Abstract:
A catadioptric imaging system combines a rectifying mirror, a lens system and subsequent image processing. This approach can produce a small form factor desktop document imaging system capable of producing high-quality, high-resolution images of paper documents.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to catadioptric imaging systems, including for example offset systems for imaging documents. 
     2. Description of the Related Art 
     To achieve a high-resolution image of a paper document, current desktop document imaging systems typically either perform some form of mechanical scanning, use a large working focal distance, and/or operate on-axis (i.e., the document is centered on and perpendicular to the optical axis of the imaging system). Current desktop document imagers can generally be classified into two broad categories: traditional document scanners and “tall” document imagers. 
       FIG. 1  shows an example of a traditional document scanner. The document is placed on the scan surface and a scan mechanism is mechanically scanned across the document. The device relies on mechanical scanning to create a two-dimensional image using a one-dimensional image capture device. The mechanical flatbed document scanner has several disadvantages. First, the scanning process is slow and noisy. Second, the document scanner is bulky, which reduces its portability and makes it unattractive as a desktop accessory. Third, the device consumes more power due to the mechanical scanning process. 
       FIG. 2  shows an example of a “tall” document imager. In these systems, the document image is captured using a two-dimensional image capture device located sufficiently far away from the paper document. Such a system requires a fairly long working distance from the document, so the camera&#39;s optical axis can be both perpendicular to and centered on the document. Reducing the operating height for such a perpendicular imaging system typically requires a wide angle imaging system, which would introduce severe barrel distortions at large field angles (i.e. along the outer boundaries of the document). 
     Some imaging systems, such as whiteboard capture imaging systems, have an optical subsystem which is not perpendicular to the document surface, allowing a lower system height. These imaging systems, however, create undesirable keystoning of the optical image. This keystoning is unacceptably deleterious for many paper document imaging applications, as it introduces a variable image resolution over the document. The foreshortening of the image can reduce the document scanning resolution considerably. 
       FIG. 3  illustrate the deleterious effects of distortion.  FIG. 3   a  visualizes a two-dimensional sensor array (grid) overlayed on a severely distorted image of a document, such as would be produced by a conventional wide-angle lens system. While the sensor may achieve the targeted resolution at the center of the image, at the periphery the image resolution is severely compromised due to the geometric distortion.  FIG. 3   b  shows a two-dimensional sensor array (grid) overlayed on a document image with significant keystoning. In this case, the document may achieve the targeted resolution at one edge of the document, but the foreshortening reduces the resolution dramatically at the opposite edge. 
     Thus, there is a need for document imaging systems that overcome some or all of these drawbacks. Such systems preferably should avoid mechanical scanning and should have a relatively short working distance and small overall size. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the limitations of the prior art by providing a catadioptric imaging system that combines a convex rectifying mirror, a lens system and subsequent image processing. 
     In one application, an offset catadioptric imaging system is designed to image an 8.5″×11″ object field at 300 dpi resolution or better, and preferably in color. The system includes, in order along an optical path from an object field to a corresponding image field: a convex mirror, a lens system and an image sensor. The convex mirror and the lens system act in concert to image the object field onto the image sensor. The convex mirror defines an optical axis. The system is offset in that the image sensor and the object field are positioned on opposite sides of the optical axis. An image processor provides additional correction of the image captured by the image sensor. The overall system preferably is small and portable, for example having a height of not more than 5″. 
     In one variation, the aperture stop is located between the convex mirror and the lens system. In another variation, the lens system has zoom capability, for example by moving various lens elements relative to each other. In another variation, the system also includes a light source that directs light in the opposite direction to illuminate the object field. 
     In another aspect of the invention, the convex mirror and lens system together are designed so they have an MTF that remains above zero at least out to a Nyquist frequency for the image sensor. One example of image processing is the use of Wiener filters or other types of field-dependent linear filters to correct for aberrations not compensated by the convex mirror and lens system. Image processing can also be used to correct for uneven illumination across the object field. 
     In other aspects, a catadioptric imaging system includes, in order along an optical path from an object field to a corresponding image field: a convex mirror, a lens system and an image sensor. The convex mirror (e.g., a rectifying mirror) and the lens system act in concert to image the object field onto the image sensor. The lens system includes a first negative lens group, a second positive lens group and a third negative lens group. Alternately, the lens system includes a first negative lens element, a second positive lens element, a third positive lens element and a fourth negative lens element. Example designs include four-, five- and six-element designs that meet the requirements of the document imaging application described above. The elements may or may not be rotationally symmetric and may or may not include aspheres. Image processing can be used to increase contrast, as above. 
     Other aspects of the invention include systems and applications for the above, and methods corresponding to all of the foregoing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  (prior art) shows a traditional document scanner. 
         FIG. 2  (prior art) shows a tall document imager. 
         FIGS. 3   a  and  3   b  (prior art) illustrate the effect of distortion on resolution. 
         FIG. 4  is a perspective view of an offset document imaging system according to the invention. 
         FIG. 5  is a cross-sectional view of the imaging system of  FIG. 4 , illustrating the optical operation of the system. 
         FIG. 6   a  shows a reflected image from a standard wide-angle catadioptric imaging system. 
         FIG. 6   b  shows a reflected image from a rectifying mirror. 
         FIGS. 7   a  and  7   b  define variables used to specify a perfectly-rectifying mirror. 
         FIG. 8  is a graph that shows illumination rolloff for an example system. 
         FIG. 9   a  is a cross-section of a five-element lens system according to the invention. 
         FIG. 9   b  is a table of the optical prescription for the lens system of  FIG. 9   a.    
         FIG. 9   c  are tables of the image processing filters used with the lens system of  FIG. 9   a.    
         FIG. 10  graphs MTFs of the catadioptric optical system at a red wavelength. 
         FIG. 11   a  graphs MTFs of the catadioptric optical system;  FIG. 11   b  graphs the corresponding Wiener filter responses required to restore the overall system to diffraction limited operation. 
         FIGS. 12   a  and  12   b  graph field curvature and distortion, respectively, for three different colors for the catadioptric optical system. 
         FIGS. 13   a - 13   d  are a cross-section, optical prescription and field curvature and distortion curves for a four-element lens system according to the invention. 
         FIGS. 14   a - 14   d  are a cross-section, optical prescription and field curvature and distortion curves for a five-element lens system according to the invention. 
         FIGS. 15   a - 15   d  are a cross-section, optical prescription and field curvature and distortion curves for a six-element lens system according to the invention. 
     
    
    
     The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 4  shows a perspective view of an offset document imaging system  400  according to the invention. This particular imager  400  is capable of capturing a 300 dpi full-color image of the 8.5″×11″ document  450  with a single snapshot, eliminating the need for mechanical scanning. The document  450  is placed next to the small-form factor document imaging system  400 , as opposed to the traditional geometries shown in  FIGS. 1 and 2 . 
       FIG. 5  is a cross-sectional view of the imaging system  400  of  FIG. 4 , illustrating the optical operation of the system. The imaging system  400  includes a convex mirror  410 , a lens system  420  and an image sensor  430 . The convex mirror  410  and lens system  420  together image the document  450  onto the image sensor  430 . An image processor  440  can be used to further process the image captured by the image sensor  430 . For example, field-dependent spatial filters could be used to sharpen the image. The image sensor  430  and/or image processor  440  can further communicate to other computing devices via a communications port (not shown), for example USB or a wireless connection. 
     In this design, the convex mirror  410  is rotationally symmetric about optical axis  480 . The system  400  is offset in the sense that the object field  450  is not centered on the optical axis, as is typically the case in the tall document imagers of  FIG. 1 . In the design of  FIG. 5 , the object field  450  lies entirely to one side of the optical axis  480  and the image sensor  430  lies entirely to the other side of the optical axis. 
     The mirror  410  is a rectifying mirror that substantially avoids the distortion common to refractive-only wide-angle optical systems.  FIG. 6  illustrates this effect.  FIG. 6   a  shows a reflected image used in standard wide-angle catadioptric imaging system. The image contains significant geometric distortion.  FIG. 6   b  shows a reflected image from a rectifying mirror. The distortion is significantly reduced. 
     A perfectly rectifying mirror for a pinhole camera can be designed using the approach described in U.S. Pat. No. 6,412,961, which is incorporated herein by reference. In this approach, the rotationally symmetric mirror  410  has a figure F(x) that satisfies 
                       2   ⁢       F   ′     ⁡     (   x   )           1   -         F   ′     ⁡     (   x   )       2         =         d   ⁡     (   x   )       -   x       F   ⁡     (   x   )                 (   1   )               
where a ray that leaves an object point at radius d(x) hits both the mirror and the image plane at a radius x (recall that for a pinhole camera, only one ray travels from the object point to the image point). See  FIG. 7   a.  If d(x) is a linear function, i.e., d(x)=ax where a is constant, then the system will be free of distortion.
 
     In an alternate approach, the rotationally symmetric mirror  410  has a figure F(t) that satisfies 
                         x   f     +       2   ⁢       F   ′     ⁡     (   t   )           1   -         F   ′     ⁡     (   t   )       2             1   -       x   f     ⁢       2   ⁢       F   ′     ⁡     (   t   )           1   -         F   ′     ⁡     (   t   )       2               =         d   ⁡     (   x   )       -   t       F   ⁡     (   t   )                 (   2   )               
where a ray that leaves an object point at radius d(x) hits the mirror at radius t(x) and is reflected to the image plane at a radius x, again assuming a pinhole camera. f is the focal length, which by geometry is given by F(t)=f+h+f/xt, where h is the distance along the optical axis from the object plane  450  to the image plane  430 . See  FIG. 7   b.  
 
     The mirrors defined by Eqns. (1) and (2) map equally-spaced regions in the object space  450  into equally-spaced regions on the image plane  430  for a pinhole camera. However, this is strictly true only for a pinhole camera, where only one ray propagates from each object point to the corresponding image point. In real systems, opening up the aperture will introduce aberrations that will blur the image. The lens system  420  and image processor  440  can be used to reduce this blur. As a result, it is not necessary (and may be detrimental) to use mirrors having exactly the figures defined by Eqns. (1) or (2). 
       FIGS. 8-15  describe various lens systems  420  that can be used in the document imaging system of  FIGS. 4-5 . In these examples, the system uses 180 degrees of a rotationally-symmetric mirror  410  which acts to rectify the image field. The mirror is similar in function to those described in Eqns. (1) and (2) above. However, the mirror is designed in conjunction with the lens system  420  and image processor  440  to achieve the desired final image quality. As a result, the mirror  410  does not have the exact figure given by the closed-form expressions (1) or (2). 
     The lens system  420  generally contains three lens groups. In order from the mirror  410  to the sensor  420 , they are a negative lens group  510 , a positive lens group  520  and another negative lens group  530 , although the number of lens elements in each lens group and the specific designs of the lens elements may vary from one design to the next. The term “lens element” is intended to mean a single lens, excluding for example two lenses separated by air (which would be referred to as a lens group) and also two lenses cemented together (e.g., a doublet). The aperture stop  415  is located between the mirror  410  and the lens system  420 . 
     The image processing  440  in this example includes two steps. The first step corrects for uneven illumination across the field of view. Wide angle systems suffer a certain amount of illumination rolloff. The relative illumination curve for the current design is shown in  FIG. 8 . The illumination falls to about 75 percent at the far corners of the 8.5″×11″ document. The image processing subsystem  440  applies a simple gain factor to correct this. Second, the image processing  440  restores contrast using field-dependent linear sharpening filters specially tuned to the point spread function of the catadioptric optical system. 
     This example design is based on a ⅙ inch 8 megapixel CMOS image sensor  430  manufactured by Micron. The design scales appropriately for different image sensors (e.g., by Sharp, Kodak, etc.) of the same class. The reference sensor  430  uses 1.75 micron pixels. Thus, the overall imaging system must achieve resolution out to the Nyquist rate of 280 lp/mm. Such resolution requirements are extremely challenging. For example, the optical system must be F/2.8 or faster just to ensure that the targeted 300 dpi resolution is within the diffraction limit. Larger image sensors  430  with larger pixels can be used to reduce the resolution requirements. However, if the overall system height is held constant, then the illumination tends to rolloff very significantly due to the increase in the chief ray angle. 
     In this particular design, the mirror  410  and sensor  430  preferably are separated by 5″ or less along the optical axis, so that the overall device can be a small form factor. In  FIG. 4 , the overall device housing is shown as 4.5″ total height. Small form factors allow the device to be easily portable. At this height and at a resolution of 300 dpi across an 8.5″×11″ object field, the half field of view of the entire system is approximately 60 degrees (or more, for shorter systems) and the field of view of an individual pixel in the sensor is approximately 0.02-0.03 degrees. 
       FIG. 9   a  is a cross-section of a first lens system  420  according to the invention. This lens system  420  includes the three lens groups  510 ,  520  and  530 , containing five lens elements total. The first lens group  510  is the positive lens group containing the first three lens elements. The second lens group  520  is the single negative lens and the third lens group  530  is the aspherical positive lens element which acts to minimize the chief ray angle. Object  540  is an IR filter/cover glass for the sensor  430 . This example uses only rotationally-symmetric lens elements. The third and fifth elements are aspheres. The optical prescription is given in  FIG. 9   b.    
     This design is capable of changing focal length to account for varying document height. For instance, if the document is sitting atop a stack of documents 1 cm thick, the lens system is capable of shifting both the fourth and fifth lens elements  520 ,  530  to zoom away from the document ensuring 300 dpi scanning over the entire page. 
     To achieve these difficult design requirements, the digital image processing subsystem  440  corrects certain shortcomings in the optical subsystem. In addition to the field-dependent gain that is applied to correct for uneven illumination, digital filters are also applied to the images captured by the image sensor  430  to restore contrast. In this case, 9×9 digital spatial filters are used.  FIG. 9   c  lists the filter coefficients for the sharpening filters used on the red, green and blue images, respectively near the optical axis. 
       FIG. 10  shows MTFs for the optical subsystem at the red wavelength of 0.656 μm. The MTF curves show that there is a loss of resolution due to an insufficient number of degrees of freedom in the design space to satisfy design requirements. These aberrations, however, have been balanced such that the contrast lost can be restored via digital processing.  FIG. 11   a  shows the MTF of the optical subsystem before digital processing for a few field locations. Curves  1100 ,  1110  and  1120  correspond to field(y,x) locations of (0,0), (1.60,0) and (2.27,0), respectively. There are two curves  1110  and  1120 , corresponding to the sagittal and tangential MTFs. The wavelength is the red wavelength of 0.656 μm. The Nyquist frequency is 270 lp/mm and the diffraction limit is 583 lp/mm.  FIG. 11   b  shows the corresponding digital filter spectral responses required to restore the system to an equivalent F/2.8 diffraction limited MTF. Such Wiener filters amplify the noise by an average of 2.5× over the image field. Filters  1105 ,  1115  and  1125  in  FIG. 11   b  correspond to MTFs  1100 ,  1110  and  1120  in  FIG. 11   a,  respectively. Further image processing can be used to handle cases where the paper document is not flat, which can be modeled as a defocus error.  FIGS. 12   a  and  12   b  shows graphs of the field curvature and distortion, at three different colors. The curves are labelled as R, G and B, corresponding to wavelengths of 0.656 μm, 0.588 μm and 0.486 μm, respectively. 
       FIGS. 13-15  show alternate designs for lens system  420 . In each of these figures, subfigure (a) is a cross-section of the lens system, subfigure (b) is the optical prescription, and subfigures (c) and (d) graph the field curvature and distortion, respectively. All designs have generally similar MTFs, in that the MTF before image processing is typically well below the diffraction limit but extends out to the Nyquist rate without crossing zero. As a result, image processing can be used to enhance the contrast of the captured image. 
     All three designs use three lens groups: negative  510 , positive  520  and negative  530 .  FIG. 13  shows a four-element design, which has two lens elements in the first negative lens group  510 . The first element in this design is not rotationally symmetric and this extra design freedom is used to compensate for field curvature.  FIG. 14  shows a five-element design, which can be categorized as four elements implementing the basic negative-positive-negative design, followed by a single element  1440  used primarily to correct field curvature.  FIG. 15  shows a six-element design, where the last two elements are used to correct field curvature. 
     Although the detailed description contains many specifics, these should not be construed as limiting the scope of the invention but merely as illustrating different examples and aspects of the invention. It should be appreciated that the scope of the invention includes other embodiments not discussed in detail above. For example, in alternate embodiments, the entire system can be scaled in size to be larger and/or smaller and/or to accommodate different shapes (such as A4 instead of 8.5″×11″). In one example, a larger version could be used to image a flat surface, for example for the purpose of tracking document objects or bar-code printouts. As another example, alternate embodiments are not limited to offset geometries or to 180 degrees of mirror  410 . An entire 360 degrees could be imaged. Alternately, two 8.5″×11″ object areas could be imaged simultaneously. Various other modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims. Therefore, the scope of the invention should be determined by the appended claims and their legal equivalents.