Abstract:
An illumination module includes a light source to output lamp light and a mixing light guide having a substantially rectangular cross-section. The mixing light guide includes a spatial homogenization section and an emission section. The spatial homogenization section includes a first end coupled to receive the lamp light having a first cross-section profile from the light source and a second end to deliver the lamp light with a second cross-section profile substantially conforming to the substantially rectangular cross-section of the mixing light guide. The emission section is adjacent to the second end. The emission section includes an emission surface through which the lamp light is emitted with a substantially uniform intensity profile. The emission surface faces a different direction than an input surface of the first end through which the lamp light is received into the mixing light guide.

Description:
TECHNICAL FIELD 
     This disclosure relates generally to the field of optics, and in particular but not exclusively, relates to near-to-eye optical systems. 
     BACKGROUND INFORMATION 
     A head mounted display (“HMD”) is a display device worn on or about the head. HMDs usually incorporate some sort of near-to-eye optical system to emit a light image within a few centimeters of the human eye. Single eye displays are referred to as monocular HMDs while dual eye displays are referred to as binocular HMDs. Some HMDs display only a computer generated image (“CGI”), while other types of HMDs are capable of superimposing CGI over a real-world view. This latter type of HMD can serve as the hardware platform for realizing augmented reality. With augmented reality the viewer&#39;s image of the world is augmented with an overlaying CGI, also referred to as a heads-up display (“HUD”). 
     HMDs have numerous practical and leisure applications. Aerospace applications permit a pilot to see vital flight control information without taking their eye off the flight path. Public safety applications include tactical displays of maps and thermal imaging. Other application fields include video games, transportation, and telecommunications. There is certain to be new found practical and leisure applications as the technology evolves; however, many of these applications are limited due to the cost, size, weight, field of view, and efficiency of conventional optical systems used to implemented existing HMDs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. The drawings are not necessarily drawn to scale. 
         FIG. 1  is a cross-sectional view of a near-to-eye display, in accordance with an embodiment of the disclosure. 
         FIG. 2A  is a cross-sectional side view an illumination module, in accordance with an embodiment of the disclosure. 
         FIG. 2B  is a cross-sectional bottom view an illumination module, in accordance with an embodiment of the disclosure. 
         FIG. 3  is a perspective view of an illumination module, in accordance with an embodiment of the disclosure. 
         FIG. 4  is a cross-sectional view of a light enhancement sandwich structure, in accordance with an embodiment of the disclosure. 
         FIG. 5A  is a cross-sectional side view an illumination module with a three-dimensional texture disposed in or on the emission surface of the mixing light guide, in accordance with an embodiment of the disclosure. 
         FIG. 5B  is a bottom view of an illumination module with a three-dimensional texture having a uniform pattern disposed in or on the emission surface of the mixing light guide, in accordance with an embodiment of the disclosure. 
         FIG. 5C  is a bottom view of an illumination module with a three-dimensional texture having a non-uniform pattern disposed in or on the emission region of the mixing light guide, in accordance with an embodiment of the disclosure. 
         FIG. 5D  is bottom view of an illumination module with a three-dimensional texture having a non-uniform pattern disposed in or on the emission region of the mixing light guide, in accordance with an embodiment of the disclosure. 
         FIG. 6  is a top view of a binocular HMD implemented with near-to-eye displays, in accordance with an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of an apparatus and method of operation of a head mounted display including an illumination module with mixing light guide are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
       FIG. 1  is a cross-sectional view of a near-to-eye display  100 , in accordance with an embodiment of the disclosure. The illustrated embodiment of near-to-eye display  100  includes a display module  105 , a light relay section  110 , and an eyepiece  115 . The illustrated embodiment of display module  105  includes an illumination module  120 , a beam splitter  125 , a display panel  130 , and a light absorbing coating  135 . Illumination module  120  includes a mixing light guide  140  and a light source  145 . The illustrated embodiment of light relay  110  includes small section  150 , half-wave plate polarization rotator  155 , light blocks  160 , and large section  165 . The illustrated embodiment of eyepiece  115  is a see-through eyepiece with a beam splitter  170 , a quarter-wave plate polarization rotator  175 , and end reflector  180 . Eyepiece  115  includes an eye-ward side  171  for emission of computer generated image (“CGI”) light  172  towards eye  173  and an external scene side  174  through which ambient scene light  176  enters. 
     Illumination module  120  generates lamp light used to illuminate display panel  130 , which modules image data onto the lamp light to create CGI light  172 . To provide a good quality, uniform image, the lamp light should illuminate display panel  130  with sufficiently uniform intensity. Illumination module  120  is well suited for use with near-to-eye displays and head mounted displays, since it is compact and efficient. 
     The lamp light generated by illumination module  120  is emitted from an emission surface into beam splitter  125 . In one embodiment, beam splitter  125  is a polarizing beam splitter (“PBS”) cube that substantially passes light of a first polarization (e.g., P polarization), while substantially reflecting light of a second polarization (e.g., S polarization). These two polarization components are typically orthogonal linear polarizations. The emitted light may be pre-polarized (e.g., P polarized) or unpolarized light. In either event, the P polarization component passes through the PBS cube to illuminate display panel  130  while most of any remaining portions of S polarization are reflected back onto light absorbing coating  135  (e.g., flat black paint). Thus, in the illustrated embodiment, display panel  130  is mounted in opposition to illumination module  120  with beam splitter  120  disposed in between. 
     Display panel  130  (e.g., liquid crystal on silicon panel, digital micro-mirror display, etc.) imparts image data onto the lamp light via selective reflection by an array of reflective pixels. In an embodiment using an LCOS panel, reflection by display panel  130  rotates the polarization of the incident lamp light by 90 degrees. Upon reflection of the incident lamp light, the CGI light (which has been rotated in polarization by 90 degrees to be, for example, S polarized) is re-directed by beam splitter  125  and propagates down light relay  110  along a forward propagation path towards eyepiece  115 . In an embodiment using a digital micro-minor display, beam splitter  125  may be implemented as a standard 50/50 beam splitter and the CGI light  172  may be unpolarized light. Since beam splitter  125  operates to launch the CGI light into light relay  110  and eyepiece  115 , it may be referred to as an in-coupling beam splitter or in-coupling PBS. 
     In one embodiment, CGI light  172  is directed along the forward propagation path within light relay  110  without need of total internal reflection (“TIR”). In other words, the cross sectional shape and divergence of the light cone formed by CGI light  172  is confined such that the light rays reach end reflector  180  without need of TIR off the sides of light relay  110  or eyepiece  115 . 
     Beam splitter  125 , light relay  110 , and beam splitter  170  may be fabricated of a number of materials including glass, optical grade plastic, fused silica, PMMA, Zeonex-E48R, or otherwise. The length D 1  of light relay  110  may be selected based upon the temple-eye separation of the average adult and such that the focal plane of end reflector  180  substantially coincides with an emission aperture of display panel  130 . To achieve focal plane alignment with the emission aperture of display panel  130 , both the length of light relay  110  and the radius of curvature of end reflector  180  may be selected in connection with each other. 
     In the illustrated embodiment, light relay  110  includes half-wave plate polarization rotator  155  disposed at the interface between small section  150  and large section  165 . Half-wave plate polarization rotator  155  servers to rotate the polarization of CGI light  172  by 90 degrees (e.g., convert the S polarized light back to P polarized light again). The illustrated embodiment of light relay  110  further includes light blocks  160  disposed on the edges of large section  165  that extend past small section  150 . Light blocks  160  reduce external light from leaking into light relay  110  and eyepiece  115 . Light blocks  160  may be opaque paint, a opaque collar extending around small section  150 , or otherwise. 
     The illustrated embodiment of eyepiece  115  includes a partially reflective surface formed within beam splitter  170 . In one embodiment, beam splitter  170  is partially transparent, which permits external (ambient) scene light  176  to pass through external scene side  174  and eye-ward side  171  of eyepiece  115  to reach eye  173 . A partially transparent embodiment facilitates an augmented reality (“AR”) where CGI light  172  is superimposed over external scene light  176  to the user eye  173 . In another embodiment, eyepiece  115  is substantially opaque (or even selectively opaque), which facilitates a virtual reality (“VR”) that immerses the user in the virtual environment displayed by CGI light  172 . 
     In the illustrated embodiment, beam splitter  170  is an out-coupling PBS cube configured to pass one linear polarization (e.g., P polarization), while reflecting the other linear polarization (e.g., S polarization). Thus, the out-coupling PBS passes CGI light  172  propagating along the forward propagation path to quarter wave-plate polarization rotator  175 . After passing through quarter-wave plate polarization rotator  175 , CGI light  172  is reflected back along a reverse propagation path back towards the out-coupling PBS. Thus, CGI light  172  is rotated a total of 90 degree in polarization during its double pass through quarter-wave plate polarization rotator  175  and is S polarized by the time it strikes the out-coupling PBS on the reverse propagation path. 
     In one embodiment, end reflector  180 , both reflects and collimates CGI light  172  such that CGI light  172  traveling along the reverse propagation path is substantially collimated. As previously stated, the focal plane of end reflector  180  may be configured to coincide or nearly coincide with the emission aperture of display panel  130 . Collimating (or nearly collimating) CGI light  172  helps eye  173  to focus on CGI light  172  emitted out eye-ward side  171  in a near-to-eye configuration (e.g., eyepiece  115  placed within 10 cm of eye  173  and typically less than 5 cm of eye  173 ). CGI light  172  is directed towards eye  173  due to the oblique orientation (e.g., approximately 45 degrees relative to sides  174  and  171 ) of the out-coupling PBS. In other embodiments, end reflector  180  merely reduces the divergence of CGI light  172  without fully collimating CGI light  172 . In yet other embodiments, end reflector  180  may be implemented as a flat reflective surface. In embodiments where in-coupling beam splitter  125  and out-coupling beam-splitter  170  are regular beam splitters, half-wave plate polarization rotator  155  and quarter-wave plate polarization rotator  175  may be omitted. 
       FIGS. 2A ,  2 B, and  3  illustrate an illumination module  200 , in accordance with an embodiment of the disclosure.  FIG. 2A  is a cross-sectional side view,  FIG. 2B  is a cross-sectional bottom view, and  FIG. 3  is a perspective view of illumination module  200 . Illumination module  200  represents one possible implementation of illumination module  120  illustrated in  FIG. 1 . The illustrated embodiment of illumination module  200  includes a mixing light guide  205 , a light source  210 , and a light enhancement sandwich structure  215 . The illustrated embodiment of mixing light guide  205  includes a spatial homogenization section  220  and an emission section  225 . The illustrated embodiment of light source  210  includes a housing  230 , a light shield  235 , and lamp  240 . 
     Illumination module  200  is a compact, high efficiency, uniform lamp light source for illuminating display  130 . In the illustrated embodiment, mixing light guide  205  is a single, continuous piece, solid optical waveguide that receives lamp light having a first cross-section profile  305  (see  FIG. 3 ) from lamp  240  and outputs the lamp light from an emission surface  250  having a substantially uniform intensity profile  310 . Mixing light guide  205  may be fabricated of optical plastic (e.g., PMMA, Zeonex—E48R) or other optical materials (e.g., glass, fused silica, quartz, etc.). Example dimensions for mixing light guide  205  include D 1 =10 mm, D 2 =6 mm, D 3 =4 mm, and D 4 =1 mm. Of course, other dimensions may be implemented. 
     Light source  210  launches lamp light into mixing light guide  205 . The illustrated embodiment of light source  210  includes lamp  240  disposed within a housing  230 . Housing  230  is at least partially covered over by light shield  235 . Lamp  240  may be implemented as a single white light source (e.g., white LED, white phosphorus LED, etc.). In this embodiment, if display module  105  is intended to be color display, then a color filter array (“CFA”) may be coated over display panel  130 . In one embodiment, lamp  240  is a tricolor lamp that includes three color LEDs (e.g., RGB LEDs) that cycle to produce a full color display without need of a CFA. 
     Spatial homogenization section  220  inputs the lamp light with cross-section profile  305  at a first end and spreads the lamp light out to have a second cross-section profile  315  at its opposite end that substantially conforms to the rectangular cross-section of mixing light guide  205 . Spatial homogenization section  220  performs the light spreading function via a series of internal reflections off of its exterior surfaces as the lamp light propagates towards emission section  225 . In one embodiment, total internal reflection (“TIR”) is responsible for the surface reflections. In one embodiment, spatial homogenization section  220  may be lined with a reflective coating. 
     Emission section  225  is defined by the presence of a pattern disposed in or on one or more surfaces of a portion of mixing light guide  205 . For example, in the embodiment illustrated in  FIG. 3 , four surfaces including top surface  320 , two edge surfaces  325 , and end surface  330  are covered with a pattern. The pattern operates to alter the reflective properties of the surfaces of emission section  225 , such that the lamp light begins to exit through emission surface  250 . For example, in one embodiment, top surface  320  and optionally one or more of edge surfaces  325  and end surface  330  are coated with a reflective coating having light scattering properties (e.g., diffusive white paint). The reflective light scattering properties change the angle of incident of the lamp light when it strikes emission surface  250  thereby defeating TIR in emission section  225  at emission surface  250 . As such, the lamp light begins to exit out emission surface  250 . Because the light scattering coating is reflective, the lamp light only emerges from emission surface  225 . As discussed below in connection with  FIGS. 5A-5D , various different types of patterns, as opposed to a solid uniform pattern (e.g., white diffusive paint), may be disposed on emission surface  250  and top surface  320  to further control how lamp light is emitted from emission section  225 . In the illustrated embodiment, emission surface  250  is substantially orthogonal to cross-section profiles  305  and  315 . 
     The illustrated embodiment of illumination module  200  includes light enhancement sandwich structure  215  disposed over emission surface  250 . Light enhancement sandwich structure  215  operates to reduce the divergence of the lamp light emitted from emission section  225  and, in some embodiments, polarize or substantially polarize the emitted lamp light.  FIG. 4  is a cross-sectional view of a light enhancement sandwich structure  400 , in accordance with an embodiment of the disclosure. Light enhancement sandwich structure  400  is one possible implementation of light enhancement sandwich structure  215 . The illustrated embodiment of light enhancement sandwich structure  400  includes a first brightness enhancement film (“BEF”)  405 , a second BEF  410 , and a reflective polarizing layer  415 . BEFs  405  and  410  may be implemented as thin layers have prismatic structures disposed thereon. The prismatic structures in each BEF operate to reduce light divergence along a single dimension. By layering two BEFs  405  and  410  substantially orthogonal to each other, the divergence of the emitted light cone can be reduced in both dimensions. Reflective polarizing layer  415  may be added to substantially linearly polarize the lamp light emitted from emission surface  250 . In one embodiment, layers  405 ,  410 , and  415  are held together using mechanical friction via clamping illumination module  200  to beam splitter  125 . 
       FIGS. 5A-D  illustrate illumination modules each having a three-dimensional texture disposed in or on the emission surface of the mixing light guide, in accordance with an embodiment of the disclosure.  FIG. 5A  is a cross-sectional side view of an illumination module  500 . The illustrated embodiment of illumination module  500  includes a mixing light guide  505 , a light source  510 , reflective surfaces  512 , and light enhancement sandwich structure  515 . Mixing light guide  505  includes a spatial homogenization section  520  and an emission section  525 . 
     Illumination module  500  is similar to illumination module  200  with at least the following exceptions. Illumination module  500  includes patterns disposed over emission section  525  implemented as three-dimensional (“3D”) textures  530  and  535  formed in or on emission surface  540  and on top surface  545 , respectively. Furthermore, reflective surfaces  512  are disposed over top surface  545 , end surface  550 , and edge surfaces  555  and  560  (see  FIGS. 5B-5D ). Reflective surfaces  512  may be implemented as a reflective coating (e.g., silver paint, etc.) or a four sided reflective housing (e.g., metal housing). In some embodiments, one or more of the reflective surfaces  512  overlaying end surface  550  or edge surfaces  555 ,  560  may be omitted. In some embodiments, both emission surface  540  and top surface  545  need not be overlaid with a 3D texture; rather, just one of the two surfaces (either top surface  545  or emission surface  540 ) may be textured. Furthermore, 3D textures  530  and  535  need not have identical patterns, though in some embodiments they can be. 
       FIG. 5B  is a bottom view of illumination module  500  with a 3D texture  500 A having a uniform pattern, in accordance with an embodiment of the disclosure. 3D texture  500 A may be used to implement one or both of 3D textures  530  and  535  illustrated in  FIG. 5A . 3D texture  500 A is a uniformly random pattern, which may be implemented as a rough texture that is impressed or molded into the emission surface  540  and/or top surface  545 . Alternatively, 3D texture  500 A may be implemented using paint dots formed (e.g., silk screened) onto the given surface. The surface texture serves to scatter incident light, thereby altering the angle of incidence and affecting TIR at emission surface  540 . In one embodiment, 3D texture  500 A is fabricated of reflective scattering elements (e.g., silver paint dots, patterned &amp; reflowed metal, etc.). 
       FIG. 5B  is a bottom view of illumination module  500  with a 3D texture  500 B having a non-uniform pattern, in accordance with an embodiment of the disclosure. 3D texture  500 B may be fabricated in similar manner to 3D texture  500 A; however, the pattern of 3D texture  500 B has an increasing density of scattering elements in the corners of emission surface  540  relative to the middle. The increasing density of scattering elements in the corners offsets the lower light lamp intensity in the corners to achieve a more uniform intensity profile  310  (see  FIG. 3 ) emitted from emission surface  540 . 
       FIG. 5C  is a bottom view of illumination module  500  with a 3D texture  500 C having a non-uniform pattern, in accordance with an embodiment of the disclosure. 3D texture  500 C may be fabricated in similar manner to 3D texture  500 A; however, the pattern of 3D texture  500 C has an increasing density of scattering elements with increasing distance from spatial homogenization section  520 . The increasing density of scattering elements moving towards end surface  550  offsets the lower light lamp intensity as light is released from emission surface  540  along its journey through emission section  525 . The non-uniform pattern of 3D texture  500 C improves the uniformity of intensity profile  310  (see  FIG. 3 ) emitted from emission surface  540 . 
     In one embodiment, one or both of 3D textures  530  and  535  may be fabricated using a composite pattern having generally increasing density of scattering elements in the corners (e.g., 3D texture  500 B) and generally increasing density of scattering elements with increasing distance form spatial homogenization section  520  (e.g., 3D texture  500 C). 
       FIG. 6  is a top view of a head mounted display (“HMD”)  600  using a pair of near-to-eye displays  601 , in accordance with an embodiment of the disclosure. Each near-to-eye display  601  may be implemented with embodiments of near-to-eye display  100 . The near-to-eye displays  601  are mounted to a frame assembly, which includes a nose bridge  605 , left ear arm  610 , and right ear arm  615 . Although  FIG. 6  illustrates a binocular embodiment, HMD  600  may also be implemented as a monocular HMD with only a single near-to-eye display  601 . 
     The two near-to-eye displays  601  are secured into an eyeglass arrangement that can be worn on the head of a user. The left and right ear arms  610  and  615  rest over the user&#39;s ears while nose assembly  605  rests over the user&#39;s nose. The frame assembly is shaped and sized to position an eyepiece  115  in front of a corresponding eye  173  of the user. Of course, other frame assemblies having other shapes may be used (e.g., a visor with ear arms and a nose bridge support, a single contiguous headset member, a headband, goggles type eyewear, etc.). 
     The illustrated embodiment of HMD  600  is capable of displaying an augmented reality to the user. The viewing region of each eyepiece  115  permits the user to see a real world image via external scene light  176 . Left and right (binocular embodiment) CGI light  630  may be generated by one or two CGI engines (not illustrated) coupled to a respective image source. CGI light  630  is seen by the user as virtual images superimposed over the real world as an augmented reality. In some embodiments, external scene light  176  may be blocked or selectively blocked to provide a head mounted virtual reality display or heads up display. 
     Although illumination module  120  is discussed above in the context of HMDs, it should be appreciated that it may also be applicable for use in other compact devices, such as cell phones, laptops, or other portable display devices. 
     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.