Lightweight eyepiece for head mounted display

An eyepiece includes an eyepiece frame, an in-coupling polarization beam splitter (“PBS”), an end reflector, and an out-coupling PBS. The eyepiece frame defines an air cavity and includes an illumination region for receiving computer generated image (“CGI”) light into the eyepiece frame and a viewing region to be aligned with an eye of a user. The in-coupling PBS is supported within the eyepiece frame at the illumination region to re-direct the CGI light to a forward propagation path extending along the air cavity towards the viewing region. The end reflector is disposed to reflect the CGI light back along a reverse propagation path within the eyepiece frame. The out-coupling PBS is supported at the viewing region to pass the CGI light traveling along the forward propagation path and to redirect the CGI light traveling along the reverse propagation path out of an eye-ward side of the eyepiece frame.

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'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.

DETAILED DESCRIPTION

Embodiments of a system, apparatus, and method of operation for a head mounted display (“HMD”) eyepiece. 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.

FIG. 1Aillustrates a first conventional near-to-eye optical system101using an input lens and two minors. An image source105outputs an image that is reflected by two minors110and115, which form an image near to eye120. Image source105is typically mounted above the head or to the side of the head, while minors110and115bend the image around the front of the viewer's face to their eye120. Since the human eye is typically incapable of focusing on objects placed within a few centimeters, this system requires a lens125interposed between the first minor110and image source105. Lens125creates a virtual image that is displaced further back from the eye than the actual location of minor115by positioning image source105inside of the focal point f of lens125. Optical system101suffers from a relatively small field of view limited by the extent of mirrors110and115and the bulkiness of lens125. The field of view can be marginally improved by placing minors110and115within a high index material to compress the angles of incidence, but the thickness of the waveguide rapidly increases to achieve larger fields of view.

FIG. 1Billustrates a second conventional near-to-eye optical system102using angle sensitive dichroic mirrors. Optical system102includes a single in-coupling mirror130and two out-coupling dichroic mirrors135disposed within a waveguide140. This system uses collimated input light from virtual images placed at infinity and uses total internal reflection (“TIR”) to guide the input light down waveguide140towards out-coupling dichroic mirrors135. In order to produce a useful image at eye120, each incident angle of input light should correspond to a single output angle of emitted light. Since light can potentially reflect off of output minors135on either a downward trajectory (ray segments145) or an upward trajectory (ray segments150), each input angle can potentially result in multiple output angles, thereby destroying the output image. To overcome this problem, optical system102uses angle sensitive dichroic mirrors135that pass light with incident sufficiently close to normal while reflecting light having a sufficiently oblique incidence. However, the nature of dichroic mirrors135that passes some incident angles while reflecting others, limits the field of view of optical system102and reduces the optical efficiency of the system. In addition, these dichroic mirror coatings do not provide sharp angular cutoffs, resulting in ghosting effects. The angle sensitive dichroic mirror coating requires a complicated and customized coating design, which is not cost effective.

FIG. 1Cillustrates a third conventional near-to-eye optical system103using holographic diffraction gratings. Optical system103is similar to optical system102, but uses holographic diffraction gratings150in place of minors130and135. Diffraction gratings150are inefficient reflectors, since they only reflect higher order diffractions while passing the first order diffraction, which contains the largest portion of energy in an optical wave front. In addition to being poor optical reflectors, the input and output diffraction gratings must be precisely tuned to one another, else the output image will suffer from color separation. Achieving a sufficient match between the input and output gratings150requires extreme control over manufacturing tolerances, which is often difficult and costly.

FIGS. 2 and 3illustrate an eyepiece200for use with a HMD, in accordance with an embodiment of the disclosure.FIG. 2illustrates a cross-sectional view of eyepiece200whileFIG. 3illustrates a perspective view of the same. The illustrated embodiment of eyepiece200includes an eyepiece frame205, an image source, an in-coupling polarizing beam splitter (“PBS”)210, an out-coupling PBS215, a transparent plate220, a polarization rotator225, and an end reflector. The illustrated embodiment of eyepiece frame205includes an illumination region230and a viewing region235. The illustrated embodiment of the image source includes illumination module240and display panel245. The illustrated embodiment of the end reflector includes a convex lens250and a reflective layer255. Referring toFIG. 3, in one embodiment, eyepiece200further includes transparent side sheets260.

In one embodiment, eyepiece frame205defines an air cavity and holds the internal components in place. In the illustrated embodiment, in-coupling PBS210and out-coupling PBS215are mounted within eyepiece frame205at oblique angles (e.g., 45 degrees) to forward propagation path203. Forward propagation path203extends within eyepiece frame205from illumination region230to viewing region235. In the embodiment illustrated inFIG. 3, frame205includes elongated members207and cross-members209. Cross-members209coupled between pairs of elongated members207to rigidly support the frame. In one embodiment, forward propagation path203is substantially parallel with elongated members207and substantially perpendicular to cross-members209. Eyepiece frame205may be fabricated of metal (e.g., magnesium metal) and may be further fabricated as a rigid metal wire frame. In some embodiments, protective sheathing layers (e.g., transparent side sheets260; note: only two are illustrated but all four sides may be sheathed for protection) may be fixed onto eyepiece frame205to protect the inner components. The protective sheathing may be transparent (e.g., glass, quartz, acrylic, clear plastic, PMMA, ZEONEX—E48R, etc.) in viewing region235to permit ambient scene light270to pass through ambient scene side201and eye-ward side202of eyepiece200to reach eye120. The semi-transparent nature of viewing region235permits eyepiece200to provide an augmented reality to the user by augmenting ambient scene light270with the CGI light.

Illumination module240may be implemented using a light emitting diode (“LED”) source (or multi-color LED array), which illuminates display panel245through in-coupling PBS210. Illumination module240may output unpolarized light (e.g., both P and S linear polarization components) or output polarized light (e.g., just P polarization component). Illumination module240and display panel245may be mounted to the outer side of frame205in the illumination region230in an opposing configuration on either side of eyepiece200.

In-coupling PBS210is positioned within illumination region230between illumination module240and display panel245. In-coupling PBS210may be implemented as a wire grid polarizer, a multi-layer thin film polarizer, or otherwise. In-coupling PBS210operates to substantially pass light of a first linear polarization while substantially reflecting light of a second polarization. The two linear polarizations are typically orthogonal linear polarizations. Display panel245(e.g., LCOS, LCD panel, etc.) imparts image data onto the illumination light output by illumination module240to output computer generated image (“CGI”) light via selective reflection by an array of image pixels. Reflection by display panel245rotates 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) is re-directed by in-coupling PBS210and propagates down eyepiece200along forward propagation path203. In one embodiment, the CGI light is directed down eyepiece200along forward propagation path203without need of total internal reflection (“TIR”). In other words, the cross sectional shape and divergence of the light cone formed by the CGI light is confined such that the light rays reach the end reflector without TIR off the sides (e.g., transparent side sheets260) of eyepiece200. In one embodiment, the light cone divergence angle (e.g., 15 degrees) is controlled by a blackout film patterned onto illumination module240, display panel245, or elsewhere within eyepiece frame205.

Forward propagation path203extends through the air cavity within eyepiece frame205to reflective layer255. The air cavity protected by eyepiece frame205operates as a lightweight light relay to protect the optical path. In one embodiment, the length of elongated members207is selected such that the focal plane of the end reflector substantially coincides with an emission aperture of display panel245. To achieve focal plane alignment with the emission aperture of display panel245, both the length of eyepiece frame205and the radius of curvature of reflective layer255may be selected in connection with each other.

The illustrated embodiment of viewing region235includes a reflective surface formed by out-coupling PBS215. In one embodiment, viewing region235is partially transparent, which permits external (ambient) scene light270to pass through external scene side201and eye-ward side202of eyepiece200to reach eye120. A partially transparent embodiment facilitates an augmented reality (“AR”) where the CGI light is superimposed over external scene light270to the user eye120. In another embodiment, viewing region235is substantially opaque (or even selectively opaque), which facilitates a virtual reality (“VR”) that immerses the user in the virtual environment displayed by the CGI light.

Out-coupling PBS215is configured to pass an orthogonal linear polarization (e.g., S polarization) than in-coupling PBS210passes, while reflecting the other linear polarization (e.g., P polarization). In-coupling PBS210may be implemented as a wire grid polarizer (seeFIGS. 6A and 6B), a multi-layer thin film polarizer, or otherwise. In the illustrated embodiment, polarization rotator225is a quarter wave-plate polarization rotator sandwiched between transparent plate220and convex lens250to eliminate deformities in the quarter wave plate's film embodiment and to allow for an antireflection coating of the optics past out-coupling PBS215into the end reflector. The CGI light is rotated 90 degree in polarization during its double pass through the quarter wave plate via forward propagation path203and reverse propagation path204after reflection by reflective layer225. In one embodiment, the end reflector, which includes convex lens250and reflective layer255, both reflects and collimates the CGI light such that the CGI light traveling along reverse propagation path204is substantially collimated. As previously stated, the focal plane of the end reflector may be configured to coincide or nearly coincide with the emission aperture of display panel245. Collimating the CGI light helps eye120to focus on the CGI light emitted out eye-ward side202in a near-to-eye configuration (e.g., eyepiece200placed within 10 cm of eye120and typically less than 5 cm of eye120). The CGI light is directed towards eye120due to the oblique orientation (e.g., approximately 45 degrees relative to sides201and202) of out-coupling PBS215. In other embodiments, the end reflector reduces the divergence of the CGI light without fully collimating the CGI light. In yet other embodiments, the end reflector may be implemented as a flat reflective surface.

In an embodiment where the end reflector collimates the CGI light, the eyebox (the zone within which eye120can see the CGI light) is determined by the projection of out-coupling PBS215onto eye-ward side202. The size of out-coupling PBS215is confined by the cross-sectional size and shape of eyepiece200. Referring toFIG. 3, in one embodiment, eyepiece200may have example cross-sectional dimensions D2=D3=10 mm. In other embodiments, dimensions D2and D3need not be equivalent (e.g., D2=10.2 mm and D3=7.7 mm). The overall length D1of eyepiece200may be selected based upon the temple-to-eye separation distance of a typical user and/or the focal plane distance of the end reflector. For example, the end reflector may have a radius of curvature approximately equal to 80 mm and eyepiece200may have a length D1approximately equal to 29.5 mm. Of course other ratios and dimensions may be used.

FIG. 4is a cross-sectional view of an illumination module400, in accordance with an embodiment of the disclosure. Illumination module400represents one possible implementation of illumination module240illustrated inFIGS. 2 and 3. The illustrated embodiment of illumination module400includes a lamp405, a light expansion zone410, reflective surfaces415, brightness enhancement films (“BEFs”)420and425, and a polarizer430.

Lamp405may be implemented as a single color LED, a multi-color array (e.g., RGB) of LEDs, or other light sources mounted to the side of light expansion zone410. The light emitted from lamp405illuminates expansion zone410, which uniformly spreads the light out over a larger cross-section. In embodiment, expansion zone410is implemented as a transparent polymer volume with reflective surfaces415(e.g., metal coatings) on its exposed sides. Expansion zone410is disposed on two stacked BEFs420and425. These films have optical power and operate to reduce the divergence of the lamp light. In one embodiment, BEFs420and425are micro-prism layers that are rotated 90 degrees relative to each other. In the illustrates embodiment, the stack of BEFs420and425is disposed on a polarizer430, which operates to polarize the lamp light into a single linear polarization component (e.g., P polarization) for illuminating display panel245through in-coupling PBS210. Thus, polarizer430is configured to output light having a polarization component to which in-coupling PBS210is substantially transparent. In one embodiment, components410,420,425, and430are clamped together without using glue.

FIGS. 5A & 5Billustrate cross-sectional views of an end reflector500, in accordance with an embodiment of the disclosure. End reflector500is one possible implementation of the end reflector illustrated inFIGS. 2 and 3. The illustrated embodiment of end reflector500includes a convex lens505and a concave reflector510coated over the convex end of convex lens505. Convex lens505is fabricated of a substantially transparent material (e.g., glass, quartz, acrylic, clear plastic, polycarbonate, PMMA, ZEONEX—E48R, etc.). Concave reflector510is made of a reflective material (e.g., metal coating) disposed over convex lens505. In an embodiment where eyepiece frame205defines an air cavity and forward propagation path203passes through a combination of air and solid surfaces (e.g., in-coupling PBS210, out-coupling PBS215, transparent plate220, polarization rotator225, and convex lens250), concave reflector510may be a toroidal mirror having two different radiuses of curvature R1and R2to correct for an astigmatism optical aberration. For example, R1may be approximately 81.87 mm and R2may be approximately 83.20 mm. Of course, other radius combinations may be used. In other embodiments, eyepiece200may be fabricated as a solid piece with the in-coupling and out-coupling PBS s embedded therein. In other embodiments, the end reflector may be fabricated as a simple concave mirror without a convex lens. Furthermore, concave reflector510may be an aspheric surface, a free form surface, or otherwise.

FIGS. 6A & 6Billustrate front and side views of a wire grid polarizer (“WGP”)600, in accordance with an embodiment of the disclosure. WGP600represents one possible implementation of either or both in-coupling PBS210and/or out-coupling PBS215. The illustrated embodiment of WGP600includes a plurality of metal lines605(or wires) that run substantially parallel to each other. In one embodiment, metal lines605are disposed on substrate610, which may be a clear or transparent substrate. In one embodiment, metal lines605may be embedded within substrate610or covered over by a protective layer615. Metal lines605may be fabricated of aluminum, tin, copper, or other conductive material. Substrate610(and protective layer615) may be fabricated of glass, quartz, acrylic, or other transparent materials such as Zeonex, PMMA, polycarbonate, etc. In some embodiments, substrate610may include one or more optical filter coatings (e.g., antireflective coatings, color coatings, darkening coatings, or otherwise). The pitch between adjacent metal lines605is generally selected to be below the wavelength(s) to be polarized. As a mere example, the pitch may be selected to be about 100 nm, while the thickness of metal lines605may be selected to be about 30 nm. Of course, other pitches and thickness may be selected according to the application and desired polarization characteristics.

During operation, when WGP600is illuminated with an unpolarized light, including components having a first linear polarization620and components having a second linear polarization625, the components having polarization620are substantially reflected while the components having polarization625pass through substantially unaffected. The electric field of linear polarization620excites electrons vertically along the length of metal lines605, which results in these components being radiated along a reflection path. In contrast, the electric field of polarization625excites electrons laterally across metal lines605. Since the electrons within metal lines605are confined horizontally, the components with polarization625pass through metal lines605. Of course, if WGP600is illuminated with polarized light substantially only having polarization625, then the light will substantially pass through. In contrast, if WGP600is illuminated with polarized light having substantially just polarization620, then the light will substantially reflect. In the illustrated embodiment, linear polarization620is orthogonal to linear polarization625.

The degree to which polarized light passes through WGP600, or is reflected thereby, is a function of at least the wavelength of the incident light and the grid pitch between metal lines605. Furthermore, by rotating WGP600relative to the input light (e.g., relative to CGI light), then WGP600can be made to pass or reflect either P or S polarization. Thus, in one embodiment, both in-coupling PBS210and out-coupling PBS215are fabricated with WGPs mounted within eyepiece frame205with orthogonal orientations (e.g., 90 degree relative physical orientations of the wire lines in the wire grid polarizers) thereby avoiding the need for a half-wave-plate polarization rotator disposed between in-coupling PBS210and out-coupling PBS215. In one embodiment, just in-coupling PBS210is fabricated using a WGP, while out-coupling PBS215is fabricated using a multi-layer thin film PBS, with the WGP at the in-coupling location oriented to pass an orthogonal polarization relative to out-coupling PBS215.

FIG. 7is a flow chart illustrating a process700of operation of eyepiece200to deliver a near-to-eye image to a user, in accordance with an embodiment of the disclosure. The order in which some or all of the process blocks appear in process700should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated, or even in parallel.

In a process block705, illumination module240emits lamp light being either unpolarized or having first a first linear polarization (e.g., illustrated as P polarization) selected to pass through in-coupling PBS210. Upon passing through in-coupling PBS210(process block710), unpolarized light becomes polarized prior to illuminating display panel245. Display panel245modulates image data onto the incident lamp light by selectively activating image pixels within the display panel. When an image pixel is activated, it reflects its portion of the lamp light (process block715). In so doing, the reflected CGI light has its polarization rotated by 90 degrees to a polarization that is reflected by in-coupling PBS210(e.g., illustrated as S polarization). The CGI light emitted from display panel245is reflected back onto in-coupling PBS210, which re-directs the CGI light along forward propagation path203(process block720). It should be appreciated that the designation of P and S polarizations, whereby P is transmitted and S is reflected is merely demonstrative. Other polarization orientations and transmitted/reflected combinations may be implemented.

The re-directed CGI light propagates down eyepiece frame205within the air cavity. In one embodiment, the length of eyepiece frame205merely provides a separation offset between the image source and the end reflector and need not operate to confine or guide the light wave. Thus, in these embodiments, the CGI light passes down eyepiece200without TIR and without external surface reflections. In a process block725, the CGI light passes through viewing region235along forward propagation path203. Since out-coupling PBS215is configured to substantially pass the same polarization component as reflected by in-coupling PBS210(i.e., in-coupling PBS210and out-coupling PBS215reflect orthogonal polarization components and transmit orthogonal polarization components relative to each other), the CGI light passes through out-coupling PBS215substantially without being affected.

In a process block730, the CGI light then passes through polarization rotator225along forward propagation path203. In so doing, the polarization of the CGI light is circularly polarized since the illustrated embodiment of polarization rotator225is a quarter wave-plate rotator.

In a process block735, the CGI light is reflected back along reverse propagation path204by reflective layer255. In one embodiment, reflective layer255is concave and has a shape to substantially collimate the CGI light reflected along reverse propagation path204. Collimating the CGI light has an effect of virtually displacing the CGI image at or near infinity thereby helping the human eye120to bring the CGI image into focus. Of course, the end reflector may reduce the divergence without fully collimating the light, thereby displacing the virtual image at a location less than infinity (e.g., 1 to 3 meters).

In a process block740, the reflected CGI light traveling along reverse propagation path204once again passes through polarization rotator225, causing the reversed circularly polarized CGI light to be linearly polarized at an orthogonal direction of polarization to the forward path. Thus, after passing through polarization rotator225for the second time, the CGI light has a polarization that is substantially reflected by out-coupling PBS215(e.g., illustrated as P polarization). In a process block745, the CGI light is reflected by out-coupling PBS215and redirected out of eyepiece200through eye-ward side202towards eye120.

FIG. 8is a top view of a head mounted display (“HMD”)800using a pair of near-to-eye optical systems801, in accordance with an embodiment of the disclosure. Each near-to-eye optical system801may be implemented with embodiments of eyepiece200. The near-to-eye optical systems801are mounted to a frame assembly, which includes a nose bridge805, left ear arm810, and right ear arm815. AlthoughFIG. 8illustrates a binocular embodiment, HMD800may also be implemented as a monocular HMD with only a single eyepiece.

The two near-to-eye optical systems801are secured into an eyeglass arrangement that can be worn on the head of a user. The left and right ear arms810and815rest over the user's ears while nose assembly805rests over the user's nose. The frame assembly is shaped and sized to position a viewing region235in front of a corresponding eye120of 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 HMD800is capable of displaying an augmented reality to the user. The viewing region of each eyepiece permits the user to see a real world image via external scene light270. Left and right (binocular embodiment) CGI light830may be generated by one or two CGI engines (not illustrated) coupled to a respective image source of the eyepieces. CGI light830is seen by the user as virtual images superimposed over the real world as an augmented reality. In some embodiments, external scene light270may be blocked or selectively blocked to provide a head mounted virtual reality display or heads up display.