Abstract:
A method for optical dithering and a dither device comprising a mounting structure, a refractive optic, an optic mounting structure surrounding the optic, and one or more linear drive motors. Four linear coils are preferably employed together with a control system operating in open loop mode. The dither device and method provides double resolution to a detector receiving output therefrom. The invention can operate in a single axis or in two axes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 10/429,052, entitled “Optical Dither Apparatus and Method”, to Hughes et al., filed on May 1, 2003, and the specification thereof is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention (Technical Field) 
     The present invention relates to apparatuses and method for optical dithering. 
     2. Background Art 
     Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes. 
     An increasingly popular technique in ultraviolet (“UV”), optical, and infrared (“IR”) imaging observations (including Forward Looking Infrared Radar (“FLIR”) applications) involves the use of “dithering,” or spatially offsetting a telescope or sensor by shifts that are generally small relative to the detector size, thereby moving the target to a number of different locations on the detector. Two of the main strategies involve offsets by integer pixels to facilitate the removal of bad pixels, and offsets by sub-integer pixels to improve spatial sampling of a point spread function (PSF). 
     The benefits of dithering include: (1) Dithering can reduce the effects of pixel-to-pixel errors in the flat-field or spatially varying detector sensitivity; (2) Integer shifts of a few pixels allow the removal of small scale detector defects such as hot pixels, bad columns, and charge traps from the image; and (3) Non-integral (sub-pixel) dithers allow the recovery of some of the information lost to undersampling by pixels that are not small compared to a point spread function. However, significant alignment overhead is associated with dithering, particularly in telescopic applications. 
     Dithering in the context of telescope observations is discussed in detail in A. M. Koekemoer, et al.,  HST Dither Handbook Ver.  2.0, chs. 1-3 (Baltimore, Space Telescope Science Institute, 2002). Mechanical dithering devices include BEI Precision Systems and Space Division&#39;s Model SPC40-030A1 Dither Optical Scanner. The present invention is of a low-cost, refractive, two axes mechanical dithering apparatus. 
     U.S. Pat. No. 4,961,627, entitled “Two-Axis Beam Steering Apparatus”, to Swain et al., is directed to a device for steering a light beam which requires a window having a first flat side and a second convex side. The device also requires a secondary window, which is rotatable with respect to the first window, having a concave first side and a flat second side. The device of Swain et al. further requires that the first and second windows be spaced apart by a “desired gap” (Col. 1, line 31). Still further, the device of Swain et al. requires that a flexible seal be provided and that a fluid be disposed between the two windows. As such, the device disclosed by Swain et al. is not only intricate and costly to manufacture, but is also susceptible to leaking which renders the device inoperable. 
     U.S. Pat. No. 6,332,576, entitled “Dithering Assemblies for Barcode Scanners”, to Colley et al. is directed to a device for dithering a laser beam, particularly those used for barcode scanners. The dithering device of Colley et al. uses a feedback system. However, Colley et al. disclose that the device can be “constructed without employing feedback to resonantly drive the dithering assembly provided that the drive force is sufficiently large to produce the maximum amplitude allowed by the travel stops under non-resonant conditions” (Col. 7, lines 37-41). The device of Colley et al. employs a movable mirror “for scanning an illumination beam of a barcode scanner” (Lines 2 and 3 of the Colley et al. Abstract). As such, the angle of the beam is shifted rather than simply linearly displaced. Further, the disclosure of Colley et al. is directed to moving a beam across a large span (e.g. the width of a bar code scan). 
     There is thus a present need for a method and apparatus which enables rapid linear displacement across a short distance, (e.g. ½ the distance between adjacent pixels in an optical array), without the added complexity of a feedback system. 
     SUMMARY OF THE INVENTION 
     Disclosure of the Invention 
     The present invention is of a method for optical dithering and a dither device comprising: a mounting structure; a refractive optic; a mounting structure surrounding the optic; and one or more linear drive motors. Although plastic optics can be used, and will provide desirable results, in a preferred embodiment, the optic comprises a glass selected from silicon glasses and germanium glasses. Four linear voice coils are preferably employed, together with a plurality of flexures and a plurality of mechanical stops (preferably with deformable tips). A control system commands the one or more linear drive motors, and preferably operates in open loop mode. The dither device and method preferably provides double resolution to a detector by rotation of the optic through a small angle, such that electromagnetic waves traveling there through are refracted and linearly displaced on a detector a distance of about one half to about one and one half the space between adjacent pixels of a detector, thus effectively “filling” the gap between the pixels upon alternate frame updates of the detector. Note that the tilt of the optic is typically a small angle (approximately 0.5 degrees), which results in a small displacement at the detector (approximately 0.001 inches). The optic can be rotated about a single axis or in two axes. 
     Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings: 
         FIG. 1  is an exploded view of the preferred dither apparatus of the invention; 
         FIG. 2  is a schematic diagram of a control system for the invention for a two-axis dither; 
         FIG. 3  is a graph of velocity of the invention when in use in radians/second; 
         FIG. 4  is a graph of position of the invention when in use in radians; 
         FIG. 5  is a schematic diagram of the basic elements of the present invention wherein an optic of the present invention has not been tilted; and 
         FIG. 6  is a schematic diagram depicting an optic of the present invention which has been tilted. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Best Modes for Carrying Out the Invention 
     The present invention is of a refractive two axes dither device comprising a fixed mounting structure, optic (preferably composed of a plastic or glass material, and most preferably a glass of silicon or germanium), optic mounting structure, linear drive motors (preferably four voice coils), flexures, and mechanical stops. The invention is particularly useful for dithering the Line of Sight (“LOS”) of FLIR systems in either one or two axes, thereby augmenting (doubling) resolution. In FLIR systems, the invention is preferably placed between the detector and the last reimager of the system. The primary differences are the use of an open loop servo to rotate the optic (no feedback devices are required) and the use of optical refraction to translate the image on the detector a very small distance. 
     The present invention is driven open loop (i.e., without feedback) with two square waves in quadrature. No feedback is required making control very simple and low cost. The drive coils are preferably cross-coupled. A is in series with −D and B is in series with −C for the following configuration of motors A, B, C, D, with plus signifying into the page: 
     A B 
     C D 
     (A−D)+(B−C)=top edge into page; 
     −(A−D)+(B−C)=right edge into page; 
     −(A−D)−(B−C)=bottom edge into page; and 
     (A−D)−(B−C)=left edge into page. 
     Because the invention is preferably refractive, the LOS displacement is a fraction of the window motion. The amount of image motion is preferably controlled by the window thickness and the material&#39;s index of refraction independent of the window motion. By controlling the dither of the image in this manner, the mechanical tilt can be larger than the desired image motion, thus making the dither motion relatively insensitive to mechanical tolerancing. The invention is smaller, of lower complexity, lower power, and lower cost than existing devices. 
     The preferred embodiment of the apparatus  10  of the invention is shown in front perspective exploded view in  FIG. 1 . The apparatus preferably comprises back stops  12  (for tilt adjustment, preferably tipped in rubber or like deformable material), flexures  14  which act as pivoting members, mounting base  16 , linear actuators  18 , optic housing  20 , front stop support  22 , front stops  24  (again for tilt adjustment and tipped in rubber of like deformable material). 
       FIG. 2  is a block diagram of the preferred control apparatus to operate the invention in a FLIR system so as to provide two-axis dithering.  FIG. 2  is also useful in performing a Matrixx/Matlab simulation of the invention. The invention is driven open loop, with voltage driven in quadrature. “Inv J” refers to the inverse of the inertia of the window/lens housing. “Visc Damp Flex” provides a dampening term for the flexures and “Visc Damp Rubber” for rubber tips on the stops. “Flexure Ks” provides the flexure spring rate and “Stop Ks” the stop spring rate, which is preferably set quite high as shown. The deadband block provides for approximately ½ degree of motion before a stop is hit. Output  1  provides cell velocity in radians/sec and output  2  provides position in radians. 
       FIGS. 3 and 4  illustrate the velocity and position of the apparatus provided by the control system of  FIG. 2 . Note that position and velocity settling are very fast, thereby yielding a high scan efficiency. 
     Referring now to  FIGS. 5 and 6 , optical member  26 , having refractive index of η and a thickness of t, is shown before and after rotation ( FIGS. 5 and 6  respectively). As depicted therein, beam  38  passes through member  26  with no refraction when member  26  is not rotated. Beam  38  experiences no displacement on a surface of detector  30 . When optical member  26  has been rotated θ degrees (the optical tilt), however, beam  38  is displaced distance d on a surface of detector  30 . As depicted in  FIG. 6 , β is the angle between normal of the optic and the principal axis of the optical apparatus of the present invention, and is determined by Equation 1.
 
β=sin −1 ( N /η)(sin θ)  Equation 1.
 
Further, the magnitude of displacement, d is determined by Equation 2.
 
                   d   =     t   ⁢           ⁢       (     sin   ⁢           ⁢   θ     )     ⁡     [     (     1   -       N   ⁢           ⁢   cos   ⁢           ⁢   θ       η   ⁢           ⁢   cos   ⁢           ⁢   β         )     ]                 Equation   ⁢           ⁢   2               
N is the index of refraction for surrounding medium  28 .
 
     Accordingly, the present invention provides dithering according to the above equations by tilting the optic with the liner actuators. The solution is elegant and provides for low cost, high robustness optical devices using dithering. 
     Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above are hereby incorporated by reference.