Abstract:
An imaging pixel array includes an active area of pixels, organized into rows and columns of pixels. The array also includes a plurality of dark pixel columns adjacent to the active area of pixels such that rows of pixels in the active area of pixels extend across the plurality of dark pixel columns. The plurality of dark pixel columns are composed of tied pixels. The array also includes a plurality of dark pixel rows adjacent to the active area of pixels and the plurality of dark pixel columns such that columns of pixels in the active area of pixels extend across the plurality of dark pixel rows. The plurality of dark pixel rows are composed of both optically black pixels and tied pixels on the same row.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor imagers. More specifically, the present invention relates to noise reduction and suppression of unwanted artifacts in semiconductor imagers. 
     BACKGROUND OF THE INVENTION 
     Complementary metal-oxide semiconductor (CMOS) image sensors utilize sensor arrays that are composed of rows and columns of pixels. The pixels are sensitive to light of various wavelengths. When a pixel is subjected to a wavelength of light to which the pixel is sensitive, the pixel generates electrical charge that represents the intensity of the sensed light. When each pixel in the sensor array outputs electrical charge based on the light sensed by the array, the combined electrical charges represent the image projected upon the array. Thus, CMOS image sensors are capable of translating an image of light into electrical signals that may be used, for example, to create digital images. 
     Ideally, the digital images created through the use of CMOS image sensors are exact duplications of the light image projected upon the sensor arrays. However, various noise sources can affect individual pixel outputs and thus distort the resulting digital image. Some noise sources may affect the entire sensor array, thereby requiring frame-wide correction of the pixel output from the array. One such corrective measure applied to the output of the entire sensor array is the setting of a base-line black level (described below). Other noise sources may only affect specific portions of the sensor array. For example, row-specific noise may be generated from a mismatch of circuit structures in the image sensors due to variations in the manufacturing processes of integrated circuits. The effect of row-specific noise in an image sensor is that rows or groups of rows may exhibit relatively different outputs in response to uniform input light. 
     In order to set a corrective black level and remove the effects of row-specific noise, dark rows and dark columns are used in image sensors, as demonstrated in  FIG. 1 .  FIG. 1  show an image sensor  100  that includes a pixel array  110  organized into N pixel columns and R pixel rows. The pixel array  110  contains an active area  112 , dark rows  115  and dark columns  117 . Although not shown in  FIG. 1 , dark rows  115  may also be located above the active area  112 , and dark columns  117  may also be located to the left of the active area  112 . Upon readout of a row, parallel pixel outputs from each column (i.e., N pixel outputs) are sampled and stored on a set of capacitors  120 , one row at a time. Each pixel is in turn sent through an analog signal processing block  130  before being digitized by an analog-to-digital converter  140 . The stream of digitized pixels are then processed digitally (block  150 ) before they are sent to an output buffer  170 . By monitoring the digitized data from the dark rows  115 , a feedback loop  160  is used to adjust the frame-wise black level. Generally, noise reducing processes (block  150 ) are applied to each pixel output, sequentially, either before or after the output reaches the analog-to-digital converters  140 . 
     Dark columns  117  and dark rows  115  are areas within the pixel array  110  that do not receive light or capture image data. Pixel outputs from the dark rows  115  and dark columns  117  are used to both set the black level for the entire pixel array  110  and correct row-specific noise. 
     One corrective technique is to ensure that pixels in the dark columns  117  and dark rows  115  do not receive image data by covering the pixels in the dark columns  117  and dark rows  115  with a metal plate. Pixels blocked from sensing light via a metal plate are referred to as optically black pixels. Because, theoretically, no light is sensed by the optically black pixels, the only charge generated by the optically black pixels is internal noise-induced charge. This is often referred to as dark current. Thus, one method of compensating for noise is through the calculation of average optically black pixel output values, which represent average noise values, and then subtracting these average values from the outputs of the pixels in the active area  112 . For example, an appropriate black level may be set by calculating an average optically black pixel output for the optically black pixels in the dark rows  115  (block  150 ), and then subtracting this average value from the output of every pixel in the active area  112  and dark columns  117 . Row-specific noise in pixel array  110  may also be compensated for by calculating an average optically black pixel output for each row of optically black pixels in the dark columns  117  (block  150 ). The calculated optically black pixel average for each row is then subtracted from the values of each of the active pixels in the corresponding rows of pixels. 
     In practice, because each row of pixel outputs in the pixel array  110  is read-out sequentially, the pixel outputs from the dark rows  115  are read first. From the dark rows  115 , the optically black blacklevel is calculated (block  150 ) and then applied to the successive pixel outputs from the active area  112  and the dark columns  117 . Row-specific noise is then corrected by using the already adjusted optically black pixel output values from the dark columns  117 . The output values of the optically black pixels for a given row in the dark columns  117  are averaged (block  150 ), and then the averaged optically black pixel output is subtracted from the output of each of the pixels within the respective row of the active area  112 . 
     A drawback with using optically black pixels in calculating a black level value is that optically black pixels are sensitive to more than just background or internal noise. Optically black pixels may generate charge in response to random, localized noise sources, thus artificially altering the calculated black level. For example, optically black pixels may generate excess charge as a result of pixel blooming. Blooming is caused when too much light enters a pixel, thus saturating the pixel. A pixel subject to blooming is unable to hold all of the charge generated as a result of sensed light. Consequently, any excess charge may leak from the pixel and contaminate adjacent pixels. Optically black pixels that generate excess charge as a result of blooming will result in an artificially high black level. Infrared (IR) reflections may also result in excess charge generation. IR reflections occur when IR radiation is incident on pixels within the pixel array  110  and is trapped within the image sensor  100 . The IR radiation, which also causes pixels to generate charge, may repeatedly reflect against multiple optically black pixels, thus again artificially inflating the amount of generated charge. In these cases, the black level sensed by the optically black pixels is generally higher than the ideal black level because of the charge collected from these noise sources. 
     In response to the disadvantages of using optically black pixels to set the black level value, an alternative technique for correcting noise in a pixel array  110  is to tie the photodiode of the pixels in the dark rows  115  to a fixed voltage. The fixed voltage is, in essence, a fixed black level for the pixel array  110 . The advantages of this method is that the black level calculation is not influenced by blooming, IR reflections, etc., and that every frame utilizes a constant and unchanging black level. However, tied pixels are not sensitive to any changes in dark current or other row-specific noise sources. Thus, a black level generated utilizing tied pixels may not accurately compensate for the noise caused by dark current. 
     There is, therefore, a need and desire for a method and apparatus for efficiently generating and applying a stable black level value utilizing the benefits of tied pixels and optically black pixels to the pixel outputs of a solid state imager, for example, a CMOS imager. 
     BRIEF SUMMARY OF THE INVENTION 
     An imaging pixel array is provided, that includes an active area of pixels, organized into rows and columns of pixels. The array also includes a plurality of dark pixel columns adjacent to the active area of pixels such that rows of pixels in the active area of pixels extend across the plurality of dark pixel columns. The plurality of dark pixel columns are composed of tied pixels. The array also includes a plurality of dark pixel rows adjacent to the active area of pixels and the plurality of dark pixel columns such that columns of pixels in the active area of pixels extend across the plurality of dark pixel rows. The plurality of dark pixel rows are composed of both optically black pixels and tied pixels. 
     Similarly, an apparatus for providing pixel correction values for an imager is provided, as well as a method to provide the pixel correction values for the imager. Both the apparatus and method utilize a pixel array that is composed of an active area, a plurality of dark pixel columns and a plurality of dark pixel rows. The plurality of dark pixel columns are composed of tied pixels while the plurality of dark pixel rows are composed of a both optically black pixels and tied pixels. 
     An imager and an imaging system are further provided, both utilizing a pixel array that is composed of an active area, a plurality of dark pixel columns and a plurality of dark pixel rows. The plurality of dark pixel columns are composed of tied pixels while the plurality of dark pixel rows are composed of a both optically black pixels and tied pixels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an image sensor; 
         FIG. 2  is an image sensor according to an exemplary embodiment of the invention; 
         FIG. 3  is an image sensor according to another exemplary embodiment of the invention; 
         FIG. 4  shows the operations of a black level correction circuit according to an exemplary embodiment of the invention; and 
         FIG. 5  is a processor based system that includes an image sensor according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In one exemplary embodiment of the invention, an image sensor utilizes both tied and optically black pixels to calculate a stable black level value for generated images.  FIG. 2  demonstrates an image sensor  200 , e.g., a CMOS image sensor, that includes a pixel array  210 , a set of holding capacitors  220 , an analog signal processing block  230 , a set of analog-to-digital converters  240 , a set of read-out buffers  270 , a black level correction circuit  250  and a feedback loop  260 . The pixel array  210  includes an active area  212 , dark rows  215  and dark columns  217 . Within the dark rows  215  are rows  285  of optically black pixels (i.e., optically black pixel rows  285 ) and rows  280  of tied pixels (i.e., tied pixel rows  280 ). Only tied pixels are in the dark columns  217 . Because the tied pixel rows  280  are not sensitive to dark current, the output levels of the optically black pixel rows  285  and tied pixel rows  280  will vary. The variance between optically black and tied pixel outputs may be compensated for by calculating the difference in readout levels between the optically black pixel rows  285  and the tied pixel rows  280  (black level correction circuit  250 ) and then applying the calculated difference as an additional black level correction value to the whole frame (feedback loop  260 ). For example, circuit  250  could calculate an average optically black pixel output or tied pixel output for each row of the dark rows  215 . An average optically black pixel output could then be calculated for all optically black pixel rows  285  in the dark rows  215 , and an average tied pixel output could be calculated for all tied pixel rows  280  in the dark rows  215 . Finally, the difference between the average optically black pixel output and the average tied pixel output could be calculated. The calculated difference between the average tied pixel output and the average optically black pixel output is applied as a black level correction value (feedback loop  260 ). 
     However, errors may arise when comparing pixel outputs from different rows. When a row with either optically black or tied pixels is sampled and then average values are calculated for each row, the values are affected by row-specific noise. The uncertainty introduced by row-specific noise can be overcome by averaging pixel outputs from a sufficient number of multiple optically black pixel rows  285 , and then finding the difference between this more accurate average optically black pixel output value and an averaged output from a sufficient number of tied pixel rows  280 . To effectively average out any row-specific noise, however, approximately thirty-two rows per pixel color must be sampled. The averaging over thirty-two rows can be accomplished by averaging a single row sample over thirty-two frames, although a drawback to this method is that when new gain settings are applied to the sensor, a user must wait for thirty-two frames before a new correction factor based on the new gain setting is generated. A preferable method is to establish the correction value during readout of the dark rows so that a new correction factor is computed before the first active row is read out. This, of course, suggests the necessity of introducing thirty-two physical dark rows to be read and averaged and then compared with the tied pixel output, thus obtaining a reliable value for the black level average for the given gain and integration time of the imager. However, the large number of necessary dark rows is undesirable. Not only does the large number of dark rows  215  affect the area of the pixel array  210  by increasing the array&#39;s size and expense, but the increase in rows also affects the frame rate of the imager, since read-out of each frame will take longer. 
     In an improved exemplary embodiment of the invention, as demonstrated in  FIG. 3 , an image sensor  300  contains both optically black pixels  385  and tied pixels  380  in each row of the dark rows  315  of a pixel array  310 . As depicted in  FIG. 3 , image sensor  300  also includes a set of holding capacitors  320 , an analog signal processing block  330 , a set of analog-to-digital converters  340 , a set of read-out buffers  370 , a black level correction circuit  350  and a feedback loop  360 . The pixel array  310  includes an active area  312 , dark rows  315  and dark columns  317 . As indicated above, the output from both optically black pixels  385  and tied pixels  380  will vary and must be accounted for. However, in the improved embodiment of the invention, the difference between optically black and tied pixel output need not account for row-specific noise. All pixels on a row “see” the same row-specific noise, so values from tied pixels  380  can be compared with values from the optically black pixels  385  without the need for taking many row samples to suppress row-specific noise. In theory, a single dark row could be sufficient to generate an accurate black level value. The average output from tied pixels  380  in the row results in an initial black level value; the difference between the average tied pixel output and the average optically black pixel output in the same row results in an additional corrective value. Both the tied pixel value and the additional corrective value are calculated by the black level correction circuit  350  and are summed together to generate a result which is applied to succeeding pixel outputs (feedback loop  360 ), thus setting an accurate black level. In practice, a few dark rows may be necessary for both redundancy and to further refine the calculated black level. 
     The physical organization of tied pixels  380  and optically black pixels  370  of dark rows  315  can be a checkerboard pattern, with individual pixels alternating between optically black pixels  385  and tied pixels  380 . Alternatively, dark rows  315  could be split in the middle, with optically black pixels  385  on one side of a row and tied pixels  380  on the other. In the event that rows are split, it is preferable to alternate the sides of the rows whereon the optically black pixels  385  and tied pixels  380  are located, so as to facilitate the averaging out of any noise artifacts arising from localized defects of the pixel array  310 . Any other repetitive pattern of n consecutive tied pixels followed by n consecutive dark pixels could be used, where n is an integer greater than one but less than half the length of dark rows  315 . In general, any symmetrical physical arrangement of tied pixels  380  and optically black pixels  385  of dark rows  315  is appropriate. 
     The operations of the black level correction circuit  350  are summarized in  FIG. 4 . As pixel values from the dark rows  315  are readout, the black level correction circuit  350  determines the average optically black pixel value and the average tied pixel value for each row in the dark rows  315  (block  410 ). For each row, the difference between the average optically black pixel value and the average tied pixel value is calculated (block  420 ). The processes of blocks  410  and  420  are repeated for each of the dark rows  315  (block  430 ). Once the difference between the average optically black pixel value and the average tied pixel value for each row is calculated, an average of the differences is calculated (block  440 ). The calculated average of the differences is summed with a tied pixel value to create an overall black level value (block  450 ). When pixel values from the active area  312  or dark columns  317  are readout, black level correction occurs by subtracting the overall black level value from the value of each pixel in the active area  312  and dark columns  317  (block  460 ). 
     The above-described embodiments of the invention are directed towards setting an appropriate black level by performing black level correction procedures on the analog pixel signal outputs of pixels in a pixel array. This analog black level correction is implemented by the black level correction circuits  250  and  350  and the feedback loops  260  and  360  ( FIGS. 2 and 3 ). However, the black level correction procedures may also be applied on digital pixel signal outputs. In this case, the black level correction circuits  250  and  350  act as described, but the feedback loops  260  and  360  are used to adjust the frame-wise black level after the pixel output is digitized. By applying black level correction to digital pixel outputs using noise correction modules  250 ,  350 , the black level correction may be implemented as either a hardware or a software solution. As a software solution, the black level correction may be implemented as either software integrated with the imagers  200 ,  300 , or as a stand-alone software product stored on a carrier medium and installed on a computer system. 
     A typical processor based system  1000 , which includes an imager device  1030  according to the present invention is illustrated in  FIG. 5 . A processor based system is exemplary of a system having digital circuits which could include imager devices. Without being limiting, such a system could include a computer system, camera system, scanner, machine vision system, vehicle navigation system, video telephone, surveillance system, auto focus system, star tracker system, motion detection system, or other image acquisition system. 
     A processor system, such as a camera system, for example, generally comprises a central processing unit (CPU)  1010 , for example, a microprocessor, that communicates with an input/output (I/O) device  1020  over a bus  1090 . The imager  1030  also communicates with the system components over bus  1090 . The computer system  1000  also includes random access memory (RAM)  1040 , and, in the case of an imaging system may include peripheral devices such as a removable memory  1050  which also communicates with CPU  1010  over the bus  1090 . Imager  1030  is preferably constructed as an integrated circuit which includes pixels containing a photosensor, such as a photogate or photodiode. The imager  1030  may be combined with a processor, such as a CPU, digital signal processor or microprocessor, with or without memory storage in a single integrated circuit, or may be on a different chip than the processor. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art(s) that various changes in form and detail can be made therein without departing from the spirit and scope of the present invention. Thus, the present invention should not be limited by any of the above-described exemplary embodiments.