Abstract:
An apparatus and method for concealing defective pixels in image sensors having a test mode. The image sensing apparatus includes a sensing module for capturing an image from an object, wherein the sensing module includes a plurality of pixels and a light source for detecting a defect of the pixel and wherein the light source is turned on or off for a test mode; a controller for determining whether there are any detective pixels in an image frame received from the sensing module using the light source and for storing a position about defective pixels; and an image concealment unit for comparing a position of the detected defective pixels with a position of the image frame of the object and for concealing the detected defective pixels.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an apparatus and method for concealing defective pixels in image sensors having a test mode.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    Generally, an image sensor is a semiconductor device that comprises CCD or CMOS and is the most important device for inputting visual information. This image sensor can be used in camcorders, digital cameras, scanners and other image reproduction systems.  
           [0003]    The image information can be described as light information and it can be distinguished by its luminosity and color. An image sensor is the device that converts information to electrical signals, more particularly, it converts analog electrical signals to digital signals to complete digitized image processes.  
           [0004]    An image sensor may be said to have a plurality of pixels in a two-dimensional structure and each pixel converts its light into an electrical signal depending on its brightness. By measuring its electrical signal, the amount of light which comes into each pixel can be defined and an image of pixel units can be formed by the defined values.  
           [0005]    On this occasion, the operation of each pixel is similar to that of a solar cell. That is, the brighter the light, the greater the electric charge accumulated, and the intensity of the light is defined by measuring accumulated charges within the fixed time.  
           [0006]    [0006]FIG. 1 is a schematic diagram of an actual object and the image in the sensor having an actual object  101 , a sensor chip  102 , pixel area  103  and an image of the actual object  104  in the pixel area of the sensor.  
           [0007]    At this point, a greater number of pixels in the pixel area on the drawing can improve the expression of the actual object.  
           [0008]    An image sensor can be defined into two categories, namely black and white, and color. In case of the color image sensor, the color filter of RGB (Red, Green, Blue) is put on each pixel and responds to the corresponding color. Among several methods of putting pattern of the color filter, the representative one is called “Bayer format”.  
                                           TABLE 1                                       R1   G1   R2   G2   R3   G3           G4   B1   G5   B2   G6   B3           R4   G7   R5   G8   R6   G9           G   B   G   B   G   B                      
 
           [0009]    Generally, each pixel of the image sensor can indicate only one color. However, all pixels should have all information of RGB in order to display an image. An interpolation technique is used to derive information that is not available. For example, the mathematic formation of the RGB values in the B1 pixel in the center-left of the 3×3 box shown in Table 1 by using interpolation is shown below.  
           [0010]    [Mathematic Formation 1] 
           [0011]    R=(R1+R2+R4+R5)/4  
           [0012]    G=(G1+G4+G5+G7)/4  
           [0013]    B=B1  
           [0014]    By using the above formation, the R, G, B information in the B 1  pixel can be determined.  
           [0015]    For another example, the mathematic formation of the RGB values in the G 6  pixel in the center-right of the 3×3 box of Table 1 can be determined.  
           [0016]    [Mathematic Formation 2] 
           [0017]    R=(R3+R6)/2  
           [0018]    G=G6  
           [0019]    B=(B2+B3)/2  
           [0020]    Generally, an abnormally operating pixel can be found for several reasons during the manufacturing process. Because this pixel does not respond to the light properly, it appears differently when the image is reorganized using the output value. A pixel which appears brighter or darker than the actual object is called a Defective Pixel.  
           [0021]    Among these, a defective pixel that appears brighter is called a “White Pixel” and a defective pixel that appears darker is called a “Dark Pixel.” Sensors that have defective pixels cannot be sold.  
           [0022]    The greater the number of pixels in the image sensor, the higher the price, however, the possibility of generating defective pixels also increases.  
           [0023]    Actually, the defective pixels in the image sensor are the most important element affecting the yield. In accordance with this fact, it can be beneficial if products having a defective pixel can be sold.  
           [0024]    To accomplish this, the Defective Pixel Concealment (DPC) method is used. This is method for concealing physically defective pixels. The basic theory of DPC is the same as the interpolation technique.  
           [0025]    In other words, if the pixel that is for display is the one that is defective, the value of a normal pixel around it is substituted in place of the defective pixel.  
                                           TABLE 2                                       R1   G1   R2   G2   R3   G3           G4   B1   G5   B2   G6   B3           R4   G7   R5   G8   R6   G9                      
 
           [0026]    To get the RGB value of the B2 pixel in Table 2, R=(R2+R3+R5+R6)/4, B=B2, G=(G2+G5+G6+G8)/4 is used for normal pixel B2.  
           [0027]    If B2 is a defective pixel, the value of the Blue color appears differently than an actual color. If B=(B1+B3)/2 is substituted for B=B2, the color gets more natural.  
           [0028]    This method is the DPC method of interpolation. In another method, it can be substituted as B=B1 and this is the DPC method of substitution.  
           [0029]    To describe the DPC method of interpolation, reference is made to Table 3 as shown below.  
                                           TABLE 3                                       R1   G1   R2   G2   R3   G3           G4   B1   G5   B2   G6   B3           R4   G7   R5   G8   R6   G9                      
 
           [0030]    In Table 3, the Green value of the G5 pixel becomes G=G5. If the G5 pixel is defective, it can be changed to G=(G1 30  G2+G7+G8). At this point, it can simply be substituted as G=G4 using the DPC method of substitution.  
           [0031]    Among the DPC methods above, the concealment of interpolation produces a more natural screen than the concealment of substitution, however, more hardware is required.  
           [0032]    The prior art to implement the DPC method will be described referring to FIG. 2A to FIG. 2C.  
           [0033]    [0033]FIG. 2A through FIG. 2C are schematic diagrams of the defective pixel concealment of the prior image sensor and illustrate the camera as examples. FIG. 2A is an entire diagram of the defective pixel concealment of the image sensor. FIG. 2B is a detailed diagram of a sensing module  210  in FIG. 2A and FIG. 2C is a detailed diagram of an image concealment unit  220  in FIG. 2A.  
           [0034]    Referring to FIG. 2A, the apparatus for concealing the defective pixels in the image sensors in the prior art comprises a sensing module  210 , an image concealment unit  220 , a storage unit  240  of EPROM to store information of the defective pixel positions, and a control unit  230 .  
           [0035]    The image concealment unit  220  is a circuit to process the image data transmitted from the sensor in the sensing module  210  with interpolation operation; a DPC circuit has to be included therein in order for the DPC to be processed before interpolation.  
           [0036]    [0036]FIG. 2C is a detailed diagram of the image concealment unit  220  and comprises an interpolation processor  221  for processing interpolation of normal pixels, an interpolation processor  222  for processing interpolation of defective pixels, a current pixel position detector  223  of current pixel position for managing information of the current pixel position, a recorder  224  for recording position information of defective pixels, a comparator  225  for comparing current pixel position and defective pixel position, a DPC processor  226  for processing DPC process in compliance with the result from the comparator, a DPC driver  227  for driving the DPC processing device, a selector  228  for selecting an output result of the interpolation process devices in normal or defective pixels in compliance with the result from the DPC process device  226 , and a processor  229  for LF processing separate functions of image frame other than DPC functions.  
           [0037]    Furthermore, a control unit  230  analyzes and processes the data from the image concealment unit  220  and programs the sensing module  210  and the image concealment unit  220 .  
           [0038]    The storage unit  240  of EPROM stores the position information of defective pixels in the sensor pixel area of sensing module  210 . When the camera operates in normal operating mode, the control unit  230  records the position information of defective pixels stored in the storage unit  240  to recorder  224  of defective pixel position in the image concealment unit  220 .  
           [0039]    The prior art of the apparatus for concealing defective pixels in the image sensors has the following methods.  
           [0040]    First of all, the following is the method for programming to find defective pixels.  
           [0041]    The exposure of the sensor in the sensing module  210  is set and the white side is captured under the light properly. Control unit  230  analyzes to find darker pixels in this capture as compared with other pixels, then treats those darker pixels as Dark Pixels and memorizes the position within their pixel area  103 .  
           [0042]    The exposure and light in the sensor is set in proper value and the black background is captured.  
           [0043]    Control unit  230  analyzes to find brighter pixels in this capture as compared with other pixels, then treats those brighter pixels as White Pixels and memorizes their position within the pixel area  103 .  
           [0044]    The position information of the Dark Pixels and the White Pixels is stored in the storage unit  240 .  
           [0045]    Next, the following is the method for operating actual DPC by using the position of the defective pixels stored in the storage unit  240 .  
           [0046]    When the voltage is impressed in a camera, the control unit  230  reads out the defective pixel positions in the storage unit  240  and writes into recorder  224  defective pixel positions in the image concealment unit  220  and drives DPC driver  227 .  
           [0047]    After that, whenever interpolation is processed, the values in the current pixel position detector  223  of current pixel position and the values in the recorder  224  for recording the position information of defective pixels are compared. If the values are the same, the selector  228  selects the value processed in the interpolation processor  222  and delivers the value to the control unit  230  to conceal the defective pixels in the image sensors.  
           [0048]    In this method for operating actual DPC, it memorizes the position of the defective pixel in the storage unit  240 , loads it into the recorder  224  (ASIC register), then conceals and transmits the data if the defective pixel position is detected. The reason for reading out the data from the storage unit  240  and loading it into the recorder  224  (ASIC register) is because the direct DPC method cannot be completed with the position data due to the lack of speed in the operating time of the storage unit  240 .  
           [0049]    However, the position of a defective pixel has to be found under this method. In order to complete this method, the integration time has to be constant and the defective pixel has to be found.  
           [0050]    But the prior DPC method has the following problems.  
           [0051]    A complex process is required, such as EPROM to store the defective pixel position, the test for finding the position of a defective pixel in every exposure of sensor and capturing environment and the storing the defect position into EPROM.  
           [0052]    Furthermore, this method cannot conceal a moving defective pixel.  
         SUMMARY OF THE INVENTION  
         [0053]    It is, therefore, an object of the present invention to provide an apparatus and method for concealing defective pixels of image sensors having a test mode. The present invention does not need to have an external storing device nor programs for defective pixel in every test. Also, the present invention provides recording media for reading out from the programmed computer in order to conceal a moving defective pixel of an image sensor.  
           [0054]    In accordance with an aspect of the present invention, there is provided an image sensing apparatus, comprising a sensing module for capturing an image from an object, wherein the sensing module includes a plurality of pixels and a light source for detecting a defect of the pixel and wherein the light source is turned on and off for a test mode; control means for determining whether there is any defective pixel in an image frame received from the sensing module using the light source and for storing a position about a defective pixel; and image concealment means for comparing a position of the detected defective pixel with a position of the image frame of the object and for concealing the detected defective pixel.  
           [0055]    In accordance with another aspect of the present invention, there is provided a method for concealing a defective pixel in an image sensor, comprising steps of searching for a White Pixel and temporarily storing a position of the White Pixel for a test mode; searching for a Dark Pixel and temporarily storing a position of the Dark Pixel; storing the position of the White and Dark pixels as defective pixels and receiving the image from an object for a test mode; and comparing positions of pixels, which are associated with the image from the object, with the position of the defective pixel and combining an image for the defective pixel through an interpolation.  
           [0056]    In accordance with still another aspect of the present invention, there is provided a method for concealing a defective pixel in an image sensor, comprising steps of storing a first image frame which is produced for a White pixel test mode; storing a second image frame which is produced for a Dark pixel test mode; determining whether there are any White or Dark pixels in the first and second frames, temporarily storing a position of the White or Dark pixel if there are any White or Dark pixels and recording the position of the White or Dark pixels as a position of a defective pixel and receiving an image from an object; and comparing positions of pixels, which are associated with the image from the object, with the position of the defective pixel and combining an image for the defective pixel through an interpolation.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0057]    [0057]FIG. 1 is a schematic diagram of an actual object and the image on the sensor of the present invention.  
         [0058]    [0058]FIG. 2A through FIG. 2C are schematic diagrams of defective pixel concealment according to the prior image sensor and illustrating the camera as examples.  
         [0059]    [0059]FIG. 3 is a diagram of an apparatus for concealing defective pixels in an image sensor having a test mode according to the present invention.  
         [0060]    [0060]FIG. 4A is a detailed diagram for the sensing module in an apparatus for concealing defective pixels of an image sensor having a test mode according to the present invention.  
         [0061]    [0061]FIG. 4B is a detailed diagram for the image concealment t unit in an apparatus for concealing defective pixels of an image sensor having a test mode according to the present invention.  
         [0062]    [0062]FIG. 4C is a detailed diagram for the control unit in an apparatus for concealing defective pixels of an image sensor having a test mode, as shown in FIG. 4B.  
         [0063]    [0063]FIG. 5 is a flow chart of a method for concealing defective pixels of an image sensor according to the present invention.  
         [0064]    [0064]FIG. 6A is a detailed flow chart for a decision method of a White Pixel, according to the present invention.  
         [0065]    [0065]FIG. 6B is a detailed flow chart for a decision method of a Dark Pixel, according to the present invention.  
         [0066]    [0066]FIG. 6C is a detailed flow chart of a method for normal operating mode in a method for concealing defective pixels of an image sensor and describes detailed procedure of the normal operating mode, according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0067]    Hereinafter, an image sensor according to the present invention will be described in detail referring to the accompanying drawings.  
         [0068]    An apparatus for concealing defective pixels according to the present invention has an internal device for searching for a defective pixel so that the defective pixel is found artificially.  
         [0069]    An apparatus for concealing defective pixels of the image sensor, which will be described below, can be applied to camcorders, digital cameras and scanners. However, the present invention will be described in an image sensor built in a digital camera system.  
         [0070]    [0070]FIG. 3 is a diagram of an apparatus for concealing defective pixels in the image sensor having a test mode according to the present invention. FIG. 4A is a detailed diagram for the sensing module in the apparatus for concealing defective pixels of the image sensor and describes a detailed cross-section of the sensing module  310  of FIG. 3. FIG. 4B is a detailed diagram for the image concealment unit  320  in an apparatus for concealing defective pixels of the image sensor and describes a detailed cross-section of the image concealment unit  320  of FIG. 3. FIG. 4C is a detailed diagram for the control unit  330  in an apparatus for concealing defective pixels of the image sensor and describes a detailed cross-section of the control unit  330  of FIG. 3.  
         [0071]    Referring to FIG. 3, the image sensor according to the present invention has a sensing module  310 , an image concealment unit  320  and a control unit  330 .  
         [0072]    As illustrated in FIG. 4A, the sensing module  310  has a lens holder  311 , a lens  312 , a light source  313 , an image sensor package  314 , an image sensor die  315  and a PCB board  316 . As compared with the sensing module  210  in FIG. 2B, the present invention further includes a light source.  
         [0073]    Therefore, the sensing module  310  having an apparatus for concealing defective pixels of the image sensor in the present invention has a small light source close to sensors therein shown in FIG. 4A. This light source  313  is turned on during a test mode searching for the defective pixels and turned off during a normal operating mode.  
         [0074]    During the normal operating mode, the light source  313  is located away from the path of the light transmitting through the lens because the light source  313  must not disturb the image capturing process.  
         [0075]    In FIG. 4B, the image concealment unit  320  includes a first interpolation processor  321  for processing interpolation of normal pixels, a second interpolation processor  322  for processing interpolation of defective pixels, a current pixel position detector  323  for managing information of the current pixel position, a recorder  324  for recording the position information of defective pixels, a comparator  325  for comparing current pixel positions and defective pixel positions, a DPC processor  326  for executing the DPC process in response to the result from the comparator, a DPC driver  327  for driving the DPC processor  326 , a selector  328  for selectively outputting the output from the first or second interpolation processor  321  or  322  in the normal operating mode and for receiving the image frame from the sensing module  310  in the test mode, a processor  329  for processing separate functions of image frame other than DPC functions, and a selection controller  341  for controlling the selector  328  in response to a control signal, which indicates the test or normal mode, from the control unit  330 .  
         [0076]    In FIG. 4B, the image frame from the sensing module  310  is transmitted to the control unit  330  via the image concealment unit  320  in the test mode. However, the image frame from the sensing module  310  can be transmitted directly to the control unit  330  without passing through the image concealment unit  320 .  
         [0077]    As shown in FIG. 4C, the control unit  330  can be a computer system or a micro controller, and controls and processes the data from the sensing module  310  and the image concealment unit  320 .  
         [0078]    The control unit  330  comprises an image frame storage  333  for storing the image frame from the sensor in sensing module  310 , a defective pixel detector  332  for detecting the defective pixels and storing a detecting program, a temporary storage  334  for storing the position information of the defective pixels detected from the defective pixel detector  332 , and an image controller  331  for controlling information between the defective pixel detector  332 , image frame storage  333 , temporary storage  334  and image concealment unit  320 .  
         [0079]    [0079]FIG. 5 is a flow chart of a method for concealing defective pixels of the image sensor according to the present invention.  
         [0080]    For concealing defective pixels in the image sensor, the power is applied to the apparatus for concealing defective pixels, step  510 .  
         [0081]    After that, the image controller  331  in the control unit a  330  sets the camera mode to test mode for detecting a White Pixel and stores the position information of the White Pixel in temporary storage  334 , step  520 . That is, the image controller  331  minimizes exposure of the sensor and disables interpolation function and DPC function in the image concealment unit  320 .  
         [0082]    If the image controller sets as test mode, the image control unit  330  reads out image frame from a sensor in the sensing module  310  and stores the image frame in the image frame storage  333 . The defective pixel detector  332  reads out the image data in a pixel unit stored in the image frame storage  333 , decides the state of pixel and, if the pixel is defective, stores the position of the pixel in the temporary storage  334  for defective pixels.  
         [0083]    The detecting procedure is repeated for every pixel in the image frame storage  333 .  
         [0084]    After the detecting procedure, the image controller  331  in the control unit  330  sets the camera mode to test mode for detecting a Dark Pixel and stores the position information of the Dark Pixel in the temporary storage  334 , step  530 . That is, the image controller sets exposure of the sensor to a proper value, reads out an image frame from the sensor by turning on the light source and turns the light source off after storing the image frame in the image frame storage  333 .  
         [0085]    After turning off the light source, the defective pixel detector  332  reads out the image data in a pixel unit stored in the image frame storage  333 , decides the state of pixel and, if the pixel is defective, stores the position of the pixel in the temporary storage  334  for defective pixels.  
         [0086]    The detecting procedure is repeated for every pixel in the image frame storage  333 .  
         [0087]    After finishing the above procedures in steps  520  and  530 , the image controller  331  reads out the position information from the temporary storage  334  for defective pixels, and records the position information into the recorder  324  for recording position information of the defective pixels, step  540 . The system mode of the digital camera is then set into the normal operating mode, step  550 . That is, DPC function and interpolation function is driven and the image is transmitted by a sensor in the sensing module  310 .  
         [0088]    The digital camera system in normal operating mode operates normally depending on the DPC circuit in the image concealment unit  320 . After comparing the position information of the current pixel in the current pixel position detector  323  and the position information of the defective pixel in the recorder  324 , output from the interpolation processor  322  for defective pixels is used if the same pixel is detected. If the same pixel is not detected, the images are combined by using output from the interpolation processor  321  for the normal pixel, step  560 .  
         [0089]    As illustrated in FIGS. 5 and 6A, the camera mode is set to test mode for searching for the White Pixel, step  521 , when the power is applied in the digital camera system, step  510 .  
         [0090]    The control unit  330  is programmed to minimize the exposure of the sensor in the sensing module  310  and disables interpolation function and DPC function in the image concealment unit  320 .  
         [0091]    In the test mode, one of the frames from the sensor in the sensing module  310  is read out and saved in the image frame storage  333 , step  522 .  
         [0092]    When the image frame is saved, the defective pixel detector  332  examines each pixel saved in the image frame storage  333  to decide whether it is a White Pixel, step  523 , and saves the position of pixels which are brighter than the others as White Pixels into the temporary storage  334  for defective pixels, step  524 .  
         [0093]    To identify the White Pixels, step  523 , a pixel value is compared with the total frame average of values of other pixels. The following table shows the values of each pixel.  
                               TABLE 4                           R1   G1   R2   G2   R3       G4   B1   G5   B2   G6       R4   G7   R5   G8   R6                  
 
         [0094]    First, the method for comparing with total frame average is to assume the G5 pixel is a defective pixel if G5&gt;A*V, A is in the total frame average.  
         [0095]    Next, the method for comparing values of other pixels is to test the G5 pixel as G5&gt;V* (G1+R2+G2+B1+B2+G7+R5+G8)/8.  
         [0096]    For another method, the value of the suspected pixel can be compared with two other pixels right next to the suspected pixel. For G5, this can be done by checking G5&gt;V*(B1+B2)/2.  
         [0097]    For a more simple method, the value of a suspected pixel can be compared with only one pixel right next to the suspected pixel. The G5 pixel is a defective pixel if G5&gt;V*B1.  
         [0098]    After deciding whether it is a defective pixel or not, the position information of every White Pixel is stored in the temporary storage  334 , step  524 . The steps of detecting White Pixels are repeated until all pixels have been checked, step  525 .  
         [0099]    [0099]FIG. 6B is a detailed flow chart for a decision method of the Dark Pixel illustrated in FIG. 5, step  530 .  
         [0100]    First of all, the camera mode is set to test mode for searching for the Dark Pixel, step  531 .  
         [0101]    The light source is turned on and the integration time is set. One raw data image frame of the image is read from the sensor in the sensing module  310 , stored into the image frame storage  333 , step  532 , and the light source is turned off, step  533 .  
         [0102]    The image controller  331  examines every pixel saved in the image frame storage  333 , step  534 , and saves the position information of defective pixels if the pixel is darker than others, step  535 .  
         [0103]    To decide whether a pixel is a Dark Pixel, a pixel value is compared with the total frame average of values of other pixels. The following table shows the values of each pixel.  
                               TABLE 5                           R1   G1   R2   G2   R3       G4   B1   G5   B2   G6       R4   G7   R5   G8   R6                  
 
         [0104]    First, the method for comparing with total frame average is to assume the G 5  pixel is a defective pixel if G 5 &lt;A*V, A is the total frame average.  
         [0105]    Next, the method for comparing with values of other pixels is to test the G5 pixel as G5&lt;V*(G1+R2+G2+B1+B2+G7+R5+G8)/8.  
         [0106]    For another method, the value of a suspected pixel can be compared with two other pixels right next to the suspected pixel. For G5 pixel, this can be done by checking G5&lt;V*(B1+B2)/2.  
         [0107]    For a more simple method, the value of a suspected pixel can be compared with only one pixel right next to the suspected pixel. The G5 pixel is a defective pixel if G5&lt;V*B1.  
         [0108]    After deciding whether the pixel is a defective pixel or Lo not, the position information of every Dark Pixel is stored in the temporary storage  334 . The steps of detecting Dark Pixels are repeated until every pixel has been checked, step  536 .  
         [0109]    [0109]FIG. 6C is a detailed flow chart illustrating the normal operating mode in concealing the defective pixel in the image sensor and describes detailed procedures of the normal operating mode, steps  550  and  560 , in FIG. 5.  
         [0110]    In normal operating mode, pixel position information from the sensor in the sensing module  310  and position information of defective pixels are read out and compared with each other in the current pixel position detector  323  of current pixel position and the recorder  324  for recording position information of defective pixels, step  561 .  
         [0111]    After comparison, if the position of the current pixel is identical with the position of any of the defective pixels, step  562 , a pixel is deemed a defective pixel and the outcome of the interpolation processor  322  for defective pixel is generated, step  564 .  
         [0112]    If the position of the current pixel is not identical with the position of any of the defective pixels, step  562 , a pixel is deemed a normal pixel and the outcome of the interpolation processor  321  for normal pixels is generated, step  563 .  
         [0113]    By using the outcomes from the above procedures, the image is presented through the output unit of the digital camera system after combining images, step  565 .  
         [0114]    The present invention does not require an external storage such as EPROM. Furthermore, a complex process is not required such as the test for finding the position of defective pixel in every exposure of the sensor and capturing environment.  
         [0115]    The present invention is advantageous for a moving defective pixel.  
         [0116]    The present invention has been described in sufficient detail with a certain degree of particularity. Various modifications will be apparent to one skilled in the art, and should be considered in view of the appended claims.