Abstract:
The invention relates to compact optical arrangement for a helmet mounted display. The arrangement is well suited to use with spatial light modulators which require front illumination such as LCoS modulators but can also be adapted to rear illuminated devices such as LCD&#39;s and to self luminous devices such as OLEDs. The device uses polarization and reflection to make dual use of both volumes and lenses.

Description:
CROSS REFERENCE 
     This application is related to provisional application Ser. No. 60/958,844 filed on Jul. 9, 2007 entitled “Design of a See-through Head-Mounted Display” and is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to compound optical pupil forming systems. In particular, the invention relates to reducing the volume of an optical system by using a plurality of optical components multiple times. 
     BACKGROUND OF THE INVENTION 
     There is an increased interest in using miniature image devices carried on the head of an observer, head-mounted displays (HMD), to present information to the observer in a dynamic mode. The image device cannot be viewed directly, when placed before the observer&#39;s eye, due to the closeness of the device to the observer. Optical elements are used to create a virtual image of the image device at some distance so as to be comfortably viewed by the observer. One of the challenges of building such a device is to make the optical elements compact while maintaining optical performance that will support the resolution of the imaging device. While some applications having a modest field of view that can be supported by magnifier viewer wider field of view devices using small miniature image devices, they require compound optical systems. Compound optical systems that preserve a direct view of the image source are commonly known as pupil forming systems. 
     The present disclosure relates to compound optical pupil forming systems. Such devices are more fully described by Shenker in U.S. Pat. No. 3,432,219 (&#39;219), hereby incorporated herein by reference. The devices in the &#39;219 patent consist of two main parts, a relay lens and an eyepiece lens combined to form an erecting eyepiece. 
     As illustrated herein by  FIG. 1 , Chen et al discloses in U.S. Pat. No. 5,822,127 (&#39;127), hereby incorporated by reference, the form configured for a helmet display,  100 . The light path begins at the image plane ( 102 ) and is relayed by ( 104 ,  104   a ,  104   b ) forming a real image near a fold mirror ( 106 ). The light is then collimated by ( 108 ,  108   a ) and is observed by the eye ( 122 ) at the conjugate pupil ( 120 ). When the image plane is a reflective display, additional optics are required for illuminating the display. 
     A typical illumination arrangement is illustrated, by Weissman et al., in U.S. Pat. No. 5,984,477 (&#39;477) in  FIG. 2  ( FIG. 1  in the &#39;477 patent). The &#39;477 patent is hereby incorporated by reference. 
     Referring to  FIG. 2  from Weissman et al., a system  200  includes having an image formed on the spatial light modulator (SLM)  202 , is a ferroelectric liquid crystal (FLC) device, and is projected onto the rear projection screen  204  by the lens assembly  206 . The SLM  202  is illuminated by light source  208 , which is focused onto the SLM  202  by lenses  210 ,  212  and  214  via polarizing cube beam splitter (PBS)  216 . The light source  208  may comprise a lamp or an optic fiber cable relaying light from a lamp to lens  210 . The lenses  210 ,  212  and  214  results in a low numerical aperture, collimated beam which is directed normally onto the SLM  202 . 
     The polarizing cube beam splitter  216  reflects light polarized in one direction and transmits light polarized in the orthogonal direction. Light impinging on a pixel in the OFF state is reflected back with the same polarization and re-enters the illumination system via lenses  214 ,  212  and  210 . Light impinging on a pixel in the ON state is reflected back with its plane of polarization rotated 90 degrees, and is, therefore, transmitted by the polarizing beam splitter toward the rear projection screen  204 . Ferroelectric liquid crystal devices and their operation are known in the art. 
     Polarizers  218  and  220  serve to reduce unwanted light reaching the rear projection screen  204  and consequently increase the contrast of the image. The first polarizer reduces the amount of light entering the cube  216 ; however, substantially all of the polarized light entering the cube is reflected by the polarized reflective surface inside cube  216 . Although the reflective surface is fully reflective (and not half reflective) for polarized light, the term “beam splitter” is used since splits non-polarized beams into polarized beams. 
     The image on the rear projection screen  204  is viewed through the cube eyepiece shown generally at  222  by an observer placing his or her eye at viewpoint  224 . The basic eyepiece is formed by beam splitter  226  and the spherical mirror  228  which serves to create a magnified virtual image of the rear projection screen  204  at a relatively large distance from the observer. Lenses  230  and  232  provide color correction while lens  234  helps to achieve uniform brightness at the normal viewing position. 
       FIG. 3  illustrates an embodiment described in U.S. Pat. No. 5,596,451 (&#39;451) by Handschy et al. that illustrates how a cube beam splitter may utilize all 4 sides in an optical design. 
     Illustrated as assembly  300  of  FIG. 3  herein, assembly  300  includes illumination arrangement  302 , spatial light modulator  304 , and an optics arrangement  306 . The optics arrangement  306  includes a first member, specifically a mirror  308  having a curved light reflecting surface  310  which are configured to, in cooperation with other members of optics arrangement  306 , direct light into a predetermined area  312 . Optics arrangement  306  also includes a second member, which in this embodiment is a polarizer-analyzer beam splitting cube  314 , hereinafter referred to as polarizing beam splitting cube  314 , having a plurality of external surfaces or faces. The illumination arrangement  302  is positioned in proximity to and in optical communication with a first external face  316  of cube  314 . If illumination arrangement  302  produces light which is not polarized, an auxiliary polarizer  318  is positioned between illumination arrangement  302  and face  318  of cube  314 . Illumination arrangement  302  can be readily removably attached adjacent to face  318  of cube  312  to allow for replacement or repair of this component, as indicated generally at  320 . Also, spatial light modulator  304  is positioned in proximity to and in optical communication with a second external face  322  of cube  314  and mirror  308  is positioned in proximity to a third face  324  of cube  314  and a quarter wave plate  326  is positioned between mirror  308  and face  324  of cube  48 . In this preferred embodiment of the present invention, mirror  308  and/or spatial light modulator  304  are readily adjustably attached adjacent to face  322  and/or face  324 , respectively, as indicated generally at  328 . This arrangement allows the distance between mirror  308  and face  324  of cube  314  and/or the distance between spatial light modulator  304  and face  322  of cube  314  to be adjusted within a predetermined range of distances thereby providing means for focusing the image generated by the assembly. 
     Polarizing beam splitting cube  314  includes a polarizing beam splitting film or layer  330  positioned within cube  314  such that one side of film  330  faces external faces  316  and  322  of cube  314 , and the other side of film  300  faces external face  324  and a fourth external face  332  of cube  314 . As indicated by lines  334  and  336 , which represent light provided by illumination arrangement  302 , light produced by illumination arrangement  302  is linearly polarized by auxiliary polarizer  318  such that S-polarized light is directed into film  330  within cube  314 . It is to be understood that fines  334  and  336  and all other lines subsequently used to trace light through the assemblies are illustrative only and are not intended to represent a ray trace as is commonly performed in the course of an optical design. It is also to be understood that the term S-polarized light is used in the common manner wherein it specifies that the electric vector of the light incident on a reflective surface is perpendicular to the plane of incidence, in this case the plane of the drawing. 
     Since film  330  is a polarizing beam splitting film, the majority of the S-polarized light  332  is directed into spatial light modulator  304 . Spatial light modulator  304  is a reflective spatial light modulator having a reflective surface and a light modulating medium, in this case a ferroelectric liquid crystal layer, which is switchable between different states. The reflective surface and the modulating medium cooperate to act on light in ways that form an overall pattern of reflected, modulated light, which constitutes a modulation encoding of a picture which may be viewed. For this embodiment, the S-polarized light which is directed into spatial light modulator  304  is modulated by the ferroelectric liquid crystal material such that the overall pattern of reflected, modulated light is a pattern of light of S-polarized light and P-polarized light which is orthogonally polarized to the S-polarized light. At any point in this pattern, the polarization depends on the state of the corresponding pixilated portions of the ferroelectric liquid crystal material through which the S-polarized light from illumination arrangement  302  has passed. Spatial light modulator  304  directs this modulated light back into cube  314  where the light is analyzed by polarizing beam splitting film  330 , as will be described immediately below. 
     The purpose of analyzing the pattern is to decode the polarization modulated pattern and transform it into a brightness modulated pattern which can be viewed and recognized as a display image. As indicated by line  336 , the S-polarized light from illumination arrangement  302  which spatial light modulator does not change, and therefore remains S-polarized light, is directed back toward illumination arrangement  302 . As indicated by line  334 , the S-polarized light from illumination arrangement  302  which spatial light modulator changes to P-polarized light passes through film  330  and is directed toward mirror  308  through quarter wave plate  326 . Mirror  308  reflects light  334  back through quarter wave plate  326  which, since light  334  has passed through quarter wave plate  326  twice, changes light  334  back to S-polarized light. And finally, polarizing beam splitting film  330  directs this S-polarized light out of cube  314  through external face  332  into area  312  which extends outwardly from face  332 . 
     The components of the above described arrangement are mutually disposed and the curvature of mirror  308 , which in this case is a magnifying mirror, is established so as to produce a viewable magnified image of the pattern of modulated light created at and by spatial light modulator  304 . This image is viewable when a viewer places an eye within viewing area  46  which extends outward from the fourth face  66  of cube  48  and when the eye is directed generally toward face  322  of the cube. This viewable image is made luminous by light from illumination arrangement  302  as modulated by the polarization control affected by spatial light modulator  304  in cooperation with polarizing beam splitter film  330  and auxiliary polarizer  318 , if included. 
     While the prior art completes the task of forming a virtual collimated image for a helmet display optical system the components are spread out over a physical large area. Additionally, components such as the polarizing cube beam splitter  216  in the Weissman &#39;477 patent are not used to full advantage as only three of the four available sides are utilized in the optical design. The majority of the lens elements are also used one time. There is a need for a compact optical with its total length, width and height significantly shorter than its optical path. 
     SUMMARY OF THE INVENTION 
     This disclosure provides a novel method of making use of optical elements several times thus reducing the number of elements required for the task of forming a virtual image while at the same time reducing the volume required to complete the same task by doubling back through the space the elements occupy. 
     The claimed invention includes a basic system and four exemplary embodiments. The basic principle of the disclosure is a compact pupil forming optical system where a sum of a length, a width and a height of the optical system is less than an optical path length traversing through or reflecting from a plurality of elements of the optical system. Such a system has a plurality of elements including: a light source; an illumination lens; one or more polarizers; one or more transmitting/reflecting polarizing device(s); a spatial light modulator; one more relay lens(es); one or more first surface or Mangin mirrors; an eyepiece; and having a real external exit pupil from which a virtual image is observed. The optical system uses linearly polarized light outside the one or more relay lenses and circularly polarized light within the one or more relay lenses, or a portion of the one or more relay lenses, thus enabling different optical paths for light entering and exiting the one or more relay lenses. 
     A first optical system includes a light source emitting light that passes through an illumination lens; then the light is polarized by the first linear polarizer; and the polarized light then is incident on transmitting/reflecting polarizing device. A portion of the polarized light is reflected by the transmitting/reflecting polarizing device and is then incident on a spatial light modulator where it is accepted, reflected, and coded by the spatial light modulator on a pixel by pixel basis and exits the spatial light modulator as a first or a second portion of linearly polarized light. The coded light returns to the transmitting/reflecting polarization device where the second portion of polarized light passes through the transmitting/reflecting polarizing device. 
     The second portion of linearly polarized light continues to the second polarizer, a ¼ wave retarder, which circularly polarizes the second portion of light. A second portion of circularly polarized light passes through the second relay lens and is incident on a Mangin/reflecting surface. The Mangin/reflecting surface directs the second portion of circularly polarized light back through the ¼ wave retarder a second time which causes a polarization phase shift of 90 degrees, thus the second portion of light is returned to linearly polarized light. 
     The second portion of linearly polarized light is then passed again to the transmitting/reflecting polarizing device and is reflected by the transmitting/reflecting polarizing device towards and is received by the eyepiece and thus observed by an eye in the viewing area. 
     The second portion of polarized light is “s” polarized light. In a sub-embodiment of the optical system the second portion of polarized light is “p” polarized light. 
     The polarizing transmission/reflecting device may be a polarizing coated flat plate or a polarizing beam splitter cube. 
     Four exemplary embodiments of the disclosure create various optical systems using the principles and methods described above. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following description, appended claims, and accompanying drawing where: 
         FIG. 1  illustrates a device described by Shenker in U.S. Pat. No. 3,432,219; 
         FIG. 2  is an illumination arrangement as illustrated, by Weissman, in U.S. Pat. No. 5,984,477; 
         FIG. 3  illustrates how a cube beam splitter may utilize all four sides in an optical design as described in U.S. Pat. No. 5,596,451 by Handschy et al.; 
         FIG. 4  is a schematic representation of the claimed system; 
         FIG. 5  illustrates a preferred embodiment of an optical system with multiple paths within one or more elements of a compact optical system; 
         FIG. 6  illustrates another embodiment of an optical system with multiple paths within one or more elements of a compact optical system; 
         FIG. 7  illustrates another embodiment of an optical system with multiple paths within one or more of a compact optical system; and 
         FIG. 8  illustrates another embodiment of an optical system with multiple paths within one or more elements of a compact optical system. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to reducing the volume of an optical system by using optical components multiple times and using the compact optical system within a head-mounted display or other applications requiring a compact system with a long optical path. The schematic arrangement of an optical display constructed in accordance with the disclosure is illustrated as system  400  in  FIG. 4 . 
     The system forms a virtual image which is viewed from an external position. The system consists of 4 components or subsystems; 1) an illumination system; 2) a spatial light modulator (SLM); 3) a reflective relay system; 4) a viewing eyepiece. It is the purpose of the illumination system to illuminate the SLM. A real image is formed on the surface of the SLM. The relay lens system reforms an aerial image of the real image. The aerial image is then collimated by the eyepiece, becoming a virtual image. The virtual image is then observed by the eye of an observer. 
     Within such an optical system, polarization is a primary element to be used and applied correctly. Polarized light has application in many fields. As a result several (successful) representations of polarized light have been developed. In the descriptions of our invention we designate an appreciably linearly polarized light state as either “s” or “p”. The said two states are orthogonal to each other and orthogonal to the propagation vector of the light. Any linearly polarizing surface can be oriented in such a manner with respect to the propagation vector of the light as to appreciably absorb, appreciably reflect, or appreciably transmit either “s” or “p”. In  FIGS. 4-8 , a double-headed filled arrow represents either “s” or “p” and a single-headed unfilled arrow represents propagation vector of light. 
     Schematically, the claimed system  400  as illustrated in  FIG. 4  includes a light source  402  emitting light that passes through an illumination lens  404 . The light is then “s” polarized by a linear polarizer  406 . The “s” light enters a transmitting/reflecting polarizing device  408  and is incident on the first surface of a transmitting/reflecting polarizing device  408 . The polarizing surface of transmitting/reflecting polarizing device  408  is such that the “s” light is reflected by the surface and “p” light is transmitted by the surface. The reflected “s” light is then incident on a SLM  410 . The “s” light is accepted, reflected, and coded by the SLM  410  on a pixel by pixel basis forming an image. The light from each pixel exits SLM  410  as “s” or “p” polarized light depending on the electrically switched state of each pixel. The light then returns to the transmitting/reflecting polarizing device  408 . The “s” polarized portion of the light is reflected back to the source while a “p” polarized portion of the light passes through the transmitting/reflecting polarizing device  408  and continues to a ¼ wave retarder  412  which circularly polarizes the incident linearly polarized light with preference to handedness (right-handedness or clockwise, left-handedness or counter-clockwise). The light then enters a relay lens  414  forming internal pupil  415  within the relay lens  414 . Then the circularly polarized light is reflected by a mirror  416  that reverses the direction of light and reverses the handedness of the circular polarization. The light travels back through the relay lens  414 , and passes through the ¼ wave retarder  412  which now converts the circularly polarized light into “s” linearly polarized light. The “s” polarized light is then reflected by the second surface of the transmitting/reflecting polarizing device  408  and an aerial image  417  is formed. Eyepiece  418  is placed such that it forms a real exit pupil  420 , which is a real image of the internal pupil  415 , while collimating an aerial image  417  at a comfortable viewing distance from real exit pupil  420 . 
     The illumination system can be as simple as an extended diffused light source or it can be a combination of a light source and a single or multiple lens system. The linear polarizer element can be an integral part of the light source or can be a separate external element placed before the light is incident on transmitting/reflecting polarizing device  408 . The illumination system must fill the SLM  410  with polarized light and impart the correct angles to the incident light such that the SLM  410  appears to be illuminated from all points within the real exit pupil  420 . 
     When the SLM  410  is a Ferroelectric Liquid Crystal Display (FLCD) the pixels are coded in both the time and color domain. When coded this way the switching is done at a high rate for gray scale and at least 3 times per frame rate for Red, Green, and Blue colors. In the past rotating color filter wheels were synchronized to the red, green and blue video frames and while this has worked with HMDs the bulk and complexity become problematic. The next advance was electro-optical color switches such as those manufactured by Color Link in Colorado that use color selective polarization switching to synchronize the color to the video frame. Currently red green and blue LED&#39;s are switched in synchronization to the video frames. Devices such as the Alphalight manufactured by Teledyne and the OSTAR manufactured by Osram are used to illuminate the field sequential SLMs. The first use of the transmitting/reflecting polarizing device  408  is to direct the light from the source to the SLM  410 . The transmitting/reflecting polarizing device  408  is also used to de-code the image formed on the SLM  410  and to direct the light to the relay lens  414  and again to direct the light exiting the relay lens  414  towards the eyepiece  418 . Since the output of the transmitting/reflecting polarizing device  408  viewed by the eyepiece  418  is opposite the side facing the light source, the efficiency of the polarizing surface of the transmitting/reflecting polarizing device  408  must be very high else the light source will be directly viewed by the observer at the real exit pupil  420  thus reducing the contrast of the image. To improve or maximize the image-contrast, the transmitting/reflecting polarizing device  408  must reflect all or as much as possible the linearly polarized light coming from the light source, toward the SLM  410 . 
     The function of the transmitting/reflecting polarizing device  408  can be achieved in various ways. Polarizing beam splitter (PBS) cubes suitable for the purpose are manufactured by Foreal Spectrum and by Unaxis Optics. A suitable PBS is manufactured using a Moxtek, Orem Utah, product called Proflux by bonding two Proflux wire grid polarizers together with the wire grids parallel and with a linear sheet polarizer between the two Proflux elements. The sheet polarizer is oriented to pass the polarization state nominally passed by the Proflux elements and absorb the state nominally reflected by the Proflux elements. Proflux is a transmitting/reflecting polarizing device meaning that randomly polarized light incident will be transmitted linearly polarized and reflected linearly polarized with an axis ninety degrees from the transmitted vector. Another structure that can polarize light in a similar way has been developed by Rolic in Switzerland. In this device photo alignment is used to align a liquid crystal structure after which the structure is solidified resulting in a transmitting/reflecting polarizing device. Still another method has been developed by 3M and is sold under the trade name of DEBAF. The 3M material uses the effect of bi-refringent and uniform index materials combined with Fresnel reflection to form a transmitting/reflecting polarizing device. PBS cubes can be made from any of these materials with varying results. 
     Before being incident on the reflective surface, within the relay lens, the linearly polarized light is circularly polarized with use of a ¼ wave retarder element. The material used for the retarder element is a stretched polymer which is sold by Nitto Japan and by Polotechnu also of Japan and a material sold by Farrand Optical New York. The purpose of circularly polarizing the light is to effect a 90 degree rotation of the polarization direction between the light leaving the transmitting/reflecting polarizing device  408 , going to the mirror  416 , and the light entering the transmitting/reflecting polarizing device  408 , coming from the reflecting surface of the mirror  416 . While ¼ wave retardation is used, use of ¾ wave retardation or any multiple which will result in a 90 degree rotation of the polarization direction as previously explained may be used. 
     The relay lens  414  can be thought of as consisting of two halves “folded” about a reflecting surface  416 . The first half roughly collimates the image and the second half reforms the image. Within the relay lens  414  first half, a pupil is formed at or near the reflecting surface. This internal pupil in combination with the second half of the relay lens and the eyepiece will be reimaged as a real exit pupil forming to form the viewing area. The detailed geometric requirements of forming the conjugate pupil and correcting the aberrations are well known to those skilled in the art of lens design. However, the overview is to re-image the internal pupil at the viewing area while re-imaging the aerial image at or near infinity. Thus the solution is to place the aerial image at the focus of the eyepiece and to place the pupil, within the relay, at a greater distance from the eyepiece typically two times the eyepiece focal length. 
     Optical system  500  of  FIG. 5  illustrates the light path of a preferred embodiment of the claimed optical system. A light source  502  emits light that passes through an illumination lens  504 . The light is then “s” polarized by polarizer  506 . The “s” polarized light enters a transmitting/reflecting polarizing device  508  and is incident on the beam splitter coating such that the “s” polarized light is reflected by the polarizing surface  509 . The light exits the transmitting/reflecting polarizing device  508  and is incident to SLM  510 . The light is accepted, reflected, and coded by SLM  510  on a pixel by pixel basis and the light from each pixel exits the SLM  510  as ether “s” or “p” polarized light. The light then returns to the transmitting/reflecting polarizing device  508  and is again incident on polarizing surface  509 . The portion of the light that is “s” polarized is reflected towards the light source  502 . The portion of the light that is “p” polarized passes through  509  and exits the transmitting/reflecting polarizing device  508 . The “p” polarized light continues to a ¼ wave retarder  512  which circularly polarizes the incident linearly polarized light with preference to handedness (right-handedness or clockwise; left-handedness or counter-clockwise). The light then passes through a relay lens  514  and is incident on a first Mangin reflecting surface  516  that reverses the direction of light and reverses the handedness of the circular polarization. The light travels back through the relay lens  514 , and passes through the ¼ wave retarder  512 , which now converts the circularly polarized light into “s” linearly polarized light. The light then reenters the transmitting/reflecting polarizing device  508  and continues to the polarizing surface  509  which reflects the “s” polarized light. The light exits the transmitting/reflecting polarizing device  508  and is received by a field lens  518 . The light passes through the field lens  518  and is incident upon a second transmitting/reflecting polarizing device  520  with its polarizing beam splitting surface  521  rotated 90 degrees about propagation vector of the incident light with respect to the prior polarizing beam splitting surface  509  for passing the incident polarized light. The light then exits the second transmitting/reflecting polarizing device  520 , and is incident on a second ¼ wave retarder  522  which circularly polarizes the light with preference to handedness (right or left). The light then enters lens  524 , and is reflected by a second Mangin surface  525  which reverses the direction of propagation of light and also reverses the handedness (left or right) of the circularly polarized light. The circularly polarized light is returned to the second ¼ wave retarder  522  which converts the light into linearly polarized light with polarization direction orthogonal to the one that first entered the second ¼ wave retarder  522 . The light then again enters the second transmitting/reflecting polarizing device  520 , this time to be reflected by the second polarizing beam splitter surface  521 . Exiting the second transmitting/reflecting polarizing device  520 , the light forms real exit pupil  526  for observing the collimated virtual image at or near infinity. Randomly polarized light  527  enters the optical system through the second transmitting/reflecting polarizing device  520 . When incident on the second polarizing beam splitter surface  521  the light is polarized into two orthogonal directions. One orthogonal polarized light component is directed towards field lens  518  and the other orthogonal polarized light component continues toward real exit pupil  526  for viewing. 
     As discussed above, the first transmitting/reflecting polarizing device  508  and/or the second transmitting/reflecting polarizing device  520  may be PBS cube or wire grid polarizing flat plate type to accomplish all the functions of the beam splitting surfaces  509  and/or  521 . 
       FIG. 6  illustrates the light path of another embodiment  600  of the system. In this embodiment a light source  602  emits light which passes through illumination lens  604  and is polarized by a linear polarizer  606  such that “p” polarized light enters a transmitting/reflecting polarizing device  608 . The light passes through the beam splitter coating  610  and exits the transmitting/reflecting polarizing device  608  on the side opposite the light source  602 . The light is then incident on the SLM  612 . The light is accepted, reflected and coded by the SLM  612  on a pixel by pixel basis forming an image. The light from each pixel exits the SLM  612  as either “s” or “p” polarized light. The light returns to the transmitting/reflecting polarizing device  608  and is incident on the beam splitter coating surface  610  where the light coded as “p” polarized light passes through while light coded as “s” is redirected by reflection. The redirected “s” polarized light exits the transmitting/reflecting polarizing device  608  and passes through a ¼ wave plate  614  which circularly polarizes the light with preference to handedness (right or left). The light then continues into a relay lens  616  and encounters a Mangin mirror  618  which reverses the direction of propagation of light and also reverses the handedness (left or right) of the circularly polarized light. The circularly polarized light then passes through the relay lens  616  in a reversed direction, exits relay lens  616  and passes through the ¼ wave plate  614  which now converts the circularly polarized light to “p” linearly polarized light. The light reenters the transmitting/reflecting polarizing device  608  and is incident on the beam splitting surface  610  which passes the “p” polarized light. The light exits the transmitting/reflecting polarizing device  608  opposite the relay lens  616  and continues to eyepiece  620 . The eyepiece collimates the light and forms a real exit pupil  622  for viewing the virtual image at or near infinity. As discussed above, transmitting/reflecting polarizing device  608  can be a polarizing flat plate or a PBS cube to accomplish all the functions of the beam splitting surface  610 . 
       FIG. 7  illustrates the light path of another embodiment  700  of the system. A light source  702  emits light that passes through an illumination lens  704 . The light is then “s” polarized by polarizer  706 . The “s” polarized light enters a first transmitting/reflecting polarizing device  708  and is incident on a first beam splitter coating  709 . The coating is such that the “s” polarized light is reflected by the coating. The light exits the first transmitting/reflecting polarizing device  708  and is incident on the SLM  710 . The light is accepted, reflected, and coded by the SLM  710  on a pixel by pixel basis and the light from each pixel exits the SLM  710  as either “s” or “p” polarized light. The light then returns to the first transmitting/reflecting polarizing device  708  and is again incident on the first beam splitter coating  709 . The portion of the light that is “s” polarized is reflected towards the light source  702 . The portion of the light that is “p” polarized passes through beam splitting coating  709  and again exits the first transmitting/reflecting polarizing device  708 . The “p” polarized light continues to a first ¼ wave retarder  712  which circularly polarizes the light with preference to handedness (right or left). The light then passes through a first relay lens  714  and is incident on a first Mangin reflecting surface  716  which reverses the direction of propagation of light and also reverses the handedness (left or right) of the circularly polarized light. The light is directed back through the first relay lens  714  by the first Mangin reflecting surface  716 , and then passes through the first ¼ wave retarder  712  which now converts the circularly polarized light to “s” polarized light. The light then reenters the first transmitting/reflecting polarizing device  708  and again continues to the beam splitter coating  709  which reflects the “s” polarized light. The light exits the first transmitting/reflecting polarizing device  708  and is received by a field lens  718 . The light passes through the field lens  718  and is incident on a ½ wave plate  719 , which causes the polarization direction to rotate 90 degrees. 
     The light then enters a second transmitting/reflecting polarizing device  720  with the second beam splitting surface  721  rotated 90 degrees about propagation vector of the incident light with respect to the prior beam splitting surface  709  so that the incident linearly polarized light is reflected by the beam splitting surface  721  towards a second ¼ wave retarder  722 . The light is then incident on the second ¼ wave retarder  722  and is circularly polarized with preference to handedness (right or left). The light then enters lens  724 , is partially reflected by a second Mangin reflecting surface  725  which also reverses the handedness of the circularly polarized light. The circularly polarized light is returned to the second ¼ wave retarder  722  which converts the light to linearly polarized light with polarization direction orthogonal to the one that first entered the second ¼ wave retarder  722 . The light reenters the second transmitting/reflecting polarizing device  720  and is transmitted by beam splitter surface  721 . Exiting the second transmitting/reflecting polarizing device  720 , the light forms real exit pupil  728  for observing the collimated virtual image at or near infinity. 
     Randomly polarized light (outside system light)  727  enters the optical system through lens  726  which has an equal and opposite power as that of lens  724 . The outside system light then enters lens  724  through the partially transmitting second Mangin reflecting surface  725  and continues through the second ¼ wave retarder  722 . The outside system light then enters the second transmitting/reflecting polarizing device  720 . When incident on the beam splitting surface  721 , the outside system light is linearly polarized into two orthogonal directions. One orthogonal linearly polarized light component is directed towards a ½ wave plate  719 . The second orthogonal linearly polarized component continues toward the real exit pupil  728  for viewing. 
     As discussed above, the transmitting/reflecting polarizing device  708  and/or the transmitting/reflecting polarizing device  720  may be a polarizing flat plate or a PBS cube to accomplish all the functions of the beam splitting surfaces  709  and/or  721 . 
       FIG. 8  illustrates the light path of an embodiment  800  very similar to the embodiment  700  of  FIG. 7  described above. The main difference is in the use of polarization. A light source  802  emits light that passes through an illumination lens  804 . The light is then “s” polarized by polarizer  806 . The “s” polarized light enters a first transmitting/reflecting polarizing device  808  and is incident on a first beam splitter coating  809 . The coating is such that the “s” polarized light is reflected by the beam splitter coating  809  surface. The light exits the first transmitting/reflecting polarizing device  808  and is incident on the SLM  810 . The light is accepted, reflected, and coded by the SLM  810  on a pixel by pixel basis and the light from each pixel exits the SLM  810  as either “s” or “p” polarized light. The light then returns to first transmitting/reflecting polarizing device  808  and is again incident on the first beam splitter coating  809 . The portion of the light that is “s” polarized is reflected towards the light source  802 . The portion of the light that is “p” polarized passes through the beam splitting coating  809  and again exits the first transmitting/reflecting polarizing device  808 . The “p” polarized light continues to a first ¼ wave retarder  812  which circularly polarizes the light with preference to handedness (right or left). The light then passes through a first relay lens  814  and is incident on a first Mangin reflecting surface  816  which reverses the direction of propagation of light and also reverses the handedness (left or right) of the circularly polarized light. The light is then directed back through the relay lens  814  by Mangin reflecting surface  816 , and then passes through the ¼ wave retarder  812  which now converts the circularly polarized light to “s” polarized light. The light then reenters the first transmitting/reflecting polarizing device  808  and continues to the first beam splitter coating  809  which reflects the “s” polarized light. The light exits first transmitting/reflecting polarizing device  808  and is received by field lens  818 . The light passes through field lens  818  and enters a second transmitting/reflecting polarizing device  820  with a second beam splitting surface  821  rotated 90 degrees about propagation vector of the incident light with respect to the prior beam splitting surface  809  so that the incident linearly polarized light is reflected by the beam splitting surface  821  towards a second ¼ wave retarder  822 . The light is then incident on the second ¼ wave retarder  822  and is circularly polarized with preference to handedness (right or left). The light then enters lens  824 , is partially reflected by a second Mangin reflecting surface  825  which also reverses the handedness of the circularly polarized light. The circularly polarized light is returned to the second ¼ wave retarder  822  which converts the light to linearly polarized light with polarization direction orthogonal to the one that first entered the second ¼ wave retarder  822 . The light reenters the transmitting/reflecting polarizing device  820  and is transmitted by the beam splitter surface  821 . Exiting the second transmitting/reflecting polarizing device  820 , the light forms real exit pupil  828  for observing the collimated virtual image at or near infinity. 
     Randomly polarized light (outside system light)  827  enters the optical system through lens  826  which has an equal and opposite power as that of lens  824 . The light then enters lens  824  through the partially transmitting second Mangin reflecting surface  825  and continues through to the second ¼ wave retarder  822 . The light then enters the second transmitting/reflecting polarizing device  820 . When the outside system light is incident on the second beam splitting surface  821 , the light is linearly polarized into two orthogonal directions. One orthogonal linearly polarized light component is directed towards the field lens  818 . The second orthogonal linearly polarized component continues toward real exit pupil  828  for viewing. As discussed above, the transmitting/reflecting polarizing device  808  and/or the transmitting/reflecting polarizing device  820  can be replaced by a polarizing flat plate or PBS cube to accomplish all the functions of the beam splitting surfaces  809  and/or  821 . 
     The schematic arrangement illustrated by  FIG. 4  and the embodiments illustrated in  FIGS. 5 ,  6 ,  7 , and  8  have the unique feature of having a total length, width and height dimension substantially less than the total optical length from the light source to the viewing area. Such a feature in a compact optical system permits smaller head mounted displays and reduces the weight of such displays. 
     The foregoing is provided for the purpose of illustrating, explaining and describing embodiments of the present disclosure. Further modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the spirit of the disclosure or the scope of the following claims.