Method and apparatus for optimizing image sensor noise and dynamic range

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.

1 . An imager, comprising:
 a pixel array for capturing an image and outputting an image signal corresponding to the captured image; and   a supply adjuster for adjusting a level of a first voltage supplied to a reset circuit based on an exposure level of light incident on the pixel array.   
     
     
         2 . The imager according to  claim 1 , wherein the supply adjuster independently adjusts a level of a second voltage supplied to the pixel array, during an interval when the image is captured and read. 
     
     
         3 . The imager according to  claim 1 , wherein the second voltage supplied to the pixel array is adjusted based on the exposure level. 
     
     
         4 . The imager according to  claim 1 , further comprising:
 a programmable gain loop for generating a digital image signal and including a gain control signal derived from the digital image signal for adjusting a gain setting of the programmable gain loop,   wherein the second voltage level is controlled in accordance with the gain control signal from the programmable gain loop.   
     
     
         5 . The imager according to  claim 1 , further comprising:
 an exposure level determiner for determining the exposure level of the incident light, the exposure level determiner including:
 a processor, coupled to the pixel array to receive the image signal, for determining a mean, median or maximum value of the image signal; and 
 a comparator, coupled to the processor, for comparing the determined value from the processor to a threshold value and outputting a comparison signal. 
   
     
     
         6 . The imager according to  claim 2 , further comprising:
 a voltage supply for generating a common supply voltage;   wherein the supply adjuster includes:
 a first adjuster, coupled to the voltage supply, for supplying the reset circuit with the first voltage level, and 
 a second adjuster, coupled to the voltage supply, for supplying the pixel array with the second voltage level. 
   
     
     
         7 . The imager according to  claim 6 , further comprising:
 a further adjuster, coupled to the voltage supply, for supplying a third voltage level, different from the first or second voltage levels, to the pixel array.   
     
     
         8 . The imager according to  claim 7 , wherein the further adjuster is coupled to one or more auxiliary circuits of the pixel array to reduce noise levels related to the third voltage level in response to the exposure level of the incident light on the pixel array. 
     
     
         9 . The imager according to  claim 6 , further comprising:
 a processor for determining the exposure level of the incident light based on the outputted image signal and for outputting a control signal, wherein the first and second voltage levels are individually controlled in accordance with the control signal by the first and second adjusters, respectively.   
     
     
         10 . The imager according to  claim 6 , wherein:
 the first adjuster includes a first lookup table having first adjustment values that are associated with exposure level values, receives a respective exposure level value corresponding to a current exposure level of the incident light on the pixel array and outputs the first voltage level based on a respective first adjustment value associated with the current exposure level; and   the second adjuster includes a second lookup table having second adjustment values, different from the first adjustment values, that are associated with the exposure level values, receives the respective exposure level value corresponding to the current exposure level and outputs the second voltage level based on a respective second adjustment value associated with the current exposure level.   
     
     
         11 . The imager according to  claim 6 , wherein the first and second adjusters each operates according to a respective algorithm for calculating an optimal value of the respective first and second voltage levels based on an exposure level signal received by the first and second adjusters. 
     
     
         12 . The imager according to  claim 6 , wherein the second adjuster decreases the second voltage level when the exposure level of the incident light on the pixel array is below a threshold and increases the second voltage level when the exposure level is above the threshold. 
     
     
         13 . A method for adjusting voltages for an imager, comprising:
 capturing an image with a pixel array;   outputting an image signal corresponding to the captured image;   adjusting, by a first adjuster, a level of a first voltage supplied to a reset circuit of the imager based on an exposure level of light incident on the pixel array.   
     
     
         14 . The method according to  claim 13 , further comprising
 independently adjusting, by a second adjuster, a level of a second voltage supplied to the pixel array during image-capture based on the exposure level of the light incident on the pixel array.   
     
     
         15 . The method according to  claim 13 , further comprising:
 determining one of a mean value, a median value or a maximum value of the outputted image signal for each image captured; and   adjusting the first and second voltage levels responsive to changes in the determined value.   
     
     
         16 . The method according to  claim 13 , wherein the imager includes an automatic gain control, the method further comprising controlling, based on a common gain control signal, a gain setting of the automatic gain control, the first voltage level of the reset circuit and the second voltage level of the pixel array. 
     
     
         17 . The method according to  claim 16 , wherein the adjusting of the second voltage level includes:
 decreasing the second voltage level when the gain setting increases; and   increasing the second voltage level when the gain setting decreases.   
     
     
         18 . A system, comprising:
 a pixel array for capturing images and outputting image signals corresponding to the captured images;   a supply adjuster for adjusting a level of a supply voltage supplied to the pixel array during image-capture based on an exposure level of light incident on the pixel array, and for independently adjusting a level of a reset signal supplied to a reset circuit; and   a controller for deriving the exposure level from the outputted image signals and for controlling the supply adjuster to supply the adjusted reset signal level to the reset circuit responsive to the derived exposure level.   
     
     
         19 . The system according to  claim 18 , wherein the controller determines the exposure level based on a maximum value or median value of a current image signal associated with a current captured image. 
     
     
         20 . The system according to  claim 18 , wherein the supply adjuster includes a reset supply adjuster for adjusting the reset signal level independent of the supply voltage level. 
     
     
         21 . The system according to  claim 18 , wherein the pixel array captures a subsequent image using the adjusted reset signal level and the adjusted supply voltage level.