Patent Description:
Modern computing and display technologies have facilitated the development of systems for so-called "virtual reality" or "augmented reality" experiences, wherein digitally produced images or portions thereof are presented in a wearable device to a user in a manner wherein they seem to be, or may be perceived as, real. A virtual reality, or "VR," scenario typically involves presentation of digital or virtual image information without transparency to other actual real-world visual input; an augmented reality, or "AR," scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the user.

Despite the progress made in these display technologies, there is a need in the art for improved methods and systems related to augmented reality systems, particularly, display systems. <CIT> discloses a projection display arranged to display an image to an observer using waveguide techniques to generate a display defining a large exit pupil at the point of the observer and a large field of view, whilst using a small image-providing light source device. The projection display uses two parallel waveguides made from a light transmissive material. One waveguide stretches the horizontal pupil of the final display and the other waveguide stretches the vertical pupil of the final display and acts as a combiner through which the observer views an outside world scene and the image overlaid on the scene. <CIT> discloses an eyepiece unit with optical filters including a set of waveguide layers including a first waveguide layer and a second waveguide layer. The first waveguide layer is disposed in a first lateral plane and includes a first incoupling diffractive element disposed at a first lateral position, a first waveguide, and a first outcoupling diffractive element. The second waveguide layer is disposed in a second lateral plane adjacent to the first lateral plane and includes a second incoupling diffractive element disposed at a second lateral position, a second waveguide, and a second outcoupling diffractive element. The eyepiece unit also includes a set of optical filters including a first optical filter positioned at the first lateral position and operable to attenuate light outside a first spectral band and a second optical filter positioned at the second lateral position and operable to attenuate light outside a second spectral band. <CIT> discloses an imaging system including a light source configured to generate a light beam. The system also includes first and second light guiding optical elements having respective first and second entry portions, and configured to propagate at least respective first and second portions of the light beam by total internal reflection. The system further includes a light distributor having a light distributor entry portion, a first exit portion, and a second exit portion. The light distributor is configured to direct the first and second portions of the light beam toward the first and second entry portions, respectively. The light distributor entry portion and the first exit portion are aligned along a first axis. The light distributor entry portion and the second exit portion are aligned along a second axis different from the first axis. <CIT> discloses a display device including an image projection device, a light guide plate, and a mask. The light guide plate includes deflectors arranged in a propagation direction of a light beam emitted from the image projection device and entering the light guide plate. Each deflector causes light beams to be emitted from an outgoing surface at angles different from each other in the propagation direction, the light beams emitted from locations different from each other or emitted in directions different from each other in a direction orthogonal to the longer direction of an incident surface in a displayed region of the image projection device. <CIT> discloses a system for displaying content to a user, comprising: a light source configured to project light associated with at least first and second frames of image data; a waveguide assembly configured to relay the projected light towards at least one eye of a user with a wavefront curvature for a focus level corresponding to a focal distance for the respective first or second frame of image data; and wherein the waveguide assembly comprises a first waveguide component operatively coupled to a first weak lens to modify first light associated with the first frame of image data such that the first light is perceived as coming from a first focal plane, and a second waveguide component, wherein the first waveguide component and the second waveguide component sandwich the first weak lens and are stacked along a z-axis in front of the at least one eye of the user. <CIT> is directed to an image forming unit that includes an image display device, a projection optical system, and a light guide. The light guide comprises a first diffractive optical element disposed on the light incident surface, and a reflection film disposed on the surface that faces the light incident surface.

The present invention is as set out in the appended set of claims. The present disclosure relates generally to methods and systems related to projection display systems including wearable displays. More particularly, embodiments of the present disclosure provide methods and systems for eyepiece units that include a retroreflector that improves system brightness and performance. In a particular embodiment, the retroreflector is integrated with a waveguide layer of an eyepiece to recycle light and increase the effective coupling efficiency of projected light into the waveguide layer. The disclosure is applicable to a variety of applications in computer vision and image display systems.

According to an embodiment of the present invention, a display system is provided. The display system includes projection optics configured to project a beam of light in a first direction and an eyepiece unit including a first waveguide layer disposed in a first lateral plane and including an incident light surface and an opposing surface opposite the incident light surface and an incoupling diffractive optical element disposed on the incident light surface. The incoupling diffractive optical element is configured to incouple a first portion of the beam of light and to propagate the first portion of the beam of light by total internal reflection in a second direction and transmit a second portion of the beam of light along the first direction. The eyepiece unit also includes a retroreflector disposed adjacent the opposing surface. The retroreflector is configured to retroreflect the second portion of the beam of light along a reflected direction opposite to the first direction.

According to another embodiment of the present invention, an eyepiece unit is provided. The eyepiece unit includes a waveguide layer disposed in a lateral plane and including an incident light surface and an opposing surface opposite the incident light surface and a first incoupling diffractive optical element disposed on the incident light surface. The first incoupling diffractive optical element is configured to incouple a first portion of a beam of light propagating in a first direction and to propagate the first portion of the beam of light by total internal reflection in a second direction and transmit a second portion of the beam of light along the first direction. The eyepiece unit also includes a second incoupling diffractive optical element disposed on the opposing surface. The second incoupling diffractive optical element is configured to receive the second portion of the beam of light along the first direction, incouple a third portion of the beam of light and to propagate the third portion of the beam of light by total internal reflection in a third direction, and transmit a fourth portion of the beam of light along the first direction. The eyepiece unit further includes a retroreflector disposed adjacent the opposing surface. The retroreflector is configured to retroreflect the fourth portion of the beam of light along a reflected direction opposite to the first direction.

According to an specific embodiment of the present invention, a method of operating a display system is provided. The method includes directing a beam of light in a first direction, receiving the beam of light at an incident light surface of a waveguide layer having an opposing surface opposite the incident light surface, and propagating, at a first diffractive optical element, a first portion of the beam of light into the waveguide layer in a second direction. The method also includes transmitting, at the first diffractive optical element, a second portion of the beam of light toward the opposing surface and retroreflecting the second portion of the beam of light along a reflected direction opposite to the first direction.

Numerous benefits are achieved by way of the present invention over conventional techniques. For example, embodiments of the present invention provide methods and systems that can result in higher efficiency and can recycle light that would otherwise be lost by using a retroreflector. These and other embodiments of the invention along with many of its advantages and features are described in more detail in conjunction with the text below and attached figures.

Embodiments of the present invention relate to methods and systems utilizing projection display systems including wearable displays. More particularly, embodiments of the present disclosure provide methods and systems for eyepiece units fitted with retroreflectors. The disclosure is applicable to a variety of applications in computer vision and image display systems.

<FIG> schematically illustrates light paths in a viewing optics assembly (VOA) that may be used to present a digital or virtual image to a viewer, according to some embodiments. The VOA includes a projector <NUM> and an eyepiece <NUM> that may be worn around a viewer's eye <NUM>. In some embodiments, the projector <NUM> may include a group of red LEDs, a group of green LEDs, and a group of blue LEDs. For example, the projector <NUM> may include two red LEDs, two green LEDs, and two blue LEDs. The eyepiece <NUM> may include one or more eyepiece layers. In some embodiments, the eyepiece <NUM> includes three eyepiece layers, one eyepiece layer for each of the three colors, red, green, and blue. In some embodiments, the eyepiece <NUM> may include six eyepiece layers, i.e., one set of eyepiece layers for each of the three colors configured for forming a virtual image at one depth plane, and another set of eyepiece layers for each of the three colors configured for forming a virtual image at another depth plane. In some embodiments, the eyepiece <NUM> may include three or more eyepiece layers for each of the three colors for three or more different depth planes. Each eyepiece layer includes a planar waveguide and may include an incoupling grating <NUM>, an orthogonal pupil expander (OPE) region <NUM>, and an exit pupil expander (EPE) region <NUM>. Although the eyepiece layer is referred to as a planar waveguide layer herein, it will be appreciated that diffractive and/or reflective structures can be integrated together with one or more surfaces of the eyepiece layer, resulting in a waveguide layer that is generally planar, but characterized by non-planar regions at which light can be diffracted and/or reflected.

Still referring to <FIG>, the projector <NUM> projects image light onto the incoupling grating <NUM> in an eyepiece layer <NUM>. The incoupling grating <NUM> couples the image light from the projector <NUM> into a planar waveguide propagating the image light in a direction toward the OPE region <NUM>. The planar waveguide propagates the image light in the horizontal direction by total internal reflection (TIR). The OPE region <NUM> of the eyepiece layer includes a diffractive element that couples and redirects a portion of the image light propagating in the waveguide toward the EPE region <NUM>. The EPE region <NUM> includes a diffractive element that couples and directs a portion of the image light propagating in the planar waveguide in a direction approximately perpendicular to the plane of the eyepiece layer toward the viewer's eye <NUM>. In this fashion, an image projected by the projector <NUM> may be viewed by the viewer's eye <NUM>. The part of the VOA illustrated in <FIG> may constitute a "monocle" for one eye of the viewer. The entire VOA may include two such monocles, one for each eye of the viewer.

As described above, image light generated by the projector may include light in the three colors, blue (B), green (G), and red (R). Such image light can be separated into the constituent colors, so that image light in each constituent color may be coupled to a respective waveguide in the eyepiece.

<FIG> illustrates schematically a method of color separation using an "in-line" approach according to an embodiment of the present invention. In this example, the eyepiece <NUM> may also include a blue waveguide <NUM>, a green waveguide <NUM>, and a red waveguide <NUM>. Each waveguide <NUM>, <NUM>, or <NUM> may include an ICG <NUM>, <NUM>, or <NUM>, an OPE region <NUM>, <NUM>, or <NUM>, and a EPE region <NUM>, <NUM>, or <NUM>. Here, image light in blue, green, and red colors generated by the projector subsystem <NUM> are not spatially separated from each other, and the ICGs <NUM>, <NUM>, and <NUM> in the blue, green, and red waveguides <NUM>, <NUM>, and <NUM> are laterally aligned with respect to each other. Thus, image light passes through each waveguide sequentially in a "serial" fashion. The eyepiece <NUM> may further include a first wavelength-selective optical element <NUM> positioned between the ICG <NUM> in the blue waveguide <NUM> and the ICG <NUM> in the green waveguide <NUM>, and a second wavelength-selective optical element <NUM> positioned between the ICG <NUM> in the green waveguide <NUM> and the ICG <NUM> in the red waveguide <NUM>. The first and second wavelength-selective optical elements <NUM> and <NUM> may, for instance, represent wavelength-selective optical filters (i.e., optical elements that selectively transmit light in a particular range of wavelengths) and/or wavelength-selective optical reflectors (i.e., mirrors and other optical elements that selectively reflect light in a particular range of wavelengths). A dichroic filter is one example of an optical element configured to both selectively transmit and reflect light on the basis of wavelength. In the following, the first and second wavelength-selective optical elements <NUM> and <NUM> may also be referred to as "optical filter <NUM>" and "optical filter <NUM>," respectively. Similarly, other wavelength-selective optical elements described with reference to any of <FIG> may also be referred to herein as "optical filters.

As illustrated in <FIG>, image light in all three colors is incident on the ICG <NUM> in the blue waveguide <NUM>. The ICG <NUM> in the blue waveguide <NUM> may couple a portion of the image light primarily in the blue wavelength range into the blue waveguide <NUM> to be guided toward the OPE region <NUM>. The ICG <NUM> in the blue waveguide <NUM> may also couple a small amount of green image light, and even a smaller amount of red light, into the blue waveguide <NUM>, as will be discussed further. Image light that is not coupled into the blue waveguide <NUM> is transmitted through the blue waveguide <NUM> and incident on the first optical filter <NUM>. The first optical filter <NUM> may be configured to have a high transmittance value in the green and red wavelength ranges, and a low transmittance value in the blue wavelength range. Therefore, image light transmitted by the first optical filter <NUM> and incident on the ICG <NUM> in the green waveguide <NUM> may contain primarily green image light and red image light, and very little or no blue image light.

Still referring to <FIG>, the ICG <NUM> in the green waveguide <NUM> may couple a portion of the image light primarily in the green wavelength range into the green waveguide <NUM> to be guided toward the OPE region <NUM>. The ICG <NUM> in the green waveguide <NUM> may also couple a small amount of red image light into the green waveguide <NUM>, as will be discussed further later. Image light that is not coupled into the green waveguide <NUM> may be transmitted through the green waveguide <NUM> and incident on the second optical filter <NUM>. The second optical filter <NUM> may be configured to have a high transmittance value in the red wavelength range, and a low transmittance value in the green and blue wavelength ranges. Therefore, image light transmitted by the second optical filter <NUM> and incident on the ICG <NUM> in the red waveguide <NUM> may contain primarily red image light, and very little or no green image light and blue image light. The ICG <NUM> in the red waveguide <NUM> may couple a portion of the image light primarily in the red wavelength range into the red waveguide <NUM> to be guided toward the OPE region <NUM>.

<FIG> illustrates schematically a plan view of an eyepiece <NUM> according to an embodiment of the present invention. The eyepiece <NUM> may include a blue waveguide <NUM>, a green waveguide <NUM>, and a red waveguide <NUM> stacked in adjacent lateral planes. Each waveguide <NUM>, <NUM>, or <NUM> may include an ICG region <NUM>, an OPE region <NUM>, and a EPE region <NUM>. The ICG regions <NUM> for the three waveguides <NUM>, <NUM>, and <NUM> may be disposed in the same lateral position, and are thus stacked along the same optical path. A first optical filter <NUM> may be positioned between the ICG <NUM> of the blue waveguide <NUM> and the ICG <NUM> of the green waveguide <NUM>. A second optical filter <NUM> may be positioned between the ICG <NUM> of the green waveguide <NUM> and the ICG <NUM> of the red waveguide <NUM>.

<FIG> is a schematic diagram illustrating time sequential encoding of colors for multiple depth planes, according to some embodiments. As illustrated in <FIG>, depth planes (three in this illustration) are encoded into the least significant bit (LSB) per pixel via a shader. The projector assembly discussed herein provides for precise placement of pixels for each color in a desired depth plane. Three colors are sequentially encoded for each depth plane-(R0, G0, B0 for plane <NUM>) <NUM>, (R1, G1, B1 for plane <NUM>) <NUM>, and (R2, G2, B2 for plane <NUM>) <NUM>. Illumination of each color for <NUM> provides an illumination frame rate <NUM> of <NUM> and a frame rate for all three colors and three depth planes <NUM> of <NUM> (based on <NUM> to refresh all colors and planes). In some embodiments, a single color for a single depth plane per frame may be used by only using light sources associated with that particular color for that particular depth plane.

In some embodiments, multiple depth planes can be implemented through the use of a variable focus lens that receives the sequentially coded colors. In these embodiments, there may be three eyepiece layers and the incoupling gratings may be spaced further apart such that incoupling gratings are not positioned directly across from one another about the optical axis. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

<FIG> is a cross-sectional view of an eyepiece with aligned diffractive optical elements and an integrated retroreflector according to an embodiment of the present invention. Eyepiece <NUM>, which can be used to project an image to a viewer's eye, includes a first planar waveguide <NUM>, also referred to as a first waveguide, positioned in a first lateral plane (i.e., in a first x-y plane and positioned at a first longitudinal (z-axis) position). The first planar waveguide <NUM> includes a first diffractive optical element (DOE) <NUM> disposed at a first lateral position (i.e., a first x-y coordinate position). The first DOE <NUM> may include an incoupling grating (ICG) formed either on the first surface <NUM>, which can also be referred to as an incident light surface, of the first planar waveguide <NUM> or a second surface <NUM>, which can be referred to as an opposing surface, of the first waveguide <NUM> opposite to the first surface. In some embodiments, the first DOE <NUM> may include an ICG formed either on both the first surface <NUM> of the first planar waveguide <NUM> and the second surface <NUM> of the first waveguide <NUM> opposite to the first surface. The first DOE <NUM> may be a transmission grating configured to diffract image light in the first wavelength range, e.g., blue image light, into the first planar waveguide <NUM> to be guided toward the second lateral region of the first waveguide <NUM>. The second lateral region may be a region between the ICG and an OPE. A portion of the image light that is not coupled into the first waveguide <NUM> may be transmitted through the first waveguide <NUM>.

The eyepiece <NUM> also includes a second planar waveguide <NUM>, also referred to as a second waveguide, positioned in a second lateral plane (i.e., at a second longitudinal position) adjacent to the first lateral plane. The second planar waveguide <NUM> includes a second DOE <NUM> disposed at the first lateral position below the first DOE <NUM>. The second DOE <NUM> may include an incoupling grating (ICG) formed either on the first surface <NUM> of the second waveguide <NUM>, a second surface <NUM> of the second waveguide <NUM> opposite to the first surface or both the first surface <NUM> and the second surface <NUM> opposite to the first surface. The second DOE <NUM> may be a transmission grating configured to diffract image light in the second wavelength range, e.g., green image light, into the second waveguide <NUM> to be guided toward the second lateral region of the second waveguide <NUM>. The second lateral region may be a region between the ICG and an OPE. A portion of the image light that is not coupled into the second waveguide <NUM> may be transmitted through the second planar waveguide <NUM>.

The eyepiece <NUM> also includes a third planar waveguide <NUM>, also referred to as a third waveguide, positioned in a third lateral plane (i.e., at a third longitudinal position) adjacent to the second lateral plane. The third planar waveguide <NUM> includes a third DOE <NUM> disposed at the first lateral position below the first DOE <NUM> and the second DOE <NUM> and aligned along a longitudinal direction (i.e., aligned with the z-axis). The third DOE <NUM> may include an incoupling grating (ICG) formed either on the first surface <NUM> of the third planar waveguide <NUM> or a second surface <NUM> of the third planar waveguide <NUM> opposite to the first surface. The third DOE <NUM> may be a transmission grating configured to diffract image light in the third wavelength range, e.g., red image light, into the third planar waveguide <NUM> to be guided toward the second lateral region of the third planar waveguide <NUM>. The second lateral region may be a region between the ICG and an OPE. A portion of the image light that is not coupled into the third planar waveguide <NUM> may be transmitted through the third planar waveguide <NUM>.

Although all three DOEs (e.g., the first DOE <NUM>, the second DOE <NUM>, and the third DOE <NUM>) are aligned in the embodiment illustrated in <FIG>, this is not required by the present disclosure and the DOEs can be spatially separated at different lateral positions. As an example, the first DOE <NUM> (e.g., to diffract green light) can be spatially separated from the second DOE <NUM> and the third DOE <NUM>, which can be aligned. In this example, since green light is in the middle of the visible spectrum, it is spatially separated from the blue and red light, which are not strongly diffracted in the DOEs for the other color, enabling the blue and red DOEs (e.g., the second DOE <NUM> and the third DOE <NUM>) to be spatially aligned. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

Referring to <FIG>, a first input beam <NUM> (e.g., a blue input beam) is coupled into the first planar waveguide <NUM> via incoupling DOE <NUM>. In a similar manner, a second input beam <NUM> (e.g., a green input beam) is coupled into the second planar waveguide <NUM> via incoupling DOE <NUM> and a third input beam <NUM> (e.g., a red input beam) is coupled into the third planar waveguide <NUM> via incoupling DOE <NUM>. After passing through the waveguides, light not diffracted into the waveguides is retroreflected by retroreflector <NUM> as described more fully below. As illustrated in <FIG>, the retroreflector <NUM> is positioned at the first lateral position and is aligned with the incoupling DOEs. The retroreflector may be configured to reflect image light transmitted through the three planar waveguides back toward the projector (not shown). As described below, the retroreflected light can be coupled into the waveguides as it propagates toward the projector, thereby increasing the brightness and contrast of the light field that is output to the viewer.

According to some other embodiments, the order of the planar waveguides <NUM>, <NUM>, and <NUM> may be different from that illustrated in <FIG>. Further, the eyepiece <NUM> may include fewer than three waveguides (e.g., two waveguides), or more than three waveguides (e.g., nine waveguides, three for each color), according to some embodiments. In some embodiments, the eyepiece <NUM> may include waveguides for colors other than red, green, and blue. For example, it may include waveguides for magenta and cyan, in place of or in addition to red, green, and blue.

<FIG> is a cross-sectional view of elements of a projector, an eyepiece, and a retroreflector according to an embodiment of the present invention. As illustrated in <FIG>, light used to generate virtual images produced by planar waveguide <NUM> of an eyepiece is reflected from display element <NUM> (e.g., a liquid crystal on silicon (LCOS) display). For purposes of illustration, a pixel <NUM> of the display element is illustrated as reflecting incident light. The reflected light passes through projection optics <NUM>, illustrated as a single lens in <FIG>. It will be appreciated by one of skill in the art that the illustration of the projection optics as a single lens is merely for purposes of clarity and, in practice, multiple optical elements, including lenses, stops, and the like, will be utilized in projection optics <NUM>. If an emissive display element is utilized, light emitted by the display element passes through projection optics <NUM> in a similar manner. Embodiments are not limited to an LCOS display and other display technologies, including both reflective and emissive displays, are included within the scope of the present invention.

After passing through projection optics <NUM>, light illustrated by ray <NUM> impinges on the incoupling DOE <NUM>. A first portion of the incident light is diffracted by the incoupling DOE <NUM> and propagates, based on TIR, toward the OPE and EPE of the planar waveguide <NUM> of the eyepiece as illustrated by ray <NUM>. It will be appreciated that light will generally be diffracted into multiple orders and only the diffracted order directed toward the OPE is illustrated in <FIG> for purposes of clarity. As a result of diffraction into multiple orders, a light absorber (not shown) may be integrated into the planar waveguide to absorb light diffracted into orders directed away from the OPE. A second portion of the incident light passes through the incoupling DOE <NUM> as illustrated by ray <NUM> and is retroreflected by retroreflector <NUM>.

The retroreflector <NUM> reflects the incident light, with the reflected light being illustrated by ray <NUM>, along a direction that is parallel, but opposite in direction from the incident light's original direction. Although ray <NUM> in <FIG> is laterally offset from ray <NUM>, it will be appreciated that in actual implementation, the lateral offset of the retroreflected light will be small in comparison to the beam diameter and the lateral offset if utilized in <FIG> in order to provide clarity in the illustration. In some embodiments, the lateral dimensions or size of the retroreflector will be approximately equal to the lateral dimensions of the incoupling diffractive optical element although this is not required and the lateral dimensions of the retroreflector can be greater than or less than that of the incoupling diffractive optical element. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

A number of different retroreflector designs can be utilized according to embodiments of the present invention, including a corner cube prism film, corner cube arrays, with the corner cube dimension on the order of tens of microns to several hundred microns, metallized microprism structures, and the like. Using nanoimprinting, semiconductor processing techniques, including maskless lithography, additive manufacturing techniques, or the like, an integrated retroreflector having a lateral extent, for example, substantially matched to the lateral extent of the incoupling diffractive optical element can be fabricated on the distal surface of the planar waveguide layer. Thus, a retroreflector having lateral dimensions measured along the x and y axes of <NUM> to -<NUM> and a height, measured along the longitudinal z-direction of <NUM> - <NUM>, for example, in the range of tens of microns to several hundred microns, can be utilized in various embodiments of the present invention. In these embodiments, the dimensions of the features making up the retroreflector can be a small fraction of the lateral dimensions of the retroreflector, for example, corner cubes less than <NUM> in dimension arrayed to provide a retroreflector several mm in dimension. As will be evident to one of skill in the art, the structure of the retroreflector is not limited to corner cubes, but can include other retroreflector structures including spherical reflectors, pyramidal reflectors, and the like.

As illustrated by light cone <NUM> in <FIG>, the retroreflected light passes back through projection optics <NUM> and impinges on the display element at the pixel location from which light was initially reflected. In contrast with specular reflection from the eyepiece elements, light that is retroreflected propagates at the same angle as the incident light as described above, not at an angle equal and opposite to the angle of incidence. As a result, the light propagating at the same angle will be diffracted at the same angle in the planar waveguide, thereby overlapping with the initially diffracted light represented by ray <NUM> and representing the same pixel to the user. Moreover, in contrast with specular reflection, which results in specularly reflected light impinging on a mirror-image pixel of the display element, the light retroreflected from retroreflector <NUM> impinges on the original pixel of the display element. As a result, artifacts (i.e., ghost images) are reduced or prevented by embodiments of the present invention.

Thus, the prevention or reduction of artifacts by embodiments of the present invention contrasts with systems in which a mirrored surface can be positioned adjacent the planar waveguide <NUM> and merely result in specular reflection. As illustrated in <FIG>, if ray <NUM> were incident on a mirrored surface (not shown), reflection by the mirrored surface would result in reflected ray <NUM>, which would reflect at an angle of reflection equal and opposite to the angle of incidence of ray <NUM>. It should be noted that reflected ray <NUM> is illustrated merely for comparison with an approach utilizing a mirrored surface and is not physically present in the system illustrated in <FIG>. Reflected ray <NUM> would then be imaged by projection optics <NUM> and impinge on display element <NUM> at a mirror image position <NUM> with respect to pixel <NUM>. Thus, light associated with pixel <NUM> will appear as an artifact at mirror image position <NUM>. If the pixel at the mirror image position <NUM> is reflective, reflected ray <NUM> can be reflected back toward projection optics <NUM> for eventual projection to the user. Thus, as described herein, the use of a retroreflector prevents the generation of the artifact since the retroreflected light, rather than being imaged at mirror image position <NUM>, is imaged at the original position associated with pixel <NUM>. Although a single pixel is discussed in relation to artifact generation in relation to <FIG>, it will be appreciated that the single pixel example is provided merely by way of example and for clarity and the discussion related to a single pixel is applicable to other pixels in the display as appropriate.

<FIG> is a cross-sectional view of a planar waveguide of an eyepiece and integrated retroreflector according to an embodiment of the present invention. Referring to <FIG>, image light, represented by ray <NUM> is incident on proximal surface <NUM> (i.e., adjacent to the projector) of planar waveguide layer <NUM>. The planar waveguide layer <NUM> includes an incoupling DOE <NUM>, an integrated retroreflector <NUM>, and orthogonal pupil expander (OPE) and exit pupil expander (EPE), which are illustrated by diffractive elements <NUM> and <NUM>, respectively.

A first portion of the image light represented by ray <NUM> is diffracted by the incoupling diffractive optical element <NUM> as illustrated by ray <NUM>. The remaining portion of the image light represented by ray <NUM> passes through the planar waveguide layer <NUM> and is retroreflected by retroreflector <NUM>. In the embodiment illustrated in <FIG>, the retroreflector <NUM> is imprinted (e.g., using nanoimprinting) on the distal surface <NUM> of the planar waveguide layer <NUM>. However, other optical arrangements are included within the scope of the present invention, including a retroreflector positioned a predetermined distance from the distal surface <NUM> of the planar waveguide layer <NUM>. Thus, the retroreflector can be integrated into the surface of the planar waveguide layer, resulting in a non-planar region of the planar waveguide layer, be a separate structure joined (e.g., bonded) to the planar surface of the planar waveguide layer, or be a separate structure optically coupled, but spatially separated from the planar waveguide layer. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

A first portion of the retroreflected light <NUM> is diffracted by incoupling DOE <NUM> as illustrated by ray <NUM>. It will be appreciated that light will generally be diffracted along multiple orders and only the diffracted order directed toward the OPE is illustrated in <FIG> for purposes of clarity. This diffracted light is directed toward the OPE and EPE for eventual delivery to the viewer along with the initially diffracted light represented by ray <NUM>. Since both rays <NUM> and <NUM> are associated with the same pixel of the display element, the brightness associated with this pixel will be increased. Thus, the use of the integrated retroreflector <NUM> effectively provides a double pass for light diffracted by the incoupling diffractive optical element, increasing the incoupling efficiency and effectively increasing the diffraction efficiency of the incoupling diffractive optical element.

The remainder of the retroreflected light <NUM> passes back through the projection optics and impinges on the display element at the pixel location from which light was initially reflected. The light reaching the display element can reflect off of the display element a second time (and additional subsequent times) as illustrated by ray <NUM>. The light reflected from the display element a second time can be referred to as recycled image light. This recycled image light, represented by ray <NUM>, can be used to produce diffracted rays <NUM> and <NUM>, which are associated with diffraction by incoupling diffractive optical element <NUM> on the first pass of the recycled image light through incoupling diffractive optical element <NUM> (i.e., associated with ray <NUM>) and diffraction by incoupling diffractive optical element <NUM> on the second pass of the recycled image light through incoupling diffractive optical element <NUM> after the second retroreflection from retroreflector <NUM> (i.e., associated with ray <NUM>). As illustrated in <FIG>, since rays <NUM> and <NUM> are reflected from the same pixel, they are parallel.

As will be evident to one of skill in the art, subsequent reflections from the display element will occur during the remainder of the frame time associated with each color and each depth plane as the light propagating backwards from the retroreflector illuminates the original pixel of the display element with the same cone angle as the light that was originally reflected from the original pixel. Thus, reflections representing the original pixel can be generated by reflection from the display element and increase eyepiece efficiency. In actual implementation, since the frame time is much greater than the round trip propagation time from the display element to the retroreflector, the recycling of the image light can result in diffraction of the image light into the planar waveguide until the intensity of the reflected light at the display element is negligible. As a result, the recycling of light produced by multiple reflections from the display element will result in increased image brightness for the user and increased system efficiency. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

As illustrated in <FIG>, when light diffracted by incoupling diffractive optical element <NUM> impinges on distal surface <NUM> to the left of integrated retroreflector <NUM>, the diffracted light is totally internally reflected and propagates towards diffractive elements <NUM> and <NUM>. If the diffracted light were to impinge on integrated retroreflector <NUM>, the diffracted light could be retroreflected back toward incoupling diffractive optical element <NUM>, at which point it could be diffracted back toward the projection optics, never resulting in light coupling into planar waveguide layer <NUM>. In order to prevent the undesired retroreflection of diffracted light, the thickness of the planar waveguide layer <NUM> can be increased to enable additional propagation in the lateral direction of the diffracted light before reaching distal surface <NUM>. Additionally or alternatively, the lateral dimensions of the ICG can be decreased. Thus, in some embodiments, lateral dimensions (e.g., area) of the retroreflector are selected such that light diffracted from the incoupling DOE is able to totally internally reflect in the waveguide after diffraction, not impinge on the retroreflector after diffraction and then be retroreflected back toward the incoupling DOE, thereby preventing the light from reaching the EPE/OPE portion of the planar waveguide. Moreover, if an air gap is introduced between the retroreflector and the planar waveguide layer, discussed in additional detail with respect to <FIG>, prevention of retroreflection of diffracted light can be achieved.

It should be noted that diffractive elements <NUM> and <NUM> can be implemented as a combined OPE/EPE, which can be referred to as a combined pupil expander (CPE). A CPE can be implemented in a two-sided configuration as illustrated in <FIG> or in a single-sided configuration. For example, the CPE can include an OPE component represented by diffractive element <NUM> and an EPE component represented by diffractive element <NUM>. Thus, an OPE component (e.g., including OPE gratings) can be located on one side of substrate <NUM> and an EPE component (e.g., including EPE gratings) can be located on the other side of substrate <NUM>. The OPE component and/or the EPE component can be implemented as 2D gratings or 1D gratings as appropriate to the particular application. Additional description related to the fabrication of CPEs is provided in commonly assigned <CIT>.

<FIG> is a cross-sectional view of a planar waveguide with multiple diffractive optical elements and an integrated retroreflector according to an embodiment of the present invention. Referring to <FIG>, an incoupling diffractive optical element has been integrated with both the proximal surface of the planar waveguide layer as well as the distal surface of the planar waveguide layer. Image light, represented by ray <NUM> is incident on proximal surface <NUM> (i.e., adjacent to the projector) of planar waveguide layer <NUM>. The planar waveguide layer <NUM> includes a first incoupling DOE <NUM>, a second incoupling diffractive optical element <NUM>, an integrated retroreflector <NUM>, and orthogonal pupil expander (OPE) and exit pupil expander (EPE), which are illustrated by diffractive elements <NUM> and <NUM>, respectively.

A first portion of the image light represented by ray <NUM> is diffracted by the first incoupling diffractive optical element <NUM> as illustrated by ray <NUM>. The remaining portion of the image light represented by ray <NUM> passes through the planar waveguide layer <NUM> and a second portion of the remaining portion is diffracted by the second incoupling diffractive optical element <NUM> as illustrated by ray <NUM>. Thus, the use of incoupling diffractive optical elements on both the proximal and distal surfaces of the planar waveguide layer increases the diffraction efficiency of image light toward the OPE.

Light that passes through the second incoupling diffractive optical element is retroreflected by retroreflector <NUM>. In some embodiments, the retroreflector <NUM> is imprinted (e.g., using nanoimprinting) on the distal surface <NUM> of the planar waveguide layer <NUM>. However, other optical arrangements are included within the scope of the present invention, including a retroreflector positioned a predetermined distance D from the distal surface <NUM> of the planar waveguide layer <NUM>. Thus, the retroreflector can be integrated into the surface of the planar waveguide layer, resulting in a non-planar region of the planar waveguide layer, be a separate structure joined (e.g., bonded) to the planar surface of the planar waveguide layer, or be a separate structure optically coupled but spatially separated from the planar waveguide layer. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

As illustrated in <FIG>, in some embodiments, the incoupling DOEs are positioned on both the front surface <NUM> as well as the back surface <NUM> of the planar waveguide <NUM>. In these embodiments, the retroreflector can be positioned a predetermined distance from the back surface of the planar waveguide and spatially separated from the back surface, as a result, by the predetermined distance. In these embodiments, the retroreflector may be fabricated as a separate structure from the planar waveguide and can be mechanically mounted to the planar waveguide or a support structure supporting both the planar waveguide and the retroreflector structure. The spatial separation can be filled with the ambient environment (e.g., an air gap), a spacer material, or the like. If a spacer material is utilized, the index of refraction of the planar waveguide, the incoupling DOE <NUM> on the back surface <NUM> of the planar waveguide <NUM>, the spacer material, and/or the retroreflector should be considered to avoid index matching between the spacer material and the incoupling DOE <NUM>, which can potentially reduce the efficiency of the incoupling DOE as the index of refraction difference between the incoupling DOE and the spacer material effectively washes out the DOE.

In some embodiments, the incoupling DOEs are fabricated using polymer materials. If the incoupling DOE <NUM> utilizes metal materials to provide the diffractive effect, the decrease in diffraction efficiency can be small in comparison to materials that are more closely index matched to the spacer material. In some embodiments, the retroreflector can be formed in contact with the incoupling DOE <NUM> using an additive manufacturing process. In these embodiments, the use of materials with different indices of refraction for the incoupling DOE (e.g., metal) and the spacer material (e.g., polymer) and/or the retroreflector (e.g., polymer) can result in low levels of index matching and maintenance of the desired DOE efficiency.

In a manner similar to that discussed in relation to <FIG>, a first portion of the retroreflected light is diffracted by incoupling DOE <NUM> as illustrated by ray <NUM> and a second portion, after passing through the planar waveguide layer, is diffracted by incoupling DOE <NUM> as illustrated by ray <NUM>. Since the retroreflected light <NUM> propagates along the same direction as (i.e., opposite to the direction of propagation of) the original image light, the diffraction of the retroreflected light will overlap with rays <NUM> and <NUM> associated with diffraction during the first pass through the planar waveguide layer. As will be evident to one of skill in the art, the diffraction order will differ (i.e., either positive or negative orders) depending on whether the DOE is operating in reflection or transmission. As examples, ray <NUM> can be a positive diffracted order produced by incoupling DOE <NUM> operating in transmission and ray <NUM> can be a positive diffracted order produced by incoupling DOE <NUM> operating in reflection. Ray <NUM> can be a negative diffracted order produced by incoupling DOE <NUM> operating in transmission and ray <NUM> can be a negative diffracted order produced by incoupling DOE <NUM> operating in reflection.

In comparison with the embodiment illustrated in <FIG>, the use of two incoupling diffractive optical elements enables light to be diffracted four times during a round trip pass through the incoupling diffractive optical element as illustrated by rays <NUM>, <NUM>, <NUM>, and <NUM>. Since the diffracted light directed toward the OPE and EPE for eventual delivery to the viewer is associated with the same pixel of the display element, the brightness associated with this pixel will be increased as illustrated by rays <NUM>, <NUM>, <NUM>, and <NUM>. In the embodiment illustrated in <FIG>, the use of two incoupling diffractive optical elements and the integrated retroreflector <NUM> effectively provides a double pass for light diffracted by each of the incoupling diffractive optical elements, increasing the incoupling efficiency and effectively increasing the diffraction efficiency of the incoupling diffractive optical element.

The remainder of the retroreflected light passes back through the projection optics and impinges on the display element at the pixel location from which light was initially reflected. The light reaching the display element can reflect off of the display element a second time (and additional subsequent times) and impinge on the waveguide layer <NUM>. In a manner similar to that discussed in relation to <FIG>, the light reflected from the display element a second time can be referred to as recycled image light. This recycled image light (not shown), can be used to produce diffracted rays, which are associated with diffraction by first incoupling diffractive optical element <NUM> and second incoupling diffractive optical element <NUM> on the first pass of the recycled image light through the planar waveguide layer and diffraction by the first incoupling diffractive optical element <NUM> and second incoupling diffractive optical element <NUM> on the second pass of the recycled image light through the planar waveguide layer, respectively.

<FIG> is a simplified flowchart illustrating a method of reducing artifacts in an eyepiece according to an embodiment of the present invention. The method comprises directing a beam of light in a first direction at step <NUM>. The method further comprises receiving the beam of light at an incident light surface of a waveguide layer having an opposing surface opposite the incident light surface at step <NUM>.

The method further comprises propagating, at a first diffractive optical element, a first portion of the beam of light into the waveguide layer in a second direction at step <NUM>. The method further comprises transmitting, by the first diffractive optical element, a second portion of the beam of light toward the opposing surface at step <NUM>. The method further comprises retroreflecting the second portion of the beam of light along a reflected direction opposite to the first direction at step <NUM>.

In some embodiments, the method further comprises propagating, at the first diffractive optical element, a first fraction of the second portion of the beam of light into the waveguide layer in the second direction. In some embodiments, the method further comprises transmitting, at the first diffractive optical element, a second fraction of the second portion of the beam of light along the reflected direction. In some embodiments, the method further comprises reflecting at least a portion of the second fraction in the first direction or toward the waveguide layer. In some embodiments, the method further comprises receiving the reflected portion at the incident light surface of the waveguide layer. In some embodiments, the method further comprises propagating, at the first diffractive optical element, a portion of the reflected portion into the waveguide layer in the second direction.

In some embodiments, the first diffractive optical element is disposed on the incident light surface. In some embodiments, retroreflecting the second portion of the beam of light includes retroreflecting the second portion along a reflected direction opposite to the first direction. In some embodiments, directing the beam of light in the first direction comprises reflecting a time sequential color beam from a display element.

<FIG> is a simplified cross-sectional view of a planar waveguide utilized in a multi-pupil system according to an embodiment of the present invention. Referring to <FIG>, a display element <NUM> (e.g., an LCOS display panel) is illuminated using a multi-pupil pixel layout. <FIG> is a simplified plan view of a multi-pupil layout for the planar waveguide illustrated in <FIG>. As illustrated in <FIG>, each pixel includes five arrayed light sources that are aligned with five corresponding incoupling DOEs R1, R2, G1, B1, and B2. Although a particular array pattern is illustrated in <FIG>, the present invention is not limited to this particular array pattern and other array patterns are included within the scope of the present invention. Additionally, although two incoupling DOEs corresponding to light sources associated with two primary colors (red and blue) are utilized in conjunction with a single incoupling DOE corresponding to a light source associated with the third primary color (green), this is also not required and different color combinations can be utilized as appropriate to the particular application. In <FIG>, the illustrated cross-section corresponds to bisector <NUM> illustrated in <FIG>. Thus, incoupling DOEs B1 and B2 will be disposed at locations in front of the plane of the figure and behind the plane of the figure, respectively, in the illustrated embodiment.

Referring to <FIG>, light from a first light source (e.g., a red source) corresponding to incoupling DOE R1 is reflected from the display element <NUM> to form light cone <NUM>. An exemplary ray included in light cone <NUM> is illustrated by ray <NUM>, which is imaged using projection optics <NUM> to impinge on the eyepiece <NUM> including three planar waveguides <NUM>, <NUM>, and <NUM>, each associated with a primary color. In an embodiment, planar waveguide <NUM> is associated with green wavelengths, planar waveguide <NUM> is associated with blue wavelengths, and planar waveguide <NUM> is associated with red wavelengths. Ray <NUM> passes through planar waveguides <NUM> and <NUM> and impinges on incoupling DOE R1. A fraction of the incident light is diffracted to propagate in the plane of the planar waveguide as illustrated by diffracted ray <NUM>. A fraction of the incident light also passes through the incoupling DOE R1 to impinge on retroreflector <NUM>. Retroreflected light represented by retroreflected ray <NUM> propagates in a direction opposite to that of ray <NUM> as it propagates back to display element <NUM>. The retroreflected light, including retroreflected ray <NUM>, which is shifted in the lateral direction by distance D after retroreflection, is characterized by light cone <NUM> during propagation back to display element <NUM>.

When the retroreflected light in light cone <NUM>, which is represented by retroreflected ray <NUM>, impinges on the display element, specular reflection of the retroreflected light will occur, resulting in reflected light cone <NUM>, which will be imaged by projection optics <NUM> to impinge on the eyepiece as represented by reflected ray <NUM>. Because light cone <NUM> has a center ray that is not normal to display element <NUM>, which results from the light sources being off-axis, light cone <NUM> will also have a center ray that is equal and opposite to the center ray of light cone <NUM>. The light reflected from the display element <NUM> in light cone <NUM> will pass through planar waveguides <NUM> and <NUM> and impinge on incoupling DOE R2. Accordingly, a fraction of the incident light will be diffracted to propagate in the plane of the planar waveguide (diffracted rays are not shown for purposes of clarity). A fraction of the incident light will also pass through the incoupling DOE R2 to impinge on retroreflector <NUM>, resulting in retroreflected light that will propagate back to the display element. After specular reflection from the display element, retracing of the optical path represented by ray <NUM>, retroreflected ray <NUM>, and ray <NUM> will occur during the remainder of the frame time associated with each color and each depth plane. Thus, multiple passes through incoupling DOEs R1 and R2 will result, increasing incoupling efficiency of planar waveguide <NUM>.

Referring once again to <FIG>, light from the second light source (e.g., a green source) corresponding to incoupling DOE G1 is reflected from the display element <NUM> to form light cone <NUM>. An exemplary ray included in light cone <NUM> is illustrated by ray <NUM>, which is imaged using projection optics <NUM> to impinge on the eyepiece <NUM>. Ray <NUM> impinges on incoupling DOE G1. A fraction of the incident light is diffracted to propagate in the plane of the planar waveguide (diffracted rays are not shown for purposes of clarity). A fraction of the incident light also passes through the incoupling DOE G1 to impinge on retroreflector <NUM>. Retroreflected light represented by retroreflected ray (not shown for purposes of clarity) propagates in a direction opposite to that of ray <NUM> as it propagates back to display element <NUM>.

When the retroreflected light in light cone <NUM>, which can be represented by a retroreflected ray, impinges on the display element <NUM>, specular reflection of the retroreflected light will occur. Because light cone <NUM> has a center ray that is normal to display element <NUM>, the light cone associated with light propagating from the display element to the eyepiece will overlap with the light cone associated with retroreflected light propagating from retroreflector <NUM> to the display element. After specular reflection from the display element, retracing of the optical path represented by ray <NUM> and the retroreflected ray will occur during the remainder of the frame time associated with each color and each depth plane. Thus, multiple passes through incoupling DOE G1 will result in increased incoupling efficiency of planar waveguide <NUM>.

In another embodiment, rather than utilizing a second incoupling DOE R2 for planar waveguide <NUM>, the second incoupling DOE R2 can be removed. Thus, incoupling DOE R2, as well as incoupling DOE B2 are optional in some embodiments. In this case, after specular reflection of retroreflected light, the light will pass through all three planar waveguides, be retroreflected a second time, and specularly reflect back in light cone <NUM>, and subsequently impinge on incoupling DOE R1.

<FIG> is a simplified schematic diagram illustrating an element of a retroreflector according to an embodiment of the present invention. As illustrated herein, retroreflected light is laterally shifted after retroreflection. Thus, although the directions of propagation before and after retroreflector are parallel, a lateral shift is present. Embodiments of the present invention take this lateral shift into account in designing the dimensions of the retroreflector as well as the incoupling DOEs. Referring to <FIG>, an element of a retroreflector is illustrated as corner cube <NUM>. Incident light represented by ray <NUM> impinges on corner cube <NUM> and is retroreflected as retroreflected ray <NUM>. After retroreflection, lateral shift D is present between the incoming and retroreflected rays. Because of this lateral shift, it is possible that retroreflected light may couple into an adjacent pupil (i.e., incoupling DOE), resulting in crosstalk. Accordingly, this crosstalk can be prevented by utilizing a small pitch size P between adjacent elements of the retroreflector, illustrated by adjacent corner cubes <NUM> and <NUM>. In some embodiments, the pitch size P is less than <NUM>, whereas in other embodiments, the pitch size ranges from about <NUM> to about <NUM>.

Because of the small pitch sizes utilized by embodiments of the present invention, the lateral shift resulting from retroreflector will typically not result in crosstalk since the separation between adjacent pupils is typically on the order of several hundred microns. Referring to <FIG>, the separation S between the incoupling DOEs, including, for example, incoupling DOE R1 and incoupling DOE G1 and/or incoupling DOE G1 and incoupling DOE B2 as illustrated, is on the order of <NUM> in some embodiments. Thus, for pitch sizes less than <NUM>, the lateral shift will not result in crosstalk between incoupling DOE R1 and incoupling DOE G1. Therefore, embodiments of the present invention utilize fabrication processes for the retroreflector, particularly the retroreflector elements, that are small enough to prevent crosstalk between adjacent incoupling DOEs that would otherwise result from the lateral shift that can occur during retroreflection. As described, some embodiments utilize a spacing or separation between adjacent incoupling DOEs in the lateral plane of greater than <NUM>, which coupled with retroreflector elements pitch sizes (measured in the lateral plane) of less than <NUM>, for example, in the range of tens of microns, prevents crosstalk. As will be evident to one of skill in the art, pitch sizes for the retroreflector elements that are very small, for example, on the order of the wavelength of light, may result in undesirable diffraction effects. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

Claim 1:
A display system comprising:
projection optics (<NUM>, <NUM>, <NUM>, <NUM>) configured to project a beam of light in a first direction;
an eyepiece unit (<NUM>, <NUM>, <NUM>) including:
a first waveguide layer (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) disposed in a first lateral plane and including an incident light surface and an opposing surface opposite the incident light surface;
an incoupling diffractive optical element (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, R1, R2) disposed on the incident light surface, wherein the incoupling diffractive optical element is configured to:
incouple a first portion of the beam of light and to redirect the first portion of the beam of light in a second direction, wherein the first portion of the beam of light propagates in the first waveguide layer by total internal reflection; and
transmit a second portion of the beam of light along the first direction; and
a retroreflector (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) disposed adjacent the opposing surface, wherein the retroreflector is configured to retroreflect the second portion of the beam of light along a reflected direction opposite to the first direction,
characterized in that the incoupling diffractive optical element is configured to diffract at least a portion of the retroreflected beam of light in the second direction.