Abstract:
The present invention relates to an optical system for directing light from an image source to a viewer&#39;s eye. The system includes an image source providing an image source light; a collimator for receiving the image source light and converting the received image source light into a collimated light projected along a first optical path; and a planar diffractive relay including opposing planar surfaces longitudinally oriented substantially along a second optical path, the opposing planar surfaces terminating into an input tilted surface and an output tilted surface. The collimated light is projected along the first optical path and redirected by the input tilted surface as propagated light traveling in the second optical path. The propagated light traveling in the second optical path is redirected by the output tilted surface into a third optical path directed toward the viewer&#39;s eye.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority of U.S. Provisional Patent Application Serial No. 60/307,547 filed Jul. 24, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a planar diffractive relay which may be mounted on the head or on a helmet to view a display, and more particularly, to view the display of a night vision device to provide night time viewing.  
         BACKGROUND OF THE INVENTION  
         [0003]    Night vision systems include objective lenses, image intensifier tubes and eyepieces usually assembled in a straight line. Many night vision goggles extend out in front of a viewer&#39;s face and fail to provide compact low profiles for the viewer. Excessive weight of these systems causes head or neck strain and fatigue problems.  
           [0004]    The operation of each of these night vision systems is similar. An objective lens collects light from a low illumination scene and focuses it onto a photocathode of an image intensifier tube. The photocathode converts this image into an electronic signal that is amplified and converted into an intensified image on a screen of the image tube. An eyepiece magnifies the screen image for viewing.  
           [0005]    Several devices use a single objective to image a scene on a single image intensifier and split the output (ocular) side into two paths (for each eye) using mirrors or prisms. These devices are, for example, the US Army PVS-7 binocular night vision goggle and the Simrad GN1 Night Visions Goggle, disclosed in U.S. Pat. No. 5,712,726. The PVS-7 device does not have a low profile (extends 170 mm beyond the face of the viewer). The GN1 device lacks inter-pupilary adjustment.  
           [0006]    Another device, known as the Filipovich/FJW compact see-through night vision goggles, is disclosed in U.S. Pat. No. 4,653,879. As disclosed, this device images a scene onto two separate image intensifier paths using conventional optical lenses and beam combining prisms to achieve a compact structure. The Filipovich device has a limited field of view and is heavy.  
           [0007]    Yet another device, known as the Takahashi system, is disclosed in U.S. Pat. No. 5,699,194 and U.S. Pat. No. 5,701,202. A similar device, known as the Okuyama system, is disclosed in U.S. Pat. No. 5,706,136. Both devices use a compact, aspheric beam-combiner prism to directly superimpose an electronically generated scene on a directly viewed scene. The Takahashi and Okuyama systems have two major problems. While the optical performance is very good, the surfaces required to correct the aberrations can not be manufactured by normal polishing or single point diamond machining. Special very expensive molds with non-rotationally symmetric surfaces are required. Another problem is the input signal (image on the image intensifier) is oriented at an odd angle (38 degrees relative to the line-of-sight). An image intensifier and objective lens folded into such a system results in a high profile.  
           [0008]    Another device, known as the Janeczko system, is disclosed in U.S. Pat. No. 6,088,165. The system uses a compact aspheric beam-combiner prism to superimpose an image from a folded image intensifier optical path and an image from a video input onto direct viewing by a viewer. While the Janeczko system has a lower profile (50 mm) than the PVS-7 system, the profile of the Janeczko system is still excessive.  
           [0009]    A compact head-up display is disclosed by Upatneik in U.S. Pat. No. 4,711,512. The display uses two linear diffraction gratings on a planar waveguide for relaying an image from a CRT to a viewer&#39;s eye. While the Upatneiks system may have good image quality, it requires a collimator with a physical diameter of about six times the diameter of the eye pupil (60 mm for 10 mm eye pupil). This is unacceptable from a weight and profile consideration. Collimators having a diameter of 60 mm and a focal length of 25 mm are also difficult to fabricate (F number=0.42). The efficiency of the display is also low because its diffractive gratings only cover an incident parallel ray bundle that subtends an angle of +/−4 degrees before extinction.  
         SUMMARY OF THE INVENTION  
         [0010]    To meet this and other needs, and in view of its purposes, the present invention provides an optical system for directing light from an image source to a viewer&#39;s eye. The optical system includes at least one image source providing an image source light; a collimator for receiving the image source light and converting the received image source light into a collimated light projected along a first optical path; and a planar diffractive relay including opposing planar surfaces longitudinally oriented substantially along a second optical path, the opposing planar surfaces terminating into an input tilted surface and an output tilted surface. The collimated light is (a) projected along the first optical path, (b) redirected by the input tilted surface as propagated light traveling in the second optical path, and (c) the propagated light traveling in the second optical path is redirected by the output tilted surface into a third optical path directed toward the viewer&#39;s eye.  
           [0011]    In one embodiment, the input tilted surface and one surface of the opposing planar surfaces subtend an angle greater than half of a critical angle for producing substantially total internal reflection of the propagated light traveling in the second optical path. The input tilted surface includes a length dimension sufficiently large for receiving the collimated light projected from the collimator and redirecting the collimated light for the propagation in the second optical path.  
           [0012]    In another embodiment, the output tilted surface and the one surface subtend an angle greater than half the critical angle for redirecting the propagated light into the third optical path.  
           [0013]    It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0014]    The invention is best understood from the following detailed description when read in connection with the accompany drawing. Included in the drawing are the following figures:  
         [0015]    [0015]FIG. 1 is a perspective view of a night vision system resting on a viewer&#39;s head, constructed in accordance with and embodying an embodiment of the present invention;  
         [0016]    [0016]FIG. 2 is an exploded view of an optical assembly of the night vision system shown in FIG. 1, in accordance with an embodiment of the present invention;  
         [0017]    [0017]FIG. 3 is a schematic representation of an optical system, in accordance with an embodiment of the present invention;  
         [0018]    [0018]FIG. 4 is a planar view of an image intensifier (I 2 ) objective assembly illustrating various lenses which focus and fold a night image onto an image intensifier, in accordance with an embodiment of the present invention;  
         [0019]    [0019]FIG. 5 is a schematic representation of an infrared (IR) channel depicting a planar view of various lenses for focussing an infrared (IR) image onto a micro-bolometer, and for magnifying and relaying the IR image from a flat panel display (FPD), in accordance with an embodiment of the present invention;  
         [0020]    [0020]FIG. 6 is a planar view of various lenses forming a collimator assembly for projecting collimated light onto a planar diffractive relay, in accordance with an embodiment of the present invention;  
         [0021]    [0021]FIG. 7 is an illustration of three light rays entering, propagating through and exiting a planar diffractive relay, in accordance with an embodiment of the present invention;  
         [0022]    [0022]FIG. 8 is an illustration of three light rays striking on and reflecting from a reflector surface of a planar diffractive relay, in accordance with an embodiment of the present invention;  
         [0023]    [0023]FIG. 9 is an illustration of three light rays entering and exiting a Kinoform type of diffractive lens, which is positioned adjacent to an output end of a planar diffractive relay, in accordance with an embodiment of the present invention;  
         [0024]    [0024]FIGS. 10A and 10B are planar views of holograms formed in diffractive optical elements (DOEs) used in accordance with various embodiments of the present invention;  
         [0025]    [0025]FIG. 11 is an illustration of three light rays entering and exiting a sandwiched holographic lens, which is positioned adjacent to an output end of a planar diffractive relay, in accordance with an embodiment of the present invention;  
         [0026]    [0026]FIG. 12 is an illustration of three light rays striking on and reflecting from an off-axis transmission holographic lens, which is positioned adjacent to an output end of a planar diffractive relay, in accordance with an embodiment of the present invention;  
         [0027]    [0027]FIG. 13 is an illustration of three light rays entering and exiting an off-axis reflection holographic lens, which is positioned adjacent to an output end of a planar diffractive relay, in accordance with an embodiment of the present invention;  
         [0028]    [0028]FIG. 14 is an illustration of a light ray passing through or reflecting off a Bragg reflector, which is positioned adjacent to an output end of a planar diffractive relay, in accordance with an embodiment of the present invention;  
         [0029]    [0029]FIG. 15 is an illustration of light rays entering and exiting a combination of lenses including a Kinoform type of diffractive lens, which are positioned adjacent to an output end of a planar diffractive relay, in accordance with an embodiment of the present invention; and  
         [0030]    FIGS.  16 A- 16 C,  17 A-! 7 B and  18 A- 18 I include surface details of various lenses used in the optical system illustrated in FIG. 3, in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    Referring now to FIG. 1, there is shown a perspective view of night vision system  10  resting on a viewer&#39;s head according to an embodiment of the invention. Night vision system  10  includes optical assembly  14  mounted on helmet  12  for accommodating the right eye of the viewer. Also included is another optical assembly  14  (not shown) mounted on helmet  12  for accommodating the left eye of the viewer. Both assemblies are constructed as mirror images of each other. An exploded view of optical assembly  14  is shown in FIG. 2.  
         [0032]    Referring to both, FIGS. 1 and 2, each optical assembly  14  includes image intensifier (I 2 ) objective assembly  40 , which collects visible scene light for image intensification and infrared (IR) objective assembly  38 , which collects infrared scene light for conversion into a visible image on a flat panel display (FPD). As shown, the I 2  objective assembly and the IR objective assembly are oriented along the cheek of the viewer and collect substantially similar images of a scene.  
         [0033]    I 2  objective assembly  40  includes a 90° bend for folding image light that enters the objective assembly at a substantially horizontal direction into a substantially vertical direction and bringing the folded light to image intensifier  48 . The image intensifier receives the folded light and intensifies it at a predetermined narrow band of wavelengths. The output light from the image intensifier produces a visible band of light, known as P-43 light.  
         [0034]    IR objective assembly  38  is aligned to I 2  objective assembly, by way of holding bracket  36 , and is effective in producing an electronic image of a scene on a micor-bolometer. The electronic image is processed by a processing circuit on board  44  and powered by battery supply  42 . The processed image is displayed on an FPD disposed at the bottom portion of FPD magnifier  46  (not shown in FIG. 1). The magnified image, which is displayed substantially in a horizontal direction, is folded 90° by mirror  18  and then relayed through lenses  20  to beam combiner  24 .  
         [0035]    Beam combiner  24  is positioned to receive images from two channels. A first channel includes the optical path taken by the IR image through IR objective assembly  38 , FPD magnifier  46 , mirror  18  and lenses  20 . A second channel includes the optical path taken by the visible image through I 2  objective assembly  40  and image intensifier  48 . Beam combiner  24  combines the visible (I 2 ) and IR images, and provides both to collimator  28 .  
         [0036]    As shown, collimator  28  is oriented horizontally and substantially parallel to I 2  objective assembly  40  and IR objective assembly  38 . The light inputted to collimator  28  is collimated into parallel beams of light and projected toward planar optic assembly  32 , including planar diffractive relay  30 . The planar diffractive relay may be a thin plate having a substantially planar cross-section. As will be explained, the planar diffractive relay internally reflects the incoming light rays as they strike each opposing surface of the plate. In this manner, the light rays are propagated from an input end of the plate (adjacent the collimator) toward an output end of the plate (adjacent the viewer&#39;s eye). As best shown in FIG. 1, planar diffractive relay  30  is oriented perpendicularly to the projected lines-of-sight of collimator  28 , I 2  objective assembly  40  and IR objective assembly  38 , and is horizontally aligned between the viewer&#39;s eye and collimator  28 .  
         [0037]    Although not shown in FIGS. 1 and 2 (but explained below), a first diffractive optical element (DOE) may be positioned between the input end of relay  30  and collimator  28 . The first DOE may convert parallel light rays projected by collimator  28  into a converging light bundle, and may also bend the converging light rays by approximately 90°, with respect to the projected line-of-sight of collimator  28 . The bent light rays (bundles) may be projected down the thin planar cross-section of relay  30 . A second DOE may be positioned between the output end of relay  30  and the viewer&#39;s eye. The second DOE may take diverging light rays propagating down planar relay  30  and bend them a second time by approximately 90° from the direction of propagation. The light rays bent the second time may also be converted into parallel (collimated) light rays by the second DOE for viewing.  
         [0038]    To complete description of FIG. 2, housing  26  is adapted to receive collimator  28 , beam combiner  24 , mirror  18 , lenses  20 , FPD magnifier  46  and image intensifier  48 . Cover plate  22  prevents extraneous light from impinging on beam combiner  24 . Tubular housing  50  is adapted to receive I 2  objective assembly  40  and image intensifier  48 . Housing covers  16  and  34  are adapted to enclose the various components of optical assembly  14 . Also shown are switch  52  (for example) which may provide an on/off control of the IR sensor circuits, and switch  54  (for example) which may provide image intensity control of the I 2  channel.  
         [0039]    Referring now to FIG. 3, there is shown a schematic representation of optical assembly  14 , designated as optical system  70 , in accordance with an embodiment of the invention. Visible light rays  95  and FIR (far-infrared) light rays  97  are reflected and emitted, respectively, from object  72 . I 2  objective assembly  74  images light rays  95  onto photocathode  83  of image intensifier  82 . The image is intensified at the output face of the image intensifier as light rays  99 .  
         [0040]    FIR light rays  97  enter IR objective assembly  76  and are processed by electronic circuit  78  to form a visible image on FPD  79 . Light rays  101 , emitted from FPD  79 , is magnified and relayed toward beam combiner  84  by way of FPD magnifier and relay  80 .  
         [0041]    Light rays  99  and  101  are combined by beam combiner  84  and sent to collimator  86 . It will be appreciated that light rays  99  and  101  may each be alternatively formed from a CRT, a laser scanner, an image tube, or a projected image source. These image sources may be displayed on a flat panel or a curved panel.  
         [0042]    Light rays  99  and  101  may be converted from diverging light rays into parallel light rays by collimator  86 . After being projected by collimator  86 , the light rays may enter, in sequence, first DOE  88 , planar diffractive relay  90  and second DOE  92 . As will be explained, first DOE  88  may be disposed at the input end of relay  90  and second DOE  92  may be disposed at the output end of relay  90 . As will also be explained, the combination of the first and second DOEs and relay  90  is effective in redirecting the propagation direction of light rays  99  and  101  by approximately 180°, as the light rays travel from collimator  86  toward the viewer&#39;s eye  94 . It will further be appreciated that in alternative embodiments either the first DOE or second DOE may be eliminated.  
         [0043]    Components of optical system  70 , shown schematically in FIG. 3, will now be described by reference to FIGS.  4 - 6 . Referring first to FIG. 4, there is shown  12  objective assembly  74 . As shown, the  12  objective includes several spherical and/or aspherical lenses, typically made of glass, which focus and provide a 90° fold to light rays entering at lens  110  and exiting at roof prism  124 . The I 2  objective assembly has a high aperture of F/1.25 and a field-of-view of substantially 40° on a 16 mm format. The lenses may be moved in relation to each other to provide a variable focus range from 25 cm to infinity.  
         [0044]    In the embodiment shown in FIG. 4, the I 2  objective assembly includes five single lenses, two double lenses and roof prism  124 . On the input side of roof prism  124 , there is shown, in sequence, singlet  110 , doublet  112 , singlet  114 , singlet  116 , doublet  118 , singlet  120  and singlet  122 . Prism  124  provides a 90° fold for the light rays entering the objective assembly.  
         [0045]    A specific construction of an embodiment of I 2  objective assembly  74  of the present invention is described by referring to FIG. 4 in combination with FIGS. 16A through 16C. As one skilled in the art may ascertain, FIGS.  16 A-  16 C are Zemax prescription data including surface details of the objective assembly, lens data, radius data, thickness, aperture radius, wavelength data, refractive indices data, and general operating condition data for the overall arrangement of the objective lens assembly. It will be appreciated that the reference designations, shown in FIGS. 16B and 16C, under the column titled “Comment”, correspond to the reference designations of lenses shown in FIG. 4, for example. Object  72  is shown in FIG. 3, for example.  
         [0046]    [0046]FIG. 4 also shows image intensifier  82  operating to intensify an image formed by I 2  objective assembly  74  on photocathode  83 . The photocathode is disposed at an input face of image intensifier  82 . The image intensifier converts the image formed at the input face into a one-to-one mapping of visible light formed at its output face  125 .  
         [0047]    Turning next to the IR channel, various lenses for focussing, magnifying and relaying the incoming IR light rays are shown in FIG. 5. As shown, IR objective  76  includes lenses  130  and  132  which forms an image through window  134  onto micro-bolometer  135 . The image is processed by electronic circuit  78  to form a visible image on FPD  79 . Lenses  130  and  132  may be fixed to project an image at infinity. The IR objective includes a field-of-view of substantially 30° on a 16 mm format.  
         [0048]    The image formed on FPD  79  is magnified and relayed by FPD magnifier and relay  80  toward beam combiner  84 . FPD magnifier and relay  80  includes seven single lenses and a 90°-fold mirror  144 . On the entrance side of mirror  144 , there is shown, in sequence, singlet  136 , singlet  138 , singlet  140  and singlet  142 . On the output side of mirror  144 , there is shown, in sequence, singlet  146 , singlet  148  and singlet  150 .  
         [0049]    A specific construction of an embodiment of IR objective  76  of the present invention is described by referring to FIG. 5 in combination with FIGS. 17A and 17B. As one skilled in the art may ascertain, FIGS.  17 A- 17 B are Zemax prescription data. It will be appreciated that the reference designations, shown in FIG. 17B, under the column titled “Comment”, correspond to the reference designations of lenses shown in FIG. 5, for example. Object  72  is shown in FIG. 3, for example.  
         [0050]    A specific construction of an embodiment of FPD magnifier and relay  80  is described by referring to FIG. 5 in combination with FIGS. 18A through 18I. As one skilled in the art may ascertain, FIGS.  18 A- 18 I are Zemax prescription data. The data are not only prescription data of FPD magnifier and relay  80  (FIG. 5, for example), but also includes prescription data of beam combiner  84 , collimator  86 , planar relay  90  and DOEs  88  and  92  (as shown in FIG. 6, for example). It will be appreciated that the reference designations, shown in FIGS.  18 B- 18 D, under the column titled “Comment”, correspond to the reference designations of lenses shown in FIGS. 5 and 6, for example. Object  94  (viewer&#39;s eye) is shown in FIG. 3, for example.  
         [0051]    Turning now to FIG. 6, there is shown collimator  86  disposed between beam combiner  84  and planar diffractive relay  90 . As shown, collimator  86  is common to the I 2  channel and the IR channel, and is adapted to convert diverging light (specular or diffuse) from any source into collimated light. The source may be an FPD, a CRT, a laser scanner or an image tube. In the embodiment shown in FIGS.  4 - 6 , the sources include FPD  79  and photocathode  83 .  
         [0052]    Collimator  86  includes five lenses. As shown, doublet lens  160  is positioned adjacently to beam combiner  84 , and singlet lens  168  is positioned adjacently to first DOE  88 . Between lenses  160  and  168 , there is shown, in sequence, singlet lens  162 , doublet lens  164  and singlet lens  166 . A specific construction of an embodiment of collimator  86  is described by referring to FIG. 6 in combination with FIGS.  18 A- 18 I.  
         [0053]    Also shown in FIG. 6 is planar diffractive relay  90 , which may be made of optically transparent material, including glass, crystal or plastic. Planar diffractive relay  90  may be approximately the size of a microscope slide, for example 5 mm thick×80 mm long×30 mm wide. The thickness of planar diffractive relay  90  may vary from 5 mm to 20 mm. As best shown in FIG. 1 (referenced as relay  30 ), planar relay  90  is disposed directly in front of the viewer&#39;s eye. Because the planar relay may be very thin and the collimator may be oriented along the cheek of the viewer, the present invention achieves an optical system having a forward projection (as measured from the eyeball) of less than 45 mm. A conventional optical system typically includes a is forward projection of 160 mm (as measured from the eyeball).  
         [0054]    As will be explained, the optical paths, in addition to planar diffractive relay  90 , may include first DOE  88  disposed at input end  170  and second DOE  92  disposed at output end  172 . As will also be explained, first DOE  88  redirects light projected from collimator  86  toward planar relay  90  and second DOE  92  redirects light propagating within planar relay  90  toward the eye of the viewer.  
         [0055]    Planar relay  90  includes planar surfaces  178  and  180  substantially parallel to each other. At input end  170 , planar surfaces  178  and  180  terminate at angled (or tilted) surface  174 . At output end  172 , planar surfaces  178  and  180  terminate at angled (or tilted) surface  176 . In one embodiment of the invention, angled surfaces  174  and  176  may each be coated with a reflective material, so that each may behave as a reflector to incident light. The reflective material may be deposited by an evaporation process with a typical thickness of about five microns.  
         [0056]    A specific construction of an embodiment of planar relay  90 , first DOE  88  and second DOE  92  are shown in FIG. 6 in combination with FIGS. 18A through 18I. As one skilled in the art may ascertain, FIGS.  18 A- 18 I are Zemax prescription data and include prescription data of the following visual optic train: FPD magnifier and relay  80  (FIG. 5), beam combiner  84 , collimator  86 , DOE  88 , planar relay  90 , and DOE  92  (as shown in FIG. 6, for example).  
         [0057]    Referring now to FIG. 7, three light rays  186 ,  188  and  190  are shown entering, propagating through and exiting planar relay  90 . The three light rays (+20, 0 and −20 degrees field-of-view), after being converted into parallel light rays by collimator  86  (FIG. 6), enter planar relay  90  at input end  170 . The three light rays are first directed to strike reflector surface  174 , then are reflected off the surface to undergo substantially total internal reflection (TIR) within planar relay  90 . The three light rays are reflected sideways to the original direction of propagation. The light rays propagate through the length of planar relay  90 , as they bounce back and forth off planar surfaces  178  and  180 . At the output end, the light rays strike reflector surface  176  and are reflected in a direction opposite to the direction the light rays originally entered the planar relay. The light rays exit the planar relay at output end  172  and form a pupil which is used by an external device, such as a viewer&#39;s eye, camera or video camera/sensor. It will be appreciated that the path of the light rays shown in FIG. 7 are actually perpendicular to the page but, for ease of illustration, are shown in the plane of the page.  
         [0058]    Reflector surfaces  174  and  176  each forms an angle greater than  
         Θ   c     2                         
 
         [0059]    degrees with planar surface  178 , as best shown in FIG. 8. It will be appreciated that θ c  is defined as a critical angle that provides total internal reflection for light ray  188  (for example) as it strikes planar surface  178  (for explanation purpose the light ray is shown directed toward the input end of planar relay  90 ). It will also be appreciated that a similar angle may be formed between reflector surface  174  and planar surface  178  (FIG. 7). Furthermore, θ c  is a theoretical angle, as a true total internal reflection is not achievable. Typically, losses of less than 10% and preferably less than 1% may be tolerated and may be included in the phrase ‘a substantially total internal reflection.’ 
         [0060]    While reflector surface  176  subtends an angle that is greater than  
         Θ   c     2                         
 
         [0061]    degrees with planar surface  178 , the size of the angle is limited, however, to allow the length (L) of reflector surface  176  to be sufficiently long to cover the full field-of-view of the light rays entering and exiting planar relay  90 . In this manner, the light rays at input end  170  and output end  172  may strike input reflector surface  174  and output reflector surface  176  only once.  
         [0062]    In the embodiment shown in FIGS.  6 - 8 , planar relay  90  substantially confines the light rays within its planar cross-section by the principal of total internal reflection. No light ray may escape whose slope angle is greater than arcsin  
         (     1   N     )     ,                         
 
         [0063]    where N is the refractive index of the planar relay. This angle may vary between  42  degrees for BK-7 glass (N approximately 1.5) and 33 degrees for SFL-57 glass (N approximately 1.85), for example.  
         [0064]    In another embodiment, planar relay  90  may be made of two parallel reflective plates (for example  178  and  180 ) which form a hollow space between the plates. The medium in the hollow space may be air.  
         [0065]    Various embodiments of a diffractive optical element (DOE) for placement at input end  170 , output end  172  or both input and output ends of planar relay  90  will now be described.  
         [0066]    At the input end, a DOE converts the parallel light rays from collimator  86  into a converging bundle of light. For example, three light rays (+20, 0 and −20 degrees field-of-view) are refracted to become a narrower bundle (for example +10.7, 0 and −10.7 degrees, if the refractive index is 1.85). As the light rays bend (approximately 90 degrees with respect to the collimator&#39;s projected line-of-sight) and propagate through the thin cross-section of planar relay  90 , an intermediate image is formed within the planar relay.  
         [0067]    It will be appreciated that the DOE at the input end of planar relay  90  includes characteristics to form the intermediate image, which reduces the overall dimensions of the planar relay. The DOE may be placed on planar surface  178  (as shown by DOE  88  in FIG. 6, for example) or on angled (tilted) surface  174  (explained below). When placed on tilted surface  174 , the DOE effectively reduces the amount of deflection angle through which the DOE must work to diffract and bend the light. The tilted surface also allows the DOE to work in a region in which an incident parallel ray bundle may subtend an angle of +/− 10 degrees before extinction.  
         [0068]    At the output end, a DOE converts the diverging light bundle propagating through planar relay  90  into a parallel or collimated light for presentation to the viewer&#39;s eye. It will be appreciated that, after the intermediate image is formed within planar relay  90 , the light bundle begins to diverge, the DOE at the output end, therefore, converts the diverging light bundle into collimated light. The DOE may be placed on planar surface  178  (as shown by DOE  92  in FIG. 6, for example) or on angled (tilted) surface  176  (explained below). When placed on tilted surface  176 , the DOE effectively reduces the amount of deflection angle through which the DOE must work.  
         [0069]    As will be explained, the DOE (at the input end or output end) may be a non-linear, unsymmetrical diffractive grating. The grating may be of on-axis type or off-axis type, and may be transmissive or reflective in nature.  
         [0070]    One embodiment of a DOE is shown in FIG. 9. As shown, DOE  192  may be a Kinoform-type of diffractive lens. The Kinoform lens may include, for example, a diameter K of 30 mm and a cross-sectional thickness of 5 mm. The Kinoform lens may be bonded to planar surface  178  with epoxy  194 , such as Norland No. 61 or an equivalent. A closed air-space  196  may be formed by planar surface  178 , epoxy  194  and Kinoform lens  192 . The air-space may be 0.1 mm in the thickness dimension.  
         [0071]    The Kinoform lens may be fabricated by precision machining or molding in a conventional manner so that each of the surfaces between major steps (not shown) is continuous. A planar view of an on-axis type of grating (or hologram) that may be formed on a surface of the Kinoform lens is illustrated in FIG. 10A and is generally designated as  200 . As shown in FIG. 10A, hologram  200  includes concentric circles (for example  202 ,  204 ) of constant phase. It will be appreciated that, between each two adjacent concentric circles, a continuous step-shaped surface may be fabricated to form the diffractive properties of the Kinoform lens.  
         [0072]    When placed at output end  172  of planar relay  90 , as shown in FIG. 9, Kinoform lens  192  collimates the diverging light rays ( 186 ,  188 ,  190 ), which are folded by reflector surface  176 , into parallel light rays for presentation to the viewer&#39;s eye. When placed at the input end of planar relay  90  (not shown), Kinoform lens  192  converges the light rays projected from collimator  86  to form the intermediate image, which effectively reduces the overall size of the optics.  
         [0073]    Another embodiment of a DOE is shown in FIG. 11. As shown, DOE  220  may be a sandwiched holographic lens, which includes hologram layer  222  and epoxy layer  224 . The index of refraction of hologram layer  222  is matched to the index of refraction of epoxy layer  224 . The sandwiched holographic lens may include, for example, a diameter S of 30 mm, and a cross-sectional thickness of 2.25 mm (hologram layer  222  may be 2.0 mm thick and epoxy layer  224  may be 0.25 mm thick). Epoxy layer  224  may be Norland No. 61 or an equivalent.  
         [0074]    A planar view of hologram layer  222  is shown in FIG. 10A. As shown, the hologram (designated as  200  in FIG. 10A) includes concentric circles of constant phase and may be similar to the hologram described for the Kinoform lens. Similar to the Kinoform lens, the sandwiched holographic lens may be placed at input end  170  or output end  172  (FIG. 6) and may advantageously reduce the overall size of the optics.  
         [0075]    Still another embodiment of a DOE is shown in FIG. 12. As shown, an off-axis transmission holographic lens  230  includes tilted surface  232 , which is pasted onto tilted surface  176  of planar relay  90  at output end  172 . Holographic lens  230  may include a holographic pattern recorded in a gelatin or polymer substrate (volume hologram).  
         [0076]    A planar view (viewed toward tilted surface  232 ) of an exemplary hologram recorded in holographic lens  230  is shown in FIG. 10B and is generally designated as hologram  210 . Hologram  210  includes de-centered concentric circles of constant phase (for example, de-centered concentric circles  212 ,  214 ).  
         [0077]    Yet another embodiment of a DOE is shown in FIG. 13. As shown, an off-axis reflection holographic lens  244  includes tilted surface  246 , which is pasted onto tilted surface  248  of planar relay  240  at output end  250 . Reflection holographic lens  244  may include a holographic pattern similar to hologram  210  of FIG. 10B, as viewed toward tilted surface  246 . It is noted that the holographic pattern on tilted surface  246  reflects some light bundles ( 186 ,  188 ,  190 ) (diverging in planar relay  240 ) onto planar surface  242 . The light bundles are then reflected from planar surface  242 , and transmitted through holographic lens  244  toward the viewer&#39;s eye.  
         [0078]    It will be appreciated that off-axis holographic lens  230  and off-axis holographic lens  244  may each be placed at the input end and output end of the planar relay.  
         [0079]    Another embodiment of a DOE is shown in FIG. 14. As shown, Bragg reflector  260  includes tilted surface  262 , which is pasted onto tilted surface  176  of planar relay  90  at output end  172 . Bragg reflector  260  includes a volume reflection hologram that may be an on-axis type (FIG. 10A) or off-axis type (FIG. 10B). As known in the art, the Bragg angle changes across the hologram. When the impinging light beam has an on-Bragg wavelength, the light beam is diffracted. When the impinging light beam has an off-Bragg wavelength, however, the light beam is transmitted through the surface. This is illustrated in FIG. 14 with three light beams originating from point source  274 . Beam  268  is transmitted through tilted surface  262  as an off-Bragg wavelength at location  264  and is diffracted at an on-Bragg wavelength at location  266 . Beam  270  and beam  272 , however, strike tilted surface  262  as on-Bragg wavelengths at location  264  and  266 , respectively, and are, consequently, diffracted, as shown.  
         [0080]    It will be appreciated that Bragg reflector  260  provides a method for assuring that incident beams are diffracted only once as they impinge on tilted surface  262 .  
         [0081]    Yet another embodiment of a DOE is shown in FIG. 15. As shown, lens  92  (described, for example, in FIGS.  18 A- 18 I) and Kinoform lens  192  are sandwiched together at output end  172  of planar relay  90 . Although not shown, a similar combination of lenses may be formed at the input end of planar relay  90 . The combined lenses diffract light beams  186  and  190 , propagating through planar relay  90 , into collimated light beams impinging on pupil  94 .  
         [0082]    The DOEs, illustrated in FIGS.  9 - 15 , do not diffract light with the same efficiency. The diffraction characteristics of the DOE are dependent on the wavelength of light, the polarization of light, and the angle in which the light strikes the grating or hologram. Advantages of volume holograms (FIGS.  12 - 14 ) are that these holograms diffract in-band wavelengths and transmit out-of-band wavelengths (allow a see-through optical system). The Bragg reflector (FIG. 14) diffracts on-Bragg light waves and transmits the other light waves. The light may bounce multiple-times off the grating or hologram until the correct Bragg angle is encountered. In this manner, the Bragg reflector acts as a spatial filter, diffractng out only the correct wavelength. Other light continues to propagate and is absorbed, minimizing flare and stray light. Angular variation of Bragg angles across the grating at the output end (viewer&#39;s eye) may be optically fabricated. (Bragg angle variation at the input end, or collimator side, are more complex and difficult to fabricate).  
         [0083]    Reasons for selecting a volume reflection hologram at the output end may include the following: A volume reflection hologram typically diffracts only to a single order and allows see-through. Because a volume reflection hologram allows multiple bounces for off-angle rays, a thinner planar relay may be used. A volume reflection hologram provides wider angular bandwidth (larger effective pupil), narrower spectral bandwidth (better see-through) and lower sensitivity to beam polarization (better efficiency) than a transmission volume hologram. A volume reflection hologram may also be fabricated holographically.  
         [0084]    Reasons for selecting a blazed reflection grating (FIGS.  9 - 11 ) at the input end may include the following: A blazed reflection grating diffracts to a single order. Because of broad angular bandwidth, the need for varying the blazed angle across the grating is reduced. A blazed reflection grating may be fabricated by ruling with continuous tone or binary masks, or may be fabricated holographically.  
         [0085]    It will be appreciated that the inventors have discovered that the chromatic aberration produced by the DOEs may be offset by chromatic aberration produced by the collimator. The DOEs produce negative chromatic aberration. In one embodiment of the invention, the refractive lenses of the collimator produce positive chromatic aberration, offsetting the negative chromatic aberration of the DOEs.  
         [0086]    As described, the present invention need not be limited to night vision systems, but may be utilized in applications including a virtual reality viewer for computer generated images, in addition to the application of night vision devices for ground troops aviators and vehicle drivers.  
         [0087]    It should be understood that the embodiments described herein are merely exemplary, and that a person skilled in the art may make variations and modifications without departing from the spirit and the scope of the invention. All such variations and modifications are intended to be within the scope of the invention as defined in the appended claims.