Abstract:
An aperture plate includes an opening and a surface adjacent to the opening. The opening passes electromagnetic energy such as light to a reflector that is aligned with the opening and that directs the electromagnetic energy to a location. The surface reflects incident electromagnetic energy away from the location in a direction that is outside of the range of directions. Such an aperture plate insures that electromagnetic energy, e.g., light, strikes only the desired portions of the reflector, and that peripheral light that is outside of the aperture opening is reflected away from the location, e.g., display screen, toward which the reflector directs the electromagnetic energy. Furthermore, because such an aperture plate is mounted near the reflector, the alignment tolerances are typically less stringent than for an aperture plate mounted near the energy source.

Description:
BACKGROUND OF THE INVENTION 
   Scanned beam displays and image-capture devices have been developed to produce high-resolution images. As shown in  FIG. 1 , a modulated source  10  of light, which may include a modulated point source of light, is formed into a source beam  13  by optics, e.g., a focus lens  12 , and directed by the lens onto a moving scan mirror  14 , which reflects the beam onto an image curve or plane, such as a display screen  16 , to create a viewable image. The mirror  14  may also scan the beam  13  directly into a viewer&#39;s eye (not shown in  FIG. 1 ) to create the image directly on the viewer&#39;s retina. One application of this latter technique is for use in a head-worn personal display system, such as described in U.S. Pat. No. 5,467,104, entitled VIRTUAL RETINAL DISPLAY (VRD), which is incorporated by reference. Furthermore, the mirror  14  may be a bi-axial microelectromechanical scanner (MEMS) that scans the beam in a raster pattern. Such a MEMS is described, for example, in U.S. Pat. No. 5,629,790 entitled MICROMACHINED TORSIONAL SCANNER, which is incorporated herein by reference. 
   Still referring to  FIG. 1 , an aperture  18  may be placed away from the scan mirror  14 , here intermediate the lens  12  and the mirror, to block unwanted light as indicated at  20  and to confine the beam  13  to a clear-optical-quality area of the mirror  14 . Ideally, the beam  13  has a cross section equal to the cross section of the clear-optical-quality surface of the scanning mirror  34  at the scanning mirror. Generally, the wider the beam  13  at the scanning mirror  34 , the smaller the achievable cross section of the beam at the image display  16 , and hence the greater the resolution that the displayed image can have. 
   Typically, the cross section of the beam is expanded to be no larger than the area of the clear-optical-quality surface of the mirror  34 , which is the area of the mirror having high optical quality. This is because the often poor-optical-quality perimeter of the mirror  34  and structure that supports the mirror (e.g., MEMS torsion arms) may scatter or otherwise reflect light from the periphery of the beam  13  (i.e., light that strikes beyond the boundary of the clear-optical-quality surface of the mirror) into the image, and thus create visible image artifacts. Because the mirror-support structures often have surfaces that lie in planes that are parallel to the planes that the mirror  34  moves through, these structures often cause bright spots in portions of the image. For example, if a mirror-support structure is coplanar with the at-rest (i.e., zero) scan position of the mirror  14 , then peripheral beam light reflected by the structure may cause the center of the image to appear brighter than the periphery of the image. The perceived brightness of the image is the eyes&#39; integration of the brightness of the scan beam  13  over the area of the image. That is, the scanning of the beam  13  “spreads out” the brightness of the beam over the area of the image. But because light reflected from the mirror-support structures is not scanned, the eyes do not integrate this light over the area of the image. Consequently, this reflected light may cause bright spots in the image. Even a relatively small amount of this unwanted reflected light can cause a visible artifact in the viewed image. 
   One way to limit or eliminate the peripheral light from the beam  13  that is outside the clear-optical-quality surface of the mirror  14  is to use the aperture plate  18 , which is an opaque plate that defines an opening  22  through which the beam  13  propagates. The placement of the aperture plate  18  involves a number of considerations. One placement is as shown in  FIG. 1 , in the beam path between the lens  12  and mirror  14 . The aperture plate  18  thus allows only a certain portion of the beam  13  to propagate through to the mirror. The size of the opening  22 , which defines the size of the beam  13 , can be calculated based on the distance along the optical path from the source  10  to the mirror  14  and on the focal length of the lens  12  so that the beam  13  fills the optical-quality surface of the mirror  34  completely but does not extend beyond this surface. Although the aperture plate  18  is shown between the mirror  14  and the lens  12 , the aperture plate can also be located between the source  10  and the lens. 
   However, there are imaging systems where the placement of the aperture plate is more constrained. For example, if the system includes multiple sources  10  of light such as in certain types of color display or image capture systems, then the distance that the aperture plate can be from the sources on the source side of the lens  12  is limited by where the light from one source  10  overlaps the light from an adjacent source. That is, light from one source  10  may “leak through” the aperture-plate opening for another source—the aperture plate typically has one opening per source. The phenomenon of light from one source “leaking through” the aperture-plate opening for another source is often called cross talk. Many imaging systems include more than one light source. A color imaging system often uses different colored sources for creating full-color images. Furthermore, some systems include multiple sources of each primary color. For example, such systems may include 13 blue sources  10  to create 13 blue scan beams, 13 red sources  10  to create 13 red scan beams, and 26 green sources  10  to create 26 green scan beams. And these light sources  10  are often LEDs, not lasers. Because many LEDs radiate light over a range of angles, cross talk can be even more of a problem where LEDs are used, and thus aperture-plate placement in such systems may be constrained. 
   To prevent beam cross talk in scanned-beam systems having multiple sources  10 , aperture plates are often placed in one of two locations: 1) close enough to the sources  10  so that light from one source does not propagate through the aperture opening of another source; and 2) close to the scan mirror (much closer than shown in  FIG. 1 ) so that the aperture plate will not interfere with proper beam formation. 
   Locating an aperture plate close to the sources  10  may require precision in manufacture. For example, the distances between adjacent sources  10  may be on the order of 300 microns, and the diameters of the aperture-plate openings may be on the order of 10 microns. The alignment tolerance of the aperture openings relative to the sources  10  may be on the order of 6 microns. While it may be possible to manufacture an imaging system with such an aperture-plate alignment precision, the cost and difficulty may be prohibitive. Also, such designs may also impose restrictions on design of a corresponding lens assembly. 
   An aperture plate located close to the scan mirror may reflect the light from the periphery of the beams into the viewer&#39;s field of view, and thus cause artifacts in the image as discussed above. In such a location, the aperture plate is or is approximately parallel with the mirror in its rest position. Therefore, as discussed above, the plate may reflect peripheral light from the beams into the center of the image, and thus cause the center of the image to appear brighter than the periphery of the image. Even if the plate has an anti-reflective coating, it may still reflect enough of the peripheral light to create a visible artifact in the image. 
   SUMMARY OF THE INVENTION 
   An aperture plate includes an aperture opening and a surface adjacent to the opening. The opening passes electromagnetic energy to a reflector that is aligned with the opening and that directs the electromagnetic energy to a location. The surface deflects incident electromagnetic energy away from the location. 
   The aperture plate blocks at least a portion of electromagnetic energy, e.g., a light beam, from striking undesired portions of the reflector, and the plate surface deflects peripheral energy in one or more directions that are different from the direction or directions in which the reflector directs the energy. Furthermore, because such an aperture plate is mounted near the reflector, the alignment tolerances may be relaxed relative to an aperture plate mounted near a light source. This may reduce the cost and/or difficulty of manufacturing an imaging system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration of a conventional imaging system. 
       FIG. 2  is an exploded isometric view of an aperture structure according to one embodiment of the invention combined with a MEMS scan mirror assembly. 
       FIG. 3  is a diagrammatic view of an aperture plate and scan mirror according to the embodiment of  FIG. 2  receiving and reflecting a source beam of light to create an image in a target field of view. 
       FIGS. 4A and 4B  are plan and end views, respectively, of an embodiment of the aperture plate according to another embodiment of the invention. 
       FIGS. 5A and 5B  are related diagrammatic illustrations of how unwanted peripheral light is reflected away from the scanned field of view by an aperture plate similar to that of  FIGS. 4A and 4B  but with different channel angles. 
       FIG. 6  is an isometric view of an aperture plate according to another embodiment of the invention. 
       FIG. 7  is an isometric view of an aperture plate according to yet another embodiment of the invention. 
       FIG. 8  is a diagrammatic illustration of an embodiment of the invention employing an aperture plate and scanning mirror in a multiple-light-source color VRD system. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 2 , an aperture plate  24  has a surface treated to absorb peripheral light or direct peripheral light away from a viewer&#39;s field of view. By directing the peripheral light away from the viewer&#39;s field of view, the plate  24  may eliminate or reduce the intensity of image artifacts, such as viewable bright spots. Treatments include providing a patterned surface. 
   With further reference to  FIG. 2 , the exemplary aperture plate  24  has formed on one face an angled surface structure  26 , which surrounds a central aperture opening  28 . A MEMS mirror  34  is contained within a housing  38 , which has an open or transparent top  40  onto which the aperture plate  24  is mounted with the aperture opening  28  in optical alignment with the mirror  34  and the angled surface structure  26  facing away from the mirror toward a source beam of light as illustrated in  FIG. 3 . When so assembled, the plane of a face  30  (opposite structure  26 ) of the aperture plate  24  is substantially parallel to the mirror  34  in its rest position, and is mated to the MEMS housing top  40  and secured by suitable means such as epoxy adhesive. As is well known, the MEMS mirror  34  has attached torsion arms (not shown) and related parts for moving the mirror in a bi-axial motion such that the mirror sweeps the reflected scan beam in a raster pattern across the field-of-view of the image-capture device or the display target (e.g., a screen or retina, not shown in  FIG. 2 ). 
   Referring to  FIG. 3 , the operation of a multiple-source light scan display system or image capture system  50 , which includes the components of  FIG. 2 , is discussed according to an embodiment of the invention. In the system  50 , the mirror  34  and the aperture plate  24  are canted to the mean optical axis  52  of a beam  54  formed from one or more sources of light—i.e., the beam  54  may be a combination of multiple beams, each propagated from a respective light source. Alternatively, a plurality of beams may be combined by a beam combiner and the beam  54  may be a combined beam from the light sources. Here for example the canting is at an angle α=60°. As a result of this canted orientation, the MEMS mirror  34 , when in its rest position, reflects the beam  54 —the reflected beam  54  has a center ray  56 —onto a field-of-view or screen  58 , which may be a viewer&#39;s retina. The mirror  34  rotates back and forth about two orthogonal axes to sweep the reflected beam  54  in two dimensions about the rest-position angle to scan an image on the screen  58  or to sequentially illuminate spots in a field-of-view  58 , which may be useful, for example, for capturing an image therefrom. The volume that the beam  54  sweeps out is the scan path, field of view (FOV) which is the same as the where the screen  58  is a viewer&#39;s retina. 
   The opening  28  of the aperture plate  24  passes the desired main portion of the incident beam  54  to the clear quality area of the mirror  34 , and the patterned surface structure  26  of the aperture plate absorbs and/or deflects peripheral portions of the beam away from the screen  58 . More specifically, the angled surface structure  26  of the aperture plate  24  intercepts the blocked peripheral light  64  and is configured and oriented so that any unwanted deflections  66  off this otherwise anti-reflective structure are directed away from the screen or field-of-view  58 . 
   The surface structure  26  of the exemplary aperture plate  24  includes an array of saw-tooth-shaped channels  27  having alternating valleys  27   a  and ridges  27   b , and side walls  27   c  and  27   d  (see also  FIG. 2 ) that meet at right angles. The side walls  27   c  are wider than the side walls  27   d  and are angled roughly 90 degrees relative to the beam axis  52  to directly deflect peripheral light from the beam  54  back toward the source as indicated by arrows  66 . The depth (d) of the channels  27  is, for example, less than about ¼ to ¾ of the thickness (t) of the relatively thin aperture plate  24 . The number of channels  27  can be decreased or increased. By increasing the number of channels  27 , there will be a corresponding decrease in the depth (d) so that the surface may appear as textured with many fine shallow channels. As discussed further below, the surface structure  26  has substantially no flat surface areas (i.e., side walls  27   d  and  27   c ) that are parallel to the plane of the mirror  34  while the mirror is in any of its beam-sweep positions. Consequently, substantially no light reflected by the surface structure  26  strikes the screen  58 . 
   Alternatively, the sidewalls  27   c  and/or  27   d  can be set to a different angle to deflect peripheral light away from the screen or field-of-view  58 . According to a second alternative, the sidewalls  27   c  and/or  27   d  may be formed at a variable angle to distribute stray light more evenly across the screen or field-of-view  58 . Such an arrangement may be used to advantage, for example, by increasing the brightness of the illuminated field-of-view in an image capture system using otherwise wasted light, thus aiding in aiming. According to another alternative, the surface structure  26  of aperture plate  24  may be formed to direct non-scanned light to form one or more aiming features (e.g., an “X” or a dot at the center of the field-of-view, an “L” defining each corner, or a frame surrounding the field-of-view) to aid in positioning an image capture or display device. Because such projected features create a substantially DC scattered light signal, such light can be prevented from affecting a captured image by AC-coupling the detection system. One skilled in the art will recognize that a variety of other angles and orientations may be implemented depending upon the particular configuration and system. 
   One way to form the channels  27  in the aperture plate  24  is to wet etch the channels, such as when the plate is formed from silicon. Wet-etched surfaces are often highly specular and may permit accurate control of where the stray light is diverted away from the screen  58 . Other fabrication processes can be used, however. Such processes include molding as discussed below for the embodiments of  FIGS. 6 and 7 , micromachining, casting, or other appropriate fabrication techniques. 
   As discussed above, the exemplary surface structure  26  of the aperture plate  24  can substantially reduce or eliminate residual reflections from the aperture plate onto the screen  58 . Without the channels  27 , the surface of the aperture plate  24  would be parallel to the plane of the mirror  34  in its resting position, and, therefore, reflections from the aperture plate might cause visible artifacts in the center of the image. Furthermore, if the flat surface of the aperture plate  24  were parallel to any plane through which the mirror  34  rotates while sweeping the beam  54 , then the plate might cause visible artifacts in the corresponding portion of the image. And, as discussed above, merely providing an anti-reflective coating and diffuse scattering on such a flat surface when parallel to the mirror may not prove sufficient for some applications. Such an approach relies primarily on increases in light absorption and diffuse scattering, and may present fabrication difficulties or reduce image quality, such as contrast ratio). Anti-reflective coatings with reflectances as low as or lower than 10% can cause visible artifacts in the scanned image. The channels  27  can reduce the level of reflectance of peripheral light onto the screen  58  by an order of 100, 1000, or more. The channels  27  may additionally be treated with a suitable anti-reflective coating to further reduce peripheral light onto the screen  58 . Suitable anti-reflective coatings for the aperture plate  24  include fine ground Si that can be further blackened according to known processes. 
   Still referring to  FIGS. 2 and 3 , the scan mirror  34  is located slightly below the opening  28  of the aperture plate  24 , such as by a fraction of a millimeter to a few millimeters. The opening  28  has substantially the same shape as the mirror, in this case circular or round, and has the same or approximately the same size as the clear-optical-quality region of the mirror so that the aperture plate  24  covers the low-optical-quality perimeter of the mirror. 
   Mounting of the aperture plate  24  on the MEMS housing  38  is adjusted to provide enough clearance (the fraction to a few millimeters clearance discussed in the preceding paragraph) above the mirror  34  to give it room to scan without hitting the aperture plate. The aperture plate  24  is thin enough so that reflections off the side of the aperture opening  28  do not cause visible artifacts in the image. Although not shown in  FIGS. 2 and 3 , the sidewall of the opening  28  can also include one or more channels  27  or other surface structures to deflect any unwanted light away from the screen  58 . 
   Because the aperture opening  28  has approximately the same size as the mirror  34 , the alignment tolerances of the aperture plate  24  relative to the mirror are less stringent than the alignment tolerances of an aperture plate on the source side of the focusing optices (e.g., the lens  12  of  FIG. 1 ), particularly where there are multiple sources. Consequently, the manufacture of the system  50  is typically less complex and costly than a system that includes a source-side aperture plate. 
   Still referring to  FIGS. 2 and 3 , other embodiments of the system  50  and of the aperture plate  28  are contemplated. For example, although the axes of the channels  27  are shown normal to the page of  FIG. 3 , the aperture plate  24  can be rotated about the opening  28  to any other orientation without degrading the ability of the sidewalls  27   c  and  27   d  to deflect peripheral light away from the screen  58 . Furthermore, although the aperture plate  24  is shown canted 60 degrees relative to the beam axis  52 , the channels  27  may be modified such that the plate (and mirror rest position) can be canted at other angles relative to the beam axis. Furthermore, the angle at which the side walls  27   c  and  27   d  meet may be altered. 
     FIGS. 4A and 4B  are plan and end views respectively of an aperture plate  74  having an aperture opening  78  according to another embodiment of the invention. The aperture plate  74  is similar to the plate  24  of  FIGS. 2 and 3  except that the surface channels have a different shape. Specifically, the aperture plate  74  has an array of contiguous V-shaped channels  76  having valleys  76   a  and peaks  76   b  connected by pairs of symmetrical, orthogonal sidewalls  76   c . Peripheral light from the beam  54  ( FIG. 3 ) striking the aperture plate  74  beyond the opening  78  is deflected away from the image (the screen  58  of  FIG. 3 ) regardless of the angle of incidence of the peripheral light as discussed above in conjunction with  FIGS. 2 and 3 . 
   Referring to  FIGS. 5A and 5B , one can change the angle at which the channel sidewalls  76   c  meet, and the aperture plate  74  still deflects peripheral light away from the image, as long as none of the sidewalls  78   c  are parallel to any plane that the mirror  34  ( FIG. 3 ) rotates through while sweeping the beam  54 . 
     FIG. 6  is a perspective view of an aperture plate  94  in accordance with another embodiment of the invention. The plate  94  has annular channels  96  that are concentric with an aperture opening  98 . Other than being annular and having annular sidewalls  99 , the channels  98  can have the cross-sectional shapes of the channels  26  of  FIGS. 2 and 3 , the channels  76  of  FIGS. 4 and 5 , or any other shape suitable to deflect peripheral light away from the image. That is, in this embodiment, none of the sidewalls  99  lie in a plane parallel to any of the planes through which the mirror  34  rotates while sweeping the beam  54  ( FIG. 3 ). The plate  94  may be made by injection molding of a suitable plastic and can enable a low-cost aperture plate that may be attached to a scan mirror housing such as the MEMS housing  38  of  FIG. 2 . Furthermore, the annular topology of the concentric channels  96  smoothly meets the boundary wall of the circular aperture opening  98  to provide a high-quality aperture edge definition. Referring to  FIGS. 2-6 , the ability to injection mold an aperture plate such as the plates  24 ,  74 , and  94  affords flexibility of design in that many different shapes consistent with the above explanations can be readily designed and then molded to cause maximum deflection of peripheral light. Many optical plastics having known properties are available. Light absorbing dyes are likewise available for mixing with the raw plastic material before molding to increase the absorbing of the peripheral light, and thus reduce the reflection of the peripheral light, by the aperture plate. 
     FIG. 7  shows an aperture plate  104  according to yet another embodiment. The plate  104  is similar to the plates  24 ,  74 , and  94  of  FIGS. 2-6  except that instead of straight or annular channels, the plate  104  has an array of close packed three-face retroreflectors  106  formed in the surface structure surrounding the aperture opening  108 . Each retroreflector has three mutually orthogonal faces so that multiple reflections of the peripheral beam light off these faces cause the peripheral light to be deflected back toward the source, and thus away from the image, independently of the angle of incidence of the incoming beam  54  ( FIG. 3 ). The aperture plate  104  may be injection molded as discussed above in conjunction with  FIG. 6 , or other fabricating techniques may used as discussed above. 
   Referring to  FIG. 8  a scanning beam display system  120  has an aperture plate  124  with channels  127  mounted on the top of a MEMS mirror housing  138  according to an embodiment of the invention. The plate  124  deflects peripheral light away from a viewer&#39;s eye  160 , and may be similar to one of the aperture plates  24 ,  74 ,  94 , and  104  of  FIGS. 2-7 , respectively. The system  120  is a color retinal scanner and further includes a plurality of light source LEDs  141 ,  142  and  143  of different wavelengths (e.g., red, green, and blue) for generating separate scan beams. The resulting scanned image is perceived as a mixture of wavelengths in the viewer&#39;s eye  160 , thus appearing as a full-color image. Modulating the LEDs  141 ,  142  and  143  are electronics  150  including data storage buffers, control circuitry and processors, and modulator drivers. The electronics  150  further include control and scan drivers for causing the periodic movement of bi-axial MEMS scan mirror  139 . The electronics are known and are not therefore further described in detail. Optional apertures and/or baffles may be located at  154  and  156  to limit the amount of peripheral beam light that strikes the aperture plate  124 . Beam forming optics such as one or more lenses  152  form and direct the resulting source beams onto the MEMS scan mirror  139  through the opening  128  of the aperture plate  124 , and a reflected scan beam  158  is projected into a viewer&#39;s eye  160 . Scanning motion and modulation of the sources  141 ,  142 , and  143  create what the viewer perceives as an image. The channels  127  on the body of the aperture  124  adjacent the aperture opening  128  deflect peripheral light away from the eye  160 , thus substantially eliminating artifacts and thus enhancing resolution of the perceived image. While this exemplary embodiment scans directly into the viewer&#39;s eye  160 , in some applications, the beam may scan onto an intermediate element, such as a screen or exit pupil expander, and/or may be coupled to the eye with an optical train. 
   Still referring to  FIG. 8 , although three LEDs  141 ,  142 , and  143  are shown, the display system  120  may include more or fewer than three LEDs. For example, the system  120  may include multiple red LEDs from which the lens  152  forms respective red scan beams, multiple green LEDs from which the lens forms respective green scan beams, and multiple blue LEDs from which the lens forms respective blue scan beams. 
   In the preceding detailed description, the invention is described with reference to specific embodiments. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. For example, although the above embodiments disclose scanned displays, the aperture structure at the scan mirror is also used to advantage in scan imagers. In such an imager, the scanned beam having peripheral source reflection reduced or eliminated by the surface structure around the aperture opening, illuminates an object and light reflected from the object is sensed by photodetectors and stored. Additionally, although there are descriptions of some specific materials and particular structures, such descriptions merely provide suitable examples and are not intended as a limitation. Other scanning approaches may also use the above aperturing at the scan mirror, including acousto-optic scanners, electro-optic scanners, spinning polygons, or some combinations thereof and successive single axis MEMS mirrors to cause in combination the fast and slow axes of a raster. In general, placing the aperture plate at the mirror is used to advantage in many types of mirror scanning apparatus where it is difficult to place beam blocking apertures at the beam source. Thus other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Furthermore, the aperture plate may be used in other optics and non-optic applications such as in cameras and telescopes.