Abstract:
An object of this invention is to obtain a high-quality image free from any variation between image sensing regions. To achieve this object, an image sensing apparatus according to this invention includes an image sensing element which is formed on a semiconductor substrate on which at least one of a semiconductor layer, a color filter layer, and a microlens layer is formed by a plurality of divisional exposure operations, and a correction device which corrects variations in a signal output from the image sensing element between a plurality of partial image sensing regions formed by the plurality of divisional exposure operations.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of application Ser. No. 10/664,508, filed Sep. 16, 2003, the entire disclosure of which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to an image sensing apparatus which senses an object image. 
       BACKGROUND OF THE INVENTION 
       [0003]    An example of the arrangement of a conventional digital still camera will be described with reference to  FIG. 7 . 
         [0004]    In  FIG. 7 , when the photographer operates a camera operation switch  201  (comprised of, e.g., the main switch and release switch of a camera), an overall control CPU  200  detects a change in the state of the camera operation switch  201 , and starts supplying power to other circuit blocks. 
         [0005]    An object image within the photographing frame range is formed on an image sensing element  204  via main photographing optical systems  202  and  203 , and converted into an analog electrical signal. The analog electrical signal from the image sensing element  204  is subjected to analog processing by a CDS/AGC circuit  205 , converted into a predetermined signal level, and converted into a digital signal for each pixel by an A/D converter  206 . 
         [0006]    A driver circuit  207  controls horizontal driving and vertical driving of the image sensing element  204  on the basis of a signal from a timing generator  208  which determines the driving timing of the whole system. The image sensing element  204  then outputs an image signal. 
         [0007]    Similarly, the CDS/AGC circuit  205  and A/D converter  206  also operate on the basis of timings from the timing generator  208 . 
         [0008]    Reference numeral  209  denotes a selector which selects a signal on the basis of a signal from the overall control CPU  200 . An output from the A/D converter  206  is input to a memory controller  215  via the selector  209 , and all signal outputs are transferred to a frame memory  216 . In this case, all pixel data of photographing frames are temporarily stored in the frame memory  216 . For sequential shooting or the like, all pixel data of photographed images are written in the frame memory  216 . 
         [0009]    After the end of write in the frame memory  216 , the contents of the frame memory  216  which stores pixel data are transferred to a camera digital signal processor (DSP)  210  via the selector  209  under the control of the memory controller  215 . The camera DSP  210  generates R, G, and B color signals on the basis of pixel data of each image stored in the frame memory  216 . 
         [0010]    Before normal photographing, the generated R, G, and B color signals are periodically (every frame) transferred to a video memory  211 , obtaining a viewfinder display or the like by a monitor display  212 . 
         [0011]    When the photographer designates photographing (i.e., image recording) by operating the camera operation switch  201 , pixel data of one frame are read out from the frame memory  216  in accordance with a control signal from the overall control CPU  200 , subjected to image processing by the camera DSP  210 , and temporarily stored in a work memory  213 . 
         [0012]    Data in the work memory  213  is compressed by a compression/decompression unit  214  on the basis of a predetermined compression format. The compressed data is stored in an external nonvolatile memory  217  (generally, a nonvolatile memory such as a flash memory is used). 
         [0013]    To observe photographed image data, data which is compressed and stored in the external memory  217  is decompressed into normal data of each pixel via the compression/decompression unit  214 . The decompressed data of each pixel is transferred to the video memory  211 , allowing to observe the photographed image via the monitor display  212 . 
         [0014]    In this manner, in a general digital camera, an output from the image sensing element  204  is converted into actual image data via the signal processing circuit in almost real time, and the result is output to the memory or monitor circuit. 
         [0015]    In such digital camera system, compatibility with a silver halide film such as a  135  format film camera system is important particularly for an interchangeable lens type single-lens reflex camera. 
         [0016]    The lens can be used as far as the mount is common. However, the photographing view angle, i.e., focal length suffers a difference in size between the image sensing element and the film. 
         [0017]    At present, the size of the image sensing element which can be manufactured at once is limited owing to the manufacturing apparatus, i.e., so-called stepper. Also in terms of cost, the image sensing element is generally smaller than the film. Considering the same photographing sense as that for the film, and particularly photographing with a wide-angle lens, an image sensing element equal in size to a silver halide film is desirable. 
         [0018]    As one measure,  FIG. 8  schematically shows one image sensing element such as a CCD which is constituted by joining in three-divisional exposure (to be referred to as joint exposure hereinafter). 
         [0019]    In  FIG. 8 , one image sensing element is divided into three, left, center, and right regions. The regions are exposed to individual masks and finally joined into one image sensing element. In  FIG. 8 , joint exposure is executed in a vertical structure of a semiconductor layer, on-chip color filer layer, and on-chip microlens layer to constitute an image sensing element equal in size to the film. 
         [0020]      FIG. 9  is a view for explaining a semiconductor layer when the image sensing element in  FIG. 8  is a CCD. In this CCD, the charges of pixels generated in a photodiode  190  are transferred at once to vertical CCDs  191  at a predetermined timing. The charges in the vertical CCDs  191  on all lines are transferred to horizontal CCDs  192 ,  193 , and  194  at the next timing. 
         [0021]    In the arrangement shown in  FIG. 9 , the horizontal CCDs  192 ,  193 , and  194  transfer charges to a common amplifier  195  every transfer clock. An amplified output is read out via common CDS/AGC circuits  196  and  198 . 
         [0022]    Such CCD can be used similarly to a general image sensing element as far as joint exposure is successful. 
         [0023]    Even in the CCD of  FIG. 9  which looks like a general image sensing element, a shift by joint exposure actually exists in each vertical structure of a semiconductor layer, on-chip color filter layer, and on-chip microlens layer. The output level varies between the three regions. 
         [0024]    Especially, the on-chip color filter layer and on-chip microlens layer readily shift, and the influence appears as a step in the gain direction. In particular, the on-chip color filter layer is exposed for each color, the shift varies, and the step varies between colors. 
         [0025]    As shown in  FIG. 10 , the regions may twist in the plane (two-dimensionally), resulting in a very complicated shift. 
       SUMMARY OF THE INVENTION 
       [0026]    The present invention has been made to overcome the conventional drawbacks, and has as its object to obtain a high-quality image free from any variation between image sensing regions. 
         [0027]    To solve the above problems and achieve the above object, according to the first aspect of the present invention, an image sensing apparatus is comprising an image sensing element which is formed on a semiconductor substrate on which at least one of a semiconductor layer, a color filter layer, and a microlens layer is formed by a plurality of divisional exposure operations, and a correction device which corrects variations in a signal output from the image sensing element between a plurality of partial image sensing regions formed by the plurality of divisional exposure operations. 
         [0028]    According to the second aspect of the present invention, an image sensing apparatus comprises an image sensing element on which color filters of a plurality of colors for sensing an object image are formed, and a correction device which divides an image sensing region of the image sensing element into a plurality of partial image sensing regions, and corrects variations between the partial image sensing regions by using a different correction value for each color. 
         [0029]    Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part hereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a block diagram for explaining the arrangement of a digital camera according to the first embodiment of the present invention; 
           [0031]      FIGS. 2A to 2D  are views for explaining block division according to the first embodiment of the present invention; 
           [0032]      FIG. 3  is a table for explaining a correction value table according to the first embodiment of the present invention; 
           [0033]      FIG. 4  is a view for explaining an expanded correction value table according to the first embodiment of the present invention; 
           [0034]      FIG. 5  is a block diagram for explaining the arrangement of a digital camera according to the second embodiment of the present invention; 
           [0035]      FIGS. 6A to 6C  are views for explaining a correction value table according to the second embodiment of the present invention; 
           [0036]      FIG. 7  is a block diagram for explaining the arrangement of a conventional digital camera; 
           [0037]      FIG. 8  is a view for explaining joint exposure; 
           [0038]      FIG. 9  is a view for explaining a semiconductor image sensing element by joint exposure; and 
           [0039]      FIG. 10  is a view for explaining an example of a shift by joint exposure. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]    An image sensing signal correction method and image sensing apparatus according to preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
       First Embodiment 
       [0041]      FIG. 1  is a block diagram showing an image sensing apparatus (digital camera) according to the first embodiment of the present invention. 
         [0042]    This image sensing apparatus is basically constituted by adding a block correction unit  6  and a correction nonvolatile memory  7  for storing a correction value to the arrangement of  FIG. 7 . 
         [0043]    In  FIG. 1 , reference numeral  1  denotes an image sensing element which is driven and controlled similarly to a general element though the image sensing element  1  is constituted by joint exposure. The image sensing element  1  is driven by a driver  2  and operates at a predetermined frequency. A timing generator  3  is a timing generation circuit which outputs a vertical sync signal VD and horizontal sync signal HD. The timing generator  3  simultaneously supplies timing signals to circuit blocks. 
         [0044]    A pixel signal from the image sensing element  1  is input to a CDS/AGC circuit  4 , and subjected to processing such as known correlated double sampling to remove reset noise contained in an output from the CCD or the like. The output is amplified to a predetermined signal level. The amplified pixel signal is converted into a digital signal by an A/D converter  5 , thus obtaining a digital pixel signal. 
         [0045]    The digital pixel signal is sent to the block correction unit  6  where a shift by joint exposure is corrected. The block correction unit  6  loads a correction value for each block from the correction nonvolatile memory in accordance with an instruction from an overall control CPU, and performs correction (to be described later) for the digital pixel signal in accordance with a timing signal from the timing generator  3 . 
         [0046]    Information of a photographing lens  20  is transferred to an overall control CPU  18  via a mount  19  (camera &amp; lens mount). To switch a correction value for each block by the photographing lens, the overall control CPU  18  sends an instruction to this effect to the block correction unit  6 . This will also be described later. 
         [0047]    The corrected pixel signal is input to a memory controller  10  via a selector  8 , and all signal outputs are transferred to a frame memory  11 . In this case, all pixel data of photographing frames must be temporarily stored in the frame memory  11 . For sequential shooting or the like, all pixel data of photographed images are written in the frame memory  11 . 
         [0048]    After the end of write in the frame memory  11 , the contents of the frame memory  11  which stores pixel data are transferred to a camera digital signal processor (DSP)  9  via the selector  8  under the control of the memory controller  10 . The camera DSP  9  generates R, G, and B color signals on the basis of pixel data of each corrected image stored in the frame memory  11 . 
         [0049]    When the photographer designates photographing (i.e., image recording) by operating a camera operation switch  17 , pixel data of one frame are read out from the frame memory  11  in accordance with a control signal from the overall control CPU  18 , subjected to image processing by the camera DSP  9 , and temporarily stored in a work memory  13 . 
         [0050]    Data in the work memory  13  is compressed by a compression/decompression unit  15  on the basis of a predetermined compression format. The compressed data is stored in an external nonvolatile memory  16  (generally, a nonvolatile memory such as a flash memory is used). 
         [0051]    To observe photographed image data, data which is compressed and stored in the external memory  16  is decompressed into normal data of each pixel via the compression/decompression unit  15 . The decompressed data of each pixel is transferred to a video memory  12 , allowing to observe the photographed image via a monitor display  14 . 
         [0052]    The arrangement of  FIG. 1  assumes that the burden of signal processing in the camera DSP  9  is not increased. If the system has a margin for signal processing in the camera DSP  9 , shift correction may also be performed in the camera DSP. 
         [0053]    Correction of a shift caused by joint exposure will be explained. 
         [0054]    As described above, an on-chip color filter layer and on-chip microlens layer readily shift, and the influence appears as a step in the gain direction. In particular, the on-chip color filter layer is exposed for each color, the shift varies, and the step varies between colors. 
         [0055]    To solve this problem, the first embodiment executes correction with a correction value obtained by storing or calculating in advance a correction value for each block which subdivides a divided exposure region and includes a plurality of pixels. 
         [0056]      FIGS. 2A to 2D  are views for explaining division into blocks. 
         [0057]    As shown in  FIG. 2A , the image sensing element undergoes joint exposure at two, right and left joints, and one image sensing element is constituted by joining three, left, center, and right regions. As shown in  FIG. 2B , the image sensing element is divided into a plurality of blocks by using the right and left joints as division boundaries. In this case, the image sensing element is divided into six in the vertical direction×11 in the horizontal direction. Blocks as shown in  FIG. 2C  are formed from the entire image sensing element, and serve as a correction value table. The first embodiment adopts a correction value corresponding to each color of the on-chip color filter. For example, for a general Bayer filter, a correction value table having four planes is prepared, as shown in  FIG. 2D . 
         [0058]    As the number of blocks increases, the precision is considered to increase. However, this increases the memory area of the system, and thus the number of blocks must be determined in consideration of the balance with the effect. 
         [0059]    The form of the correction value will be explained. 
         [0060]    The correction value is one in the gain direction. Pixel signals on two sides at a joint are multiplied by correction values so as to eliminate any steps at the right and left joints. 
         [0061]    As correction values to be obtained, the first embodiment obtains correction values in the right and left regions by using as a reference a signal in a divided exposure region including the optical axis of a photographing optical system, i.e., the central region in this embodiment. 
         [0062]    This is because the center of the frame is designed to be directly used. The center of the frame often provides an object to be observed, and is determined to be kept unchanged in order to fully exploit the performance of the image sensing optical system. 
         [0063]    A shift is so corrected as to eliminate any step at a joint. This purpose is primarily achieved. In other words, other portions are processed by other correction operations. An independent correction value is used in the boundary direction at the joint, and the same correction value as that at the end of the boundary is used in a direction perpendicular to the boundary direction. 
         [0064]    From this viewpoint, one correction value table is shown in  FIG. 3 . 
         [0065]    In  FIG. 3 , all correction values in the central region are 1. In the right and left regions, blocks along each joint are assigned values optimal for them. In a direction perpendicular to the joint (horizontal direction in  FIG. 3 ), the same correction value as that of a block along the joint is set. With this setting, even if, e.g., the peripheral light quantity changes at the periphery, a natural change can be maintained. 
         [0066]    The correction value is switched for a target lens when a shift cannot be satisfactorily corrected unless the correction value is changed owing to, e.g., the pupil distance or F-number of a photographing lens. 
         [0067]      FIG. 4  is a view for explaining this. Especially in an interchangeable lens for a single-lens reflex camera system, the pupil distance changes depending on the focal length. To cope with this, three correction values are prepared for a standard lens, telephoto lens, and wide-angle lens, and stored in the correction value nonvolatile memory. When the photographing lens  20  is mounted on the camera, an optimal correction value table is sent to the correction calculation region of the block correction unit  6  on the basis of lens identification information (lens ID) obtained via the mount  19 . Satisfactory shift correction can always be achieved. 
         [0068]    Instead of changing the correction value on the basis of the lens ID as information on a mounted lens, the correction value may be changed or calculated on the basis of information on the pupil distance or F-number. 
       Second Embodiment 
       [0069]    In the first embodiment, the read system of the image sensing element  1  has only one channel. In the second embodiment, a read system capable of high-speed read has two channels. 
         [0070]      FIG. 5  is a block diagram showing an image sensing apparatus (digital camera) in which the read system of an image sensing element  1  has two channels. 
         [0071]    Since importance is placed on high-speed processing, two CDS/AGC circuits  4 - 1  and  4 - 2  and two A/D converters  5 - 1  and  5 - 2  are added to the block diagram of  FIG. 1 . 
         [0072]    The image sensing element  1  adopts various read methods by the two systems. Signals may be read out divisionally from right and left regions at the center of the element surface, as disclosed in Japanese Patent Laid-Open No. 2000-253305 as shown in  FIG. 6A . Instead, signals may be alternately read out every line ( FIG. 6B ). Signals may be read out by subdividing the region within the image sensing element, and multiplexed into two channels in externally outputting the signals ( FIG. 6C ). 
         [0073]    In these cases, a shift generated by the difference in read system must be coped with. 
         [0074]    The measure changes depending on a combination with the arrangement of an on-chip color filter. For example, when the Bayer array is employed, the methods in  FIGS. 6A and 6B  can be realized even by one set of block correction tables. 
         [0075]    The method of  FIG. 6C  requires two sets of block correction values unless the shift between a plurality of systems is corrected by any method. This is because a shift occurs between the two read channels even with the same color of the on-chip color filter. 
         [0076]    As described above, according to the first and second embodiments, the signal difference between a plurality of regions can be corrected when the image signal region of an image sensing element in an image sensing apparatus is formed by joint exposure and the sensitivity becomes nonuniform in the two-dimensional direction. 
         [0077]    Even if nonuniformity changes depending on the optical factor of a photographing optical system such as the exit pupil position or F-number, a signal difference can be more properly corrected. 
         [0078]    In read by many systems, the signal difference between a plurality of regions including the shift between a plurality of read channels can be corrected. 
         [0079]    The above embodiments can obtain a high-quality image free from any variation between image sensing regions. 
         [0080]    The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention the following claims are made.