Abstract:
There is provided an optical system, including a light-transmitting substrate ( 20 ) having at least two major surfaces ( 26 ) and edges, an optical prism ( 54 ) having at least a first ( 58 ), a second ( 56 ) and a third ( 60 ) surface, for coupling light waves having a given field-of-view into the substrate by total internal reflection, at least one partially reflecting surface located in the substrate, the partially reflecting surface being orientated non-parallelly with respect to the major surfaces of the substrate, for coupling light waves out of the substrate, at least one of the edges ( 50 ) of the substrate is slanted at an oblique angle with respect to the major surfaces, the second surface of the prism is located adjacent to the slanted edge of the substrate, and a part of the substrate located next to the slanted edge is substantially transparent, wherein the light waves enter the prism through the first surface of the prism, traverse the prism without any reflection and enter the substrate through the slanted edge.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to substrate-guided optical devices, and particularly to devices which include a plurality of reflecting surfaces carried by a common light-transmissive substrate, also referred to as a light-guide element. 
         [0002]    The invention can be implemented to advantage in a large number of imaging applications, such as portable DVDs, cellular phones, mobile TV receivers, video games, portable media players or any other mobile display devices. 
       BACKGROUND OF THE INVENTION 
       [0003]    One application for compact optical elements concerns head-mounted displays (HMDs), wherein an optical module serves both as an imaging lens and a combiner, wherein a two-dimensional image source is imaged to infinity and reflected into the eye of an observer. The display source may originate directly from a spatial light modulator (SLM), such as a cathode ray tube (CRT), a liquid crystal display (LCD), an organic light emitting diode array (OLED), a scanning source or similar devices, or indirectly, by means of a relay lens, an optical fiber bundle, or similar devices. The display source comprises an array of elements (pixels) imaged to infinity by a collimating lens and transmitted into the eye of the viewer by means of a reflecting, or partially reflecting, surface acting as a combiner for non-see-through and see-through applications, respectively. Typically, a conventional, free-space optical module is used for these purposes. As the desired field-of-view (FOV) of the system increases, however, such a conventional optical module becomes larger, heavier and bulkier, and therefore, even for a moderate performance device, is impractical. This is a major drawback for all kinds of displays and especially in head-mounted applications, wherein the system should necessarily be as light and as compact as possible. 
         [0004]    The strive for compactness has led to several different complex optical solutions, all of which, on the one hand, are still not sufficiently compact for most practical applications, and, on the other hand, suffer major drawbacks with respect to manufacturability. Furthermore, the eye-motion-box (EMB) of the optical viewing angles resulting from these designs is usually very small - typically less than 8 mm. Hence, the performance of the optical system is very sensitive, even for small movements of the optical system relative to the eye of a viewer, and does not allow sufficient pupil motion for comfortable reading of a text from such displays. 
         [0005]    The teachings included in Publication Nos. WO 01/95027, WO 03/081320, WO2005/024485, WO2005/024491, WO2005/024969, WO2005/124427, WO2006/013565, WO2006/085309, WO2006/085310, WO2006/087709, WO2007/054928, WO2007/093983, WO2008/023367, WO2008/129539, WO2008/149339 and WO2013/175465, all in the name of Applicant, are herein incorporated by reference. 
       DISCLOSURE OF THE INVENTION 
       [0006]    The present invention facilitates the exploitation of very compact light-guide optical element (LOB) for, amongst other applications, HMDs. The invention allows relatively wide FOVs together with relatively large EMB values. The resulting optical system offers a large, high-quality image, which also accommodates large movements of the eye. The optical system disclosed by the present invention is particularly advantageous because it is substantially more compact than state-of-the-art implementations and yet it can be readily incorporated even into optical systems having specialized configurations. 
         [0007]    A broad object of the present invention is therefore to alleviate the drawbacks of prior art compact optical display devices and to provide other optical components and systems having improved performance, according to specific requirements. 
         [0008]    In accordance with the present invention, there is provided an optical system, comprising a light-transmitting substrate having at least two major surfaces and edges; an optical prism having at least a first, a second and a third surface, for coupling light waves having a given field-of-view into the substrate by total internal reflection; at least one partially reflecting surface located in the substrate, the partially reflecting surface being orientated non-parallelly with respect to the major surfaces of said substrate, for coupling light waves out of the substrate; at least one of the edges of the substrate is slanted at an oblique angle with respect to the major surfaces; the second surface of the prism is located adjacent to the slanted edge of the substrate, and a part of the substrate located next to the slanted edge is substantially transparent, characterized in that the light waves enter the prism through the first surface of the prism, traverse the prism without any reflection and enter the substrate through the slanted edge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention is described in connection with certain preferred embodiments, with reference to the following illustrative figures so that it may be more fully understood. 
           [0010]    With specific reference to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention. The description taken with the drawings are to serve as direction to those skilled in the art as to how the several forms of the invention may be embodied in practice. 
           [0011]    In the drawings: 
           [0012]      FIG. 1  illustrates a span of optical rays which are coupled into an LOE, according to the present invention; 
           [0013]      FIG. 2  illustrates a span of optical rays which illuminates the input aperture of an LOE; 
           [0014]      FIG. 3  illustrates a prior art side view of an exemplary coupling-in mechanism comprising a prism optically attached to one of the major surfaces of the LOE; 
           [0015]      FIG. 4  is an another schematic diagram illustrating a side view of a prior art exemplary coupling-in mechanism comprising a prism optically attached to one of the major surfaces of the LOE; 
           [0016]      FIG. 5  illustrates a span of optical rays illuminating the input aperture of an LOE wherein one of the edges of the LOE is slanted at an oblique angle with respect to the major surfaces; 
           [0017]      FIG. 6  is a schematic diagram illustrating another system with a span of optical rays illuminating the input aperture of an LOE, wherein one of the edges of the LOE is slanted at an oblique angle with respect to the major surfaces; 
           [0018]      FIG. 7  is a schematic diagram illustrating an embodiment of an optical system coupling-in input light waves from a display light source into a substrate, having an intermediate prism attached to the slanted edge of the LOE, in accordance with the present invention; 
           [0019]      FIG. 8  illustrates another embodiment of an optical system coupling-in input light waves from a display light source into a substrate, having an intermediate prism attached to the slanted edge of the LOE, in accordance with the present invention; 
           [0020]      FIG. 9  is a schematic diagram illustrating a device for collimating input light waves from a display light source, by utilizing a polarizing beamsplitter, in accordance with the present invention, and 
           [0021]      FIG. 10  is a schematic diagram illustrating a device for collimating input light waves from liquid crystals on silicon (LCOS) light source, in accordance with the present invention and 
           [0022]      FIGS. 11A and 11B  are two embodiments showing a top view of eyeglasses according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    The present invention relates to substrate-guided optical devices, in particular, compact HMD optical systems. Usually, a collimated image having a finite FOV is coupled into a substrate. As illustrated in  FIG. 1 , the image inside an LOE or, hereinafter, a substrate  20  contains a span of plane waves having central waves  14  and marginal waves  16  and  18 . The angle between a central wave  14  of the image and the normal to the plane of the major surfaces  26 ,  32  is α in . The FOV inside the substrate  20  is defined as 2·Δα. Consequentially, the angles between the marginal waves  16  and  18  of the image and the normal to the plane of the major surfaces are α in +Δα and α in −Δα, respectively. After several reflections off the surfaces  26 ,  32  of the substrate  20 , the trapped waves reach an array of selectively reflecting surfaces  22 , which couple the light waves out of the substrate into an eye  24  of a viewer. For simplicity, only the rays of the central waves  14  are plotted as being coupled-out from the substrate. 
         [0024]    The object of the present invention is to find a light wave coupling-in mechanism which is different to the coupling-in mechanism of the prior art and having more compact dimensions. In  FIG. 2 , there is illustrated a span of rays that have to be coupled into substrate  20 , with a minimal required input aperture  21 . In order to avoid an image with gaps or stripes, the points on the boundary line  25 , between the edge of the input aperture  21  and the lower surface  26  of the substrate  20 , should be illuminated for each one of the input light waves by two different rays that enter the substrate from two different locations: one ray  30  that illuminates the boundary line  25  directly, and another ray  31 , which is first reflected by the upper surface  32  of the substrate before illuminating the boundary line  25 . The size of the input aperture  21  is usually determined by two marginal rays: the rightmost ray  34  of the highest angle of the FOV, and the leftmost ray  36  of the lowest angle of the FOV. 
         [0025]    A possible embodiment for coupling the marginal rays into the substrate  20  is illustrated in  FIG. 3 . Here, the input light waves source  38 , as well as a collimating module  40 , e.g., a collimating lens, are oriented at the required off-axis angle compared to the major surfaces  26 ,  32  of the substrate  20 . A relay prism  44  is located between the collimating module  40  and the substrate  20  and is optically cemented to the lower surface  26  of the substrate  20 , such that the light rays from the display source  38  impinge on the major surface  26  at angles which are larger than the critical angle, for total internal reflection inside the substrate. As a result, all the optical light waves of the image are trapped inside the substrate by total internal reflection from the major surfaces  26  and  32 . Although the optical system illustrated here is simple, it is still not the most compact coupling-in mechanism. This is an important point for optical systems which should conform to the external shape of eyeglasses, as well as to hand-held or other displays. 
         [0026]    In order to minimize the dimensions of the collimating module  40 , the aperture D T  of the input surface  46  of the coupling-in prism  44  should be as small as possible. As a result, the dimensions of the coupling-in prism would also be minimized accordingly, while the coupled rays of the entire FOV will pass through the coupling-in prism  44 . 
         [0027]    As illustrated in  FIG. 4 , in order for the rightmost ray  34  of the highest angle of the FOV to pass through the prism  44 , the aperture D L  of the output surface  21  of the prism  44  must fulfil the relation 
         [0000]        D   L ≧2 d ·tan (α in +Δα)   (1)
 
         [0028]    wherein d is the thickness of the substrate  20 . 
         [0029]    In addition, in order for the leftmost ray  36  of the lowest angle of the FOV to pass through the prism  44 , the angle α sur1  between the left surface  48  of the prism  44  and the normal to the major surface  26  of the substrate  20  must fulfil the relation 
         [0000]      α sur1 ≦α in −Δα  (2)
 
         [0030]    For minimizing the chromatic aberrations of the optical waves passing through the prism  44 , it is advantageous to orient the input surface  46  of the coupling-in prism  44  to be substantially normal to the central wave  14  of the image. As a result, the angle α sur2  between the entrance surface  46  of the prism  44  and the normal to the major surface  26  of the substrate  20  is 
         [0000]      α sur2 =90°−α in    (3)
 
         [0031]    Taking the inequality of Eq. 2 to the limit, in order to minimize the dimensions of the prism  44  yields the following internal angles of the prism: the angle between the surfaces  46  and  21  is α in ; the angle between surface  48  and  21  is 90°−α in +Δα. Consequentially, the angle between surfaces  46  and  48  is 90°−Δα. Utilizing these values yields 
         [0000]    
       
         
           
             
               
                 
                   
                     D 
                     T 
                   
                   = 
                   
                     
                       
                         D 
                         L 
                       
                       
                         sin 
                          
                         
                           ( 
                           
                             
                               90 
                                
                               ° 
                             
                             - 
                             
                               Δ 
                                
                               
                                   
                               
                                
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                           ) 
                         
                       
                     
                     · 
                     
                       sin 
                        
                       
                         ( 
                         
                           
                             90 
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                             ° 
                           
                           - 
                           
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                             α 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0032]    Taking the inequality of Eq. 1 to the limit and inserting it in Eq. 4 yields 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           D 
                           T 
                         
                         = 
                           
                          
                         
                           
                             
                               2 
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                                 · 
                                 
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                                    
                                   
                                     ( 
                                     
                                       
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                             · 
                             
                               cos 
                                
                               
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                   ( 
                   5 
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         [0033]    Although the optical system illustrated in  FIGS. 3 and 4  seems to be simple, it is still not the most compact coupling-in mechanism, since it is important for such optical systems to conform to the external shape of displays such as eyeglasses or hand-held displays. 
         [0034]      FIG. 5  illustrates an alternative embodiment of coupling light waves into the substrate through one of its edges. Here, the light waves-transmitting substrate  20  has two major parallel surfaces  26  and  32  and edges, wherein at least one edge  50  is oriented at an oblique angle with respect to the major surfaces and wherein α sur3  is the angle between the edge  50  and the normal to the major surfaces of the substrate. Usually the incoming collimated light waves are coupled directly from the air, or alternatively, the collimating module  40  ( FIG. 3 ) can be attached to the substrate  20 . As a result, it is advantageous to couple the central wave  14  normal to the slanted surface  50  for minimizing chromatic aberrations. Unfortunately, this requirement cannot be fulfilled by coupling the light directly through surface  50 . Usually, even for coupled images having a moderate FOV, the angle α in  ( FIG. 3 ) between the central wave  14  of the image and the normal to the plane of the major surfaces has to fulfil the requirement α in≧ 50°. As a result, if the central wave  14  is indeed normal to the slanted surface  50 , then the relation α sur3 ≦40° must be fulfilled. Consequentially, the outcome will be the fulfillment of the relations in the system α sur3 &lt;α in  and, for a comparatively wide FOV, even α sur3 &lt;&lt;α in +Δα. 
         [0035]      FIG. 6  illustrates the complex situation wherein the maximal angle between the trapped rays and the major surfaces  26 ,  32  is larger than the angle between the input surface  50  and the major surfaces. As illustrated, the points on the boundary line  25 , between the edge of input aperture  50  and the lower surface  26  of substrate  20 , are illuminated only by the leftmost ray  35  of the wave that directly illuminates the boundary line  25 . The other marginal ray  34 , which impinges on the edge  51  of the input surface  50 , is first reflected by the upper surface  32  prior to illuminating the lower surface at a different line  52  which is located at a distance Δx from the boundary line  25 . As illustrated, the gap Δy is not illuminated at all by the trapped rays of the marginal wave  34 . Consequentially, dark stripes will appear and the coupled-out waves and the image quality will be significantly inferior. 
         [0036]    This situation is solved by the embodiment shown in  FIG. 7 . An intermediate prism  54  is inserted between the collimating module  40  ( FIG. 3 ) and the slanted edge  50  of the substrate. One of the prism&#39;s surfaces  56  is located adjacent to the slanted edge  50  of the substrate  20 . In most cases, the refractive index of the intermediate prism should be similar to that of the substrate  20 . Nevertheless, there are cases wherein a different refractive index might be chosen for the prism, in order to compensate for chromatic aberrations in the system. The incoming light waves are coupled directly from the air, or alternatively, the collimating module  40 , can be attached to the intermediate prism  54 . In many cases, the refractive index of the collimating module  40  is substantially different than that of the substrate  20 , and accordingly, is different from that of the prism  54 . Therefore, for minimizing the chromatic aberrations, the input surface  58  of the prism  54  should be oriented substantially normal to the central light wave of the incoming ray. In addition, the leftmost ray of the lowest angle of the FOV should pass through the prism  54 . As a result, the conditions of Eqs. (2) and (3) should be fulfilled also for the configuration of  FIG. 7 . To eliminate the undesired phenomena of dark stripes as described with reference to  FIG. 6 , the relation 
         [0000]      α sur3 ≧α in +Δα  (6)
 
         [0000]    must be satisfied, namely, the angle between the slanted edge of the substrate and the normal to the major surfaces of the substrate is larger than the highest angle of the FOV. Accordingly, the aperture D S  of the output surface  56  of the prism  54  must fulfil the relation 
         [0000]    
       
         
           
             
               
                 
                   
                     D 
                     s 
                   
                   ≥ 
                   
                     d 
                     
                       cos 
                        
                       
                         ( 
                         
                           
                             α 
                             
                               i 
                                
                               
                                   
                               
                                
                               n 
                             
                           
                           + 
                           
                             Δ 
                              
                             
                                 
                             
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                             α 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0037]    Apparently, since the light waves enter the prism  54  through the entrance surface  58  of the prism, directly cross the prism without any reflections and enter the substrate through the slanted edge  50 , the expansion of the active area D p  of the entrance surface  58  in relation to the aperture D s  of the exit surface  56 , is minimal. In addition, as described above, in order for the leftmost ray  36  ( FIG. 4 ) of the lowest angle of the FOV to pass through the prism  54 , the angle α sur1  between the left surface  60  of the prism  54  and the normal to the major surface  26  of the substrate must also fulfil the relation of Eq. (2), namely, the angle between the surface  60  of the prism  54  and the normal to the major surfaces of the substrate, is smaller than the lowest angle of the FOV. Therefore, when the relations of Eqs. (2), (6) and (7) are fulfilled, the coupled-in light waves from the entire FOV will completely cover the major surfaces of the substrate without any stripes or gaps. 
         [0038]    As illustrated in  FIG. 8 , by taking the inequalities of Eqs. (2), (6) and (7) to the limit, the internal angles of the prism  54  are: the angle between the surfaces  56  and  58  is 2α in −90°+Δα and the angle between surface  56  and  60  is 180°−2α in . Consequentially, the angle between surfaces  58  and  60  is 90°−Δα. Utilizing these values yields 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           D 
                           P 
                         
                         = 
                           
                          
                         
                           
                             
                               d 
                               
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                                  
                                 
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                                       α 
                                       
                                         i 
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                             · 
                             
                               cos 
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         [0039]    wherein D P  is the active area of the input surface  58  of the intermediate prism  54 . 
         [0040]    Therefore, by comparing Eqs. (5) and (8), the relation between the active areas D P  and D T  of the input surfaces of the prisms  54  and  44  of the prior art system of  FIG. 4 , respectively, is: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       D 
                       P 
                     
                     
                       D 
                       T 
                     
                   
                   = 
                   
                     
                       
                         sin 
                          
                         
                           ( 
                           
                             α 
                             
                               i 
                                
                               
                                   
                               
                                
                               n 
                             
                           
                           ) 
                         
                       
                       · 
                       
                         cos 
                          
                         
                           ( 
                           
                             α 
                             
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                         sin 
                          
                         
                           ( 
                           
                             
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                                 i 
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                                 n 
                               
                             
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                       · 
                       
                         cos 
                          
                         
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                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0041]    Apparently, for a narrow FOV, that is, Δα&lt;&lt;α in , the improvement is negligible. However, for a relatively wide FOV the active area D P  of the prism  54  should be reduced considerably compared to the active area D T  of the prism  44 . For example, for Δα=12° and α in =52° the reduction ratio of Eq. (9) has a significant value of D P /D T≈ 0.7. 
         [0042]    In the embodiment illustrated in  FIG. 3 , the collimating module  40  is shown to be a simple transmission lens, however, much more compact structures utilizing reflective lenses, polarizing beamsplitters and retardation plates may be employed. In such a structure, the fact that in most microdisplay light sources, such as LCDs or LCOS light sources, the light which is linearly polarized, is exploited by optical component  61 , as illustrated in  FIG. 9 . As shown, the s-polarized input light waves  62  from the display light source  64 , are coupled into a light-guide  66 , which is usually composed of a light waves transmitting material, through its lower surface  68 . Following reflection-off of a polarizing beamsplitter  70 , the light waves are coupled-out of the substrate through surface  72  of the light-guide  66 . The light waves then pass through a quarter-wavelength retardation plate  74 , reflected by a reflecting optical element  76 , e.g., a flat mirror, return to pass again through the retardation plate  74 , and re-enter the light-guide  66  through surface  72 . The now p-polarized light waves pass through the polarizing beamsplitter  70  and are coupled out of the light-guide through surface  78  of the light-guide  66 . The light waves then pass through a second quarter-wavelength retardation plate  80 , collimated by a component  82 , e.g., a lens, at its reflecting surface  84 , return to pass again through the retardation plate  80 , and re-enter the light-guide  66  through surface  78 . The now s-polarized light waves reflect off the polarizing beamsplitter  70  and exit the light-guide through the exit surface  86 , attached to the intermediate prism  54 . The reflecting surfaces  76  and  84  can be materialized either by a metallic or a dielectric coating. 
         [0043]    In the embodiment illustrated in  FIG. 9 , the display source can be an LCD panel, however, there are optical systems, especially wherein high brightness imaging characteristics are required, where it is preferred to utilize an LCOS light source device as a display light source. Similar to LCD panels, LCOS light source panels contain a two-dimensional array of cells filled with liquid crystals that twist and align in response to control voltages. With the LCOS light source, however, the cells are grafted directly onto a reflective silicon chip. As the liquid crystals twist, the polarization of the light is either changed or unchanged following reflection of the mirrored surface below. This, together with a polarizing beamsplitter, causes modulation of the light waves and creates the image. The reflective technology means that the illumination and imaging light beams share the same space. Both of these factors necessitate the addition of a special beamsplitting optical element to the module, in order to enable the simultaneous operations of the illuminating, as well as the imaging, functions. The addition of such an element would normally complicate the module and, when using an LCOS light source as the display light source, some modules using a frontal coupling-in element or a folding prism, would become even larger. For example, the embodiment of  FIG. 9  could be modified to accommodate an LCOS light source by inserting another beamsplitter between the display source  64  and the beamsplitter  66 . However, this modified version may be problematic for systems with a comparatively wide FOV, wherein the focal length of the collimating module is shorter than the optical path of the rays passing through the of double beamsplitter configuration. 
         [0044]    To solve this problem, as seen in  FIG. 10 , a modified optical component  90  is provided, wherein only one reflecting surface  84  is located adjacent to surface  78  of the light-guide  66 . Hence, the optical path through this light-guide  66  is much shorter. As shown, the s-polarized light waves  92 , emanating from a light source  94 , enter the prism  96 , reflect off the polarizing beamsplitter  98  and illuminate the front surface of the LCOS light source  100 . The polarization of the reflected light waves from the “light” pixels is rotated to the p-polarization and the light waves are then passed through the beamsplitter  98 , and consequentially, through a polarizer  102  which is located between the prisms  96  and  66  and blocks the s-polarized light which was reflected from the “dark” pixels of the LCOS light source  100 . The light waves then enter the prism  66  and pass through the second beamsplitter  70 , are coupled out of the prism through surface  78  of the prism  66 , pass through a quarter-wavelength retardation plate  80 , collimated by a collimating lens  82  at its reflecting surface  84 , return to pass again through the retardation plate  80 , and re-enter the prism  66  through surface  78 . The now s-polarized light waves reflect off the polarizing beamsplitter  70  and exit the prism  66  through the exit surface  86 , which is attached to the intermediate prism 54. 
         [0045]    Returning now to  FIG. 9 , wherein the viewer&#39;s eye  24  is located at the same side of the slanted edge  50 , the dimensions of the optical prism  66  are substantially extended over the lower major surface  26  of substrate  20  and only slightly extended over the upper surface  32 . This slight extension can be completely eliminated with a proper design, for instance, by slightly increasing the angle α sur3  of the slanted edge  50 . 
         [0046]    For the embodiment which is illustrated in  FIG. 10 , however, the optical component  90  is substantially extended over the lower surface  26  of the substrate  20 , as well as over the upper surface  32 . 
         [0047]    As illustrated in  FIG. 11A , this unique configuration may be preferred for optical systems wherein a collimating module is composed of the optical component  90  of  FIG. 10 , having prisms  66  and  96 . Optical component  90  is installed between the eyeglasses frame  104  and the substrate  20 . In this case, the viewer&#39;s eye  24  is located on the opposite side of the slanted edge  50  of the substrate  20 . The light waves are coupled into the substrate  20  through the slanted edge  50  towards the major surface  32 , from which surface  32 , it bounces towards the partially reflecting surfaces  22  and from there exit the substrate through the major surface  32  towards the viewer&#39;s eye  24 . Even though there is a front extension  106  of the optical component  90  to the front part of the eyeglasses, the rear extension  108  of the prism  96  is minimal, and the entire optical component  90 , can easily be integrated inside the frame  104  of the eyeglasses. 
         [0048]    Seen in  FIG. 11B  is a modification based on the optical module illustrated in  FIG. 9 , wherein the viewer&#39;s eye  24  is located on the same side of the slanted edge  50  of the substrate  20 . The light waves emanating from the optical component  90  are coupled into the substrate  20  through the slanted edge  50 , enter the substrate  20  towards the major surface  26 , from which surface it bounces towards the major surface  32  and from there it continues towards the partially reflecting surfaces  22 , and exit the substrate though the major surface  32  towards the viewer&#39;s eye  24 . 
         [0049]    It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.