ADJUSTABLE FOCAL LENGTH ILLUMINATOR FOR A DISPLAY PANEL

Abstract

An illuminator for a display panel includes a slab of transparent material for propagating illuminating light between outer surfaces of the slab, an out-coupler supported by the slab for out-coupling portions of the illuminating light along one of the outer surfaces of the slab, and a tunable microlens array for forming an array of light spots from the out-coupled illuminating light portions downstream of the focusing element for illuminating pixels of the display panel. The array of light spots may be repeated at a distance from the tunable microlens array due to Talbot effect. The display panel may be illuminated in a color-sequential manner, and the tunable microlens array may be used to adjust the focal plane position for each color channel individually.

Description

REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Patent Application No. 63/286,381 entitled “Display Applications of Switchable Gratings”, and U.S. Provisional Patent Application No. 63/286,230 entitled “Active Fluidic Optical Element”, both filed on Dec. 6, 2021 and incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to illuminators, visual display devices, and related components, modules, and methods.

BACKGROUND

Visual displays provide information to viewer(s) including still images, video, data, etc. Visual displays have applications in diverse fields including entertainment, education, engineering, science, professional training, advertising, to name just a few examples. Some visual displays such as TV sets display images to several users, and some visual display systems such s near-eye displays (NEDs) are intended for individual users.

An artificial reality system generally includes an NED (e.g., a headset or a pair of glasses) configured to present content to a user. The near-eye display may display virtual objects or combine images of real objects with virtual objects, as in virtual reality (VR), augmented reality (AR), or mixed reality (MR) applications. For example, in an AR system, a user may view images of virtual objects (e.g., computer-generated images (CGIs)) superimposed with the surrounding environment by seeing through a “combiner” component. The combiner of a wearable display is typically transparent to external light but includes some light routing optic to direct the display light into the user's field of view.

Because a display of HMD or NED is usually worn on the head of a user, a large, bulky, unbalanced, and/or heavy display device with a heavy battery would be cumbersome and uncomfortable for the user to wear. Consequently, head-mounted display devices can benefit from a compact and efficient configuration, including efficient light sources and illuminators providing illumination of a display panel, high-throughput ocular lenses and other optical elements in the image forming train.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with the drawings, in which:

FIG. 1A is a side cross-sectional view of an illuminator of this disclosure;

FIG. 1B is a side cross-sectional view of a Talbot embodiment of the illuminator of FIG. 1A, superimposed with lateral optical power density distribution of the illuminating light;

FIG. 2 is a Talbot distribution of optical power density in a substrate of the display panel illuminated by the illuminator of FIG. 1A, in accordance with an embodiment;

FIG. 3 is a side cross-sectional view of a lightguide illuminated with light of three color channels at different angle of incidence, illustrating a color-specific lateral shift of light spots focused by the focusing element of the illuminator of FIG. 1A;

FIGS. 4A to 4D are side cross-sectional view of a tunable microlens array of this disclosure in a color-sequential illumination configuration;

FIG. 5 is a side cross-sectional view of a microlens array with a tunable liquid crystal (LC) layer;

FIG. 6 is a side cross-sectional view of a tunable LC droplet microlens array;

FIG. 7A is a plan view of an array of tunable Pancharatnam-Berry phase (PBP) LC microlenses;

FIG. 7B is a frontal view of a single PBP LC microlens of the array of FIG. 7A;

FIG. 7C is a magnified schematic view of LC molecules in an LC layer of the tunable PBP LC microlens of FIG. 7B;

FIG. 8 is an exploded vide of a display apparatus with tunable the positions of the array of light spots/optical power density peaks of the illuminating light;

FIG. 9 is a flow chart of a method for illuminating a display panel in accordance with this disclosure;

FIG. 10 is a view of wearable display of this disclosure having a form factor of a pair of eyeglasses; and

FIG. 11 is a three-dimensional view of a head-mounted display (HMD) of this disclosure.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. All statements herein reciting principles, aspects, and embodiments of this disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are not intended to imply sequential ordering, but rather are intended to distinguish one element from another, unless explicitly stated. Similarly, sequential ordering of method steps does not imply a sequential order of their execution, unless explicitly stated.

In a visual display including an array of pixels coupled to an illuminator, the efficiency of light utilization depends on a ratio of a geometrical area occupied by pixels to a total area of the display panel. For miniature displays often used in near-eye and/or head-mounted displays, the ratio can be lower than 50%. The efficient backlight utilization can be further hindered by color filters on the display panel, which on average transmit no more than 30% of incoming light. On top of that, there may exist a 50% polarization loss for polarization-based display panels such as liquid crystal (LC) display panels. All these factors considerably reduce the light utilization and overall wall plug efficiency of the display, which is undesirable.

In accordance with this disclosure, light utilization and wall plug efficiency of a backlit display may be improved by providing a lightguide illuminator coupled to a microlens array. The microlens array is disposed downstream of the slab lightguide to concentrate the out-coupled wide light beam into an array of tightly focused light spots. In displays where the illuminator emits light of primary colors, e.g. red, green, and blue, the colors and locations of focused spots of illuminating light may be matched to that of the color filters of the display. Furthermore, upon illumination with color-interleaved arrays of focused spots, the color filters may be omitted altogether. For polarization-based displays, the polarization of the emitted light may be matched to a pre-defined input polarization state. Matching the spatial distribution, transmission wavelength, and/or the transmitted polarization characteristics of the pixels of the display panel enables one to considerably improve the useful portion of display light that is not absorbed or reflected by the display panel on its way to the eyes of the viewer, and consequently to considerably improve the display's wall plug efficiency.

The microlens array used to provide the array of illuminating spots matching the array of pixels may have a strong chromatic focal shift, especially for high numerical aperture of the microlenses used to provide a wide viewing angle of the display panel. Furthermore, in some embodiments, a light interference effect termed Talbot effect may be utilized to have the illuminating light cross a comparatively thick substrate of the display panel while preserving the high numerical aperture of the focused light spots. A Talbot distance, i.e. a distance where a peaked illuminating pattern is repeated, depends on wavelength. The Talbot distance wavelength dependence may result in light spots of different colors forming at different depths into the display panel substrate.

In accordance with this disclosure, the color dependence of the focal length and/or the color dependence of Talbot distance may be overcome by providing an array of tunable microlenses, i.e. microlenses having a tunable or switchable focal length. The lightguide may be fed with light of different color channels in a color-sequential manner, and the focal length of the microlens array may be tuned or switched in sync with sequencing the colors, enabling all colors of the illuminating light to be focused effectively into a display panel. Throughout the specification, the terms “tunable” and “switchable”, when applied to lenses or other focusing elements, are used interchangeably.

In accordance with the present disclosure, there is provided an illuminator comprising a slab of transparent material, the slab including opposed first and second surfaces for propagating illuminating light in the slab by a series of internal reflections from the first and second surfaces. An out-coupler is supported by the slab for out-coupling portions of the illuminating light from the slab at the first surface. A tunable microlens array is coupled to the first surface for forming an array of light spots from the out-coupled illuminating light portions at an adjustable distance from the first surface.

The illuminator may further include a multi-color light source for providing the illuminating light of a color channel of a plurality of color channels, and an in-coupler for in-coupling the illuminating light into the slab. The in-coupler may be configured to in-couple different color channels of the plurality of color channels at different angles, whereby a lateral position of light spots of the array of light spots depends on the color channel of the illuminating light. The tunable microlens array may include at least one of: a liquid crystal layer with a variable liquid crystal orientation; a tunable liquid crystal microlens array; an array of switchable Pancharatnam-Berry phase microlenses; etc.

In accordance with the present disclosure, there is provided a display apparatus comprising a display panel including a pixel array on a substrate, and an illuminator described above. In operation, the an array of light spots may be formed on the pixel array. In some embodiments, light of the array of light spots propagates through the substrate and produces an array of optical power density peaks at the pixel array due to Talbot effect. The pixel array may include a plurality of interleaved color sub-pixel arrays, each color sub-pixel array corresponding to a color channel of a plurality of color channels of an image to be displayed by the display apparatus.

The illuminator may include a multi-color light source for providing the illuminating light of a color channel of the plurality of color channels, and an in-coupler for in-coupling the illuminating light into the slab. The in-coupler may be configured to in-couple different color channels of the plurality of color channels at different angles, whereby a lateral position of light spots of the array of light spots depends on the color channel of the illuminating light. In operation, light of the array of light spots may propagate through the substrate and produce an array of optical power density peaks at the pixel array due to Talbot effect. A lateral position of optical power density peaks of the array of optical power density peaks may be matched to a lateral position of a corresponding color sub-pixel sub-array of the plurality of interleaved color sub-pixel arrays.

In some embodiments, the display apparatus may further include a controller operably coupled to the multi-color light source and the tunable microlens array. The controller may be configured to operate the multi-color light source to provide the illuminating light in a color-sequential manner, and to tune the tunable microlens array to adjust the distance depending on a current color channel of the illuminating light. The in-coupler may include a tiltable reflector for varying an angle of incidence of the illuminating light onto the slab.

In accordance with the present disclosure, there is further provided a method for illuminating a display panel comprising a pixel array on a substrate. The method includes propagating illuminating light in a slab of transparent material by a series of internal reflections from opposed first and second surfaces of the slab; out-coupling portions of the illuminating light from the slab at the first surface using an out-coupler; focusing the out-coupled illuminating light portions at a distance from the first surface using a tunable microlens array; and tuning the tunable microlens array to form an array of light spots for illuminating the pixel array of the display panel.

The method may further include propagating light of the array of light spots through the substrate to produce an array of optical power density peaks at the pixel array due to Talbot effect. The method may further include operating a multi-color light source to provide the illuminating light in a color-sequential manner, and tuning the tunable microlens array to adjust the distance depending on a current color of the illuminating light. The method may further include in-coupling different color channels of the plurality of color channels at different angles, whereby a lateral position of light spots of the array of light spots depends on a color channel of the illuminating light.

In embodiments where the pixel array comprises a plurality of interleaved color sub-pixel arrays, the method may further include propagating light of the array of light spots through the substrate to produce an array of optical power density peaks at the pixel array due to Talbot effect, and matching a lateral position of the optical power density peaks of the array of optical power density peaks to a lateral position of a corresponding color sub-pixel sub-array of the plurality of interleaved color sub-pixel arrays.

Referring now to FIG. 1A, an illuminator 100 is configured for illuminating a display panel 102. The display panel 102 includes a substrate 122 supporting an array of pixels 124 (e.g. one-dimensional or two-dimensional array) and an optional black grid 130. The display panel 102 may include other layers and substrates, which are omitted in FIGS. 1A and 1B for brevity of illustration. The illuminator 100 includes a slab 104 of transparent material, e.g. a plano-parallel slab of glass, plastic, transparent oxide, transparent crystalline material, or another suitable material. The slab 104 includes first 111 and second 112 opposed surfaces, which in this example are outer surfaces of the slab 104 disposed parallel to XY plane, for propagating illuminating light 106 in the slab 104 by a series of internal reflections, e.g. total internal reflections, from the first 111 and second 112 opposed surfaces. The series of internal reflections is illustrated schematically with a zigzag dashed line, which represents the propagation path of the illuminating light 106.

The illuminating light 106 is emitted by a light source 108. The illuminating light 106 is coupled into the slab 104 by an in-coupling grating 110 or by another suitable in-coupler such as a prism, a slanted surface, etc. Portions 114 of the illuminating light 106 propagating in the slab 104 are out-coupled through the first surface 111 by an out-coupler, e.g. an out-coupling grating 116, supported by the slab 104. The out-coupling grating 116 may be a smooth and flat, continuous grating, and may be disposed in the slab 104 or on the slab 104, as shown in FIG. 1A. The slab 104 with its out-coupling grating 116 operates as a pupil-replicating lightguide providing multiple offset light portions 114 of the illuminating light 106. The out-coupled light portions 114 form a nearly-collimated wide light beam that impinges onto a tunable microlens array 118, which forms an array of light spots 120 (FIG. 1B). In other words, the array of light spots 120 is formed by focusing the out-coupled illuminating light portions 114 by the tunable microlens array 118. The light spots 120 may be formed at an adjustable distance or depth, i.e. at an adjustable Z-coordinate in FIGS. 1A and 1B, by tuning the focal distance of the tunable microlens array 118.

In embodiments where the substrate 122 of the display panel 102 is thin enough, the light spots 120 may illuminate the pixels 124 of the display panel 102 directly. In embodiments where the substrate 122 is too thick for the light spots 120 to be smaller than the pixel 124 width and/or to be formed with a sufficient angle of convergence at the pixel center, the tunable microlens array 118 may be configured to provide an array of optical power density peaks 128 from the array of light spots 120 at a z-distance from the array of light spots 120 by utilizing Talbot effect. The Talbot effect causes a peaky optical power density distribution to reproduce itself at a distance from the array of light spots 120 for sufficiently spatially coherent light. In FIG. 1B, the array of optical power density peaks 128 illuminates the pixels 124. Positions of individual optical power density peaks 128 are coordinated with positions of individual pixels 124 of the display panel 102, with one optical power density peak 128 illuminating one pixel 124 in this example.

The Talbot effect that reproduces the optical power density distribution at a higher plane spaced apart from the original plane of a peaky optical power density distribution is illustrated in FIG. 2. This figure shows a map 200 of optical power density through the substrate 122 of the display panel 102, with horizontal axis (i.e. X-axis in FIG. 2) representing a lateral coordinate on the tunable microlens array 118, and a vertical axis (i.e. Z-axis in FIG. 2) representing the thickness dimension of the substrate 122 of the display panel 102. The tunable microlens array 118 may be configured to form the array of light spots 120 at a focal plane 202 disposed some 0.09 mm into the substrate 122. The lateral (XY) optical power density distribution is repeated at a Talbot plane 204, forming the array of optical power density peaks 128 at the Talbot plane 204 with a same pitch as at the focal plane 202. The array of pixels 124 of the display panel 102 may be disposed at the Talbot plane 204. Such a configuration allows the efficiency of light utilization to be considerably increased due to most of the illuminating light 106 propagating through the pixels 124 without being absorbed by the black grid 130 between the pixels 124 (FIGS. 1A and 1B).

One method to provide an array of color-dispersed light spots for illuminating of a color display panel including a plurality of interleaved color sub-pixel arrays is to pre-tilt light beams of individual color channels impinging onto the pupil-replicating lightguide. Referring to FIG. 3 for a non-limiting illustrative example, the slab 104 is illuminated by a multi-color light source 306 providing light of red (R), green (G), and/or blue (B) color channels in-coupled by the in-coupling grating 110 into the slab 104 at slightly different in-coupling angles. The in-coupling angles are exaggerated in FIG. 3 for clarity. A light beam of the R color channel is shown with long-dash lines, a light beam of the G color channel is shown with solid lines, and a light beam of B color channel is shown with short-dash lines. To provide different incidence angles for the in-coupled light of different colors, the multi-color light source may include a plurality of laterally offset laser diodes and/or light-emitting diodes of different colors coupled to a common collimator. The multi-color light source 306 may provide light of different colors simultaneously or in a color-sequential manner.

The light beams of the R, G, B in-coupled color channels propagate in the slab 104. The light beams are out-coupled by the out-coupling grating 116 at angles corresponding to the in-coupling angles of the R, G, B light beams into the slab 104. Since the light beams of the R, G, B in-coupled color channels are out-coupled at different angles, the out-coupled light beams of the R, G, B color channels are focused by the tunable microlens array 118 at offset locations forming color-interleaved sub-arrays of R, G, B light spots, as illustrated in FIG. 3. Each one of the color-interleaved sub-arrays of R, G, B light spots corresponding to light of a particular one of the plurality of R, G, B color channels. The configuration of FIG. 3 may be used for illuminating the interleaved color sub-pixel arrays of a color display panel directly, or by using the Talbot effect.

The utilization of Talbot effect for color panel illumination requires accounting for a dependence of z-position Z_T of a Talbot order N on wavelength, which may be defined by the following equation:

$\begin{array}{cc}{Z}_T=\frac{{\mathrm{Na}}^2}{\lambda },& \left(1\right)\end{array}$

where N is an integer denoting a Talbot order, a is the length of a Talbot period and λ is wavelength of light. The z-positions Z_T of different color channels (i.e. different wavelengths) are different for a same Talbot order N. The focal length tunability of the microlens array 118 enables the Z-coordinate of the focal plane of the color-interleaved sub-arrays of R, G, B light spots to be dynamically varied. This may be useful to precisely focus the R, G, B light spots onto the pixel array of the display panel being illuminated. In color-sequential illumination configurations with only illuminating color present at any given moment of time, the Z-coordinate of the focal plane may be adjusted in a color-selective manner, enabling a great degree of flexibility in compensating the inherent dependence of Talbot distance on wavelength represented by Eq. (1) above, as well as accounting for a chromatic focal shift of microlens arrays due to chromatic dispersion of the microlens material.

The latter point is illustrated in to FIGS. 4A to 4D. In FIG. 4A, the microlens array 118 is not tuned as the color composition of the illuminating light is changed. Due to chromatic dispersion of the microlens material, light 406B of blue color is focused more strongly than light 406G of green color and light 406R of red color. The microlens array 118 may be tuned to adjust the focal distance depending on a current color channel of the illuminating light. This is illustrated in FIGS. 4B to 4D, where the microlens array 118 is tuned to adjust all focal lengths f to be equal, that is, the focal length for the blue color light 406B equal to the focal length for the green color light 406G equal to the focal length for the red color light 406R. The tunability of the microlens array 118 enables compensation of any chromatic focus shifts, however caused, be it due to material dispersion, Talbot distance dependence on wavelength as given by Eq. (1) above, or for any other reason, provided a sufficient tuning range of the microlens array 118.

Several illustrative non-limiting examples of tunable microlens arrays usable in illuminators of this disclosure will now be considered.

Referring first to FIG. 5, a microlens array 518 may be used as the tunable microlens array 118 of the illuminator 100 of FIGS. 1A and 1B. The microlens array 518 includes an array of refractive microlenses 509 made of isotropic or anisotropic material, e.g. isotropic or anisotropic polymer. The refractive microlenses 509 are disposed in a liquid crystal (LC) cell defined by first 501 and second 502 opposed transparent electrodes. The LC cell is filled with a nematic LC fluid, forming an LC layer 505. The LC layer 505 includes rod-like LC molecules 504 that may be oriented in an electric field caused by applying voltage to the first 501 and second 502 electrodes. The nematic LC fluid is birefringent with an optic axis parallel to LC director. The LC director indicates a local orientation of the LC molecules 504. Typically, an effective refractive index is higher for light linearly polarized along the LC director. In operation, application of voltage to the first 501 and second 502 electrodes causes the LC molecules 504A of the LC layer 505 to become vertically oriented, i.e. oriented along Z-axis as shown in FIG. 5, causing a reduction of the effective refractive index of the LC layer 505 surrounding the refractive microlenses 509, which causes their effective focal length to be tuned.

Turning to FIG. 6, a tunable LC microlens array 618 may be used as the tunable microlens array 118 of the illuminator 100 of FIGS. 1A and 1B. The tunable LC microlens array 618 of FIG. 6 may include an array of LC microlenses 600 including round droplets (e.g. hemispherical droplets) of oriented LC molecules 604 immersed into an isotropic polymer substrate 603. The refractive index of the isotropic polymer substrate 603 may be matched to an ordinary index of refraction of the LC fluid in the droplets, or may be different from both the ordinary and extraordinary indices of refraction of the LC fluid in the droplets. The LC molecules 604 may be oriented e.g. along X-axis as shown on a left-side portion of FIG. 6. When illuminated with a light beam 606 linearly polarized along x-axis, the microlens 600 will focus the light beam 306 due to the focusing property of a curved interface 611 between the LC droplets and the polymer substrate 603, the curved interface 611 having a non-zero refractive index step, similarly to the microlens array 518 of FIG. 5. When the LC molecules 604 become oriented along Z-axis, e.g. by applying a strong voltage to a pair of optional transparent electrodes 614 and 615, the curved interface 611 has a zero refractive index step since the refractive index of the isotropic polymer substrate is matched to an ordinary index of refraction of the LC fluid. Accordingly, the light beam 606 will remain non-focused as illustrated on the right-side portion of FIG. 6. When the applied voltage is insufficient to orient the LC molecules 604 along Z-axis, or when the refractive index of the isotropic polymer substrate 603 is different from both the ordinary and extraordinary indices of refraction of the LC fluid in the droplets, applying the electric field will change the focal length of the LC microlenses 600, while retaining some focusing/defocusing power of the LC microlens array 618.

Referring now to FIG. 7A, a Pancharatnam-Berry phase (PBP) microlens array 718 may be used as the tunable microlens array 118 of the illuminator 100 of FIGS. 1A and 1B. The PBP microlens array 718 includes an array of PBP LC microlenses 700 formed in a liquid crystal (LC) layer 702 including LC molecules 704 (FIGS. 7A, 7B). The LC molecules 704 are disposed in XY plane at a varying in-plane orientation depending on the distance r from the lens center. The orientation angle ϕ(r) of the LC molecules 704 in the liquid crystal layer of each PBP LC microlens 700 is given by

$\begin{array}{cc}\varphi \left(r\right)=\frac{\pi r^2}{2f_0{\lambda }_0}& \left(2a\right)\end{array}$

where f₀ is a desired focal length and λ₀ is wavelength. The optical phase delay in each PBP LC microlens 700 is due to Pancharatnam-Berry phase, or geometrical phase effect. An optical retardation R of the liquid crystal layer having a thickness t is defined as R=tΔn, where Δn is the optical birefringence of the liquid crystal layer. At the optical retardation R of the LC layer of λ_∥/2, i.e. half wavelength, the accumulated phase delay P(r) due to the PBP effect can be expressed rather simply as P(r)=2ϕ(r), or, by taking into account Eq. (2a) above,

$\begin{array}{cc}P\left(r\right)=\frac{\pi r^2}{f_0{\lambda }_0}& \left(2b\right)\end{array}$

It is the quadratic dependence of the PBP P(r) on the radial coordinate r that results in the focusing, or defocusing, function of each PBP LC microlens 700. Each PBP LC microlens 300B has the azimuthal angle ϕ continuously and smoothly varying across the surface of the LC layer (FIG. 7C). Accordingly, the mapping of the azimuthal angle to PBP, i.e. P(r)=2ϕ(r) when R=λ−/2, allows for a more drastic phase change without introducing discontinuities at a boundary of 2π modulo.

The phase delay relationship as defined by Eq. (2b) may be erased by application of an electric field across the LC layer 702, making the PBP LC microlens 700 switchable. By combining the switchable PBP LC microlens 700 with a constant refractive microlens, and/or by providing a stack of the switchable PBP LC microlenses 700 of different optical powers, the tunability of the PBP LC microlens array 718 may be expanded.

Referring to FIG. 8, a display apparatus 850 is a non-limiting illustrative embodiment of application of an illuminator of this disclosure in a visual display. The display apparatus 850 includes the display panel 102 having the array of pixels 124 defined by the black grid 130 and supported by the substrate 122, and an illuminator 800 coupled to the display panel 102 for illuminating the array of pixels 124 through the substrate 122.

The illuminator 800 is an embodiment of the illuminator 100 of FIG. 1A, and includes similar elements. The illuminator 800 of FIG. 8 includes a slab 804 of transparent material, an in-coupling grating 810 and an out-coupling grating 816 supported by the slab 804. The in-coupling grating 810 is a polarization volume hologram (PVH) grating that diffracts circularly polarized light of a first handedness while transmitting through a circularly polarized light of a second, opposite handedness.

In operation, a light source 808 emits a light beam 806 that is circularly polarized at the second handedness. The light beam 806 propagates through the PVH in-coupling grating 810 and the slab 804 and impinges onto a microelectromechanical system (MEMS) 840 including a tiltable reflector 842. The light beam 806 is reflected by the tiltable reflector 842 and impinges again onto the PVH in-coupling grating 810. Since handedness of a circularly polarized light reverses upon reflection, the reflected light beam 806 is diffracted by the PVH in-coupling grating 810, which in-couples the light beam 806 into the slab 804 to propagate in the slab 804 by a series of internal reflections from opposed outer surfaces of the slab 804, as illustrated with a solid arrowed zigzag line.

Out-coupled portions 814 of the light beam 806 are focused by a tunable microlens array 818, which may include any of the tunable microlenses described above with reference to FIGS. 4A-4D to FIGS. 7A-7C. It is to be noted that the out-coupled portions 814 originate from an out-coupled wide beam that is broken into individual portions or sub-beams by microlenses 819 of the tunable microlens array 818, not necessarily by the X-period of zigzag reflections. The individual portions are focused into light spots 820, similarly to the illuminator 100 of FIG. 1A. The array of light spots 820 is converted into an array of optical power density peaks 828 by Talbot effect in an optical stack including the substrate 122 of the display panel 102, as explained above with reference to FIGS. 1A, 1B, and FIG. 2.

The illuminator 800 of FIG. 8 further includes a controller 880 operably coupled to the light source 808, the tunable microlens array 818, and the MEMS 840. The controller 880 is configured to tilt the tiltable reflector 842 by a controllable angle to make the array of optical power density peaks 828 coincide with the array of pixels 124 defined by the black grid or black matrix 130. For example, initially the controller 880 may tilt the out-coupled portions 814 to shifted positions 814′ shown with dashed lines. At the shifted positions 814′, the light spots 820 are shifted from a nominal position. The shifted light spots 820 cause the array of optical power density peaks 828 to also be shifted to positions 828′ shown with dashed lines, and no light passes through the black grid 130. The controller 880 may tune the angle of tilt of the tiltable mirror 842 to bring the array of optical power density peaks 828 to the positions overlapping with the pixels 124, and the light passes through the pixels 124. Herein the term “pixels” also includes color sub-pixels of a color display panel.

For a color display panel 102 including a plurality of interleaved color sub-pixel arrays, each color sub-pixel array corresponding to a color channel of a plurality of color channels of an image to be displayed by the display apparatus 850, the light source 808 may be a multi-color light source for providing the illuminating light of a color channel of the plurality of color channels. The MEMS 840 may be a part of an in-coupler for in-coupling the illuminating light 806 into the slab 804. The controller 880 may cause the multi-color light source to output light of different color channels one after another in a color-sequential manner. The controller 880 may operate the MEMS 840 such that different color channels are in-coupled at different angles, whereby a lateral position of the light spots 820 and the optical power density peaks 828 depends on the color channel of the illuminating light 806, and can be matched to a lateral position of a corresponding color sub-pixel sub-array of the display panel 102. The controller 880 may tune the tunable microlens array 818 to adjust the focusing distance depending on a current color channel of the illuminating light, to compensate for Talbot distance wavelength dependence and/or chromatic aberration/chromatic focal shift of the tunable microlens array 818.

Referring now to FIG. 9 with further reference to FIG. 8, a method 900 for illuminating a display panel including a pixel array on a substrate includes propagating (906) illuminating light in a slab of transparent material e.g. the slab 804 (FIG. 8), by a series of internal reflections, such as total internal reflections (TIRs), from opposed surfaces of the slab. Portions of the illuminating light are out-coupled (FIG. 9; 908) from the slab at one of the slab's outer surfaces using an out-coupler. The out-coupler may include a grating disposed on or within the slab such as a surface-relief grating, a PVH grating, etc. The illuminating light portions are focused (910) at a distance from the surface by a microlens array, e.g. the tunable microlens array 818 of FIG. 8. The focused light may be propagated further (912) through the substrate of the display panel to form an array of optical power density peaks due to Talbot effect. The microlens array may be tuned (914) to adjust the location of the focused light spots and/or the optical power density peaks to match the plane of the display panel (sub)pixels.

For color display panels, the method 900 may include operating (902) a multi-color light source to provide the illuminating light in a color-sequential manner, i.e. one color channel after another. The microlens tuning 914 may be performed in coordination with changing light color of the illuminating light, by adjusting the focusing distance so as to offset color-dependent focusing effects as explained above. Different color channels may be in-coupled (904) at different angles, whereby a lateral position of light spots depends on the color channel of the illuminating light. A lateral position of the optical power density peaks may be matched (916) to a lateral position of a corresponding color sub-pixel sub-array of the color display panel. In FIG. 9, optional steps are shown with dashed rounded boxes.

Referring to FIG. 10, a virtual reality (VR) near-eye display 1000 includes a frame 1001 supporting, for each eye: an illuminator 1030 including any of the illuminators disclosed herein; a display panel 1010 including an array of display pixels; and an ocular lens 1020 for converting the image in linear domain generated by the display panel 1010 into an image in angular domain for direct observation at an eyebox 1012. A plurality of eyebox illuminators 1006, shown as black dots, may be placed around the display panel 1010 on a surface that faces the eyebox 1012. An eye-tracking camera 1004 may be provided for each eyebox 1012.

The purpose of the eye-tracking cameras 1004 is to determine position and/or orientation of both eyes of the user. The eyebox illuminators 1006 illuminate the eyes at the corresponding eyeboxes 1012, allowing the eye-tracking cameras 1004 to obtain the images of the eyes, as well as to provide reference reflections i.e. glints. The glints may function as reference points in the captured eye image, facilitating the eye gazing direction determination by determining position of the eye pupil images relative to the glints images. To avoid distracting the user with the light of the eyebox illuminators 1006, the latter may be made to emit light invisible to the user. For example, infrared light may be used to illuminate the eyeboxes 1012.

Turning to FIG. 11, an HMD 1100 is an example of an AR/VR wearable display system which encloses the user's face, for a greater degree of immersion into the AR/VR environment. The HMD 1100 may generate the entirely virtual 3D imagery. The HMD 1100 may include a front body 1102 and a band 1104 that can be secured around the user's head. The front body 1102 is configured for placement in front of eyes of a user in a reliable and comfortable manner. A display system 1180 may be disposed in the front body 1102 for presenting AR/VR imagery to the user. The display system 1180 may include any of the display devices and illuminators disclosed herein. Sides 1106 of the front body 1102 may be opaque or transparent.

In some embodiments, the front body 1102 includes locators 1108 and an inertial measurement unit (IMU) 1110 for tracking acceleration of the HMD 1100, and position sensors 1112 for tracking position of the HMD 1100. The IMU 1110 is an electronic device that generates data indicating a position of the HMD 1100 based on measurement signals received from one or more of position sensors 1112, which generate one or more measurement signals in response to motion of the HMD 1100. Examples of position sensors 1112 include: one or more accelerometers, one or more gyroscopes, one or more magnetometers, another suitable type of sensor that detects motion, a type of sensor used for error correction of the IMU 1110, or some combination thereof. The position sensors 1112 may be located external to the IMU 1110, internal to the IMU 1110, or some combination thereof.

The locators 1108 are traced by an external imaging device of a virtual reality system, such that the virtual reality system can track the location and orientation of the entire HMD 1100. Information generated by the IMU 1110 and the position sensors 1112 may be compared with the position and orientation obtained by tracking the locators 1108, for improved tracking accuracy of position and orientation of the HMD 1100. Accurate position and orientation is important for presenting appropriate virtual scenery to the user as the latter moves and turns in 3D space.

The HMD 1100 may further include a depth camera assembly (DCA) 1111, which captures data describing depth information of a local area surrounding some or all of the HMD 1100. The depth information may be compared with the information from the IMU 1110, for better accuracy of determination of position and orientation of the HMD 1100 in 3D space.

The HMD 1100 may further include an eye tracking system 1114 for determining orientation and position of user's eyes in real time. The obtained position and orientation of the eyes also allows the HMD 1100 to determine the gaze direction of the user and to adjust the image generated by the display system 1180 accordingly. The determined gaze direction and vergence angle may be used to adjust the display system 1180 to reduce the vergence-accommodation conflict. The direction and vergence may also be used for displays' exit pupil steering as disclosed herein. Furthermore, the determined vergence and gaze angles may be used for interaction with the user, highlighting objects, bringing objects to the foreground, creating additional objects or pointers, etc. An audio system may also be provided including e.g. a set of small speakers built into the front body 1102.

Embodiments of the present disclosure may include, or be implemented in conjunction with, an artificial reality system. An artificial reality system adjusts sensory information about outside world obtained through the senses such as visual information, audio, touch (somatosensation) information, acceleration, balance, etc., in some manner before presentation to a user. By way of non-limiting examples, artificial reality may include virtual reality (VR), augmented reality (AR), mixed reality (MR), hybrid reality, or some combination and/or derivatives thereof. Artificial reality content may include entirely generated content or generated content combined with captured (e.g., real-world) content. The artificial reality content may include video, audio, somatic or haptic feedback, or some combination thereof. Any of this content may be presented in a single channel or in multiple channels, such as in a stereo video that produces a three-dimensional effect to the viewer. Furthermore, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, for example, create content in artificial reality and/or are otherwise used in (e.g., perform activities in) artificial reality. The artificial reality system that provides the artificial reality content may be implemented on various platforms, including a wearable display such as an HMD connected to a host computer system, a standalone HMD, a near-eye display having a form factor of eyeglasses, a mobile device or computing system, or any other hardware platform capable of providing artificial reality content to one or more viewers.

The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments and modifications, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.

Claims

1. An illuminator comprising:

a slab of transparent material, the slab comprising opposed first and second surfaces for propagating illuminating light in the slab by a series of internal reflections from the first and second surfaces;

an out-coupler supported by the slab for out-coupling portions of the illuminating light from the slab at the first surface; and

a tunable microlens array coupled to the first surface for forming an array of light spots from the out-coupled illuminating light portions at an adjustable distance from the first surface.

2. The illuminator of claim 1, further comprising:

a multi-color light source for providing the illuminating light of a color channel of a plurality of color channels; and

an in-coupler for in-coupling the illuminating light into the slab, wherein the in-coupler is configured to in-couple different color channels of the plurality of color channels at different angles, whereby a lateral position of light spots of the array of light spots depends on the color channel of the illuminating light.

3. The illuminator of claim 1, wherein the tunable microlens array comprises a liquid crystal layer with a variable liquid crystal orientation.

4. The illuminator of claim 1, wherein the tunable microlens array comprises a tunable liquid crystal microlens array.

5. The illuminator of claim 1, wherein the tunable microlens array comprises an array of switchable Pancharatnam-Berry phase microlenses.

6. A display apparatus comprising:

a display panel comprising a pixel array on a substrate; and

an illuminator coupled to the display panel for illuminating the pixel array through the substrate, the illuminator comprising: a slab of transparent material, the slab comprising opposed first and second surfaces for propagating illuminating light in the slab by a series of internal reflections from the first and second surfaces; an out-coupler supported by the slab for out-coupling portions of the illuminating light from the slab at the first surface; and a tunable microlens array coupled to the first surface for forming an array of light spots from the out-coupled illuminating light portions at a distance from the first surface.

7. The display apparatus of claim 6, wherein in operation, the an array of light spots is formed on the pixel array.

8. The display apparatus of claim 6, wherein in operation, light of the array of light spots propagates through the substrate and produces an array of optical power density peaks at the pixel array due to Talbot effect.

9. The display apparatus of claim 6, wherein the pixel array comprises a plurality of interleaved color sub-pixel arrays, each color sub-pixel array corresponding to a color channel of a plurality of color channels of an image to be displayed by the display apparatus, the illuminator further comprising:

a multi-color light source for providing the illuminating light of a color channel of the plurality of color channels; and

an in-coupler for in-coupling the illuminating light into the slab, wherein the in-coupler is configured to in-couple different color channels of the plurality of color channels at different angles, whereby a lateral position of light spots of the array of light spots depends on the color channel of the illuminating light.

10. The display apparatus of claim 9, wherein in operation, light of the array of light spots propagates through the substrate and produces an array of optical power density peaks at the pixel array due to Talbot effect, wherein a lateral position of optical power density peaks of the array of optical power density peaks is matched to a lateral position of a corresponding color sub-pixel sub-array of the plurality of interleaved color sub-pixel arrays.

11. The display apparatus of claim 9, further comprising a controller operably coupled to the multi-color light source and the tunable microlens array and configured to:

operate the multi-color light source to provide the illuminating light in a color-sequential manner; and

tune the tunable microlens array to adjust the distance depending on a current color channel of the illuminating light.

12. The display apparatus of claim 9, wherein the in-coupler comprises a tiltable reflector for varying an angle of incidence of the illuminating light onto the slab.

13. The display apparatus of claim 6, wherein the tunable microlens array comprises a liquid crystal layer with a variable liquid crystal orientation.

14. The display apparatus of claim 6, wherein the tunable microlens array comprises a tunable liquid crystal microlens array.

15. The display apparatus of claim 6, wherein the tunable microlens array comprises an array of switchable Pancharatnam-Berry phase microlenses.

16. A method for illuminating a display panel comprising a pixel array on a substrate, the method comprising:

propagating illuminating light in a slab of transparent material by a series of internal reflections from opposed first and second surfaces of the slab;

out-coupling portions of the illuminating light from the slab at the first surface using an out-coupler;

focusing the out-coupled illuminating light portions at a distance from the first surface using a tunable microlens array; and

tuning the tunable microlens array to form an array of light spots for illuminating the pixel array of the display panel.

17. The method of claim 16, further comprising propagating light of the array of light spots through the substrate to produce an array of optical power density peaks at the pixel array due to Talbot effect.

18. The method of claim 16, further comprising:

operating a multi-color light source to provide the illuminating light in a color-sequential manner; and

tuning the tunable microlens array to adjust the distance depending on a current color of the illuminating light.

19. The method of claim 18, further comprising in-coupling different color channels of the plurality of color channels at different angles, whereby a lateral position of light spots of the array of light spots depends on a color channel of the illuminating light.

20. The method of claim 19, wherein the pixel array comprises a plurality of interleaved color sub-pixel arrays, the method further comprising:

propagating light of the array of light spots through the substrate to produce an array of optical power density peaks at the pixel array due to Talbot effect; and

matching a lateral position of the optical power density peaks of the array of optical power density peaks to a lateral position of a corresponding color sub-pixel sub-array of the plurality of interleaved color sub-pixel arrays.