Abstract:
The invention relates to an apparatus and method, for capturing an electronic image using a CMOS imager having an electronic shutter and a reduced dark current component in its image output signal. The dark current is reduced by—reading out each line of the CMOS imager in normal and reversed order and subsequent processing.

Description:
[0001]    The invention relates to an apparatus and method, and particularly an apparatus and method for capturing an electronic image. 
       BACKGROUND 
       [0002]    CMOS image sensors may use a rolling shutter, in which pixels in a pixel array are electronically shuttered line by line. The term “electronic shuttering” relates to the electronic control of a pixel to define an exposure period in which the pixel is exposed to light for the capture of an image. The end of the exposure period may be defined by the readout of the pixel. Alternatively, the exposure period may be terminated prior to readout. The total time from the start of the exposure period to the end of readout is known as the integration time. 
         [0003]    Sensors which use a rolling shutter distort images of moving objects because the object moves between the times at which successive lines are electronically shuttered. The distortion may take the form of time-displacement artefacts, as shown in  FIG. 1  and  FIG. 2 . 
         [0004]    A mechanical shutter can be used to overcome the problem, but adds to the cost, complexity and size of the image-capturing apparatus. 
         [0005]    Another way to avoid the problem is to use a global shutter, in which all pixels are electronically shuttered simultaneously. This is usually followed by a readout stage in which pixels are read out line by line. Simultaneous readout is not economical since it requires a separate readout structure and line from each pixel to output or memory. 
         [0006]    The global shutter method suffers due to increased dark-current error. Dark current is the current generated in a pixel when no light is incident on the pixel. When the exposure period begins, charge starts to generate at a rate depending on the amount of light striking the pixel. This charge is generated only during the exposure period. However, charge is also accumulated due to dark current, leading to an inaccurate reading. The accumulation of charge due to dark current occurs throughout the integration time, i.e. it continues during and after the exposure period and does not end until the integration value of the pixel is read out. As the pixels are read out line by line, the integration time for the last pixel is longer than for the first pixel, and so the dark-current error gradually increases from one end of the image to the other. This is unlike a rolling shutter in which every pixel has an identical integration time. Furthermore, the integration time when using a global shutter can be higher than when using a rolling shutter, leading to a more significant dark-current error. 
         [0007]    The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge. 
       SUMMARY 
       [0008]    An apparatus for capturing an electronic image may comprise
       a pixel array comprising a plurality of light-sensitive pixels arranged to be exposed to light to capture an electronic image;   readout circuitry arranged to read out light-intensity values from first and second pixel groups in the pixel array at different times to capture light-intensity values for the image, each pixel group comprising one or more pixels;   wherein the apparatus comprises noise-compensation circuitry arranged to compensate for dark current in the pixels of the pixel array by using light-intensity values obtained at the different times from respective first and second pixel groups.       
 
         [0012]    A pixel group may comprise a single pixel, a line of pixels in the array (i.e. the whole or a part of a row or column), more than one line, or any other collection of pixels. 
         [0013]    In any embodiment, pixels may be electronically shuttered simultaneously over the whole pixel array to implement a global shutter, or may be electronically shuttered in groups (e.g. line by line) to implement a rolling shutter. However, the apparatus offers greater advantages when a global shutter is implemented, since time-displacement artefacts are avoided. 
         [0014]    In a first embodiment, the readout circuitry may be arranged to read out the light-intensity values from the first and second pixel groups in a first predetermined group order in a first reading, and subsequently to reread light-intensity values from the first and second pixel groups in a second predetermined group order in a second reading, the second predetermined group order being the reverse of the first predetermined group order. 
         [0015]    While only two groups are mentioned, it is to be understood that the invention is applicable to any number of groups being two or higher. For example, the readout circuitry may be arranged to read out the light-intensity values from a plurality of pixel groups in a first predetermined group order in a first reading, and subsequently to reread light-intensity values from a plurality of pixel groups in a second predetermined group order in a second reading, the second predetermined group order being the reverse of the first predetermined group order. 
         [0016]    In the case that a group comprises more than one pixel, the pixels in the group are read out simultaneously, for example in a line-by-line manner using a shift register. In this case, a group order is the order in which groups are read out. In the case that a group comprises a single pixel, a group order is the order in which individual pixels are read out. Groups may be read out in consecutive order moving from one end of the pixel array to the other, or in any other order. 
         [0017]    In many implementations, the dark-current error increases at a constant rate in each pixel across the pixel array. The dark-current error in respect of the pixel group which is read last in the first reading will be higher than that in respect of the pixel group which is read first in the first reading. After the first reading, this would result in a dark-current error which gradually increases from one end of the image to the other, as discussed above. However, by rereading the light-intensity values from pixel groups in reverse order, the dark-current error which results from the second reading increases in the reverse direction to that of the first reading. This allows various operations to be readily performed in order to even out the dark-current error and/or to remove it. 
         [0018]    The noise-compensation circuitry may be arranged to calculate an average light-intensity value for a pixel in a respective group from the respective light-intensity values obtained for the pixel in the first and second readings. This operation results in a dark-current error which is even across the image. Although the dark-current error still exists, it is preferable for it to be even rather than gradually increasing. 
         [0019]    The noise-compensation circuitry may be arranged to subtract a dark-current error from the average light-intensity value for the pixel to obtain a noise-compensated light-intensity value for the pixel. 
         [0020]    The pixel array may comprise one or more optically-shielded light-sensitive pixels (or “black” pixels). The readout circuitry may be arranged to read out values from the or each black pixel in the first and second readings. The noise-compensation circuitry may be arranged to calculate an average value from the values obtained from the or each black pixel in the first and second readings, the average value representing a dark-current error. 
         [0021]    Alternatively, the apparatus may comprise a mechanical shutter, in which case a dark frame can be captured, and removed from a primary image. 
         [0022]    In a second embodiment, the readout circuitry may be arranged to obtain a first dark-current error associated with reading the light-intensity values from the first pixel group, and to obtain a second dark-current error associated with reading the light-intensity values from the second pixel group, the first and second pixel groups being spaced apart from other intermediate pixel groups. The noise-compensation circuitry may be arranged to interpolate the first and second dark-current errors to calculate one or more dark-current errors for the intermediate pixel groups. 
         [0023]    In this way, the dark current error for each group can readily be calculated in order to allow for operations to remove it to be performed. 
         [0024]    The noise-compensation circuitry may be arranged to subtract the dark-current error from the light-intensity value of a pixel to obtain a noise-compensated light-intensity value for a pixel. 
         [0025]    The pixel array may comprise one or more optically-shielded light-sensitive pixels arranged to be used by the readout circuitry to obtain the first and second dark-current errors. 
         [0026]    The apparatus may be arranged such that the first and second pixel groups can be exposed to light simultaneously. Additionally or alternatively, the apparatus may be arranged such that the first and second pixel groups can be exposed to light at different times. The apparatus may be arranged such that a pixel group can be exposed to light and its light-intensity values read by the readout circuitry before a subsequent pixel group is exposed to light. 
         [0027]    The apparatus may comprise a digital camera. Alternatively, the apparatus may comprise a module or a functional block for a digital camera. 
         [0028]    A method of capturing an electronic image may comprise
       exposing a plurality of light-sensitive pixels in a pixel array to light to capture an electronic image;   reading light-intensity values from first and second pixel groups in the pixel array at different times to capture light-intensity values for the image, each pixel group comprising one or more pixels;   compensating for dark current in the pixels of the pixel array by using light-intensity values obtained at the different times from respective first and second pixel groups.       
 
         [0032]    The method may comprise reading the light-intensity values from the first and second pixel groups in a first predetermined group order in a first reading, and subsequently rereading light-intensity values from the first and second pixel groups in a second predetermined group order in a second reading, the second predetermined group order being the reverse of the first predetermined group order. 
         [0033]    The method may comprise calculating an average light-intensity value for a pixel in a respective group from the respective light-intensity values obtained for the pixel in the first and second readings. 
         [0034]    The method may comprise subtracting a dark-current error from the average light-intensity value for the pixel to obtain a noise-compensated light-intensity value for the pixel. 
         [0035]    The method may comprise obtaining a first dark-current error associated with reading the light-intensity values from the first pixel group, and obtaining a second dark-current error associated with reading the light-intensity values from the second pixel group, the first and second pixel groups being spaced apart from other intermediate pixel groups; and
       interpolating the first and second dark-current errors to calculate one or more dark-current errors for the intermediate pixel groups.       
 
         [0037]    The method may comprise subtracting the dark-current error from the light-intensity value of a pixel to obtain a noise-compensated light-intensity value for a pixel. 
         [0038]    An apparatus for capturing an electronic image may comprise
       a pixel array comprising a plurality of light-sensitive pixels arranged to be exposed to light to capture an electronic image;   readout circuitry arranged to read out light-intensity values from a plurality of pixel groups in the pixel array in a first predetermined group order in a first reading, and subsequently to reread light-intensity values from the plurality of pixel groups in a second predetermined group order in a second reading, the readout circuitry being arranged to read each group at a different time to other groups in each of the first and second readings, each pixel group comprising one or more pixels, the second predetermined group order being the reverse of the first predetermined group order.       
 
         [0041]    An apparatus for capturing an electronic image may comprise
       a pixel array comprising a plurality of light-sensitive pixels arranged to be exposed to light to capture an electronic image;   readout circuitry arranged to read out light-intensity values from one or more pixel groups in the pixel array to capture light-intensity values for the image, the or each pixel group comprising one or more pixels, the readout circuitry being arranged to obtain a first dark-current error before reading the light-intensity values and to obtain a second dark-current error after reading the light-intensity values; and   noise-compensation circuitry arranged to interpolate the first and second dark-current errors to calculate a dark current error for the or each pixel group.       
 
         [0045]    The apparatus may comprise a plurality of pixel groups, the readout circuitry being arranged to read each group at a different time to other groups. 
         [0046]    A method of capturing an electronic image may comprise
       reading out light-intensity values from a plurality of pixel groups in a pixel array in a first predetermined group order in a first reading, and subsequently rereading light-intensity values from the plurality of pixel groups in a second predetermined group order in a second reading, the method comprising reading each group at a different time to other groups in each of the first and second readings, each pixel group comprising one or more pixels, the second predetermined group order being the reverse of the first predetermined group order.       
 
         [0048]    A method of capturing an electronic image may comprise
       reading out light-intensity values from one or more pixel groups in a pixel array to capture light-intensity values for an image, the or each pixel group comprising one or more pixels, obtaining a first dark-current error before reading the light-intensity values and obtaining a second dark-current error after reading the light-intensity values; and interpolating the first and second dark-current errors to calculate a dark current error for the or each pixel group.       
 
         [0050]    An apparatus for capturing an electronic image may comprise
       an array comprising a plurality of light-sensitive means arranged to be exposed to light to capture an electronic image;   means for reading out light-intensity values from first and second groups of light-sensitive means in the array at different times to capture light-intensity values for the image, each group comprising one or more light-sensitive means;   means for compensating for dark current in the light-sensitive means of the array by using light-intensity values obtained at the different times from respective first and second groups.       
 
         [0054]    The means for reading out light-intensity values may be arranged to read out the light-intensity values from the first and second groups in a first predetermined group order in a first reading, and subsequently to reread light-intensity values from the first and second groups in a second predetermined group order in a second reading, the second predetermined group order being the reverse of the first predetermined group order. 
         [0055]    The means for reading out light-intensity values may be arranged to obtain a first dark-current error associated with reading the light-intensity values from the first group, and to obtain a second dark-current error associated with reading the light-intensity values from the second group, the first and second groups being spaced apart from other intermediate groups. The means for compensating for dark current may be arranged to interpolate the first and second dark-current errors to calculate one or more dark-current errors for the intermediate groups. 
         [0056]    An apparatus for capturing an electronic image may comprise
       an array comprising a plurality of light-sensitive means arranged to be exposed to light to capture an electronic image;   means for reading out light-intensity values from a plurality of groups of light-sensitive means in the array in a first predetermined group order in a first reading, and subsequently to reread light-intensity values from the plurality of groups in a second predetermined group order in a second reading, the means for reading out light-intensity values being arranged to read each group at a different time to other groups in each of the first and second readings, each group comprising one or more light-sensitive means, the second predetermined group order being the reverse of the first predetermined group order.       
 
         [0059]    An apparatus for capturing an electronic image may comprise
       an array comprising a plurality of light-sensitive means arranged to be exposed to light to capture an electronic image;   means for reading out light-intensity values from one or more groups of light-sensitive means in the pixel array to capture light-intensity values for the image, the or each group comprising one or more light-sensitive means, the means for reading out light-intensity values being arranged to obtain a first dark-current error before reading the light-intensity values and to obtain a second dark-current error after reading the light-intensity values; and   means for interpolating the first and second dark-current errors to calculate a dark current error for the or each group.       
 
         [0063]    A computer program product may comprise computer-executable code which, when run on a computer, causes the computer
       to expose a plurality of light-sensitive pixels in a pixel array to light to capture an electronic image;   to read light-intensity values from first and second pixel groups in the pixel array at different times to capture light-intensity values for the image, each pixel group comprising one or more pixels;   to compensate for dark current in the pixels of the pixel array by using light-intensity values obtained at the different times from respective first and second pixel groups.       
 
         [0067]    The computer-executable code, when run on a computer, may further cause the computer
       to read the light-intensity values from the first and second pixel groups in a first predetermined group order in a first reading, and subsequently to reread light-intensity values from the first and second pixel groups in a second predetermined group order in a second reading, the second predetermined group order being the reverse of the first predetermined group order.       
 
         [0069]    The computer-executable code, when run on a computer, may further cause the computer
       to obtain a first dark-current error associated with reading the light-intensity values from the first pixel group, and to obtain a second dark-current error associated with reading the light-intensity values from the second pixel group, the first and second pixel groups being spaced apart from other intermediate pixel groups; and   to interpolate the first and second dark-current errors to calculate one or more dark-current errors for the intermediate pixel groups.       
 
         [0072]    A computer program product may comprise computer-executable code which, when run on a computer, causes the computer
       to read out light-intensity values from a plurality of pixel groups in a pixel array in a first predetermined group order in a first reading, and subsequently to reread light-intensity values from the plurality of pixel groups in a second predetermined group order in a second reading, each group being read at a different time to other groups in each of the first and second readings, each pixel group comprising one or more pixels, the second predetermined group order being the reverse of the first predetermined group order.       
 
         [0074]    A computer program product may comprise computer-executable code which, when run on a computer, causes the computer
       to read out light-intensity values from one or more pixel groups in a pixel array to capture light-intensity values for an image, the or each pixel group comprising one or more pixels, to obtain a first dark-current error before reading the light-intensity values and to obtain a second dark-current error after reading the light-intensity values; and interpolate the first and second dark-current errors to calculate a dark current error for the or each pixel group.       
 
         [0076]    An apparatus comprising
       means for capturing an electronic image using an array of light-sensitive elements arranged to be exposed to light to capture the electronic image;   means for reading out light-intensity values from first and second groups of the light-sensitive elements at different times to capture light-intensity values for the image, each light-sensitive element group comprising one or more light-sensitive element;   wherein the apparatus comprises means for noise-compensation arranged to compensate for dark current in the light-sensitive elements by using light-intensity values obtained at the different times from respective first and second groups of light-sensitive elements.       
 
         [0080]    Any circuitry of the invention may include one or more processors, memories and bus lines. One or more of the circuitries described may share circuitry elements. 
         [0081]    The present invention includes one or more aspects, embodiments and/or features of said aspects and/or embodiments in isolation and/or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0082]    A description is now given, by way of example only, reference being made to the accompanying drawings, in which:— 
           [0083]      FIG. 1  shows a picture of a moving object distorted by time-displacement artefacts; 
           [0084]      FIG. 2  shows a picture of a moving object distorted by time-displacement artefacts; 
           [0085]      FIG. 3  is a schematic diagram of apparatus according to the invention; 
           [0086]      FIG. 4  is a graph showing the removal of dark-current error from an image; 
           [0087]      FIG. 5  illustrates a readout method for an exemplary pixel array. 
       
    
    
     DETAILED DESCRIPTION 
       [0088]      FIG. 3  is a schematic diagram of apparatus  100  for capturing an electronic image. The apparatus  100  may form part of a digital camera, for example, or any other electronic device in which image-capture capability is desirable, for example a mobile telephone or personal digital assistant. 
         [0089]    The apparatus  100  includes a pixel array  102  comprising a plurality of light-sensitive pixels, for example those designated by the numerals  104   n   1 - 104   n   7 , arranged to be exposed to light to capture an electronic image, and a plurality of optically-shielded or “black” pixels, for example those designated by the numerals  104   n   8 . The term “n” designates one of the lines from “a” to “h” shown in  FIG. 3 . The apparatus  100  further includes readout circuitry  106 , noise-compensation circuitry  108 , a row decoder  110 , a shift register  112  and electronic shuttering circuitry  116 . Each component is coupled to the other components via a bus  114 . Although each component is shown to be coupled to each other component by the bus  114 , it is to be understood that the components may be connected differently, for example by only coupling components together when such coupling is required. Furthermore, the components may comprise parts or functions of a single processing unit. 
         [0090]    The black pixels  104   n   8  are masked from the light to which the other pixels  104   n   1 - 104   n   7  are exposed but in other respects the black pixels  104   n   8  are identical to the other pixels  104   n   1 - 104   n   7 . Thus, the output of the black pixels  104   n   8  can be used to determine the dark-current error in the pixels  104   n   1 - 104   n   7 . 
         [0091]    The row decoder  110  is used to address pixels  104   n   1 - 104   n   8  in the pixel array  102 . The shift register  112  is arranged such that light-intensity values from the pixels  104   n   1 - 104   n   8  can be read out line by line, i.e. in rolling order. 
         [0092]    The capture of an electronic image using the apparatus  100  consists of at least two stages: an electronic shuttering stage followed by a readout stage. In the electronic shuttering stage, the electronic shuttering circuitry  116  resets every pixel  104   n   1 - 104   n   8  in the array  102  simultaneously and exposes the pixels  104   n   1 - 104   n   8  to light. At the end of a predetermined exposure period following the reset, the electronic shuttering circuitry  116  ends the exposure of every pixel  104   n   1 - 104   n   8  in the array  102  simultaneously. Thus, a global shutter is implemented. During the exposure period, the pixels are exposed to light so as to capture an image. The arrangement whereby light is conveyed to the pixels is not described herein and any appropriate known arrangement (or future development) may be used. In the readout stage, the integrated values (or light-intensity values) are transferred out of the pixel array  102  and into memory. The apparatus may comprise sampling circuitry, amplification circuitry and/or analogue-to-digital conversion circuitry in order to extract the light-intensity values from the pixel array  102 . However, the details of those circuits are not described herein, and any appropriate known circuitry (or future development) may be used. The details of the readout stage will now be described with respect to first and second embodiments. 
         [0093]    In a first embodiment, the readout circuitry  106  is arranged to perform two readings of light-intensity values of pixels  104   n   1 - 104   n   8  in the pixel array  102 : a first reading and a second reading. 
         [0094]    In the first reading, the readout circuitry  106  reads out consecutive lines of pixels  104   a ,  104   b ,  104   c , . . . ,  104   h  by successively moving the values of lines of pixels  104   a ,  104   b ,  104   c , . . . ,  104   h  into the shift register  112 , starting with a line ( 104   a ,  104   h ) at one end of the pixel array  102  and finishing with a line ( 104   h ,  104   a ) at the other end. Following the readout of each line, the values are shifted one by one from the shift register  112  into the readout circuitry  106  and stored in memory (not shown) of the readout circuitry  106 . In a variant, the memory is an additional component of the apparatus  100  separate to the readout circuitry  106 . 
         [0095]    Subsequently, in the second reading, the readout circuitry  106  rereads light-intensity values from the lines of pixels  104   a ,  104   b ,  104   c , . . . ,  104   h  in the reverse order to that in which the lines were read out in the first reading. Thus, the line which was read out last in the first reading is read out first in the second reading, and the line which was read out first in the first reading is readout last in the second reading. So, for example, if in the first reading line  104   a  was read first and line  104   h  was read last, line  104   h  would be read first and line  104   a  would be read last in the second reading. Again, the values are shifted into memory. In this embodiment, by shifting the values of whole lines of pixels  104   n   1 - 104   n   8  simultaneously into the shift register  112 , the pixels in any one line are read out simultaneously. 
         [0096]    In a variant, the pixels  104   a   1 ,  104   a   2 ,  104   a   3  . . . ,  104   a   8 ,  104   b   1 ,  104   b   2 , . . .  104   h   1 ,  104   h   8  are individually read out from the array  102  one by one. In this case, the pixels are read out individually in the second reading in the reverse order to that in which they were read out in the first reading. For example, in a first reading  104   a   1 ,  104   a   2 ,  104   a   3 , . . .  104   a   8 ,  104   b   1 , . . .  104   b   8 , . . .  104   h   1 , . . . ,  104   h   8 , and in a second reading  104   h   8 ,  104   h   7 , . . .  104   h   1 ,  104   g   8 , . . .  104   g   1 , . . .  104   a   8 , . . .  104   a   1 . In another embodiment, the first reading may be done using the sequence  104   a   1 ,  104   a   2 ,  104   a   3 , . . .  104   a   8 ,  104   b   8 , . . . ,  104   b   1 , . . . ,  104   c   1 , . . .  104   c   8 , . . . ,  104   g   1 , . . . ,  104   g   8 ,  104   h   8 , . . .  104   h   1 , and in the second reading  104   h   1 , . . .  104   h   8 ,  104   g   8 , . . . ,  104   g   1 , . . . ,  1048   c   8 , . . .  104   c   1 ,  104   b   1 , . . . ,  104   b   8 ,  104   a   8 , . . . ,  104   a   1 . 
         [0097]    The noise-compensation circuitry  108  is coupled to the readout circuitry  106 . (In a variant, the readout circuitry  106  and noise-compensation circuitry from part of a single processing unit.) The noise-compensation circuitry  108  is arranged to receive the light-intensity values readout from the pixel array  102  by the readout circuitry  106 , and to calculate a first/second average light-intensity value for each pixel  104   n   1 - 104   n   8  from the light-intensity values obtained for the pixels  104   n   1 - 104   n   8  in the first and second readings. By this averaging process, the dark-current error in the light-intensity values is evened out across the pixel array  102 , owing to the fact that the pixels were read out in the second reading in the opposite direction to the first reading. The noise-compensation circuitry  108  is further arranged to calculate the dark-current error using the average values obtained from the black pixels  104   n   8  and to subtract the dark-current error from the average light-intensity values of each pixel  104   n   1 - 104   n   7  to obtain a noise-compensated light-intensity value for each pixel  104   n   1 - 104   n   7 . This is explained further in relation to  FIG. 4 . 
         [0098]      FIG. 4  is a graph showing the evening (averaging) out and removal of dark-current error from an image using the apparatus of the first embodiment. The horizontal axis represents pixels  104  in the pixel array  102 . The left-hand end of the horizontal axis indicates pixels read out first in the first reading and last in the second reading, and the right-hand end indicates pixels read out last in the first reading and first in the second reading. The vertical axis represents the light-intensity value read out from the pixels  104 . Trace  41  represents the ideal image signal to be captured by the apparatus  100 . During the first reading, dark-current error accumulates in each pixel  104  of the array  102  at the same rate and is represented by the trace  42 . The trace  42  increases in a linear manner towards the right-hand end of the graph as the pixels are read out in rolling order in that direction. At the end of the first reading, the dark-current error has increased to a level e dc1 . The light-intensity values read out during the first reading, which include the ideal image signal in addition to a dark-current error, are represented by trace  43 . The trace  43  is similar to the ideal trace  41  except that it is offset from the horizontal axis by the dark-current error represented by trace  42 . During the second reading, dark-current error continues to accumulate in each pixel at the same rate and is represented by the trace  44 . At the end of the second reading, the dark-current error has increased to a level e dc2 . In this case, the second reading begins immediately after the first reading has finished, such that the dark-current error e dc2  is substantially double the dark-current error e dc1 . The light-intensity values read out during the second reading are represented by trace  45 . The trace  45  is similar to the ideal trace  41  except that it is offset from the horizontal axis by the dark-current error represented by trace  42  and additionally by trace  44 . The averaging process performed by the noise-compensation circuitry  108  results in values shown by the trace  46 . As can be seen in the trace  46 , an offset due to dark current error still exists, but the offset is even across the pixel array  102 , unlike the traces  43  and  45  in which the offset increases from one end to the other. The offset in trace  46  is e dc1  across the full pixel array. The varying offset in traces  43  and  45  represents the gradually-increasing dark-current error described above. Trace  47  represents the averaged light-intensity values after the dark-current error has been subtracted by the noise-compensation circuitry  108 . As can be seen, trace  47  is similar to ideal trace  41 . 
         [0099]    In a second embodiment, the readout circuitry  106  is arranged to obtain a first dark-current error from one or more of the black pixels  104   n   8  before reading out the other pixels  104   n   1 - 104   n   7 . The readout circuitry then reads out values from the pixels  104   n   1 - 104   n   7 , line by line (possibly pixel by pixel, as discussed previously), before obtaining a second dark-current error by reading out one or more black pixels  104   n   8  a second time following the readout of the entire array  102 . The noise-compensation circuitry  108  is arranged to interpolate the first and second dark-current errors to calculate a dark current error for each pixel  104   n   1 - 104   n   7 , and to subtract the dark-current error from the light-intensity value of each pixel  104   n   1 - 104   n   7  to obtain a noise-compensated light-intensity value for each pixel  104   n   1 - 104   n   7 . 
         [0100]      FIG. 5  illustrates the readout method of the second embodiment for an exemplary pixel array comprising three lines of pixels designated  104   a   1 - 104   a   7 ,  104   b   1 - 104   b   7 ,  104   c   1 - 104   c   7  and one or more black pixels  104   n   8 . The vertical axis represents dark-current error while the horizontal axis represents the lines  104   a   1 - 104   a   7 ,  104   b   1 - 104   b   7 ,  104   c   1 - 104   c   7  of pixels in the array and the black pixels  104   n   8 . The black pixels  104   n   8  are read out first. Then, the lines  104   a   1 - 104   a   7 ,  104   b   1 - 104   b   7 ,  104   c   1 - 104   c   7  are read out in consecutive order before the black pixels  104   n   8  are read out a second time. The two readings obtained from the black pixels  104   n   8  represent the first and second dark-current errors e dc1  and e dc2 . Dark-current errors for lines  104   a   1 - 104   a   7  to  104   c   1 - 104   c   7  can readily be calculated by interpolating the first and second dark-current errors, as shown in  FIG. 5 . 
         [0101]    The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. All possible permutations and combinations of the claims are within the present disclosure whether or not specifically disclosed in that combination or permutation. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention. 
         [0102]    It will be appreciated that the aforementioned read-out circuitry, noise compensation circuitry etc. may have other functions in addition to mentioned functions, (of reading out and noise compensation in these cases), and that these functions may be performed by the same circuit. 
         [0103]    While there have been shown and described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto. Furthermore, in the claims means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.