Abstract:
A near-eye display for an augmented reality display system couples a self-emitting microdisplay to a transmissive waveguide through an input coupler. Light from the self-emitting microdisplay propagates along the waveguide and through an output coupler for generating a virtual image that is visible together with a real-world scene through the waveguide within an eyebox located in front of the output coupler. A light sensor detects light levels within different portions of the real world scene visible within the eyebox. Drive electronics control the light output from individual pixels of the self-emitting microdisplay to preserve desired local contrast values between the virtual image and the real-world scene.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to compact display systems, particularly such systems designed to produce magnified virtual images of generated patterns that are overlaid upon a real-world view, produced by microdisplay engines and to such systems that can be arranged for near-eye viewing within head-mounted frames or that can be arranged for viewing within prescribed exit pupils. 
       BACKGROUND 
       [0002]    Augmented reality systems, which add computer enhancements to an individual&#39;s otherwise unobstructed field of view (FOV), are featured in applications ranging from enterprise to defense to entertainment. Various attempts have be made to produce portable (wearable) devices, such as glasses or safety goggles, capable of presenting high resolution, dynamic digital information within the user&#39;s unobstructed field of view of the world. High dynamic range photonic level environments to which the human eye adopts using its pupils, lens, and eyelids present additional challenges. Whether for head-up display (HUD) applications or full mixed and augmented reality training simulations, small, inexpensive, ruggedized solutions are needed. 
         [0003]    Various approaches have been tried to present digital image information to users. The information can be provided in an immersive environment to the user or overlaid atop the user&#39;s otherwise unimpeded view of the world. A conventional approach uses bulky optics, glass lenses, and beam splitters that approach the size of a football helmet, especially if a large field of view is desired. Additionally, these bulk optics approaches have difficulty providing the user with high dynamic range virtual images at the same time as unimpeded views of the world. 
         [0004]    To circumvent the drawbacks of bulky optics, compact near-eye displays can use plate-shaped (usually planar) waveguides for transmitting angular image information to users&#39; eyes as virtual images. Image information is generally input near one end of the waveguides and is output near another end of the waveguides. The image information propagates along the inside of the waveguides as a plurality of angularly related beams that are internally reflected along the waveguide. Diffractive optics (e.g. gratings) and refractive optics (e.g. prisms) are often used to couple image information into the waveguides. Rays having incident angles that are totally internally reflected within the waveguides are then output by matching diffractive or refractive output couplers forming an exit pupil that can be aligned with the users&#39; eyes. Often the waveguide and associated diffractive optics at the output end of the waveguides are at least partially transparent so that the user can also view the ambient environment through the waveguides. Such transparency becomes useful especially when (a) such image information is not being conveyed by the waveguides, (b) the image information does not fill the entire field of view, or (c) additional virtually reality content is overlaid over the ambient environment. 
         [0005]    Due to their planar thin geometries, diffraction gratings provide a compact means to couple light into and out of waveguides. With proper design, the virtual image output maintains the same angles as the virtual image input, resulting in planar waveguides that translate image content along the length of the waveguide. In addition, intermediate gratings can be used to expand the exit pupil to sizes that are considerably larger than the size of the entrance pupil. Gratings placed between the input and output gratings can be designed to expand the exit pupil in one or two dimensions using so-called 1D (one-dimensional) or 2D (two-dimensional) expanders. In the case of 1D expanders, light that is injected into the waveguide maintains the angular relationship of coupled light along the direction of propagation, except the image content is output over a wider exit pupil. Such beam expanders can also be used to reduce the dimension of the injection optics along the propagation direction, thereby reducing bulk and cost. However, 1D (one dimensional) expanders do not expand the pupil in the orthogonal direction. Larger injection optics can be used to reduce this problem but this approach requires larger optical components, which can significantly increase system cost and bulk. 2D (two dimensional) pupil expanders have significant advantages since they allow the use of injection optics which can be smaller in two dimensions, and can thereby be much lighter and fit into form factors ideal for near-eye displays. 
         [0006]    To couple image content into waveguides, injection optics often use spatial light modulator microdisplays that modulate light incident of each display pixel along with projection optics to create virtual images. However, transmissive spatial light modulators used in this manner can be optically inefficient thereby increasing power requirements of the light source. Consequently, illumination sources such as light emitting diodes (LED&#39;s) must be driven with higher currents, increasing power consumption and heating. Reflective spatial light modulators such as liquid crystal on silicon (LCoS) or DLP (Digital Light Processing) can be optically more efficient and are used in a number of applications such as digital projectors. However, since transmissive or reflective systems modulate incident light rather than emit light, they require additional optics that project, condense, and split output beams from the LED sources. Although much effort in the industry has been devoted to miniaturizing “engines” that integrate displays, sources, projection optics, beam splitters, polarizers, heat sinks, etc., state-of-the-art dimensions are still undesirably large for near-eye displays (i.e., on the order of 25 mm×25 mm×8 mm or larger). Additional drawbacks associated with current engine technology negatively affect cost, size, weight, and power. Since these displays only modulate incident light, the light source must remain turned on regardless of image content. For example, a bright full-screen virtual image and a simple arrow that takes up only 5% of the display pixels will consume approximately the same power. 
         [0007]    Self-emitting displays can circumvent many of the aforementioned problems. Inorganic and organic LED arrays (OLED) produce light on a pixel-by-pixel basis to produce the desired image. Self-emitting pixel-addressable displays such as OLED consume power depending on how many pixels are addressed and specific brightness of each pixel addressed. This pixel-power addressable approach can substantially decrease power consumption. Contrast ratio can also be improved since when pixels are turned off, no light is emitted and background light is largely limited to other system reflections or flare, which in practice can be nearly eliminated. Another significant advantage of self-emitting pixel-addressable displays is that such displays do not require projection optics to illuminate the spatial light modulator display. Therefore, no internal illumination beam projection optics, beam splitters, etc. are required. 
         [0008]    In spite of their many advantages, self-emitting displays, such as OLED&#39;s, are generally considered impractical for use with diffractive waveguide-based near-eye displays. Self-emitting devices are typically Lambertian emitters, emitting light over large extended angles that are inefficiently coupled into the projection optics. Although fast optics can improve optical efficiency to some extent, fast optics can negatively affect the angular distribution of the light coupled into the waveguide, or result in poor coupling efficiency. Additionally, the quantum efficiency of OLED devices is low, resulting in relatively low brightness. For these reasons, displays of choice for diffractive waveguides that include 2D expanders continue to be spatial modulator displays illuminated by separate light sources. For these reasons, there are currently no viable microdisplay systems that incorporate diffractive waveguides and self-emitting displays having high optical output and low power consumption with compact geometries and form factors. 
       SUMMARY 
       [0009]    The invention in one or more of its preferred embodiments features a highly compact, power efficient, near-eye display system incorporating high-power “pixel power” adjustable self-emitting microdisplays that can be physically coupled on waveguides and can include diffractive input and output couplers and 2D pupil expanders, along with sensors and electro-optical windows, which together provide for a low-power display with virtual to real-world contrast management. Unlike waveguide-based near-eye displays thus far found in the prior art, preferred versions of the invention do not require image projection optics that use separate light sources and spatial light modulators. Digital imagery coupled by an input grating into the waveguide is preferably produced by a self-emitting display that is pixel-power addressable- that is, the display requires power only when pixels are activated. As a result, power consumption of the display is based on image content unlike current waveguide near-eye displays that require constant illumination of transmissive or reflective spatial light modulators. 
         [0010]    Another significant advantage of the pixel-power addressable characteristics in the self-emitting microdisplays envisioned for the preferred embodiments of the invention is a significant improvement of contrast ratios. Other approaches that project light emanating from a uniformly illuminated spatial light modulator require high on-to-off ratios from the device. Widely used LCoS microdisplays suffer from low contrast ratios, often exhibiting ratios significantly lower than 500:1. Higher contrast is possible but at the expense of reduced illumination and the addition of complex compensators, including polarizers, polarization beam splitters, and other elements. In the case of the envisioned self-emitting displays, when power is not applied to a pixel, the pixel is off—resulting in no light emission and very high contrast. 
         [0011]    High output self-emitting displays that produce narrower angular distribution output than Lambertian sources can be used to increase the percentage of light that can be coupled into the waveguide, as well as the image brightness at the exit pupil. Improved optical efficiency also results in lower power consumption since the image source can be run at lower level. 
         [0012]    Another significant advantage of pixel-power addressable display sources is that the collective average pixel power can be adjusted to maximize the virtual image contrast against the real-world view. The average and localized power output of the pixel-power addressable display sources can be adjusted as a function of the real-world scene to adjust the brightness of the virtual scene as a function of brightness levels within the real-world scene. 
         [0013]    2D (two dimensional) expander gratings allow the use of small displays and associated projection optics while still achieving a sufficiently large exit pupil for the user. The absence of a pupil expanding embodiment or even 1D pupil expanders requires that large virtual image inputs beams be projected onto an input coupler (e.g., grating), thereby increasing system bulk. Additionally, large area input gratings used without pupil expanders or even 1D expanders occupy additional areas on the waveguide, increasing waveguide dimensions, materials, and manufacturing cost. 
         [0014]    Embodiments of the invention enable a degree of miniaturization not possible with current technologies. In the prior art, the pixelated image is generally projected using a separate, projection display “engine” that must be mechanically positioned and aligned relative to the waveguide&#39;s input grating. In contrast, preferred embodiments of this invention feature an integral virtual image generating engine that is monolithically attached to the planar waveguide. As a result, system compactness is significantly increased, there is less need to fold the various optical subsystems, and the mechanical robustness of the system is greatly enhanced. All these attributes enable the design of near-eye robust eyewear that resembles conventional glasses, safety glasses for industrial applications, and other desirable forms of wearable virtual reality systems. The significant size reduction of the optical system also frees up additional space for associated electronics subsystems, wireless capability, gesture monitors, light sensors, digital image capture sensors and electronics, and more. 
         [0015]    Further, with the addition of sensors to measure the real-world scene characteristics like brightness in one-dimensional or two-dimensional space, further embodiments of this invention enable an active contrast control allowing the pixel-power-addressable display to adjust both local and overall display intensities and hence allows for optimum contrast to power ratios and contrast matching based on the scene illuminations, for instance, a cloudy day versus a bright sunny day. In each case, the average pixel power (instead of the entire display power) can be adjusted for optimum performance as a function of the scene differences. 
         [0016]    With the addition of electrochomic windows in front of the waveguides, scene brightness compared to the virtual image brightness can be further controlled, allowing for lower power consumption and better virtual to scene matching. Local concentrations of light within the transmitted real-world image can be adjusted by the electrochromic window to preserve desired contrast with the generated virtual image throughout the field of view 
         [0017]    In addition to controlling the overall or local brightness of the real image through a modulator or as an alternative thereof, the virtual image of the pixel-power-addressable display can be controlled on a pixel-by-pixel basis to achieve a desired contrast overlaid upon the real-world scene. That is, the desired contrast can be maintained throughout the field of view despite variations in the illumination levels within the real-world scene. Sensors can monitor the incoming light from the real-world scene throughout the field of view and the output power of the pixels generating the overlaid virtual image can be adjusted in accordance with the sensed incoming light levels of their immediate backgrounds or surroundings. Thus, pixels overlaying areas within the visible real world scene that are relatively dim can be driven at lower power levels than pixels overlaying areas within the visible real world scene that are relatively bright to maintain desired levels of contrast for the overlying virtual images. 
         [0018]    Embodiments of this invention contemplate not only small, low-power display engine optics for wearable displays but also closed-loop active scene/virtual display modulation engines to best control the power consumption and virtual-to-scene image matching. 
         [0019]    Near-eye displays envisioned in various embodiments of this invention offer simplicity, compactness, lower weight, lower power consumption, reduced localized heating, and greater robustness, greater contrast ratios, and potential lower cost, as well as a greater range of applications over what has been possible in the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0020]      FIG. 1  is a diagrammatic back view (i.e., facing the user&#39;s eye) of a waveguide including input and output coupling gratings and beam expander grating. 
           [0021]      FIG. 2  is a diagrammatic back view of the waveguide including the input and output coupling gratings and the beam expander grating, as well as a self-emitting microdisplay system positioned over the input coupler grating, and a trajectory of a coupled ray propagating through the beam expander grating and output coupler grating. 
           [0022]      FIG. 3  is a cross-sectional view of the input coupler including the waveguide, a grating structure, mirror-based folded optics, projection optics, and a self-emitting microdisplay. 
           [0023]      FIG. 4  depicts and off-axis example of  FIG. 3  that uses a doublet for the imaging optics. 
           [0024]      FIG. 5  is a cross-sectional view of the input coupler including the waveguide, a grating structure, prism-based folded optics, projection optics, and self-emitting microdisplay. 
           [0025]      FIG. 6 a    shows a front view of a compact and power efficient 2D or 3D near-eye display system, and  FIG. 6 b    shows a perspective view where the self-emitting display with drive electronics that are mounted compactly in each temple of the glasses. 
           [0026]      FIG. 7  shows a top view of view of a compact and power efficient 2D or 3D near-eye display system with an electro-optical scene brightness modulator and sensors. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]      FIG. 1  shows a waveguide  10  that includes an input coupler grating  11 , a 2D pupil expander grating  12 , and output coupling grating  13  supported by an optically transparent plane-parallel substrate  20 . Gratings  11 ,  12 , and  13  can be replicated on the substrate  20  using a polymeric material of similar refractive index to the substrate or created as part of the substrate itself. The gratings can be patterned using microlithographic, holographic, stamping, and etching methods well known in the art. Although the refractive index of substrate  10  and gratings  11  through  13  could be similar to standard glass (e.g. 1.5), higher refractive indices such as 1.7 are preferred to maximize the range of angles and number of modes that can be coupled into the waveguide  10 . Both plastic and glass substrates can serve as acceptable substrates as long as the two (front and back) surfaces are highly parallel and are sufficiently transmissive to propagate coupled modes without significant loss across the waveguide. As an example, waveguide  10  can have a length of approximately 60 millimeters for reaching the eye position and a height of approximately 30 millimeters and a thickness of 1 to 2 mm to support the propagation of the desired modes. Other configurations such as stacked waveguides, curved waveguides, grating pitches tailored to desired wavelengths, are well known in the prior art. It should be noted that this invention is not limited to any specific waveguide geometry or configuration. 
         [0028]      FIG. 2  shows a waveguide  30  similar to the waveguide of  FIG. 1  but with the addition of a self-emitting microdisplay system  21 . The self-emitting microdisplay system  21  is positioned directly over input grating  11  and emits light in the direction of the grating. By controlling a number of grating and system properties, output rays from  21  can be efficiently coupled into the waveguide. For example, assume that a ray  31  as shown in  FIG. 2  is coupled into the waveguide and propagates in the direction of the 2D expander grating  12 . The 2D expander grating pattern is angularly oriented relative to the direction of the propagation direction of the coupled rays. As a result, it produces a large number of multiply reflected orders  32  which expand the narrow input beam  31  into a broader width labeled “w” in  FIG. 2 . Since the image input to input grating  11  is composed of many rays ranging in position and angle, a large and practically continuous set of coupled orders propagates toward output grating  13 . If the output grating  13  matches diffractive characteristics of input grating  11 , output rays will preserve angles of the incident rays illuminating the input grating  11 . To view images, the observer places his or her eye within an eyebox in front of the output grating  13  and thereby receives the image content originally injected by the self-emitting microdisplay system  21 . 
         [0029]    A cross-sectional side view of the region around the input coupling is shown in  FIG. 3 . Waveguide substrate  20  supports a layer  41  having a refractive index approximately equal to the substrate  20 . A grating pattern area  42  imprinted or replicated using methods well known in the art defines the dimensions of input grating  11 . Although  FIG. 3  depicts a vertical grating pattern for convenience, the grating structure (not to scale and greatly magnified) can be tilted from vertical alignment to improve coupling efficiency. 
         [0030]    The self-emitting microdisplay system  40  shown in  FIG. 3  is positioned directly over grating pattern area  42 . The system  40  includes positive imaging optics  43 , a folded optics mirror  44 , and a self-emitting microdisplay  45 . Folded optics mirror  44  is an optional feature that can be included to reduce the dimensions of the self-emitting microdisplay system  40 . Depending on imaging requirements, imaging optics  43  can include a positive singlet, a doublet, and have additional elements, as well as elements corrected for chromaticity. The focal length of the imaging optics  43  is preferably chosen such that microdisplay  45  resides at approximately the focal plane of the imaging optics  43 . Although  FIG. 3  shows a system with its optical axis normal to waveguide  20 , in some cases a tilt from normal incidence would be desirable. In such cases, elements of the system  40  would be respectively tilted to achieve a good virtual image.  FIG. 4  illustrates an off-axis example that uses a doublet lens  44  for the imaging optics. 
         [0031]    Further integration and size reduction is possible using other folded optics approaches.  FIG. 5  depicts a more highly folded system that includes a prism-shaped reflecting structure  51 , positive imaging optics  52 , and a self-emitting microdisplay  53 . In this case, light emanating from self-emitting microdisplay  53  follows optical path  54  through two reflections. Preferably, positive imaging optics  52  have a focal length coinciding with the total optical path length of  54 . Either mirror-based, plastic-based, or glass-based elements may be used for the reflecting structure  51 . In addition to the smaller form factor, the self-emitting microdisplay  53  can be mounted coplanar to the waveguide surface. The coplanar surface mounting can improve manufacturability and result in greater physical integrity. Furthermore, a planar heat sink  55  can be integrated directly onto the waveguide to further improve compactness and mechanical robustness. The planar heat sink  55  can be slightly shimmed away from the waveguide to provide a small air gap needed to sustain TIR (total internal reflection) conditions. If the heat sink  55  is directly attached to the waveguide (e.g. glued), the input grating  56  can be designed such that the primary coupled modes travel in a direction away from the heat sink  55  (i.e., toward the left in  FIG. 5 ). Although the grating in  FIG. 5  resides on the waveguide surface adjacent to positive imaging optics  52 , in some cases, it may be desirable to place input grating  56  on the opposite face of the waveguide substrate  20  for performing in a similar manner as a reflection grating. 
         [0032]    The embodiments of this invention enable extremely compact, planar, and power efficient near-eye display systems. The pixel-power addressable characteristics of the self-emitting microdisplays provide benefits including lower power consumption, decreased heating of the display and discomfort to the user, relaxed requirements on heat sinking and thermal management which could otherwise increase system bulk, and lower requirements for battery power resulting in more compact or longer-lasting batteries. 
         [0033]    As an example,  FIG. 6 a    depicts a near-eye display system  60  in which two waveguide substrates  66  are used to deliver digital image content to the viewer&#39;s eyes within eyeboxes  68 . Similar to the embodiments shown in  FIGS. 1-5 , each substrate  66  includes input gratings  61 ,2D beam expander gratings  62 , output gratings  63 , and self-emitting microdisplay systems  64  similar to those shown in  FIGS. 3-5 . Drive electronics  65  provide input to the self-emitting microdisplay systems  64  corresponding to the digital image content being displayed for each eye. Output of the electronics can be formatted and output for 2D content or 3D (stereoscopic) content. Pixel-addressable power requirements of the self-emitting microdisplays  64  require power only as needed to generate illumination corresponding to the output power of pixels composing the images. The drive electronics uses some power for clocking and image formatting functions but this amount of power is generally inconsequential compared to drive power provided to the emitting elements.  FIG. 6 b    shows an alternative configuration for a compact near-eye display system. In this case, self-emitting microdisplay systems  64  and associated drive electronics  65  are mounted along the temple of the glasses  80 , configuring the system in such a way to resemble conventional eye-wear. Other configurations of the self-emitting microdisplay systems  64  and drive electronics  65  are possible, for example using mirrors to guide the optical path to best match the specific glasses being used. 
         [0034]    Another embodiment arranged to further reduce power consumption is shown in  FIG. 7 . In addition to the aforementioned components of the near-eye display system, a light sensor  71  is included to measure ambient illumination. Since waveguide  72  is a transmissive element capable of providing the viewer with an unobstructed field of view within the real world, ambient illumination can significantly affect the perceived contrast ratio of the virtual reality digital content and, in some cases, overwhelm the digital content. Conversely, too high power input to the self-emitting display system  73  can overwhelm a darker ambient environment such as nighttime or indoors conditions. In order to allow effective superposition of digital virtual reality content onto the viewer&#39;s unobstructed field of view within eyebox  88  regardless of ambient illumination, the measured power of ambient illumination measured by the light sensor is preferably digitized using A/D system  74 , and these data are input to control electronics  75 . The power input to the self-emitting display system  73  can be optimized  73  through the use of known algorithms and/or look-up tables  74 . This process can happen dynamically in response to both ambient illumination and the digital content  76  to be displayed by the near-eye display system. Since near-eye displays enabled by this invention can be wearable and useful in many settings, dynamic adjustment of the self-emitting display power is very useful as the user moves from one environment to another, e.g. turning a corner from a sunlit to a shadowed sidewalk or entering an elevator from a brightly lit office environment. The dynamic change of input signal to  73  can also result in additional power savings. 
         [0035]    Enhanced power savings and an improved user experience can also be achieved using an electro-optical light modulator  77  that transmits a controlled amount of ambient illumination transmitted through the waveguide to the user&#39;s eyes. Liquid crystal displays, electro-optical ceramics (e.g. PLZT), electrochromic, electrophoretic modulators are well-known examples of electro-optical light modulators that can be utilized. Control electronics  75  can be used to adjust the modulating effects of the light modulator  77  in response to the output of a light sensor  71 . An output digital signal  78  based on both the ambient illumination and virtual reality content can be converted to an analog signal by D/A converter  79  and provided as input to light modulator  77 . Additional enhancement of this process can be achieved using a two-dimensional image sensor such as CMOS sensor or CCD sensor for  71  that determines ambient illumination levels within the image space. For example, the direction of a bright reflection from a car spotlight, or indoor lamp may be detected using the image sensor for  71 . Based on the location of this ambient illumination relative to the virtual reality content, the signal input to the waveguide can be modified to match spatially the illumination source and compensate for the ambient illumination if needed. In addition, if a pixelated light modulator display  77  is used instead of a simple uniform filter, the spatial optical density of the modulator can be further controlled to selectively filter ambient light depending on position within the display field. The ability to reduce transmitted ambient light, thereby reducing power requirements by the self-emitting display system  73  provides additional power savings to the system. 
         [0036]    Having thus described various embodiments of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and various alterations, improvements, and modifications will be apparent to those of skill in the art in accordance with the overall teaching of this invention.