Abstract:
A method and apparatus for optimizing the voltage supply of an image sensor pixel array to minimize pixel noise and maximize dynamic range is disclosed. The voltage supply is adjusted in response to the exposure level of the pixel array when it captures an image. The voltage supply is increased in higher exposure levels to expand the dynamic range of the pixel array. In lower exposure levels, when the full dynamic range of the pixel array is not utilized, the voltage supply is decreased to lower pixel noise level and reduce its effect on image quality.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention is directed towards the field of image sensors, and more specifically, towards optimizing noise and dynamic range in the image sensors. 
       BACKGROUND OF THE INVENTION 
       [0002]    An image sensor uses an array of pixels to capture an image when the image sensor is exposed to light.  FIG. 1  shows a block diagram of an illustrative prior art pixel array  103 . Pixel array  103  includes auxiliary circuitry such as drivers, buffers, and multiplexers for the signals in the array. A voltage supply  105  supplies the pixel array  103  with power. At the beginning of an exposure period, a reset signal  107  is asserted to reset some or all of the pixels in the pixel array  103 . Consequently, the pixels are charged to a reset voltage, which is typically a function of the voltage supply  105 . As the pixel array  103  is exposed to incident light  109 , the voltages at each pixel decrease. 
         [0003]    At the end of an exposure period, the final voltage of each pixel is compared to its original reset voltage. These voltage swings represent the captured image, and are proportional to the exposure level of the pixel array  103 . Large voltage swings indicate a high exposure level, which means that the pixel array  103  was exposed to bright light or had a long exposure period. Conversely, small voltage swings indicate a low exposure level, which means that the pixel array  103  was exposed to dim light or had a short exposure period. The voltage swings are read from the pixel array  103  as image signals  111 . 
         [0004]    A higher voltage supply increases the dynamic range of a pixel array, because each pixel has a larger reset voltage, and thus a bigger range for the voltage swing. A larger dynamic range allows the pixel array to capture a more faithful image when the exposure level is high. However, both pixel temporal noise and dark current noise (hereinafter, collectively referred to as just “noise” or “pixel noise”) have been found to increase along with the voltage supply when the pixel array is created with complimentary metal oxide silicon (CMOS) technology. The noise distorts the image captured by the pixel array. 
       SUMMARY OF THE INVENTION 
       [0005]    In accordance with a preferred embodiment of the present invention, a method and apparatus are described for optimizing the voltage supply of an image sensor pixel array. The voltage supply is varied in response to the exposure level of the pixel array when it captures an image. The voltage supply is increased when exposure levels are higher, to increase the reset voltage and expand the dynamic range of the pixel array. When the exposure levels are lower and the full dynamic range of the pixel array is not utilized, the voltage supply is decreased to lower the reset voltage, thus lowering the noise level and reducing its effect on image quality. 
         [0006]    In one embodiment of the present invention, the exposure level is determined by checking the gain of a programmable gain amplifier (PGA) that amplifies the signals from the pixel array, before the signals are digitized by an analog-to-digital converter (ADC). A gain control block controls the gain of the PGA to match the signal range from the pixel array to the input range of the ADC to minimize quantization error. A high PGA gain indicates lower signal levels from the pixel array, whereas a low PGA gain indicates higher signal levels from the pixel array. The gain of the PGA is thus an indicator of the exposure level. 
         [0007]    In an alternate embodiment of the present invention, the exposure level is determined by comparing the mean signal value from the pixel array to a threshold value. When the mean signal value is above the threshold value, then the pixel array has a high exposure level. When the mean signal value is below the threshold value, then the pixel array has a low exposure level. Alternatively, the exposure level can be determined by comparing the median or maximum signal value from the pixel array to a threshold value. 
         [0008]    In another embodiment of the present invention, the pixel array has more than one voltage supply. One or more of the voltage supplies is changed in response to the exposure level of the pixel array to optimize the noise level and dynamic range of the pixel array. 
         [0009]    In another embodiment of the present invention, the pixel array may be designed so that its reset voltage is not a function of a voltage supply to the pixel array. In such configurations, the reset voltage may also be optimized independently of the voltage supply to reduce noise levels in response to the exposure level of the pixel array. 
         [0010]    Further features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying exemplary drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates a block diagram of a prior art pixel array. 
           [0012]      FIG. 2  illustrates a block diagram of a system for optimizing the voltage supply of a pixel array in response to exposure levels, made according to the present invention. 
           [0013]      FIG. 3A  illustrates one possible implementation for the exposure level determiner in  FIG. 2   
           [0014]      FIG. 3B  shows an alternate implementation for the exposure level determiner. 
           [0015]      FIG. 4  illustrates a possible implementation for the variable voltage source. 
           [0016]      FIG. 5  illustrates a pixel array having multiple voltage supplies. 
           [0017]      FIG. 6  illustrates a process flow chart according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    When the pixel array has a high exposure level, the pixel noise is negligible because the image signals are large compared to the pixel noise floor. The large signal-to-noise ratio results in high image quality under high exposure levels. However, the voltage swings of the pixel array may be relatively small under low exposure levels. The signal-to-noise ratio is lower in these conditions and results in poorer image quality. Therefore, the voltage supply to the pixel array is varied in response to its exposure level to optimize the noise levels and dynamic range of the pixel array. 
         [0019]      FIG. 2  illustrates a block diagram of a system  201  for optimizing the voltage supply of a pixel array in response to its exposure level, made according to the present invention. A pixel array  203  is used to capture an image, represented by image signals  211 . An exposure level determiner  207  determines the exposure level of the image signals  211  and generates an exposure level indicator  209  for feedback to the supply adjuster  206 . A supply adjuster  206  adjusts the voltage from a voltage supply  205  to provide an optimized voltage supply (Array Vdd  204 ) to the pixel array  203 . Array Vdd  204  is selected for the optimal balance between noise level and dynamic range at the exposure level indicated by exposure level indicator  209 . 
         [0020]    For example, when the exposure level determiner  207  indicates that the pixel array  203  has a high exposure level, the supply adjuster  206  increases Array Vdd  204 . This allows for greater dynamic range in the pixel array  203 . When the exposure level determiner  207  indicates that the pixel array  203  has a low exposure level, the supply adjuster  206  decreases Array Vdd  204 . Decreasing Array Vdd  204  does not hurt the dynamic range of the pixel array  203  in low exposure levels, since the voltage swings at each pixel are smaller. Decreasing Array Vdd  204  also reduces the amount of pixel noise, thus improving the signal-to-noise ratio and the quality of images captured under low exposure levels. The criteria for distinguishing low exposure levels from high exposure levels will vary from system to system, depending on factors such as length of exposure time, the pixel sensitivity, intensity of the ambient light, and other system variables. Generally, however, when the image signals  211  are higher than a reference value, the pixel array  203  has a high exposure level. When the image signals  211  are lower than a reference value, the pixel array  203  has a low exposure level. 
         [0021]      FIG. 3A  illustrates one possible implementation for the exposure level determiner  207  in  FIG. 2 . The inputs to the exposure level determiner  207  are the image signals  211 . The image signals  211  are read from the pixel array  203  and amplified by a programmable gain amplifier (PGA)  301  when needed. Whether amplification is needed or not is discussed further below. Next, the amplified image signals  302  are processed by an analog-to-digital converter (ADC)  303 , which converts the amplified image signals  302  into digital form (digitized image signals  304 ). 
         [0022]    Whenever analog signals are digitized, quantization errors occur which introduce additional noise into the digitized signals. If the quantization noise is comparable to or larger than the noise present on the analog signal being digitized, then the quantization noise will degrade the overall signal-to-noise ratio. To minimize the effect of quantization noise, the analog signal may be amplified, such that the signal amplitude is maximized (without exceeding the ADC input range) before the addition of quantization noise. This minimizes the effect of the added quantization noise on the signal-to-noise ratio. Therefore, the PGA  301  amplifies weak image signals to better match the range of the ADC  303 . A gain control block  305  analyzes the digitized image signals from the ADC  303  to determine if amplification is needed. For example, if the mean level of the digitized image signals  304  does not meet a target value, the gain control block  305  adjusts the gain setting  306  of the PGA  301  accordingly. 
         [0023]    The gain setting  306  of the PGA  301  is therefore an indicator of the exposure levels of the image signals  211 . A high gain indicates that the image signals  211  needed to be amplified a considerable amount for input to the ADC  303 . Therefore, the pixel array  203  had a low exposure level. Conversely, a low gain indicates that little or no amplification was needed for the image signals  211 , and indicates that the pixel array  203  had a high exposure level. The exposure level indicator  209  output from the exposure level determiner  207  is just the gain setting  306  of the PGA  301 . 
         [0024]      FIG. 3B  shows an alternate implementation for the exposure level determiner  207 . The mean value of the image signals  211  is calculated by a mean value calculator  311 . A comparator  307  compares the mean signal value to a threshold value  309 . When the mean signal value is above the threshold value  309 , then the pixel array has a high exposure level. When the mean pixel value is below a threshold value  309 , then the pixel array has a low exposure level. Alternatively, the comparator  209  can compare the median or maximum signal value from the image signals  211  to a threshold value  309 . The exposure level indicator  209  output from this exposure level determiner  207  is simply the output of the comparator  307 . Other methods may also be used to determine the exposure level of the pixel array. 
         [0025]      FIG. 4  illustrates a possible implementation for the supply adjuster  206 , using a voltage control block  401  and a voltage regulator  403 . Regardless of how the exposure level determiner  207  is implemented (i.e. the implementation of  FIG. 3A ,  3 B, or any other implementation), the exposure level indicator  209  will be representative of the exposure level in which the image  211  was captured. The voltage control block  401  generates a voltage reference  405 , based on the exposure level indicator  209 . The optimal value for the voltage reference  405  is one that minimizes pixel noise in the pixel array  203  without compromising its dynamic range. These optimal values can be determined for the system beforehand and stored in a look-up memory table within the voltage control block  401 . 
         [0026]    Alternatively, an algorithm may be developed for calculating the optimal value for the voltage reference  405 , based on the exposure level indicator  209 . This algorithm may be implemented in hardware circuitry or software within voltage control block  401 . An exemplary algorithm would be a comparison function. The voltage control block  401  could include a comparator that compares the exposure level indicator  209  to a threshold value. If the exposure level indicator  209  is greater than the threshold value, then the voltage reference  405  is increased. If the exposure level indicator  209  is less than the threshold value, then the voltage reference  405  is decreased. 
         [0027]    The voltage regulator  403  regulates Array Vdd  204  to match the optimal voltage reference  405 . The voltage regulator  403  has an operational amplifier (op-amp)  407  that drives the gate of a transistor  409 . The negative input of the op-amp  407  is connected to the drain of the transistor  409 , while the source of the transistor  409  is connected to the voltage supply  205 . The voltage regulator  403  is a well-known circuit in the art, and the implementation illustrated here is just one of many possible designs. 
         [0028]    In some image sensors, the auxiliary circuitry for a pixel array (such as the drivers, buffers, multiplexers, etc.) may derive its power from one or more distinct voltage supplies. Each of these voltage supplies may also be optimized to reduce noise levels in response to the exposure level of the pixel array.  FIG. 5  illustrates a pixel array  203  having multiple voltage supplies  205 A,  205 B, and  205 C. Each voltage supply is adjusted by a supply adjuster  206 A,  206 B, and  206 C, respectively, to optimize the voltage supply for the exposure level indicated by the exposure level indicator  209 . 
         [0029]    In another embodiment of the present invention, the pixel array may be designed so that its reset voltage is not a function of a voltage supply to the pixel array. However, the noise level of the pixel array remains dependent on the reset voltage—the noise increases with the reset voltage. In such configurations, the reset voltage may also be optimized independently of the voltage supply to reduce noise levels. For example, the reset voltage is a function of the reset signal  208  in some image sensors. A reset voltage adjuster, similar to the supply adjuster  206 , can be used to adjust the reset signal  208  in response to the exposure level of the pixel array. 
         [0030]      FIG. 6  illustrates a process flow chart according to the present invention. First, in step  601 , an image is captured on a pixel array. Next, in step  603 , the image is analyzed to determine its exposure level. If the exposure level is low, then a voltage supply of the pixel array is lowered. If the exposure level is relatively high, then the voltage supply can be increased. After adjustment, the next image can be captured and the process begins again at step  601 . When the reset voltage is not a function of the voltage supply, the reset voltage may also be adjusted independently of the voltage supply. 
         [0031]    Although the present invention has been described in detail with reference to particular preferred embodiments, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims that follow.