Abstract:
A system of non-uniformity correction (NUC) for a pixel in an imaging array includes: a storage module for storing (a) a first gain coefficient for correcting a gain error of the pixel and (b) multiple damping factors, including a first damping factor, for adjusting the first gain coefficient, in response to multiple respective light levels that the pixel senses during operation. Also included is a NUC corrector module for receiving a first intensity value from the pixel in response to a first light level. The NUC corrector module extracts the first gain coefficient and a first damping factor from the storage module, and then corrects the first intensity value of the pixel using the (a) first gain coefficient and (b) first damping factor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/483,938, filed May 9, 2011, the contents of which are incorporated by reference herein in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made under Dakota Project Contract No. 13635723, 22D and the United States Government may have certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates, in general, to image processing. More specifically, the present invention relates to correcting an image due to non-uniformity of light emitting elements, or pixels of an imaging sensor. 
     BACKGROUND OF THE INVENTION 
     Imaging sensors typically include a plurality of light detecting elements, also known as picture elements or pixels. The pixels are usually arranged in an array. When the array is exposed to a subject of interest, each pixel captures a certain amount of light to form an image. Imaging sensors may include non-uniformities that are inherent in the composition of the sensor. For the image to be properly viewed or evaluated, the non-uniformities should be corrected. 
     Each pixel in the array stores one or more values related to a characteristic of the radiation, such as color, brightness, etc., otherwise known as characteristic data. However, due to manufacturing issues, installation problems, material limitations and defects, a portion of the pixels in the array may not capture and store the characteristic data correctly. Some pixels may be considered good but may still need an adjustment to the data that is stored. Therefore, characteristic data of all the pixels in the array need to be adjusted by one or more correction components. 
     Generally, the array of pixels is tested and evaluated before field usage or product distribution to determine which of the correction components may need to be applied to the pixels. The correction components include a gain coefficient and an offset coefficient. The gain coefficient is a value that may be multiplied by the characteristic data in order to correct the data. In various embodiments, the gain coefficient may have a range of values from 0 to approximately 2. The offset coefficient is another value that may be added to the characteristic data to provide correction to each pixel. In various embodiments, the offset coefficient may have a range of values from approximately −512 to approximately 511. 
     Some pixels in the array may need their characteristic data corrected by the gain coefficients, some pixels may need correction by the offset coefficients, some pixels my need both the gain coefficient and offset coefficient corrections. If a pixel is good and needs no correction, as an example, the gain coefficient may have a value of 1 and the offset coefficient may have a value of 0. 
     In general, an offset coefficient and a gain coefficient are applied to every pixel in the array, so that each pixel will respond to light uniformly. This process is known as non-uniformity correction (NUC) of a pixel. A method used to calculate the NUC offset and gain coefficients involves recording flat field images at various light levels. Normally, two different light levels are selected and a pixel&#39;s response to the two different light levels is calculated as a slope and compared to the average slope of the entire sensor. 
     The method of calculating NUC gain coefficients is linear. Pixels, however, respond non-linearly to different light levels. The inventor discovered that less non-uniformity is observed at light levels near those used to calculate the NUC gain coefficients, but the error/non-uniformity of a non-uniformity corrected pixel may be quite dramatic at extreme ADU levels. The effect of this is that if the average video level of a scene is near the midpoint of the display range, pixels at the upper boundary of the display range may be displayed inaccurately, because the amount of gain applied is inappropriate for this video level. 
     Accordingly, an improved method is still needed to provide non-uniformity correction of pixels in an imaging array that overcomes the aforementioned problems. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a system of non-uniformity correction (NUC) for a pixel in an imaging array. The system includes a storage module for storing (a) a first gain coefficient for correcting a gain error of the pixel and (b) multiple damping factors, including a first damping factor, for adjusting the first gain coefficient, in response to multiple respective light levels that the pixel senses during operation. Also included is a NUC corrector module for receiving a first intensity value from the pixel in response to a first light level. The NUC corrector module extracts the first gain coefficient and a first damping factor from the storage module. The NUC corrector module corrects the first intensity value of the pixel using the (a) first gain coefficient and (b) first damping factor. 
     The system further includes an NUC calibration module for calculating the first gain coefficient and the multiple damping factors, and providing the first gain coefficient and the multiple damping factors to the storage module. The first gain coefficient is determined by capturing a flat field image at a predetermined light level. The multiple damping factors are determined by capturing multiple flat field images at a plurality of further light levels, and calculating a change in gain at each of the plurality of further light levels. 
     The NUC calibration module is configured for calculation during a calibration mode, and configured for operation during an operating mode. The NUC calibration module is configured to determine a first offset coefficient for adjusting an offset value of the pixel. The storage module is configured to receive and store the first offset coefficient from the NUC calibration module. The storage module includes a look-up table (LUT) of the multiple respective light levels corresponding to the multiple damping factors of the pixel. The storage module stores (a) a second gain coefficient for correcting a gain error of another pixel and (b) multiple damping factors, including a second damping factor, for adjusting the second gain coefficient, in response to multiple respective light levels that the other pixel senses during operation. The NUC corrector module corrects a second intensity value of the other pixel using the (a) second gain coefficient and (b) second damping factor. 
     The NUC corrector module includes a subtractor, a multiplier and an adder for (a) the subtractor reducing a value of 1 by the first gain coefficient to provide a reduced value, (b) the multiplier multiplying the first damping factor by the reduced value to provide a multiplied value, and (c) the adder adding or subtracting the multiplied value from the value of 1 to provide a corrected gain of the pixel. The NUC corrector module also includes an offset adjustment value which is multiplied by the corrected gain of the pixel to provide a final output data of the pixel. 
     Another embodiment of the present invention is a method for non-uniformity correction (NUC) of a pixel in an imaging array. The method includes the steps of: 
     storing a gain coefficient of the pixel; 
     storing a plurality of damping factors for a corresponding plurality of video levels of the pixel; 
     determining a video level of the pixel; and 
     correcting the pixel, at the determined video level, by adjusting the gain coefficient of the pixel using a stored damping factor that corresponds to the determined video level of the pixel. 
     The method also includes the steps of: 
     storing an offset coefficient for the pixel; and 
     correcting the pixel using the stored offset coefficient for the pixel. 
     Yet another embodiment of the present invention is a method for non-uniformity correction (NUC) of a pixel in an imaging array. The method includes the steps of: 
     determining a video level of the pixel; 
     selecting a predetermined NUC gain for the pixel; 
     modifying the predetermined NUC gain by a damping factor to provide a damped gain; 
     correcting the video level of the pixel using the damped gain. 
     The method further includes the steps of: 
     storing at least one function that mathematically expresses multiple gain values of the pixel as a function of respective video levels of the pixel; 
     selecting a gain value from the function based on the video level determined for the pixel; 
     using the selected gain value to calculate the damped gain; 
     subtracting the selected gain value from a value of 1 to provide a subtracted value; 
     multiplying the subtracted value by the damping factor of the pixel to provide a multiplied value; 
     adding or subtracting the multiplied value from the value of 1 to provide a calculated damped gain; 
     adding an offset value to the video level of the pixel; and 
     multiplying the offset value with the calculated damped gain to provide the corrected video level of the pixel. 
     The step of storing includes: storing a plurality of functions that mathematically express multiple gain values of the pixel as a function of the respective video levels of the pixel; and the step of selecting includes: selecting the gain value from the plurality of functions that is adjacent to the predetermined NUC gain for the pixel. 
     It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be understood from the following detailed description when read in connection with the accompanying figures: 
         FIG. 1  is a non-uniformity correction (NUC) system, in accordance with an embodiment of the present invention. 
         FIG. 2  is a flow diagram used by the non-uniformity correction (NUC) system  100  shown in  FIG. 1 , in accordance with an embodiment of the present invention. 
         FIG. 3  is a plot of average gain as a function of average video level (in ADU), depicting examples of variations in gain as the pixel video levels are changed. 
         FIG. 4  is a functional diagram showing how correcting the non-uniformity of a pixel is accomplished by the NUC corrector module of the system shown in  FIG. 1 , in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As will be explained, the present invention calculates a damping factor, which is applied to each gain coefficient used to correct each respective pixel in an imaging array. The damping factor is applied to each gain coefficient based on the video level stored by the respective pixel in a look up table (LUT). By applying a damping factor to the gain coefficient of each pixel, a much lower level of non-uniformity is achieved. 
     Referring to  FIG. 1 , there is shown a non-uniformity correction (NUC) system, designated as  100 , in accordance with an embodiment of the present invention. As shown, system  100  includes NUC calibrator module  112 , gain/offset module  122 , damping adjustment module  124  and NUC corrector module  114 . Also included is lens assembly  106  receiving light reflected from object  102  by using light source  104 . The imaging array  108  provides pixel intensities in response to the light received by lens assembly  106 . An analog-to-digital converter (ADC)  110  outputs digital data representing the intensities sensed by the pixels in imaging array  108 . 
     As shown, the digital data is sent to NUC corrector module  114  and to NUC calibrator module  112 . During normal operation, a frame of data is sent to the NUC corrector module, where pixel data is corrected and then outputted as final image data. During calibration, on the other hand, a frame of data is sent to NUC calibrator module  112  for calibrating the correction coefficients, prior to sending them to NUC corrector module  114 . 
     The NUC calibrator module includes a gain and offset calculator, designated as  118 , and a separate damping adjustment calculator, designated as  120 . Gain and offset coefficients are calculated for every pixel in imaging array  108  and then stored in gain/offset memory module  122 . In addition, a damping factor is calculated for different video levels that are sensed by each pixel of imaging array  108 . The damping factors are stored in damping adjustment memory module  124 . 
     The damping adjustment memory module  124  may include a look-up table (LUT)  126 . For each pixel (for example, pixel n), there are multiple video levels (for example, video level 1, 2, 3, . . . , j ) that correspond to multiple damping values (for example, damping value 1, 2, 3, . . . , j ), respectively. These values are calculated by damping adjustment calculator module  120  by exposing imaging array  108  to different light intensities transmitted from light source  104 . 
     Accordingly, the present invention provides a gain coefficient; an offset coefficient; and damping factors as a function of video input levels for each pixel of imaging array  108 . The intensity value sensed by each pixel is, thus, corrected using the two coefficient values and the appropriate damping factor in NUC corrector module  114 . 
     An example of a method used by the non-uniformity correction (NUC) system  100  is shown in  FIG. 2 . As shown, a flat field image (for example, a black light level) is captured by step  200 . Step  202  calculates the offset correction to be used for each pixel in imaging array  108 . The offset correction is stored by step  204  in gain/offset memory module  122 . Using another flat field image at a predetermined light level, step  206  captures the intensity of each pixel value in imaging array  108 . A gain correction coefficient is calculated for each pixel by step  208  and stored as a gain correction coefficient in gain/offset memory module  122  by step  210 . 
     Using multiple light levels, by way of light source  104 , step  212  captures a flat field image for each light level. For each pixel, step  214  determines the change of gain between each light level and a light level producing unity gain for each pixel. The change in gain becomes the damping value for each light level. This value is stored in damping adjustment module  124 . Accordingly, a look-up table (LUT) may be formed, for each pixel (for example, pixel n), which includes multiple video levels, namely, video level 1, 2, 3, . . . j , that correspond to multiple damping values, namely, damping value 1, 2, 3, . . . j . 
     Referring to  FIG. 3 , there is shown an example of curves depicting average gain as a function of average video level (in ADU). As shown, groups of pixels are grouped into different regions as a function of NUC gain, in order to correctly adjust the intensity output of a pixel at normal temperature and light level. An average gain for each slope band may be compared to the same group from images taken under different lighting conditions. The slope bands demonstrate that NUC does vary with video levels. For example, at an average video level of 605 ADU, there are four different average gain values for the four groups of pixels, respectively. The average gain at 605 ADU varies from 0.097 up to 1.22. Much of this effect is corrected by the present invention by adjustment based on the ADU value, which is applied as a damping factor to the gain and offset coefficients of each pixel. It will be appreciated that as the average video level is increased all the pixels tend to converge upon an average gain of approximately unity (1). 
     Referring now to  FIG. 4 , there is shown a functional diagram, generally designated as  114 , for correcting the non-uniformity of a pixel in imaging array  108 . It will be appreciated that the elements shown in the functional diagram may be implemented by NUC corrector module  114  of system  100 . As shown, for each pixel n, a digital intensity value, represented by DATA n , is inputted to summer  402 . Also provided into summer  402  is the stored offset coefficient correction value for pixel n, namely, OFFSET n . The intensity value, DATA n , is added to the offset coefficient value, OFFSET n , and the result is outputted onto line  404 . 
     The stored gain coefficient correction value for pixel n, namely GAIN n , is subtracted from unity (1), and inputted into NUC corrector module  114 . Also inputted into the NUC corrector module from LUT  126  is a damping factor, DAMP n , at the actual video level of the pixel, namely, VIDEO LEVEL j . This value, DAMP n (VIDEO LEVEL j ), is then multiplied by 1-GAIN n  using multiplier  406 . The output data from multiplier  406 , by way of line  408 , is either subtracted from or added to the value of unity (1) using adder  410 . If the output data from multiplier  406  is positive (because GAINn is smaller than one), then the output of adder  410  is less than one, due to the change in sign on line  408 . If the output data from multiplier  406  is negative, however, (because GAINn is greater than one), then the output of adder  410  is also greater than one, due to the change in sign on line  408 . The output data from adder  410  is provided, by way of line  412 , to multiplier  414 . Multiplier  414  receives the offset corrected data, by way of line  404 , and multiplies this value by the corrected gain placed on line  412 . The final data for pixel n is outputted as OUTPUT DATA n . The process shown in  FIG. 4  is repeated for every pixel in imaging array  108 . It will be appreciated that the correct gain values placed on line  412  are the same gain values stored in module  122  of  FIG. 1 . 
     It will be appreciated that if the damping factor is not used by the present invention, then multiplier  414  would simply multiply the corrected gain value arriving on line  412  (using only the gain coefficient) with the offset data arriving on line  404  and thus provide the final output data for pixel n. The present invention however, advantageously provides the ADU value of pixel n, which then triggers a call to LUT  126  for reporting the appropriate damping factor for pixel n. This appropriate damping factor then corrects the gain coefficient of pixel n. 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.