the eye respectively according to the configurations described in Figures 8A and 8C.

[0048] Figures 8E and 8F show different configurations, with different duplication numbers of the Eye-Box.

[0049] Fig. 8G schematically illustrates the embodiment described in relation to Figures 8A to 8F, in a reflection configuration, as described in relation to Fig. 2C.

[0050] Figures 9A to 9D show another variant allowing the size of the Eye-Box to be increased.

[0051] Fig. 9A and Fig. 9C show configurations in which the eye is respectively positioned facing two angular directions.

[0052] Fig. 9B and 9D illustrate images perceived by the eye respectively according to the configurations described in Figures 9A and 9C. PRESENTATION OF SPECIFIC IMPLEMENTATION METHODS

[0053] Figure [Fig. 2A] schematically illustrates the main elements of a device 1 according to the invention. The device includes a screen 10, containing 10 pixels. In [Fig. 2A], point B1 designates a pixel. The screen 10 is a directional screen. The term directional screen designates 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.

[0054] Thus, each pixel 10 emits a diverging light wave, in a spectral emission band, propagating along an emission axis Aj. The emission axis Aj is inclined at an emission angle yj with respect to a direction perpendicular to the screen. The respective spectral emission bands of each pixel may be identical or different from each other.

[0055] The device includes a combiner 30. The term combiner refers to a component that combines both a transparency optical function, within a spectral band of transparency, and a reflection optical 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.

[0056] Figure 2A schematically illustrates an "unfolded" optical scheme, in which the combiner 30 is shown operating in transmission mode. 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 diverging wave. Weakly diverging means a wave whose divergence angle is less than 1°. This makes it possible to form, at the eye, an image "at a great distance," i.e., a distance greater than 2 m.

[0057] 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.

[0058] 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 layer A holographic image is deposited on a substrate, such as a spectacle lens. A holographic combiner is highly 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.

[0059] The holographic combiner 30 defines an object focal plane and an image focal plane. The pixels of the screen 10 are arranged in the object focal plane of the combiner.

[0060] According to the embodiment shown in [Fig. 2A], the emission axes of all the pixels of the screen converge towards the same virtual point AL. In [Fig. 2B], the emission axes of two different pixels are shown. The virtual point AL is such that its image, by the combiner 30, is positioned at a point C3. Point C3 corresponds to a position at which the user places the pupil P of their eye O. Thus, the beam from each pixel is collimated by the combiner and directed towards point C3, centered with respect to it. It is understood that the emission angle associated with each pixel is adjusted so as to converge towards the virtual point AL.

[0061] 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 virtual point AL. It follows that the respective emission angles of different pixels 10 are different from each other and converge towards the virtual point AL. Each emission angle can be defined with respect to an optical axis A0, passing through the center of the combiner C2 and through the previously defined point C3. In the example shown, the optical axis A0 passes through the center of the screen, although this is not necessary.

[0062] In Figure 2A, the notations respectively denote the sizes of The beam of the light wave, relative to the beam propagation axis, from the light wave emitted by pixel 10, in a plane parallel to the plane along which the combiner extends, passing through points B1, C2, and C3. The screen 10 extends along the object focal plane of the combiner, being perpendicular to the optical axis A0. The collimated light wave resulting from each pixel 10 reaches point C3 at an angle of inclination α with respect to the optical axis A0. The distance between pixel 10 and the optical axis A0 is denoted dy.

[0063] The collimated light wave resulting from the combiner is focused by the eye O 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 B2. The position of B2 is defined by the angle of inclination 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.

[0064] The combiner allows the generation of a collimated wave at the level of the eye so When a user looks at a distant object (the eye-to-object distance being large compared to the size of the eye), such as a mountain peak, they can also perceive the image on the screen, which generates augmented reality-type information, such as the name of the peak and its altitude. The concept 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 more than 100 times the size of the eye, approximately 2 meters from the observer).

[0065] Let Zv be the distance between the virtual point Al and the point F positioned on the screen. Let f be the focal length of the combiner (distance between C2 and the screen).

[0066] The emission angle Vf of each pixel 10; and the tilt angle ai are such that:

[0067] r= tair'(ÿ)(l)

[0068] and

[0069] a tan-1( ^)(2)

[0070] The size of the eye-box, which depends on the divergence angle / 3 of each pixel, is such that:

[0071] eb = 2x / xtan( / i.) (3)

[0072] Preferably, each pixel of the directional screen is configured so as to have the same divergence angle.

[0073] Figure 2C shows a so-called folded configuration, in which the combiner acts as a reflector. The combiner is integrated into the lens V of a telescope. The screen 10 is fixed to the mount M of the telescope. Each pixel of the screen has an emission axis converging towards the same point AL. The collimated beams resulting from the combiner 30 and propagating towards the pupil of the eye are shown.

[0074] 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 y; with respect to the direction D, and forming a cone whose half-angle at the apex, called the divergence angle, is noted [3]; - 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.

[0075] 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 line. Unlike the configuration described in US9632317, the light source 11 line 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.

[0076] 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 to 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 periodic pattern, in silicon dioxide (SiO2), each inclusion being made of a material whose refractive index is electrically tunable, for example, a liquid crystal. When the wavelength of the light is 532 nm, the period of the pattern of the diffraction grating 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, 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 can be made of a transparent conductive material, for example ITO (indium tin oxide). Each electrode can 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, we mean a photosensitive substrate capable of recording a hologram. The holographic film is assumed to be thin enough to be considered the emission surface. The holographic film can be a photopolymer such as a photoresin or a suspension of light-sensitive compounds such as silver halide.

[0077] The layers are formed on a transparent substrate. This could, for example, be a glass or polycarbonate substrate.

[0078] 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].

[0079] 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.

[0080] 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 of the screen. The term "intersects" 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, illuminated by an extraction of light propagating in the light guide.

[0081] 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 Y2 and Y4 defined for pixels 102 and 104. The angular emission characteristics can be defined for each pixel independently of the other pixels.

[0082] 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 activatable, so as to extract all or part of the light propagating in a light guide. Each The 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 dissipates the extracted light, preventing stray light from passing through the screen 10. The use of modulators allows the intensity of simultaneously activated screen pixels to be adjusted.

[0083] 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.

[0084] 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).

[0085] The fiber forming the reference beam 42 reproduces 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.

[0086] 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.

[0087] Figures 5A to 5D schematically illustrate different layers mentioned previously. 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 previously. Figure 5C shows a holographic layer, comprising the holographic elementary zones 14; as well as the absorber 17.

[0088] 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, lin1 and lin2, 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.

[0089] 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 µm apart.

[0090] 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.

[0091] Figure 6C shows the deposition of a SiO2 layer in which SiN light guides 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 in the visible range.

[0092] In [Fig.6D], 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.

[0093] 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 (made of glass or transparent plastic material), is arranged on the liquid crystal layer. See [Fig. 0E].

[0094] 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.

[0095] 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.

[0096] Figure 7A details the operation of the holographic combiner 30, which behaves equivalently to a converging lens. In Figure 7A, the combiner conjugates point A' with point A. Points F and M lie in the object focal plane. The light waves emitted by points F and M are reflected to infinity by the combiner. 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 from each other.

[0097] 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 lens reflects a converging beam towards a point A'.

[0098] 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. 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 a lens of eyeglasses or on the visor of a virtual reality headset.

[0099] According to a sizing example: - ER (Eye Relief - eye relief, corresponding to the distance between the combiner and the eye): 20 mm; - distance Zv; 20 mm - focal length of the 30:50 mm combiner; - Screen size: 13 mm x 13 mm; - field of observation: 30°, value obtained by applying (2) to the pixels furthest from the optical axis.

[0100] Each line or column of the screen can have 1920 pixels of 7 pm on each side, which is a realistic dimension, while providing an acceptable spatial resolution of the image formed on the retina.

[0101] As previously described, in relation to expression (3), the size of the Eye Box depends on the divergence angle [3; assigned to each pixel. Taking into account a divergence angle of 3°, the eye box is a square with sides of 5 mm, which is acceptable.

[0102] Figures 8A to 8E describe a variant of the configuration shown schematically in [Fig. 2A]. According to this variant: - the pixels 10? of the screen 10 are segmented into groups of pixels; - the emission axes A; pixels of the same group of pixels converge towards the same sighting point associated with the group of pixels: on [Fig.8A], two different sighting points Al, A2 have been represented. - two different pixel groups are associated with two different aiming points, at least one aiming point associated with a pixel group being distant from the optical axis.

[0103] According to this embodiment, a displacement of the eye relative to the device is taken into account. By rotating, the eye scans a wide angular range, while maintaining good visual quality.

[0104] It is assumed that the user's eye forms a well-defined image in an angular field of 10° around the axis of vision, the latter being perpendicular to the pupil and centered with respect to the latter.

[0105] The objective of this variant is to perform a duplication of the eye box. In Figure 8A, the pupil of the eye is placed at a point C3p. Figure 8B shows the visual field corresponding to this position. Two different sighting points, A1 and A2 respectively, are shown, from two pixels B1 and B2. The emission angle F of pixel B1 converges to the virtual point A1 described in connection with Figure 2A. The emission angle of pixel B2 converges to a virtual point A2, distinct from point A1. In [Fig. 8A], the beam propagation diagram from the two pixels B1 and B2 is shown.

[0106] The spatial position of the pixels on the screen fixes the angles ai and a2 according to (2)

[0107]

[0108] , .[ dy{ \ and d 1 ■ a, - tan 1 = tan 1 ~r \ f / 2 \ f Where dyi and dy2 are the distances between the pixels and the optical axis defined by the combiner.

[0109] The two pixels B1 and B2 are close together on the screen, with distance differences of a few tens of microns, we have ai ~ a2. The emission axes of the two pixels are different: Their respective emission angles F] and are defined so that pixel B1 targets point A1 and pixel B2 targets point A2.

[0110] We have: [YES] t -ipyAet t = ^2 = tan

[0112] hy and hy^ are the distances, considered in the combiner plane, between the pixel and its aiming point. Zi^ and Zv2 are the distances along the optical axis, between the focal point and the points Al and A2.

[0113] For pixel Bl, the targeted point Al is on the optical axis of the combiner. Therefore, hyi = dyt.

[0114] For pixel B2, the target point A2 is not on the optical axis of the combiner: hy2 ~ 0.

[0115] According to this embodiment, each pixel B of the same target pixel group The Aidont point in the image, as represented by the combiner, is a C3 point; corresponding to a potential position of the user's pupil. The index i is an integer ranging from 1 to I, where I corresponds to the number of duplications of the Eye Box.

[0116] Pixels B1 and B2 have the same content and form a "macropixel". A macropixel is a group of pixels with the same content but aimed at different viewing points, such that the pixels of the macropixel contribute to forming the same image at different positions on the retina. Each pixel of a macropixel is intended to form the same image pixel on each image formed at the level of the retina.

[0117] Figure 8B shows an example of the visual field of an image projected onto the eye. In this example, the image contains a list of European capital city names. The image produced by pixel B1 is located at point B1' at angular coordinate ab. This image represents a foveal rendering in which, beyond an angle of approximately 5°, the image projected onto the retina is perceived by the brain with low resolution. The image emitted by pixel B2 does not enter the eye, or only very partially. Therefore, point B2' has not been shown in Figure 8B.

[0118] Figure 8C shows the same device as discussed in relation to Figures 8A and 8B. The eye position corresponds to point C32. This may be due to the optical system being used by another user with a different interpupillary distance. It may also be due to movement of the device on the user's head.

[0119] Figure 8D shows an example of the visual field of an image projected into the eye When the pupil's position corresponds to point C32, the signal from pixel B1 no longer enters the eye. The signal from pixel B2 enters the eye and produces an image pixel B2' located at angle a2 of the visual field. Since ai ~ a2 and this principle of pixel duplication is reproduced across the entire screen, the image perceived by the observer does not change, or changes imperceptibly.

[0120] According to this variant, the screen is covered with partially redundant pixels, forming the previously mentioned pixel groups. The pixels in the same pixel group target the same point Ai, which allows for the definition of a pupil position C3i. The pixels in the different pixel groups target different points Ai, which allows for the definition of different pupil positions C3i.

[0121] The pixels are configured such that the image formed at point C32 is a replica of the image formed in the eyebox centered around point C3p. Thus, the eye can move from point C3i to point C32 perceiving the same image. In other words, the different groups of pixels, each targeting a different sighting point, are configured to form the same image. According to this embodiment, the user perceives the same image whether their pupil is in the first position C3i or in the second position C32.

[0122] The higher the redundancy, the lower the image resolution, but the greater the ease of adjusting the optical system thanks to the numerous duplications of the Eye Box. Figures 8E and 8F represent the distributions of the Eye Box (EB) in the plane of the eye's entrance pupil. The choice of the number of EB replications will depend on the choice of beam divergence [3], which is imposed by an energy conservation criterion in the optical system.

[0123] In [Fig. 8E], the high value of [3] allows for a EB (shown in light gray dotted line) slightly larger than the size of the eye's pupil (represented in dark gray). In this case, the EB was duplicated in a 3x3 format. Compared to a screen without EB replication, there is a loss of image resolution by a factor of 9.

[0124] In [Fig. 8F], the size of the EB is slightly smaller than the pupil size of the eye. A replication method favoring the horizontal direction was chosen (a choice consistent with human morphology). This results in a 5x3 duplication, representing a resolution loss of a factor of 15 compared to an image formed with a screen without EB replication.

[0125] Fig. 8G illustrates a folded configuration, similar to Fig. 2C. Fig. 8G shows beams converging towards two different points of the eye.

[0126] Figures 9A to 9D illustrate another variant. In [Fig. 9A] and 9C, the beam propagation scheme from two pixels B1 and B2 is shown. The same notation as in Figures 8A and 8C is used. Two points are shown. The two pixels B1 and B2 have different viewing angles, A1 and A2 respectively. The image produced by pixel B1 is located at point B1' at angular coordinate ab. The image produced by pixel B2 is located at point B2' at angular coordinate a2. Unlike the previous case, the positions of these pixels are differentiated so that the angles ai and a2 of projection of image pixels in the eye are different.

[0127] On the image formed on the retina, pixel B1 forms an image pixel B1' at the limit of the 10° angular field (apparent angle α equals 10°). To view the pixels of the image displayed by the screen beyond point B1', for example, the image pixel located at point B2', the user rotates their eye by an angle θ: see [Fig. 9C]. Thus, the center of the pupil is no longer point C3b but moves towards point C32. The emission angle of pixel B2 is determined so as to target not point A1, but point A2, determined to allow continuity between the viewing angle of the image and the axis of the user's pupil. Such an embodiment makes it possible to form a "wide-angle" image on the screen, which the user can follow by rotating their eye. The points targeted by the screen pixels are defined so as to allow a progressive rotation of the user's pupil.

[0128] Angle a2 targets an angular position in the visual field that is beyond the foveal area: the eye cannot perceive it with good resolution when the pupil is located at point C3i. Angle ai targets an angular position in the visual field that is in the periphery of the foveal area: the eye perceives it with satisfactory resolution.

[0129] In [Fig. 9A], the pupil of the eye is located at a point C3 situated on the optical axis. In [Fig. 9C], the pupil of the eye is located at a point C32 angularly offset from the optical axis by an angle 0.

[0130] If the beam from pixel B2 passed through point C3b, the resulting image pixel B2' would be rendered with good energy efficiency. However, this would be inefficient because the eye would not perceive this pixel with good resolution.

[0131] To improve resolution at this viewing angle a2, the eye will naturally orient its gaze axis towards this angular direction. This results in a rotation of the eye within its orbit and therefore a spatial displacement of the pupil. With this displacement, the center of the pupil shifts from point C3 to point C32.

[0132] According to this variant: - The screen has a first group of pixels, similar to pixel Bl, whose emission axes converge towards the first virtual point AL. The first group of pixels allows the formation of a first part of the image in the eyebox centered around point C3i. See [Fig.9B]. - The screen contains a second group of pixels, similar to pixel B2, whose emission axes converge towards the second virtual point A2. The second group of pixels allows us to form a second part of the image when the pupil is centered around point C32: See [Fig.9D]

[0133] According to this embodiment, the user perceives the first part of the image when his pupil occupies a first position (point C3i), and the second part of the image when his pupil occupies the second position (point C32).

[0134] The first part of the image and the second part of the image are complementary: they correspond to two different parts of a wide-field image displayed by the device. Unlike the embodiment described in connection with Figures 8A to 8G, the Eye Box is not duplicated, but rather extended spatially.

[0135] This embodiment allows observation of the image while rotating the eye. It takes advantage of the fact that visual acuity is optimal in a central region of the retina, called the fovea. To cover the entire image, the eye rotates so that at two different angular positions, the fovea perceives two different parts of the image projected by the screen.

[0136] Such a variant allows the image to be formed in two parts: a first part around point C3i, and a second part, angularly shifted, around point C32. The eye can perceive each part of the image under the effect of a rotation. This variant can be generalized to n different viewing points, n being greater than or equal to 2.

[0137] The invention can be integrated into a pair of glasses, or a visor, or a virtual reality headset.

Claims

Demands

1. Device (1) for projecting an image onto an eye (0), the device comprising: - a light emitter, configured to emit light waves 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 combiner has an object focal plane; - the light emitter has a directional screen (10), comprising different pixels (10), each pixel being configured to emit a diverging light wave along a predefined emission axis, the light wave propagating at a predefined divergence angle (|3|) with respect to the emission axis; - the directional screen is disposed in the object focal plane of the combiner;- The optical combiner is configured to receive each light wave emitted by a pixel and form a collimated light wave propagating towards a position likely to be occupied by the pupil of the eye; - the respective emission axes of different pixels of the directional screen converge towards the same aiming point, downstream of the combiner; - the image of the aiming point, by the combiner, corresponds to the position likely to be occupied by the pupil of the eye.

2. Device according to claim 1 wherein 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 modulated, 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 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; 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.

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. 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, supplying the 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 claim 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, wherein: - the combiner extends around an optical axis (Ao); - the screen pixels are segmented into pixel groups; - the emission axes of the pixels of the same pixel group converge towards the same aiming point associated with the pixel group; - two different pixel groups are associated with two different aiming points, at least one aiming point associated with a pixel group being distant from the optical axis.

9. Device according to claim 8, wherein: - the screen comprises a first group of pixels, whose emission axes converge towards a first sighting point, the first group of pixels being configured to form a first part of an image when the pupil of the eye occupies a first position; - the screen comprises a second group of pixels, whose emission axes converge towards a second sighting point, different from the first sighting point, the second group of pixels being configured to form a second part of the image when the pupil of the eye occupies a second position, angularly offset with respect to the first position.

10. Device according to claim 8, wherein: - the screen has a first group of pixels, whose emission axes converge towards a first point of view, the first group of pixels being configured to form an image when the pupil of the eye occupies a first position; - the screen has a second group of pixels, whose emission axes converge towards a second point of view, different from the first point of view, the second group of pixels being configured to form the image when the pupil of the eye occupies a second position, different from the first position.

11. Device according to claim 10, wherein: - the pixels of the screen are segmented into macropixels, the pixels of the same macropixel being configured to display the same content; - the emission axes of the pixels of the same macropixel aim at different aiming points.

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

13. Device according to claim 12, 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.

14. Device according to claim 13, wherein the holographic combiner forms a reflector in the or each spectral emission band.

Images