Patent Publication Number: US-2005116141-A1

Title: Method, assemblage, and scanner for optically sampling light by a photosensitive device

Description:
TECHNICAL FIELD  
      This invention relates to imaging technologies and, more particularly, to a method, assemblage, and scanner for optically sampling light by a photosensitive device.  
     BACKGROUND  
      Image scanners convert a visible image on a document, photograph, or other medium into an electronic form suitable for copying, storing, or processing by a computer. Reflective image scanners typically have an illumination source that directs light onto a document surface to be imaged. Light is reflected from the document surface, through an optics system, and onto an array of photosensitive devices. The photosensitive devices convert received light intensity into an electronic signal. Transparency image scanners pass light through a transparent image, such as a photographic positive slide, through an optics system, and onto an array of photosensitive devices. A photosensitive device, such as a charge-coupled device (CCD), used in a scanner is frequently implemented as an array of photosensitive elements. The photosensitive device may comprise a one- or two-dimensional array of photosensitive elements.  
      The smallest area of a document image that is sampled by an individual element of a photosensitive device is referred to as a pixel. A pixel also refers to a set of numbers in a data set that electrically defines the image pixel. For example, a common specification of reflective and transparent image scanners is “pixels-per-inch” as measured on the surface of the document being scanned. Photosensor arrays employ numerous individual photosensitive elements (or alternatively sets of photosensitive elements when implemented in a color scanner) that respectively measure light intensity from a single area of the document, each element thereby defining one pixel on the document being scanned. The optical sampling rate, or resolution, is the number of samples optically captured from one scan line divided by the length of the scan line.  
      For black-and-white and grayscale scanners, there is a one-to-one correspondence between one pixel on the document being scanned, one sensor element, and one numerical intensity measurement. For color scanners, at least three sensor elements are employed to sense all the colors for one pixel on the document image and three corresponding numerical intensity values are used to represent all colors for one pixel on the document image.  
      Two general types of photosensor arrays are employed in image scanners: contact image sensors (CIS) arrays and charge-coupled device arrays. CIS arrays have a length equivalent to the length of the scan line. The primary advantage of CIS arrays is that reduction optics are not required. CCD arrays typically have a length that is smaller than the length of a scan line. Reduction optics are used to focus a scan line of the image onto the CCD array.  
      It is desirable to increase the resolution of a scanned image to provide sharper images. However, increasing the resolution of a photosensor array, whether a CIS array or CCD array, requires more pixels per scan line and, therefor, more photosensor elements per scan line. Increasing the number of photosensor elements results in a larger photosensor array. One approach to reducing the photosensor array size is to reduce the size of individual photosensor elements. However, each photosensitive element receives light from a fixed pixel area on the image being scanned. Thus, the minimum size of a photosensitive element is determined by integration circuit fabrication technology of the optics system, e.g., fundamental diffraction limits. Therefore, the photosensor array size increases in proportion to an increase in resolution.  
     SUMMARY OF THE INVENTION  
      In accordance with an embodiment of the present invention, a method comprises directing light from an image onto a mask, switching a first area of the mask to an optically-conductive state, switching a remaining area of the mask to an optically-blocking state, sampling light passing through the first area of the mask by the photosensitive element, switching the first area to an optically-blocking state, switching the remaining area to an optically-conductive state, and sampling light passing through the remaining area by the photosensitive element.  
      In accordance with another embodiment of the present invention, an assemblage for sampling an image comprises a photosensitive element operable to convert light into an electrical signal, and a mask having a plurality of mask cells, each mask cell having an optically-conductive state and an optically-blocking state, a mask cell in an optically-conductive state permitting light to pass through to the photosensitive element.  
      In accordance with yet another embodiment of the present invention, an imaging device comprises a plurality of photosensitive elements arranged in a linear array, and a plurality of mask elements, each of the mask elements respectively associated with one of the plurality of photosensitive elements. Each mask element comprises a plurality of mask cells electrically switchable between optically-conductive and optically-blocking states. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of the present invention, the objects and advantages thereof, reference is now made to the following descriptions taken in connection with the accompanying drawings in which:  
       FIG. 1  is a schematic of a reduction optic device and photosensor array according to embodiments of the present invention;  
       FIG. 2  is a detailed schematic of a section of reduction optic device and photosensor array in the configuration illustrated in  FIG. 1 ;  
       FIG. 3  is a schematic of an individual pixel and a photosensitive element sampling the pixel;  
       FIG. 4  is schematic of a pixel divided into portions that are sampled by a single photosensitive element according to an embodiment of the present invention;  
       FIGS. 5A-5D  are respective schematics of mask cells in various optically-conductive states according to an embodiment of the present invention;  
       FIG. 6  is a schematic of a photosensitive array having photosensitive elements and associated mask cells in a configuration for sampling a scan line; and  
       FIG. 7  is a schematic of a mask cell implemented as a pockel cell according to an embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      The preferred embodiment of the present invention and its advantages are best understood by referring to  FIGS. 1 through 7  of the drawings, like numerals being used for like and corresponding parts of the various drawings.  
       FIG. 1  is a schematic of a reduction optic device  20  and photosensor array  30  according to embodiments of the present invention. A document  15  having an image to be scanned rests against a transparent platen  10 . A scan line of document  15  is illuminated with a light source and light reflected from (or alternatively passed through) document  15  is directed into an optic device  20 . Optic device  20  guides light reflected from document  15  onto a photosensor array  30 . The length of document  15  (L D ) (and thus the scan line length) is generally greater than the length of photosensor array  30  (L A ). Optic device  20  facilitates reduction of the scan line length and is thus referred to as a reduction optic device.  
       FIG. 2  is a more detailed schematic of a section of reduction optic device  20  and photosensor array  30  in the configuration illustrated in  FIG. 1 . A portion of a scan line  35  of document  15  comprises pixels  350 - 359 , or image areas, that are individually sampled by respective photosensitive elements  300 - 309 . As light  250 - 259  is reflected from or through respective pixels  350 - 359  of document  15 , optic device  20  guides light  250 - 259  onto element  300 - 309  of photosensor array  30 . Elements  300 - 309  perform analog-to-digital conversion of the detected light intensity and respective data sets are generated that numerically define pixels  350 - 359 .  
       FIG. 3  is a schematic of an individual pixel  350  and photosensitive element  300  for sampling pixel  35   0 . Optic device  20  functions to direct light from pixel  35   0  to photosensor array element  30   0 . With reference also to  FIG. 4 , pixel  35   0  is illustratively divided into portions, e.g., quadrants  35   01 - 35   03 , by using a mask  40 . A single photosensitive element  30   0  is used to receive the reflected light from divided portions of pixel  35   0 . The illustrative pixel  35   0  has length, L, and width, W. In the example shown in  FIG. 4 , L is equal to W and each quadrant  35   00 - 35   03  has length and width dimensions of L/2. Mask  40  comprises a plurality of mask cells  40   0 - 40   3 . In the illustrative example, each mask cell  40   0 - 40   3  respectively corresponds to quadrant  35   00 - 35   03 . Embodiments of the present invention are not, however, limited to a particular number or configuration of mask cells  40   0 - 40   3  and the mask cells may be arranged in an array or a matrix configuration. Furthermore, each mask may comprise an M×N matrix of cells, where M and N are integers. Preferably, mask cells  40   0 - 40   3  are implemented as electronically controlled light switches having two operational states: optically-conductive and optically-blocking.  
       FIGS. 5A-5D  are schematics of mask  40  illustrating operational states of mask cells  400 - 403  according to an embodiment of the present invention. For illustrative purposes, a mask cell state of optically-blocking is shown with appropriate shading. In  FIG. 5A , mask cell  400  is optically-conductive and the remaining mask cells  401 - 403  are in a state of optically-blocking. In some embodiments, no more than a single mask cell is in an optically-conductive state at any given time. Turning to  FIG. 5B , mask cell  400  is switched off and mask cell  401  is switched on (or placed in an optically-conductive state). Likewise, as shown in  FIGS. 5C and 5D , mask cells  402  and  403  are alternatively switched on while respective mask cells  400 ,  401 , and  403  and  400 - 402  are switched off, respectively.  
      In one embodiment, mask  40  is implemented by an array of electro-optic modulators such as pockel cell modulators.  FIG. 7  is an exemplary mask cell  40   0  implemented as a pockel cell modulator in accordance with an embodiment of the invention. Pockel cell modulator  400  comprises two polarizers  200  and  201  having a crystal  210  disposed therebetween. Crystal  210  has optical properties that are modifiable by the application of an electric field. Particularly, the refraction properties of crystal  210  are changed upon application of a suitable electric field across crystal  210 . Polarizers  200  and  201  are aligned at 90 degree offsets with respect to the polarization angles of polarizer  200  and  201  applied. During application of an electric field across crystal  210 , crystal  210  is birefringent and, accordingly, light passes through crystal  210  and polarizer  201  prior to incidence on the photosensitive element. Alternatively, when an electric field is not applied across crystal  210 , crystal  210  is not birefringent and the passage of light is prevented. Thus, an optically-conductive state of mask cell  40   0  is provided by an application of an electric field to crystal  210 , and an optically-blocking state of mask cell  40   0  is provided by the absence of an electric field across crystal  210 . Accordingly, mask  40  may be implemented as a 2-by-2 configuration of pockel cell modulators in an embodiment of the invention. Mask  40  is configured to sequentially cycle individual cells in an optically-conductive state by supplying an electric field across the crystal of a single pockel cell modulator while no electric field is supplied across the remaining pockel cell crystals.  
      Referring to  FIG. 6 , a photosensitive device  130  comprises a plurality of photosensitive elements  130 A- 130 N arranged in a one-dimensional or two-dimensional array. In a preferred embodiment, each photosensitive element  130 A- 130 N has an associated mask  140 A- 140 N comprised of a plurality of mask cells  140 A 0 - 140 A 3 - 140 N 0 - 140 N 3  (illustratively denoted with dashed lines). The number, X, of photosensor elements  130 A- 130 N and associated mask elements  140 A- 140 N (each with Y mask cells) is arbitrary and photosensitive devices, such as CCD arrays, often comprise thousands of individual photosensitive elements.  
      Mask  140  is configured with photosensitive device  130  such that a single mask cell of each mask element  140 A- 140 N is placed in an optically-conductive state and is optically coupled with a respective photosensitive element  130 A- 130 N at a given time during the sampling of a scan line. Preferably, individual cells  140 A 0 - 140 A 3 - 140 N 0 - 140 N 3  are cycled through a single optically-conductive state and are placed in an optically-blocking state for the remainder of a scan line sampling process. For example, mask cells  140 A 0 - 140 N 0  are preferably placed in an optically-conductive state while the remaining mask cells  140 A 1 - 140 A 3 - 140 N 1 - 140 N 3  are placed in an optically-blocking state. During the sample period in which cells  140 A 0 - 140 N 0  are optically-conductive, photosensitive elements  130 A- 130 N generate a respective data set  131 A 0 - 131 N 0  representative of the light intensity passing through cells  140 A 0 - 140 N 0 . After passage of a sample period sufficient for photosensitive elements  130 A- 130 N to generate respective samples, the optically-conductive cells  140 A 0 - 140 N 0  are switched to an optically-blocking state and another cell  140 A 1 - 140 N 1  of masks  140 A- 140 N are switched to an optically-conductive state. Photosensitive elements  130 A- 130 N generate respective data sets  131 A 1 - 131 N 1  after passage of light through cells  140 A 1 - 140 N 1  for the sample period. This process is repeated until photosensitive elements  130 A- 130 N have sampled light passing through each of the remaining mask cells  140 A 2 - 140 N 2  and  140 A 3 - 140 N 3  and generated respective data sets  131 A 2 - 131 N 2  and  131 A 3 - 131 N 3  representative thereof.  
      The sampling of a scan line is completed after each of mask cells  140 A 0 - 140 A 3 - 140 N 0 - 140 N 3  have been cycled through an optically-conductive state. The sample data set comprises X·Y samples, where X is the number of photosensitive elements in the scan array and Y is the number of mask cells in each mask element. Accordingly, a minimum scan line sample period is equivalent to the product of the sample period of the photosensitive elements  130 A- 130 N, Ts, and the number of mask cells in individual mask elements  140 A- 140 N. In the particular configuration of the illustrative example, a scan line sample period is equal to 4Ts. It should be understood that each photosensitive element and mask may be further subdivided to achieve even greater resolution, for example a 4×4 matrix of cells sub-divided from a single mask for a single photosensitive element.  
      The above-described embodiments have included preferred configurations of a mask and photosensitive element or assemblage that may be implemented in a charge-coupled device photosensor array. However, embodiments of the invention may be implemented in other photosensory devices as well. For example, an assemblage of a mask and photosensitive array may be implemented in a contact image sensory array in accordance with embodiments of the invention. Additionally, the assemblage of the mask and photosensitive array described hereinabove comprises a one-dimensional array of both photosensitive elements and mask elements. However, such a configuration is exemplary only and has been chosen to facilitate an understanding of embodiments of the invention. Embodiments of the invention include implementation in two-dimensional photosensitive arrays and corresponding two-dimensional arrays of masks.  
      Embodiments of the present invention may be used to increase the resolution of various imaging devices without increasing the number or density of photosensitive devices. Therefore, a high-resolution scanner may be implemented using low-resolution photosensitive devices.