Patent Publication Number: US-8125716-B2

Title: Near eye display prism optic assembly

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, sold, imported and/or licensed by or for the United States Government. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure relates to near eye, head or helmet-mounted display systems, weapon sight viewers, and handheld viewing devices, incorporating micro-display technologies. 
     BACKGROUND INFORMATION 
     Most prior art in the area of monolithic prisms for micro-display imaging have similar properties. Such patents describe a prism with three optically-active surfaces, which can be used with and without a corrector optic. 
     Recent developments in micro-display technology have created large-format, full-color micro-displays. A new near-eye optical imaging approach is needed to fully realize the size and resolution of these displays with wide fields of view and large eye pupils. Also to use such a device effectively in a system concept, the imaging device must allow a large range of diopter adjustment for comfort and must conserve power by not requiring digital image remapping of the displayed image. 
     Similar previous work employs a monolithic prism for presenting micro-display imagery to the eye in a compact, low-profile manner. These monolithic prisms utilize multiple internal reflections to image a micro-display to the eye. For example, U.S. Pat. No. 5,745,295 describes such an apparatus, including an image display device and an ocular optical system for displaying an image of the display device. While such known display image sources can scale to diagonal dimension of less than or equal to 15 mm, these known sources will have several performance flaws if scaled to image sources with a diagonal dimension of greater than 15 mm. This is due to the limited optical path associated with such known prism arrangements. Further, such prior art disclosures (e.g., U.S. Pat. No. 5,745,295) describe embodiments that have discontinuities in optical surface profiles which preclude scaling without extensive redesign. 
     SUMMARY 
     The present disclosure uniquely addresses solutions to the flaws in such known works, e.g., lack of focus in the absence of a digital remapping of imagery, asymmetric distortion, requirement of digital remapping of imagery, small eye relief (&lt;25 mm), small pupil (4 mm), small usable array size on the micro-display (&lt;15 mm), small field of view (&lt;40°×30°), and environmental vulnerability. In another aspect, scalable optical configurations are disclosed suitable for various display image formats with a diagonal dimension of less than 30 mm. 
     Exemplary embodiments are disclosed to allow imaging large display format sizes as needed. Such exemplary embodiments can have continuous surface profiles throughout optically active areas on prism optic, which allow large pupil sizes and the ability to be scaled to different sizes and formats. 
     Further, prisms made of molded plastic for low weight and ease of application of an aspheric surface can be utilized. All optical plastic elements require glass sacrificial windows to protect the delicate plastic material during normal use, decreasing the usable eye relief of such a device. The corrector optic is chosen in an exemplary embodiment to deliver the 25 mm eye relief for ease of use while providing environmental protection for the prism. Such an exemplary embodiment can address the need of a pupil up to 6 millimeters in diameter, a large eye relief of 25 millimeters, and display diagonals of up to 24.6 millimeters simultaneously in a package that will fit on a helmet mount, weapon sight, or handheld viewer and will be protected from environmental hazards. Additionally, such an exemplary embodiment can be telecentric and have negligible chromatic aberration and distortion, which can allow it to be used as a full-color photopic, digital display device without any digital image re-mapping for a large range of diopter focus. 
     Low and symmetric distortion can be attained by making the surface nearest to the display flat and controlling magnification nearer to the pupil. Low and nearly-symmetric distortion make this a scalable design because the image does not warp when scaled. 
     The corrector group is selected such that it sufficiently balances the backward curving field and provides excellent chromatic correction. This radially-symmetric correction is applied near to the pupil, and therefore has little effect on distortion. The prism is configured in conjunction with this corrector group to produce negligible distortion effect at the object plane that is extremely symmetric in comparison to prior art. 
     Such a unique configuration can also allow a telecentric optical path in object space such that the focus of the eyepiece can be adjusted +1 to −2 diopters without any significant magnification errors or warping of imagery. Prior art does not specifically teach this. Rather, prior art requires warping tables to digitally remap the imagery at nominal focus and any different diopter settings. 
     Such a micro-display technology as variously embodied can create much larger format arrays, which can have slightly smaller pixels. Also, uses of such devices requiring larger fields of view can be satisfied. The present disclosure addresses these concerns, including the needs for a large-format display. 
     Accordingly, one aspect of the present disclosure relates to an optical configuration that is reproducible, uses a prism optic in conjunction with a glass collimating optic, and presents imagery of a micro-display to the eye. 
     Another aspect of the present disclosure is to provide focus ability, low-power implementation, large field of view, exit pupil, eye relief for a large-format, and full-color micro-display. 
     In one exemplary embodiment, lightweight, plastic prism Optic is used with an aspheric surface in conjunction with a glass corrector optic to magnify the image of a micro-display and very comfortably present the immersive imagery to a user. 
     Such an exemplary embodiment can nominally provide a 40×30 degree, rectangular field of view of a 24.6-millimeter-diagonal, full-color, micro-display at a 25 millimeter eye relief for an axial exit pupil that is 10 millimeters in diameter, decreasing to a 6 millimeter exit pupil for the corner field angle. Furthermore, the optical features can be scaled to fit several display sizes at multiple fields of view. Such an embodiment as exemplified can be scalable to have the same field of view and eye relief on a 30-millimeter-diagonal display. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is provided in reference to the attached drawings wherein: 
         FIG. 1  is a schematic representation of an exemplary embodiment of prism optic assembly. 
         FIG. 2  is an exemplary housing and focus mechanism shown with such a prism optic assembly. 
         FIG. 3  is a plot of the orthoscopic, near-radially-symmetric distortion profile that is achievable with such an exemplary prism optic assembly. 
         FIG. 4  is a plot of the astigmatic field curves showing the flat field curvature that is achievable with such an exemplary prism optic assembly. The vertical lines on the plots denote the equivalent focus positions of +0.25 and −0.33 diopters. 
         FIG. 5  is a plot of the lateral color correction that is achieved with such an exemplary prism optic assembly. 
         FIG. 6  is an exemplary plot of the average of radial and tangential modulation transfer function at selected points within the full 40-degree by 30-degree field of view of an exemplary embodiment. 
         FIG. 7  illustrates an exemplary optical design prescription according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description is provided in reference to an exemplary embodiment shown in  FIG. 1 . 
       FIG. 1  illustrates an optical schematic of an exemplary embodiment of Prism Optic Assembly (POA). This figure shows an exemplary layout of the integrated optical components and depicts a representation of the optical path from a micro-display  1  to the eye  4  in a horizontal view of a vertical cross-section as shown. 
     The POA  10  comprises an optical member  2  and a corrector lens grouping  3 . The optical member  2  has three adjacent optical surfaces, two of which are curved surfaces; the other surface is flat. The volume between these surfaces together with their mutually orthogonal edge faces is filled with an optical polymer. Such a prism optic is ideally comprised of an optical plastic that can be diamond turned or molded to create the 3 optical surfaces and mechanical mounting features. However, this prism can also be molded or ground in glass or some other optical material that transmits visible radiation. 
     Light rays emitted from the flat panel display surface  1  enter the optical member  2  through the first transmitting surface  5  and are subsequently reflected by the first reflecting surface  6 . The first reflected light rays are again reflected by the second reflecting surface  7 , whereupon they are led to the observer&#39;s eye pupil  4  via a second transmitting surface  8  and the respective transmitting optical surfaces of the corrector lens grouping  3 . The first reflection at surface  6  is enabled by total internal reflection (TIR). A back-coated mirror accomplishes the second reflection at surface  7 . Surfaces  6  and  8  are, in reality, one in the same surface. They are differentiated solely on the basis of the reciprocating ray surface sequence through the optical member  2 . 
     The optical surfaces of optical member  2  are tilted and decentered with respect to a reference axis  9 , which is the axial principal ray that passes through the center of the eye pupil  4 . 
     In order to correct the POA for the image errors induced by tilted and decentered surfaces, the curved surfaces of optical member  2  are anamorphic, wherein the surface geometries are independent in the X- and Y-axes. The sag of an anamorphic surface is given by the expression 
     
       
         
           
             
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     k x  and k y =conic constant in x and y, 
     α I =aspheric coefficients on powers of x, 
     β I =aspheric coefficients on powers of y. 
     Anamorphic surfaces are not rotationally symmetric, however, they are bi-laterally symmetric (about the Y-axis). 
     The optical surfaces of the corrector lens grouping  3  are spheres, all of which are centered on the reference axis  9 . 
     With such an exemplary embodiment, because some of the surfaces of the POA are tilted and decentered about a reference axis, such first-order properties that are normally derived from paraxial ray tracing would be inapplicable, and therefore are invalid. Therefore, it was necessary to use exact ray tracing to calculate attributes such as focal length, numerical aperture, image height, entrance and exit pupil positions, etc. 
     Adjusting diopter focus on this optical assembly can be achieved in two ways: by moving the corrector optic  3 , or by moving the micro-display  1 . 
       FIG. 2  is an exemplary housing and focus mechanism shown with such a prism optic assembly. Adjustment of diopter focus of the exemplary embodiment is also shown with respect to  FIG. 2 . Diopter focus is achieved by moving micro-display  1  and its associated back-light and electronics together as micro-display assembly  12  in the direction indicated by arrow  11 . Micro-display assembly  12  is oriented perpendicular to reference axis  19 , which is the axial principal ray that passes through the center of the eye pupil in object space. Micro-display assembly  12  is centered about reference axis  19 . Drive motor  13  can actuate linear drive mechanism  14  to move the micro-display assembly  12  along the length of reference axis  19  to achieve diopter focus. Another linear drive mechanism  15  can operate simultaneously to linear drive mechanism  14 , and a rail that lies parallel to  14  and  15  control the orientation of micro-display assembly  12 , e.g., to be always perpendicular to reference axis  12  throughout the full range of diopter focus. 
     In yet another aspect, minor adjustment to diopter focus can be achieved by moving corrector optic  3  along reference axis  9 , e.g., in the directions indicated by arrow  18 . In this another aspect of the exemplary embodiment, corrector optic  3  has a positive optical power, and therefore may not efficiently allow diopter adjustment. The corrector optic can move, e.g., 2 mm to achieve a diopter change of 0.5 D. However, corrector optic  3  can better effectively adjust the magnification of the prism optic assembly. For applications that require precision tuning of magnification, corrector optic  3  can be positioned along reference axis  9 , e.g., through the use of machined lens spacers and precision-machined lens housings to achieve fine-tuning of the focal length of the prism optic assembly. Focal length of the prism optic assembly can be adjusted in this manner over a range, e.g., of 0.5 mm with negligible effect to system performance. 
     The corrector optic  3  also provides an environmental seal for the prism optic  2  when this assembly is mounted in a package. An exemplary embodiment of the mounted prism optic assembly is shown in  FIG. 2 . Such an exemplary embodiment would have the prism optic  2  comprised of an optical plastic, which can be susceptible to scratching, moisture, and other environmental hazards. In this exemplary embodiment, corrector optic  3  is comprised of glass material. Corrector optic  3  can be sealed to the housing at location  17  either with adhesive or elastomeric seal to form an airtight seal around its annular contact area with the lens housing  16 . Such an exemplary corrector optic  3  can provide a ruggedized protection for the prism optic  2 . 
     The corrector optic  3  allows this optical assembly to have a long eye relief, large field of view, and large optical pupil, while still remaining compact in size. Such an exemplary embodiment can be used in handheld, weapon-mounted, or head-mounted scenarios, and can be configured to mount on a headgear, e.g., a helmet, which is the most size-limiting employment of a head-mounted optic. 
     The optical assembly as described uses a flat surface closest to the micro-display, a correcting optic, and an aspheric surface that are designed in conjunction to provide a very low-magnitude, symmetric optical distortion. 
     Optical distortion is a common aberration in prior art, and is defined as a change in focal length with field angle; or more commonly, an aberration that alters the shape of an image. Prism optics inherently exhibit non radially-symmetric distortion due to their use of off-axis, tilted optical surfaces with optical power. An exemplary embodiment of the present disclosure will exhibit optical distortion as shown in  FIG. 3 . The line grid in  FIG. 3  represents a perfect image with 0% distortion. The small “X” symbols denote the distorted location of the grid intersections and represent the distorted image as formed by the prism optic assembly. The maximum distortion in an exemplary embodiment of this disclosure is less than 1.32%. This maximum occurs at the extreme corners of the field of view, at 25-degree field angle from the optical axis. This amount of distortion is not noticeable by the human eye, and is considered to be orthoscopic, meaning free of distortion. The distortion exhibited by the exemplary embodiment is nearly radially-symmetric about the viewing optical axis, which creates imagery that is very similar to that provided by more conventional lens optics. Even if the magnitude of the distortion was increased to become noticeable in alternative exemplary embodiments, the nearly axially-symmetric profile would still provide a high level of comfort for the user of this device.  FIG. 1  indicates the features employed by the exemplary embodiment of the current disclosure to uniquely control distortion to levels below human perception. Corrector optic  3  applies radially symmetric optical power and balances negative field curvature, thereby reducing the optical power required by the surfaces of the prism. Furthermore, optical surface  5  on the prism is flat, and contributes no optical power. Optical surface  5  is located near to the focal plane, and therefore any optical power on this surface contributes almost completely to field curvature and distortion. The exemplary embodiment eliminates optical power on this sensitive surface to reduce distortion and control distortion symmetry. 
     An exemplary embodiment of this optical assembly provides a 25-millimeter eye relief when viewing a 40-degree horizontal by 30-degree vertical field of view on a 19.2×15.36 millimeter, color micro-display. The designed optical pupil is 4 millimeters in diameter, and provides equivalent axial optical performance when positioned at any X-Y location within a 6 millimeter circle centered on the assembly&#39;s optical axis at the eye pupil plane. Also, the optical assembly provides adequate optical performance for all off-axis fields at X-Y displacements of the 4 millimeter pupil that decrease with field angle down to 1 millimeter when viewing the corner field on the micro-display. The pupil size of this invention is a feature that provides for a very comfortable, immersive viewing of the micro-display imagery. This comfort level makes the exemplary embodiment(s) ideal for an immersive display in commercial applications and as well as field mobility and targeting applications. 
       FIG. 4  shows exemplary astigmatic field curves of the exemplary embodiment. The curves depict focal performance better than what the human eye can perceive. The exemplary embodiment achieves this performance by using corrector optic  3  (as shown in  FIG. 1 ) to balance the negative field curvature induced by the prism optic. The vertical axes of the plots in  FIG. 4  indicate the radial field angle within the image relative to an axial position in the center of the field of view. The horizontal axes denote positions along the optical axis in object space, centered about the ideal focal plane at position 0 (zero) The dashed curves represent the ideal focus position that the exemplary embodiment creates in object space. The long dash is labeled “S” and represents the sagittal (radial) focus. The short dash is labeled “T” and represents the tangential focus. The difference between the sagittal and tangential foci is known as “astigmatism”. The solid vertical lines in the plots in  FIG. 4  represent the human eye&#39;s imperceptible accommodation limits. The human eye can accommodate to focus within −0.33 diopters and +0.25 diopters of its nominal focus position subconsciously with no effort, and no defocus within these bounds is perceivable. The plots show that for field angles in the positive vertical (denoted +Y), negative vertical (denoted −Y), and the horiztonal (denoted +X, and is symmetric and equal to the performance in the negative horizontal, so is not listed) directions, the focus of the exemplary embodiment is perfect within the human ability to perceive. 
       FIG. 5  is a plot of the exemplary embodiment performance with respect to lateral chromatic aberration. The exemplary embodiment uses the corrector optic together with the prism optic to balance chromatic aberration throughout the photopic spectrum for full-color performance. The photopic human eye responds primarily to all wavelengths from 0.47 μm to 0.65 μm. The vertical axis in  FIG. 4  depicts the field angle relative to an axial position in the center of the field of view. The horizontal axis shows radial distance from the focus of 0.55 μm light. This axis depicts the lateral focus position for select colors within the photopic spectrum. The vertical lines in  FIG. 5  show limits to human perception of chromatic aberration in an optical system, within which aberrations are imperceptible. The vertical solid line indicates the diffraction limited blur spot diameter, or “Airy diameter”. This diffraction-limit is the absolute minimum measurable unit with any optical measurement device. The vertical long-dash line indicates the diameter of a 15-micron pixel, which is common in the art. 
     Chromatic aberration is an image blur caused by an optic focusing separate color wavelengths differently from one another. The primary chromatic aberrations are axial and lateral. Axial chromatic aberration occurs when each color focuses at a different distance along the optical axis, and appears as circular blur spot. The spot typically shows either a blue or red ring around a white spot. Lateral chromatic aberration occurs when each color focuses at a different field height, and appears as an oblong, rainbow-colored spot. Lateral chromatic aberration is typically corrected by using several different glass types with different properties. Lateral chromatic aberration is the most challenging chromatic aberration to correct in a prism optic assembly due to the use of a prism optic that is made entirely of the same material. The exemplary embodiment employs the corrector optic and its glass material properties along with the prism optic together to balance lateral chromatic aberration for full-Color performance. 
       FIG. 6  shows the modulation transfer function (MTF) of the exemplary embodiment. MTF is a commonly accepted performance metric for optical systems. MTF is a measure of image blur in that is describes modulation between the brightest and darkest areas between two equal blur spots created by an optical system at a particular location in an image, at a particular distance from each other. The particular distance from each other is measured in spatial frequency, with typical units of cycles per millimeter (cy/mm). MTF is measured in percent, with perfect modulation of 100%, and cutoff with no modulation at 0%.  FIG. 6  is a 2-dimensional representation of the entire image area presented by the exemplary embodiment for a full field of view of 40-degrees horizontal by 30 degrees vertical. The numbers in the chart are the average MTF values (in units of percent) at that particular location within the image field of view. The average MTF is a weighted average of MTF results for each color within the photopic spectrum, and an equal-weighted average of sagittal (radial) and tangential MTF results. Sagittal and tangential MTF are simply descriptions of blur in the vertical and horizontal directions, respectively. The human eye essentially sees an average of all these results as a common blur, and so this measurement is an effective description of performance. The MTF results for the exemplary embodiment in  FIG. 6  are sampled at 34 cy/mm, which corresponds to the maximum spatial frequency visible when using the exemplary embodiment with a micro-display that is made up of 15-micron pixels. 
     Shown in  FIG. 7  is an exemplary optical design prescription. The columns are described as follows: “Surf No.” indicates the Surface number within the optical path as traced from the pupil of the eye to the micro-display focal plane. “Radius of Curvature” describes the surface curvature of each the surfaces in the optical path. Radius of curvature is given in units of millimeters. The variables in the “Radius of Curvature” column are coefficients that are used in the following formula to describe the sag (z) of each surface along that surface&#39;s optical axis. 
             z   =             c   x     ⁢     x   2       +       c   y     ⁢     y   2           1   +       1   -       (     1   +     k   x       )     ⁢     c   x   2     ⁢     x   2       -       (     1   +     k   y       )     ⁢     c   y   2     ⁢     y   2               +       ∑     i   =   1     16     ⁢       α   i     ⁢     x   i         +       ∑     i   =   1     16     ⁢       β   i     ⁢       y   i     .                 
Note that in the formula: C x =1/R x , and c y =1/R y . Aspheric surfaces are described with multiple constants that can be placed into the formula to calculate the sag of the aspheric surface to be machined or molded to re-create the proper surface curvature of the exemplary embodiment.
 
     Further, “Surface Separation” as shown in  FIG. 7  provides the thickness in millimeters along the optical axis between the vertices of each of the surfaces. The prism surfaces do not contain values in this column, as they do not lie on the optical axis. “Refractive Index (Displacement)” first gives the refractive index of the optical material for each surface at the 0.55 μm wavelength. Secondly, this column provides the Y and Z locations in 3-dimensional space, relative to the pupil (surface  1 ) in millimeters. “Abbe No. (Tilt Angle)” first provides the Abbe number of the optical material for each surface in the exemplary embodiment. Secondly, this column provides the tilt angle for each optical surface, signified by the symbol theta (θ) and given in units of degrees. The optical prescription in  FIG. 7  depicts the exemplary embodiment in a position that images at the −1 diopter setting. Diopter changes are Made by adjusting the position of surface  9 , as shown in  FIG. 7 . 
     In summary, exemplary embodiments as shown in  FIG. 1  can use a 3-surface prism  2  and corrector optic  3  to image a microdisplay device  1  to the eye. Further, a flat surface  5  can be disposed near a display plane to allow distortion to be uniquely controlled by a corrector optic  3  and the two anamorphic aspheric surfaces (e.g.,  6 / 8  and  7  of  FIG. 1 ) such that it is negligible, or orthoscopic. A centered, symmetric corrector lens  3  and the flat surface  5  of the prism  2  allow us to uniquely achieve low distortion and telecentricity. 
     Further, the display focal space can be uniquely telecentric with a maximum chief ray angle of 0.4 deg across an entire (e.g., 19.2×15.36 mm rectangular) display area; allowing for no apparent magnification or performance change throughout a large range of travel for diopter-focus adjustment. For example, 3.25 diopters of adjustment can be made before there is even 1 pixel of magnification change. The flat surface  5  and corrector optic  3  features interact with the anamorphic asphere surfaces to allow such an exemplary embodiment to achieve the unique attributes of, e.g., long eye relief (25 mm), large display area (19.2×15.36 mm with 40-deg×32-deg FOV), and scalability to different format sizes. 
     Accordingly, such a prism-optic assembly can achieve: 
     Nearly symmetric distortion; 
     negligible orthoscopic (negligible . . . &lt;1.32%) distortion (see, e.g.,  FIG. 2 ); 
     one flat surface (see, e.g.,  5  of  FIG. 1 ) at surface closest to display; 
     long eye relief (25 mm+); 
     large display area (19.2×15.32 mm); 
     telecentric (only 0.4 deg variation in field angle across display . . . good diopter focus); 
     diopter range without digital warping; 
     scalable to other formats and sizes; 
     diopter range +1 to −2 D, and maybe larger; 
     corrected chromatic aberration;
         6 mm eye pupil;       

     biconic Zernicke anamorphic asphere; 
     doublet corrector element; 
     3 surface prism; and/or 
     all surfaces are continuous throughout optical area. Centered, symmetric corrector lens (e.g.,  3  of  FIG. 1 ) and the flat, display-side surface (e.g.,  5  of  FIG. 1 ) of the disclosed exemplary prism (e.g.,  2  of  FIG. 1 ) allow us to uniquely achieve low distortion and telecentricity. These exemplary features, along with the disclosed anamorphic asphere surface, can help to achieve the long eye relief, large display area, and scalability. 
     The invention has been described in an illustrative manner. It is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than limitation. Many modifications and variations of the invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the invention may be practiced other than as specifically described.