Patent Publication Number: US-11656452-B2

Title: Apparatus for optical see-through head mounted display with mutual occlusion and opaqueness control capability

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/558,241, filed on Sep. 2, 2019, which is a continuation of U.S. patent application Ser. No. 16/196,886, filed on Nov. 20, 2018 now U.S. Pat. No. 10,451,883, which is a division of U.S. patent application Ser. No. 15/977,593, filed on May 11, 2018 now U.S. Pat. No. 10,175,491, which is a division of U.S. patent application Ser. No. 15/833,945, filed on Dec. 6, 2017 now U.S. Pat. No. 10,048,501, which is a division of U.S. patent application Ser. No. 15/607,335, filed on May 26, 2017 now U.S. Pat. No. 9,874,752, which is a division of U.S. patent application Ser. No. 15/277,887, filed on Sep. 27, 2016 now U.S. Pat. No. 9,726,893, which is a division of U.S. patent application Ser. No. 13/857,656, filed on Apr. 5, 2013 now U.S. Pat. No. 9,547,174, which claims priority to U.S. Provisional Application No. 61/620,574, filed on Apr. 5, 2012 and U.S. Provisional Application No. 61/620,581, filed on Apr. 5, 2012, the disclosures of which are incorporated herein by reference in their entirety. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     This invention was partially made with government support under SBIR contract No. W91CRB-12-C-0002 awarded by the U.S. ARMY. The government has certain rights in the invention. 
    
    
     FIELD OF THE TINE INVENTION 
     The present invention relates generally to Head Mounted Displays, and more particularly, but not exclusively, to optical see-through head-mounted displays with opaqueness control and mutual occlusion capability in which real objects may be occluded by computer-rendered virtual objects situated in front or vice versa. 
     BACKGROUND OF THE INVENTION 
     Over the past decades, Augmented Reality (AR) technology has been applied in many application fields, such as medical and military training, engineering design and prototyping, tele-manipulation and tele-presence, and personal entertainment systems. See-through Head-Mounted Displays (ST-HMD) are one of the enabling technologies of an augmented reality system for merging virtual views with a physical scene. There are two types of ST-HMDs: optical and video (J. Rolland and H. Fuchs, “Optical versus video see-through head mounted. displays,” In Fundamentals of Wearable Computers and Augmented Reality, pp. 113-157, 2001.). The major drawbacks of the video see-through approach include: degradation of the image quality of the see-through view; image lag due to processing of the incoming video stream; potentially loss of the see-through view due to hardware/software malfunction. In contrast, the optical see-through HMD (OST-HMD) provides a direct view of the real world through a beamsplitter and thus has minimal affects to the view of the real world. It is highly (preferred in demanding applications where a user&#39;s awareness to the live environment is paramount. 
     Developing optical see-through HMDs, however, confronts complicated technical challenges. One of the critical issues lies in that the virtual views in an OST-HMD appear “ghost-like” and are floating in the real world due to the lack of the occlusion capability.  FIG.  1    shows a comparison illustration of the augmented view seen through a typical OST-HWID ( FIG.  1   a   ) and the augmented view seen through an occlusion capable OST-HMD (OCOST-HMD) system ( FIG.  1   b   ), In the figure, a virtual car model is superimposed on a solid platform which represents a real object. Without proper occlusion management as shown in  FIG.  1   a   , in a typical AR view, the car is mixed with the platform and it is difficult to distinguish the depth relationship of the car and the platform. On the contrary, with proper occlusion management as shown in  FIG.  1   b   , the car blocks a portion of the platform and it can be clearly identified that the car seats on the top of the platform. Adding occlusion capability to the AR display enables realistically merging virtual objects into the real environment. Such occlusion-enabled capability may generate transformative impacts on AR display technology and is very appealing for many augmented-reality based applications. 
     An OCOST-HMD system typically comprises of two key sub-systems. The first is an eyepiece optics that allows a user to see a magnified image displayed on a microdisplay; and the second is a relay optics that collects and modulates the light from an external scene in the real world, which enables the opaqueness and occlusion control on the external scene when presenting to the viewers. The key challenges of creating truly portable and lightweight OCOST-HMD system lies in addressing three cornerstone issues: (1) an optical scheme that allows the integration of the two subsystems without adding significant weight and volume to the system. (2) a proper optical method that maintains the parity of the coordinate system of the external scene; (3) an optical design method that enables the design of these optical subsystems with an elegant form factor, which has been a persisting dream for HMD developers. Several occlusion-capable optical ST-HMD concepts have been developed (U.S. Pat. No. 7,639,208 B1 − Kiyokawa, K., Kurata, Y., and Ohno, H., “An Optical See-through Display for Mutual Occlusion with a Real-time Stereo Vision System,” Elsevier Computer &amp; Graphics, Special Issue on “Mixed Realities—Beyond. Conventions,” Vol, 25, No. 5, pp. 2765-779, 2001. K. Kiyokawa, M, Billinghurst, B. Campbell, E. Woods, “An Occlusion-Capable Optical See-through Head Mount Display for Supporting Co-located Collaboration,” ISMAR 2003, pp, 133-141). For example, Kiyokawa et. al. developed ELMO series occlusion displays using conventional lenses, prisms and minors. Not only because of the number of elements being used, but also more importantly due to the rotationally symmetric nature of the optical systems, the existing occlusion-capable OST-HMDs have a helmet-like, bulky form factor. They have been used exclusively in laboratory environments due to the heavy weight and cumbersome design. The cumbersome, helmet-like form factor prevents the acceptance of the technology for many demanding and emerging applications. 
     SUMMARY OF THE INVENTION 
     This invention concerns an optical see-through head mounted display (OST-HMD) device with opaqueness control and mutual occlusion capability, The display system typically comprises of a virtual view path for viewing a displayed virtual image and a see-through path for viewing an external scene in the real world. In the present invention, the virtual view path includes a miniature image display unit for supplying virtual image content and an eyepiece through which a user views a magnified virtual image. The see-through path comprises of an objective optics to directly capture the light from the external scene and firm at least one intermediate image, a spatial light modular (SLM) placed at or near an intermediate image plane in the see-through path to control and modulate the opaqueness of the see-through view, and an eyepiece optics through which the modulated see-through view is seen by the viewer. In the see-through path, the objective optics and eyepiece together act as a relay optics for passing the light from the real world to viewer&#39;s eye. To achieve a compact form factor and reduce the viewpoint offset, the see-through path is folded into two layers through several reflective surfaces, a front layer accepting the incoming light from an external scene and a back layer coupling the light captured by the front layer into a viewer&#39;s eye. The see-through path is merged with the virtual image path by a beamsplitter so that the same the eyepiece is shared by both paths for viewing displayed virtual content and the modulated see-through image. The microdisplay and the SLM are optically conjugate to each other through the beamsplitter, which makes the pixel level occlusion manipulation possible. In the present invention, the eyepiece, the objective optics, or both may be rotationally symmetric lenses or non-rotationally symmetric freeform optics. In one of its significant aspects, the present invention may utilize freeform optical technology in eyepiece optics, objective optics or both to achieve a compact and lightweight OCOST-HMD design. 
     The reflective surfaces for folding the optical paths may be planar mirrors, spherical, aspherical, or freeform surfaces with optical power. In another significant aspect of the present invention, some of the reflective surfaces may utilize freeform optical technology. Some of the reflective surfaces may also be strategically designed to be an integral part of the eyepiece or objective optics where the reflective surfaces not only facilitate the folding of the optical path for achieving compact display design but also contribute optical power and correct optical aberrations. In an exemplary configuration, the present invention may use a one-reflection or multi-reflection freeform prism as an eyepiece or objective optics where the prism is a single optical element comprises of refractive surfaces and one or more than one reflective surfaces for folding the optical path and correcting aberrations. 
     In another significant aspect of the present invention, the objective optics in the see-through path forms at least one accessible intermediate image, near which an SLM is placed to provide opaqueness control and see-through modulation. In the present invention, either a reflection-type SLM or a transmission-type SLM may be used for modulating the see-through view for occlusion control. A longer back focal distance for the objective optics is required for a reflection-type SLM than a transmission-type SLM. A reflection-type SLM may have the advantage of higher light efficiency than a transmission-type SLM. 
     In another significant aspect of the present invention, the see-through path may form an odd or even number of intermediate images. In the case of an odd number of intermediate images, an optical method is provided to invert and/or revert the see-through view in the see-through path. For example, depending on the number of reflections involved in the see-through path, examples of the possible methods include, but not limited to, inserting an additional reflection or reflections, utilizing a roof mirror surface, or inserting an erection prism or lens. In the case of an even number of intermediate images, no image erection element is needed if there is no parity change in the see-through view. For instance, multiple-reflection freeform prism structure (typical more than 2) may be utilized as eyepiece or objective optics, or both, which allow folding the see-through optical path inside the objective and/or eyepiece prism multiple times and form intermediate image(s) inside the prisms which eliminates the necessity of using an erection roof reflective surface. The potential advantage of eliminating the erection prism is that the approach may lead to a more compact design. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary and the following detailed description of exemplary embodiments of the present invention may be further understood when read in conjunction with the appended drawings, in which: 
         FIGS.  1   a  and  1   b    schematically illustrate AR views seen through an optical see-through HMD: without occlusion capability ( FIG.  1   a   ) and with occlusion capability ( FIG.  1   b   ). 
         FIGS.  2   a  and  2   b    schematically illustrate an exemplary optical accordance with the present invention shown as a monocular optical module. 
         FIG.  3    schematically illustrates a preferred embodiment in accordance with the present invention based on freeform optical technology. The embodiment comprises of a one-reflection eyepiece prism, a one-reflection objective prism, a reflection-type SLM and a roof reflective surface. 
         FIG.  4    schematically illustrates another preferred embodiment in accordance with the present invention based on freeform optical technology. The embodiment comprises of a two-reflection eyepiece prism, a four-reflection objective prism, and a reflection-type SLM. 
         FIG.  5    schematically illustrates another preferred embodiment in accordance with the present invention based on freeform optical technology. The embodiment comprises of a two-reflection eyepiece prism, a one-reflection objective prism, a transmission-type SLM and a roof reflective surface. 
         FIG.  6    schematically illustrates another preferred embodiment in accordance with the present invention based on freeform optical technology. The embodiment comprises of a two-reflection eyepiece prism, a three-reflection objective prism and a transmission-type SLM. 
         FIG.  7    schematically illustrates another preferred embodiment in accordance with the present invention based on freeform optical technology. The embodiment comprises of a two-reflection eyepiece prism, a two-reflection objective prism, a reflection-type SLM and a relay lens. 
         FIG.  8    schematically illustrates an exemplary design of an OCOST-HMD system in accordance with the present invention based on an exemplary layout in  FIG.  3   . 
         FIG.  9    illustrates the field map plot of the polychromatic modulation transfer functions (MTF) of the virtual display path of the design in  FIG.  8    at cutoff frequency 401 ps/min (line pairs per millimeter) evaluated using 3 min pupil diameter. 
         FIG.  10    schematically illustrate an exemplary design of an OCOST-HMD system in accordance with the present invention based on an exemplary layout in  FIG.  3    with the eyepiece and objective optics having identical freeform structure. 
         FIG.  11    illustrates the field map plot of the polychromatic modulation transfer functions (MTF) of the virtual display path of the design in  FIG.  10    at cutoff frequency 401 ps/mm (line pairs per millimeter) evaluated using 3 mm pupil diameter. 
         FIG.  12    depicts a block diagram of an example of an image processing pipeline in accordance with the present invention. 
         FIG.  13    shows Table 1: Optical surface prescription of surface  1  of the eyepiece prism. 
         FIG.  14    shows Table 2: Optical surface prescription of surface  2  of the eyepiece prism. 
         FIG.  15    shows Table 3: Optical surface prescription of surface  3  of eyepiece prism. 
         FIG.  16    shows Table 4: Position and orientation parameters of the eyepiece prism. 
         FIG.  17    shows Table 5: Optical surface prescription of surface  4  of the objective prism. 
         FIG.  18    shows Table 6: Optical surface prescription of surface  5  of the objective prism. 
         FIG.  19    shows Table 7: Optical surface prescription of surface  6  of the objective prism. 
         FIG.  20    shows Table 8: Position and orientation parameters of the objective prism. 
         FIG.  21    shows Table 9: Surface parameters for DOE plates  882  and  884 . 
         FIG.  22    shows Table 10: Optical surface prescription of surface  1  of the freeform prism. 
         FIG.  23    shows Table 11: Optical surface prescription of surface  2  of the freeform prism. 
         FIG.  24    shows Table 12: Optical surface prescription of surface  3  of the freeform prism. 
         FIG.  25    shows Table 13: Position and orientation parameters of the freeform prism as the eyepiece. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments according to the present invention will be fully described with respect to the attached drawings. The descriptions are set forth in order to provide an understanding of the invention. However, it will be apparent that the invention can be practiced without these details. Furthermore, the present invention may be implemented in various forms. However, the embodiments of the present invention described below shall not be constructed as limited to the embodiments set forth herein. Rather, these embodiments, drawings and examples are illustrative and are meant to avoid obscuring the invention. 
     An occlusion capable optical see-through head-mounted display (OCOST-HMD) system typically comprises of a virtual view path for viewing a displayed virtual image and a see-through path for viewing an external scene in the real world. Hereafter the virtual image observed through the virtual view path is referred to as the virtual view and the external scene observed through the see-though path is referred to as the see-through view. In some embodiments of the present invention, the virtual view path includes a microdisplay unit for supplying virtual image content and an eyepiece through which a user views a magnified virtual image. The see-through path comprises of an objective optics to capture the light from the external scene and form at least one intermediate image, a spatial light modular (SLM) placed at or near an intermediate image plane in the see-through path to control and modulate the opaqueness of the see-through view, and an eyepiece through which the modulated see-through view is seen by the viewer. In the see-through path, the objective optics and eyepiece together act as a relay optics for passing the light from the real world to viewer&#39;s eye. The intermediate image in the see-through path is referred to as a see-through image, and an intermediate image modulated by the SLIM is referred to as a. modulated see-through image. An OCOST-HMD produces a combined view of the virtual and see-through views, in which the virtual view occludes portions of the see-through view. 
     A some embodiment, the present invention comprises a. compact optical see-through head-mounted display  200 , capable of combining a see-through path  207  with a virtual view path  205  such that the opaqueness of the see-through path can be modulated and the virtual view occludes parts of the see-through view and vice versa, the display comprising:
         a. a microdisplay  250  for generating an image to be viewed by a  − user, the microdisplay having a virtual view path  205  associated therewith;   b. a spatial light modulator  240  for modifying the light from an external scene in the real world to block portions of the see-through view that are to be occluded, the spatial light modulator having a see-through path  207  associated therewith;   c. an objective optics  220  configured to receive the incoming light from the external scene and to focus the light upon the spatial light modulator  240 ;   d. a beamsplitter  230  configured to merge a virtual image from a microdisplay  250  and a modulated see-through image of an external scene passing from a spatial light modulator, producing a combined image;   e. an eyepiece  210  configured to magnify the combined image;   f. an exit pupil  202  configured to face the eyepiece, where the user observes a combined view of the virtual and see-through views in which the virtual view occludes portions of the see-through view;   g. a plurality of reflective surfaces configured to fold the virtual view path  205  and see-through paths  207  into two layers.       

     In some embodiments, at least three reflective surfaces are used to fold the virtual and see-through paths into two layers. The first reflective surface (M 1 ) is located upon the front layer of the display oriented to reflect light from the external scene. The objective optics  220  is located upon the front layer of the display. The second reflective surface (M 2 ) is located upon the front layer of the display oriented to reflect light into the spatial light modulator. The spatial light modulator  240  is located at or near an intermediate image plane of the see-through path  207 , in optical communication with the objective optics  220  and the eyepiece  210  through the beam splitter  230  along the see-through path  207 . The microdisplay  250  is located at the focal plane of the eyepiece  210 , in optical communication with the eyepiece  210  through the beamsplitter  230  along the virtual view path  205 . The beam splitter  230  is oriented such that the see-through path  207  is merged with virtual view path  205  and the light from both the see-through path and the virtual view path is directed to the eyepiece  210 . The eyepiece  210  is located upon the back layer of the display. The third reflective surface (M 3 ) is located upon the back layer of the display oriented to reflect light from the eyepiece into the exit pupil  202 . 
     In some embodiments, the objective optics  220  receives tight of the external scene, and focuses the light of the external scene and forms a. see-through image upon the spatial light modulator  240 . The spatial light modulator  240  modifies the see-through image to remove portions of the image that are to be occluded. The microdisplay  250  projects a virtual image to the beam splitter  230 . The spatial light modulator  240  transmits the modified see-through image to the beam splitter  230 , where the beam splitter  230  merges the two images producing a combined image in which the virtual image occludes portions of the see-through image. The beam splitter  230  then projects the combined image to the eyepiece  210 , whereupon the eyepiece projects the image to the exit pupil  202 . 
     In some embodiments, the present invention comprises of an optical see-through head-mounted display  200 , capable of combining an external scene in the real world with a virtual view, where the opaqueness of the external scene is modulated and the digitally generated virtual view occludes parts of the external scene and vice versa. The invention comprises, a microdisplay  250  which transmits a virtual image, a spatial light modulator  240  for modifying the light from an external scene, an objective optics  220 , which captures an external scene, a beamsplitter  230  configured to merge the digitally generated virtual image from the microdisplay  250  with the modified external scene from the spatial light modulator, an eyepiece  210  magnifying the virtual image and the modified external scene and an exit pupil  202  where the user observes a combined view of the virtual image and the modified external scene. 
     In some embodiments, at least three reflective surfaces are used to fold the virtual view path  205  and the see-through path  207  into two layers. The objective optics  220  is located on the front layer of the display, while the eyepiece  210  is located on the back layer of the display. A series of mirrors may be used to guide light along the optical paths through the spatial light modulator, beam splitter and eyepiece. The spatial light modulator  240  is located at or near an intermediate image plane in the see-through path. The microdisplay  250  faces the beam splitter  230 , so that light from the microdisplay is transmitted into the beam splitter  230 . The beam splitter  230  combines light from the microdisplay and the spatial light modulator and is oriented such that the direction of light transmission from the beam splitter is facing the eyepiece  210 . The eyepiece  210  is located so that the light from the beam splitter passed through the eyepiece and is transmitted into the exit pupil. 
     In some embodiments, the objective optics  220  receives an image of the external scene, and reflects or refracts the image to the spatial light modulator  240 . The spatial light modulator  240  modifies the light from the external scene to remove portions of the image that are to be occluded, and transmits or reflects the light into the beam splitter. The microdisplay  250  transmits a virtual image to the beam splitter  230 , and the beam splitter  230  merges the two images producing a combined image in which the virtual image  205  occludes portions of the image of the external scene. The beam splitter  230  projects the combined image to the eyepiece  210 , which passes the image to the exit pupil  208 . Thus the user observes the combined image, in which the virtual image appears to occlude portions of the external scene. 
       FIG.  2    illustrates a schematic layout  200  in accordance with the present invention for achieving a compact OCOST-HMD system. In this exemplary layout  200 , the virtual view path.  205  (illustrated in dash lines) represents the light propagation path of the virtual view and comprises of a microdisplay  250  for supplying display content and eyepiece  210  through which a user views a magnified image of the displayed content; the see-through path  207  (illustrated in solid lines) represents the light propagation path of the see-through view and comprises of both objective optics  220  and eyepiece  210  acting as a relay optics for passing the light from an external scene in the real world to viewer&#39;s eye. To achieve a compact form factor and reduce the viewpoint offset, the see-through path  207  is folded into two layers in front of the viewer&#39;s eye through several reflective surfaces M 1 ˜M 3 . The front layer  215 , accepting the incoming light from an external scene, contains mainly the objective optics  220  and necessary reflective surfaces M 1  and M 2 . The back layer  217 , coupling the light captured by the front layer into a viewer&#39;s eye, mainly contains the eyepiece  210  and other necessary optical components such as additional folding mirror M 3 , In the front layer  215 , the reflective surface MI directs the incoming light from the external scene toward objective optics  220 ; and after passing through objective optics  220 , the light is folded toward the back layer  217  through the reflective surface M 2 . The objective optics  220  in the see-through path  207  forms at least one accessible intermediate image. A spatial light modulator (SLM)  240  is placed at or near the location of the accessible intermediate image, which is typically at the back focal plane of the objective optics, to provide opaqueness control and see-through modulation of the see-through view. In the present invention, a SIM is a light control device which can modulates the intensity of the light beam that passes through or is reflected by it. A SLM can be either a reflection-type SLM, e.g., a liquid crystal on silicon (LCoS) display panel or a. digital mirror device (DMD), or a transmission-type SLM, e.g., a liquid crystal display (LCD) panel. Both types of the SLM may be used for modulating the see-through view for occlusion control in the see-through path  207 .  FIG.  2 ( a )  illustrates an exemplary configuration of using a reflection-type SLM while  FIG.  2 ( b )  illustrates the use of a transmission-type SLM. Depending on the focal plane position of objective optics  220 , the SLM  240  can be placed at the position of SLM 2  with a refection-type SLM in  FIG.  2 ( a ) , or at the position of SLM 1  with a transmission-type SLM in  FIG.  2 ( b ) . The beamsplitter  230  folds the see-through path  207  and merges it with the virtual view path  205  so that the same the eyepiece  210  is shared for viewing the displayed virtual content and the modulated see-through view. The reflective surface M 3  directs the virtual view path  205  and see-through path  207  to exit pupil  202 , where the viewer&#39;s eye observes a mixed virtual and real view. The reflective surfaces M 1 ˜M 3  could be either a standing alone element (e.g. mirror) or could be strategically designed to be an integral part of the eyepiece  210  or objective optics  220 . The microdisplay  250  and SLIM  240  are both located at the focal plane of the objective optics  220  and are optically conjugate to each other through the beamsplitter  230 , which makes the pixel level opaqueness control on the see-through view possible, Though the unit assembling the SLM  240 , microdisplay  250 , and beamsplitter  230  is included in the back layer as shown in the exemplary figures, it may be incorporated into the front layer when the back focal distance of the eyepiece is larger than that of the objective optics such that it is preferred to place the combiner  − unit closer to the objective optics. The approach described above enables us to achieve a compact OCOST-HMD solution and minimal view axis shift. 
     As one of its benefits, the optical layout  200  has applicability to many types of MOD optics, including, without limitation, rotationally symmetric optics and non-rotationally symmetric freeform optics. The reflective surfaces M 1 ˜M 3  for folding the optical paths may be planar mirrors, spherical, aspherical, or freeform surfaces with optical power. Some of the reflective surfaces may utilize freeform optical technology. Some of the reflective surfaces may also be strategically designed to be an integral part of the eyepiece  210  or objective optics  220  where the reflective surfaces not only facilitate the folding of the optical paths for achieving compact display design but also contribute optical power and correct optical aberrations, In an exemplary configuration shown in  FIG.  3   , the present invention demonstrated the use of a one-reflection freeform prism as an eyepiece and objective optics where the prism is a single optical element comprises of two refractive surfaces and one reflective surface for folding the optical path and correcting aberrations. In other examples of configurations, multi-reflection freeform prisms are demonstrated. 
     In another significant aspect of the present invention, besides the intermediate image accessible to the SLM  240 , the see-through path  207  may form additional intermediate images  260  by the objective optics  220 , or eyepiece  210 , or both. For instance, multiple-reflection freeform prism structure (typically more than 2) may be utilized as eyepiece or objective optics, or both, which allow folding the see-through path inside the objective and/or eyepiece prism multiple times and form intermediate image(s) inside the prism. As a result, the see-through path  207  may yield a total odd or even number of intermediate images. The potential advantage of creating more than one intermediate image is the benefit of extended optical path length, long back focal distance, and the elimination of real-view erection element. 
     Depending on the total number of intermediate images being created and the total number of reflective surfaces being used in the see-through path  207 , a see-through view erection method may be needed to invert and/or revert the see-through view of the see-through path to maintain the parity of the coordinate system of the see-through view and prevent a viewer from seeing an inverted or reverted see-through view. As to the see-through view erection method specifically, the present invention considers two different image erection strategies. When a total even number of reflections is involved in the see-through path  207 , which induces no change to the parity of the coordinate system of the see-through view, the form of eyepiece  210  and objective optics  220  will be designed such that an even number of intermediate images is created in the see-through path  207 . When an odd number of reflections exist along with an odd number of intermediate images in the see-through path  207 , which induces parity change, one of the reflective surfaces M 1  through M 3  may be replaced by a roof mirror surface for the see-through view erection. The preferred embodiments with the view erection using a roof reflection will be discussed below in connection with  FIGS.  3  and  5   . The preferred embodiments with the view erection using an intermediate image will be discussed below in connection with  FIGS.  4 ,  6  and  7   . 
     In one of its significant aspects, the present invention may utilize freeform optical technology in eyepiece, objective optics or both to achieve a compact and lightweight OCOST-HMD.  FIG.  3    shows a block diagram  300  of an exemplary approach to a compact OCOST-HMD design in accordance with the present invention based on freeform optical technology. The eyepiece  310  in the back layer  317  is a one-reflection freeform prism comprising three optical freeform surfaces: refractive surface S 1 , reflective surface S 2  and refractive surface  53 . In virtual view path  305 , the light ray emitted from microdisplay  350 , enters the eyepiece  310  through the refractive surface S 3 , then is reflected by the reflective surface S 2  and exits eyepiece  310  through the refractive surface Si and reaches exit pupil  302 , where the viewer&#39;s eye is aligned to see a magnified virtual image of microdisplay  350 . The objective optics  320  in the front layer  315  is also a one-reflection freeform prism comprising of three optical freeform surfaces: refractive surface S 4 , reflective surface S 5  and refractive surface S 6 . In the see-through path  307 , the objective optics  320  works together with eyepiece  310  act as a relay optics for the see-through view. The incoming light from an external scene reflected by mirror  325 , enters the objective optics  320  through the refractive surface S 4 , then is reflected by the reflective surface S 5  and exits the objective optics  320  through refractive surface S 6  and forms an intermediate image at its focal plane on SLM  340  for light modulation. The beamsplitter  330  merges the modulated light in the see-through path  307  with the light in the virtual view path  305  and folds toward the eyepiece  310  for viewing. The beamsplitter  330  may be a wire-grid type beamsplitter, a polarized cube beamsplitter or other similar type beamsplitters. In this approach, the SLM  340  is a reflection-type SLM and is located at the SLM 2  position of the schematic layout  200  and is optically conjugated to the microdisplay  350  through the beamsplitter  330 . 
     In this exemplary layout  300 , the reflective surface M 2  of the schematic layout  200  is strategically designed to be an integrated part of the objective prism  320  as freeform reflective surface S 5 ; the reflective surface. M 3  of the schematic layout  200  is strategically designed to be an integrated part of the eyepiece prism  310  as freeform reflective surface S 2 ; the reflective surface M 1  of schematic layout  200  is designed as a roof type mirror  325  for view erection given that the total number of reflections in see-through path  307  is 5 (an odd number). 
     In this exemplary layout  300 , the eyepiece  310  and the objective optics  320  may have an identical freeform prism structure. The advantage of using an identical structure for the eyepiece and the objective optics is that the optical design strategy of one prism can be readily applied to the other, which helps simplify the optical design. The symmetric structure of the eyepiece and objective optics also helps correcting odd order aberrations, such as coma, distortion, and lateral color. 
       FIG.  4    shows a block diagram  400  of another exemplary approach to a compact OCOST-HMD design in accordance with the present invention based on freeform optical technology. In one exemplary implementation, the eyepiece  410  is a two-reflection prism and the objective optics  420  is a four-reflection prism. Inside the objective optics  420 , an intermediate image  460  is formed to erect the see-through view which eliminates the necessity of using an erection roof reflective surface. The potential advantage of eliminating the erection prism is that this system structure may lead to a. more compact design by folding the optical path inside the objective prism multiple times. The eyepiece  410  in the back layer  417  comprises of four optical freeform surfaces: refractive surface Sit reflective surface  52 , reflective surface S 1 ′ and refractive. surface S 3 , In the virtual view path  405 , the light ray emitted from the microdisplay  450 , enters eyepiece  410  through the refractive surface S 3 , then is consecutively reflected by the reflective surfaces S 1 ′ and S 2 , and exits the eyepiece  410  through the refractive surface SI and reaches the exit pupil  402 , where the viewer&#39;s eye is aligned to see a magnified virtual image of microdisplay  450 . The refractive surface S 1  and the reflective surface S 1 ′ may be the same physical surfaces and possess the same set of surface prescriptions. The objective optics  420  in the front layer  415  comprises of six optical freeform surfaces: refractive surface S 4 , reflective surfaces S 5 , S 4 ′,  55 ′, and S 6  and refractive surface S 7 . In the see-through path  407 , the objective optics  420  works together with the eyepiece  410  act as a relay optics for the see-through view. The incoming light from an external scene in the real world enters the objective optics  420  through the refractive surface S 4 , then is consecutively reflected by the reflective surfaces S 5 , S 4 ′, S 5 ′ and S 6 , and exits the objective optics  420  through the refractive surface S 7  and forms an intermediate image at its focal plane on SLM  440  for light modulation. The refractive surface S 4  and reflective surface S 4 ′ may be the same physical surfaces and possess the same set of surface prescriptions. The reflective surface S 5  and the reflective surface S 5 ′ may be the same physical surfaces and possess the same set of surface prescriptions. The beamsplitter  430  merges the modulated light in the see-through path  407  with the light in the virtual view path  405  and folds toward the eyepiece  410  for viewing. The beamsplitter  430  may be a wire-grid type beamsplitter, a polarized cube beamsplitter or other similar type beamsplitters. In this approach, the SLM  440  is a reflection-type SLM and is located at the SLM 2  position of the schematic layout  200  and is optically conjugated to the microdisplay  450  through beamsplitter  430 . 
     In this exemplary layout  400 , the reflective surface M 2  of the schematic layout  200  is strategically designed as an integrated part of the objective optics  420  as the reflective surface S 6 ; the reflective surface M 3  of the schematic layout  200  is strategically designed as an integrated part of the eyepiece  410  as the reflective surface S 2 ; the reflective surface M 1  of schematic layout  200  is designed as an integrated part of the objective optics  420  as the reflective surface S 5 . An intermediate image  460  is formed inside of the objective optics  410  for the real-view erection. Given that the total number of reflections in the see-through path  407  is 8 (an even number), no roof mirror is required on any reflective surfaces. 
       FIG.  5    shows a block diagram  500  of another exemplary approach to a compact OCOST-HMD design in accordance with the present invention based on freeform optical technology. This approach facilitates the usage of a transmission-type SLM. The eyepiece  510  is a two-reflection prism and the objective optics  520  is a one-reflection prism. A roof mirror  527  is placed at the top of objective prism  520  to invert the see-through view and to fold the see-through path  507  toward the back layer  517 . The eyepiece  510  in the back layer  517  comprises of four optical freeform surfaces: refractive surface S 1 , reflective surface S 2 , reflective surface S 1 ′ and refractive surface S 3 , In the virtual view path  505 , the light ray emitted from the microdisplay  550 , enters the eyepiece  510  through the refractive surface S 3 , then is consecutively reflected by reflective surfaces Sit and  52 , and exits the eyepiece  510  through the refractive surface S 1  and reaches exit pupil  502 , where the viewer&#39;s eye is aligned to see a magnified virtual image of the microdisplay  550 . The refractive surface S 1  and reflective surface S 1 ′ may the same physical surfaces and possess the same set of surface prescriptions. The objective optics  520  in the front layer  515  comprises of three optical freeform surfaces: refractive surface S 4 , reflective surface S 5  and refractive surface S 6 . In the see-through path  507 , the objective optics  520  works together with the eyepiece  510  act as a relay optics for the see-through view. The incoming light from an external scene in the real word enters the objective optics  520  through the refractive surface S 4 , then is reflected by the reflective surface S 5  and exits the objective optics  520  through the refractive surface S 6  and is folded by the mirror  527  toward the back layer  517  and forms an intermediate image at its focal plane on SLM  540  tor light modulation. The beamsplitter  530  merges the modulated light in the see-through path  507  with the light in the virtual view path  505  and folds the merged light toward the eyepiece  510  for viewing. The beamsplitter  530  may be a wire-grid type beamsplitter, a polarized cube beamsplitter or other similar type beamsplitters. In this approach, the SLM  540  is a transmission-type SLM and is located at the SLM, position of the schematic layout  200  and is optically conjugated to the micro-display  550  through the beamsplitter  530 . 
     In this exemplary layout  500 , the reflective surface M 1  of the schematic layout  200  is strategically designed as an integrated part of objective optics  520  as the reflective surface S 5 ; the reflective surface M 3  of the schematic layout  200  is strategically designed as an integrated part of the eyepiece  510  as the reflective surface S 2 ; the reflective surface M 2  of the schematic layout  200  is designed as a roof type mirror  527  for view erection given that the total number of reflections in the see-through path  507  is 5 (an odd number). 
       FIG.  6    shows a block diagram  600  of another exemplary approach to a compact OCOST-HMD design in accordance with the present invention based on freeform optical technology. This approach also facilitates the usage of a transmission type SLM. In one exemplary implementation, the eyepiece  610  is a two-reflection freeform prism and the objective optics  620  is a three-reflection freeform prism. Inside the objective optics  620 , an intermediate image  660  is formed to erect the see-through view. The eyepiece  610  in the back layer  617  comprises of four optical freeform surfaces: refractive surface S 1 , reflective surface S 2 , reflective surface S 1 ′ and refractive surface S 3 . In the virtual view path  605 , the light ray emitted from the microdisplay  650 , enters the eyepiece  610  through the refractive surface S 3 , then is consecutively reflected by reflective surfaces S 1 ′ and S 2 , and exits the eyepiece  610  through the refractive surface S 1  and reaches exit pupil  602 , where the viewer&#39;s eye is aligned to see a magnified virtual image of the microdisplay  650 . The refractive surface S 1  and the reflective surface S 1 ′ may the same physical surfaces and possess the same set of surface prescriptions. The objective optics  620  in the front layer  615  comprises of five optical freeform surfaces: refractive surface S 4 , reflective surfaces S 5 , S 4 ′ and S 6  and refractive surface S 7 , In the see-through path  607 , the objective optics  620  works together with the eyepiece  610  acting as relay optics for the see-through view. The incoming light from an external scene in the real world enters the objective optics  620  through the refractive surface S 4 , consecutively reflected by the reflective surfaces S 5 , S 4 ′ and S 6 , and exits the objective optics  620  through the refractive surface S 7  and forms an intermediate image at its focal plane on SLM  640  for light modulation. The refractive surface S 4  and the reflective surface S 4 ′ may be the same physical surfaces and possess the same set of surface prescriptions. The beamsplitter  630  merges the modulated light in the see-through path  607  with the light in the virtual view path  605  and folds toward the eyepiece  610  for viewing. The beamsplitter  630  may be a wire-grid type beamsplitter, a polarized cube beamsplitter or other similar type beamsplitters in this approach, the SLM  640  is a transmission-type SLM and is located at the SLM 1  position of the schematic layout  200  and is optically conjugated to the micro-display  650  through the beamsplitter  630 . 
     In this exemplary layout  600 , the reflective surface M 1  of the schematic layout  200  is strategically designed as an integrated part of the objective optics  620  as the reflective surface S 5 ; the reflective surface M 2  of the schematic layout  200  is strategically designed as an integrated part of the objective optics  620  as the reflective surface S 6 ; the reflective surface M 3  of the schematic layout  200  is strategically designed as an integrated part of the eyepiece  610  as the reflective surface S 2 . An intermediate image  660  is formed inside of the objective optics  610  for real-view erection. Given that the total number of reflections in the see-through path  607  is 6 (an even number), no roof mirror is required on any reflective surface, 
       FIG.  7    shows a block diagram  700  of another exemplary approach to a compact OCOST-HMD design in accordance with the present invention based on freeform optical technology. In one exemplary implementation, both the eyepiece and the objective optics are two-reflection  − freeform prisms and have nearly identical structure. The advantage of using an identical structure for the eyepiece and objective is that the optical design strategy of one prism can be readily applied to the other, which helps simplify the optical design. The symmetric structure of the eyepiece and objective prisms may also help correcting odd order aberrations, such as coma, distortion, and lateral color. The eyepiece  710  in the back layer  717  comprises of four optical freeform surfaces: refractive surface S 1 , reflective surface S 2 , reflective surface S 1 ′ and refractive surface S 3 . In the virtual view path  705 , the light ray emitted from the microdisplay  750 , enters the eyepiece  710  through the refractive surface S 3 , then is consecutively reflected by the reflective surfaces S 1 ′ and S 2 , and exits the eyepiece  710  through the refractive surface S 1  and reaches exit pupil  702 , where the viewer&#39;s eye is aligned to see a magnified virtual image of the microdisplay  750 . The refractive surface S 1  and the reflective surface S 1 ′ may the same physical surfaces and possess the same set of surface prescriptions, The objective optics  720  in the front layer  715  comprises of four optical freeform surfaces: refractive surface S 4 , reflective surfaces S 5 , S 4 ′ and refractive surface S 6 . In the see-through path  707 , the objective optics  720  works together with the eyepiece  710  acting as a relay optics for the see-through view. The incoming light from an external scene in the real world enters the Objective optics  720  through the refractive surface S 4 , consecutively reflected by the reflective surfaces S 5 , S 4 ′, and exits the objective optics  720  through the refractive surface S 6  and forms an intermediate image  760  at its focal plane. The beamsplitter  780  folds the see-through path  707  away from the back layer  715  toward the mirror  790  positioned at the focal plane of the objective optics  720 . The see-through path  707  is reflected by the mirror  790  back toward the back layer  715 . A relay lens  770  is used to create an image of the intermediate image  760  at the SLM 2  position of the schematic layout  200  for view modulation. The beamsplitter  730  merges the modulated light in the see-through path  707  with the light in the virtual view path  705  and folds toward the eyepiece  710  for viewing. In this approach, the SLM  740  is a reflection-type SLM and is optically conjugated to the microdisplay  750  through beamsplitter  730 . Due to the fact that the intermediate image  760  is optically conjugated to the SLM  740 , the positions of the SIM  740  and the mirror  790  are interchangeable. 
     In this exemplary layout  700 , the reflective surface M 1  of the schematic layout  200  is strategically designed as an integrated part of the objective optics  720  as the reflective surface S 5 ; the reflective surface M 3  of the schematic layout  200  is strategically designed as an integrated part of the eyepiece  710  as the reflective surface S 2 ; the reflective surface M 2  of the schematic layout  200  is positioned at the focal plane of the Objective optics  710  as the mirror  790  and folds the see-through path  707  toward the virtual view path  705 ; The intermediate image  760  is formed at the focal plane of the objective optics  720  for real-view erection. Given that the total number of reflections in the see-through path  707  is 8 (an even number), no roof mirror is required on any reflective surface. 
       FIG.  8    schematically illustrated an exemplary design  800  based on the exemplary approach depicted in  FIG.  3   . The design achieved a diagonal FOV of 40 degrees, which is 31.7 degrees in the horizontal direction (X-axis direction) and 25.6 degrees in the vertical direction (Y-axis direction), an exit pupil diameter (ETD) of 8 mm (non-vignetted), and an eye clearance of 18 mm. The design is based on a 0.8″ microdisplay with a 5:4 aspect ratio and a 1280×1024 pixel resolution. The microdisplay has an effective area of 15.36 mm and 12.29 mm and a pixel size of 12 m. The design used a SLM of the same size and resolution as the microdisplay. A polarized cube beamsplitter is used to combine the virtual view path and the see-through path. DOE plates  882  and  884  are used to correct chromatic aberrations. The system is measured as 43 mm(X)×23 mm (Y)×44.5 mm (Z). The viewpoint shifts between the entrance pupil  886  and exit pupil  802  are 0.6 mm in Y direction and 67 mm in Z direction, respectively. 
     An exemplary optical prescription of the eyepiece  810  is listed in the Tables 1-4. All the three optical surfaces in the eyepiece  810  are anamorphic aspheric surface (AAS), The sag of an AAS surface is defined by 
               z   =             c   x     ⁢     x   2       +       c   y     ⁢     y   2           1   +       1   -       (     1   +     K   x       )     ⁢     c   x   2     ⁢     y   2               +     AR   ⁢       {         (     1   -   AP     )     ⁢     x   2       +       (     1   +   AP     )     ⁢     y   2         }     2       +       {         (     1   -   BP     )     ⁢     x   2       +       (     1   +   BP     )     ⁢     y   2         }     3     +     CR   ⁢       {         (     1   -   CP     )     ⁢     x   2       +       (     1   +   CP     )     ⁢     y   2         }     4       +     DR   ⁢       {         (     1   -   DP     )     ⁢     x   2       +       (     1   +   DP     )     ⁢     y   2         }     5           ,         
where z is the sag of the free-form surface measured along the z-axis of a local x, y, z coordinate system, cx and c y  are the vertex curvature in x and y axes, respectively, K x  and K y  are the conic constant in x and y axes, respectively, AR, BR, CR and DR are the rotationally symmetric portion of the 4th, 6th, 8th, and 10th order deformation from the conic, AP, BP, CP, and DP are the non-rotationally symmetric components of the 4th, 6th, 8th, and 10th order deformation from the conic.
 
Table 1: Optical surface prescription of surface  1  of the eyepiece prism, See  FIG.  13   .
 
Table 2: Optical surface prescription of surface  2  of the eyepiece prism, See  FIG.  14   
 
Table 3: Optical surface prescription of surface  3  of the eyepiece prism, See  FIG.  15   
 
Table 4: Position and orientation parameters of the eyepiece prism, See  FIG.  16   
 
     An exemplary optical prescription of the objective optics  820  is listed in the Tables 5-8. All the three optical surfaces in the objective optics  820  are anamorphic aspheric surface (AAS). 
     Table 5: Optical surface prescription of surface  4  of the objective prism, See  FIG.  17   . 
     Table 6: Optical surface prescription of surface  5  of the objective prism, See  FIG.  18   . 
     Table 7: Optical surface prescription of surface  6  of the objective prism, See  FIG.  19   . 
     Table 8: Position and orientation parameters of the objective prism, See  FIG.  20   . 
     An exemplary optical prescription of the DOE plate  882  and  884  is listed in the Tables 9. 
     Table 9: Surface parameters for DOE plates  882  and  884 . See  FIG.  21   . 
       FIG.  9    shows the field map of polychromatic modulation transfer functions (MTF) of the virtual display path at cutoff frequency 401 ps/min (line pairs per millimeter) evaluated using 3 mm pupil diameter. The 40 Ips/mm cutoff frequency was determined from the pixel size of the microdisplay. The plot shows that our design has very good performance for majority fields except two upper display corners whose MTF values at cutoff frequency are little less than 15%). Across the entire FOV the distortion of the virtual display path is less than 2.9%, while the distortion of the see-through path is less than 0.5%. The total estimated weight for the optics alone is 33 grams per eye. 
       FIG.  10    schematically illustrated an exemplary design  1000  based on the exemplary approach depicted in  FIG.  3   . The design achieved a diagonal FOV of 40 degrees with 35.2 degrees horizontally (X-direction) and 20.2 degrees vertically (V-direction), an exit pupil diameter (EPD) of 8 ram (non-vignetted), and an eye clearance of 18 mm. The design is based on a 0.7″ microdisplay with a 16:9 aspect ratio and a 1280×720 pixel resolution. The design used a SLM of the same size and resolution as the microdisplay. A wire-grid plate beamsplitter is used to combine the virtual view path and the see-through path. The same freeform prism is used as the eyepiece and the objective optics. 
     An exemplary optical prescription of the freeform prism is listed in the Tables 10-15. Two surfaces in the prism are anamorphic aspheric surface (AAS) and one is aspheric surface (ASP). The sag of an ASP surface is defined by 
             z   =         cr   2       1   +       1   -       (     1   +   K     )     ⁢     c   2     ⁢     r   2               +     Ar   4     +     Br   6     +     Cr   8     +     Dr     1   ⁢   0       +     Er     1   ⁢   2       +       Fr     1   ⁢   4       ⁢     Gr     1   ⁢   6         +     Hr     1   ⁢   8       +     Jr     2   ⁢   0               
where z is the sag of the surface measured along the z-axis of a local x, y, z coordinate system, c is the vertex curvature, k is the conic constant, A through J are the 4th, 6th, 8th, 10th, 12th, 14th, 16th, 18th, and 20th order deformation coefficients, respectively.
 
Table 10: Optical surface prescription of surface  1  of the freeform prism, See  FIG.  22   
 
Table 11: Optical surface prescription of surface  2  of the freeform prism, See  FIG.  23   .
 
Table 12: Optical surface prescription of surface  3  of the freeform prism, See  FIG.  24   .
 
Table 13: Position and orientation parameters of the freeform prism as the eyepiece, See  FIG.  25   .
 
       FIG.  11    shows the field map of polychromatic modulation transfer functions (MTF) of the virtual display path at cutoff frequency 401 psi/mm (line pairs per millimeter) evaluated using 3 mm pupil diameter. The plot shows that our design has very good performance for majority fields. 
       FIG.  12    depicts a block diagram of an example of an image processing pipeline necessary for the present invention. Firstly, the depth map of the external scene is extracted using proper depth sensing means. Then, the virtual object is compared with the depth map to determine the regions where the occlusion occurs. A mask generation algorithm creates a binary mask image according to the pre-determined occlusion regions. The mask image is then displayed on spatial light modulator to block the light from the occluded region in the intermediate image of the external scene. A virtual image of the virtual object is rendered and displayed on the micro-display. The viewer observes a combined image of the virtual image and the modulated see-through image of the external scene through the display device of the present invention, 
     Compared to the prior art, the present invention features a folded image path that permits the invention to be compressed into a compact form, more easily wearable as a head-mounted display. In the prior art (U.S. Pat. No. 7,639,208 B1), the optical path is linearly arranged using rotationally symmetric lenses. As a result the prior art occlusion-type displays have a long telescope-like shape, which is unwieldy for wearing on the head. The present invention folds the image path using reflective surfaces into two layers to that the spatial light modulator, microdisplay and beamsplitter, are mounted to the top of the head, rather than linearly in front of the eye. 
     The prior art relies on only a reflection type spatial light modulator, while the present invention may use either a reflection or transmission type spatial light modulator. Moreover, the prior art requires a polarized beamsplitter to modulate the external image, while the present. invention does not necessitate polarization. 
     Since the present invention is arrange in layers, the eyepiece and the objective optics are not necessarily collinear, as in the case in the prior art. The objective optics is also not necessarily tele-centric. 
     In the prior art, due to the optics of the system the view of the world is a mirror reflection of the see-through view. The present invention the folded image path allows a roof mirror to be inserted to maintain parity between the view of the user and the external scene. This makes the present invention more functional from the user&#39;s perspective. 
     Compared to the prior art, the present invention makes use of freeform optical. technology, which allows the system to be made even more compact. The freeform optical surfaces can be designed to reflect light internally multiple times, so that mirrors may not be needed to fold the light path. 
     In the present invention, the reflective surfaces for folding the optical paths may be planar mirrors, spherical, aspherical, or freeform surfaces with optical power. A significant aspect of the present invention lies in that some of the reflective surfaces utilize freeform optical technology, which helps to boost the optical performance and compactness. In the present invention, sonic of the reflective surfaces are strategically designed to be an integral part of the eyepiece or objective optics where the reflective surfaces not only facilitate the folding of the optical path for achieving compact display design but also contribute optical power and correct optical aberrations. For example, in  FIG.  2   , the reflective surfaces M 1 ˜M 3  were shown as generic mirrors separate from the eyepiece and objective optics. In  FIG.  3   , two of the mirrors (M 2  and M 3 ) are freeform surfaces incorporated into the freeform eyepiece and Objective prisms as S 2  and S 5 . In  FIG.  4 ,  4    reflective freeform surfaces were incorporated into the freeform objective prism and 2 were incorporated into the freeform eyepiece prisms. In  FIG.  5   , 1 freeform surface was in the objective prism, 2 freeform surfaces were in the eyepiece, in addition to a roof prism. In  FIG.  6   , 3 freeform surfaces are in the objective and 2 freeform surfaces in the eyepiece. In  FIG.  7 ,  2    reflective freeform mirrors are in the objective, 2 freeform mirrors are in the eyepiece, in addition to a mirror  790  and a. beamsplitter  780 . 
     Our invention ensures that the see-through view seen through the system is correctly erected (neither inverted nor reverted). Two different optical methods were utilized in our embodiments for achieving this, depending on the number of intermediate images formed in the see-through path and the number of reflections involved in the see-through path. In the case of an odd number of intermediate images, an optical method is provided to invert and/or revert the see-through view in the see-through path. For example, depending on the number of reflections involved in the see-through path, examples of the possible methods include, but not limited to, inserting an additional reflection or reflections, utilizing a roof mirror surface, or inserting an erector lens, in the case of an even number of intermediate images, no image erection element is needed if no parity change is needed. For instance, multiple-reflection freeform prism structure (typical more than 2) may be utilized as eyepiece or objective optics, or both, which allow folding the see-through optical path inside the objective and/or eyepiece prism multiple times and form intermediate image(s inside the prism to erect the see-through view which eliminates the necessity of using an erection roof reflective surface. 
     In  FIG.  3   , only 1 intermediate image is created in the see-through path. This structure utilized a roof prism for 325 to properly create an erected see-through view. 
     In  FIG.  4   , a 4-reflection freeform prism was utilized as an objective optics, which created 2 intermediate images (one for SLM  440 , and one 460 inside the prism). Additionally, there were total 8 reflections involved in the see-through path, which leads to no parity change. Therefore, an erected  − view is created, It is worth mention that the structure of the objective and eyepiece may be exchanged for the same results. 
     In  FIG.  5   , 1 intermediate image is created in the see-through path for the SIM. This design utilized a roof prism  527  to erect the see-through view. 
     In  FIG.  6   , a 3-reflection freeform prism was utilized as an objective optics, which created 2 intermediate images (one for SLM  640 , and one 660 inside the prism). Additionally, there were total 6 reflections involved in the see-through path, which leads to no parity change. Therefore, an erected view is created, It is worth mention that the structure of the objective and eyepiece may be exchanged for the same results. 
     In  FIG.  7   , the objective optics  720  utilized only 2 reflections, the combination of the beamsplitter  780  and the mirror  790  facilitated the creation of 2 intermediate images in the see-through path (one for the SLM  740  and an additional one 760). Additionally, total 8 reflections were involved in the see-through path. Therefore, an erected see-through view was created. 
     It is very important for a see-through head mounted display to maintain the parity of the external scene which provides the users a realistic experience as their usual views without a HMD. 
     Although there has been shown and described the preferred embodiment of the present invention, it will be readily apparent to those skilled in the art that modifications may be made thereto which do not exceed the scope of the appended claims. Reference numbers recited in the claims are exemplary and for ease of review by the patent office only, and are not limiting in any way. In some embodiments, the figures presented in this patent application are drawn to scale, including the angles, ratios of dimensions, etc. In some embodiments, the figures are representative only and the claims are not limited by the dimensions of the figures. 
     The reference numbers recited in the below claims are solely for ease of examination of this patent application, and are exemplary, and are not intended in any way to limit the scope of the claims to the particular features having the corresponding reference numbers in the drawings.