Method for capturing a dark frame

Abstract

A method for controlling the exposure of an active pixel array electronic still camera includes the steps of: integrating photocurrent in each pixel during an integration time period; collecting overflow charge from all pixels in the array during the integration time period; developing an overflow signal as a function of the overflow charge; and terminating the integration time period when the overflow signal exceeds a preset threshold level selected to represent a desired reference exposure level. Apparatus for performing the method of the present invention includes circuitry for integrating photocurrent in each pixel during a integration time period; circuitry for diverting and detecting overflow charge from all pixels in the array during the integration time period; circuitry for developing an overflow signal as a function of the overflow charge; and circuitry for terminating said integration time period when the overflow signal exceeds a preset threshold level selected to represent a desired reference exposure level.

What is claimed is:  
     
         1 . A method for controlling the exposure of an active pixel array disposed on a semiconductor substrate, comprising the steps of: 
 integrating photocurrent in each pixel during an integration time period;    collecting overflow charge from all pixels in the array during said integration time period;    developing an overflow signal as a function of said overflow charge; and    terminating said integration time period when said overflow signal exceeds a preset threshold level selected to represent a desired reference exposure level.    
     
     
         2 . The method of  claim 1  wherein the step of collecting overflow charge from all pixels in said array comprises setting a charge potential barrier at a charge collection electrode of a photodiode in each pixel, said potential barrier being lower than the potential at which charge would overflow into the substrate and collecting charge overflowing said potential barrier.  
     
     
         3 . The method of  claim 2  wherein the step of developing an overflow signal comprises developing a voltage proportional to said charge having overflowed said potential barrier.  
     
     
         4 . A method for controlling the exposure of an active pixel array disposed on a semiconductor substrate, comprising the steps of: 
 resetting each pixel during a reset time period;    integrating photocurrent in each pixel during an integration time period;    collecting overflow charge from all pixels in the array during said integration time period;    developing an overflow signal as a function of said overflow charge; and    terminating said integration time period when said overflow signal exceeds a preset threshold level.    
     
     
         5 . The method of  claim 4  wherein the step of collecting overflow charge from all pixels in said array comprises setting a charge potential barrier at a charge collection electrode of a photodiode in each pixel, said potential barrier being lower than the potential at which charge would overflow into the substrate and collecting charge overflowing said potential barrier.  
     
     
         6 . The method of  claim 5  wherein the step of developing an overflow signal comprises developing a voltage proportional to said charge having overflowed said potential barrier.  
     
     
         7 . Apparatus for controlling the exposure of an active pixel array, comprising: 
 means for integrating photocurrent in each pixel during a integration time period;    means for collecting overflow charge from all pixels in the array during said integration time period;    means for developing an overflow signal as a function of said overflow charge; and    means for terminating said integration time period when said overflow signal exceeds a preset threshold level.    
     
     
         8 . The apparatus of  claim 7  wherein the means for collecting overflow charge from all pixels in said array comprises means for setting a charge potential barrier at a charge collection electrode of a photodiode in each pixel, said potential barrier being lower than the potential at which charge would overflow into the substrate, and for collecting charge overflowing said potential barrier.  
     
     
         9 . The apparatus of  claim 8  wherein the means for developing an overflow signal comprises means for developing a voltage proportional to said charge having overflowed said potential barrier.  
     
     
         10 . In an active pixel imager array including pixels each comprising a photodiode having a first terminal connected to a fixed potential, a second terminal connectable to a reference voltage source through a reset transistor having a gate, a source connected to the second terminal and a drain connected to the reference voltage source, a storage capacitor connectable to the second terminal through a transfer switch, an amplifier having an input coupled to the capacitor and an output coupled to an output line through a select switch, a method for controlling the exposure of the active pixel array, comprising the steps of: 
 turning on the reset transistor and the transfer switch of each pixel to reset each pixel during a reset time period, said reset transistor turned on by applying a first voltage to its gate;    turning off the reset transistor while leaving the transfer switch on in each pixel to integrate photocurrent in each pixel during an integration time period, said reset transistor in each pixel turned off by applying a second voltage to its gate, said second voltage selected to bias said reset transistor in each pixel off until voltage at the second terminal of the photodiode in individual pixels decreases by a set amount enough to pass charge through the pixel's reset transistor during said integration time period;    deriving an overflow signal proportional to the total charge drawn from said reference voltage source during said integration period; and    terminating said second time period when said overflow signal exceeds a preset threshold level selected to represent a desired reference exposure level.    
     
     
         11 . In an active frame storage pixel array disposed on a semiconductor substrate, the active pixel array controlled by an electronic shutter, a method for capturing a dark frame comprising the steps of: 
 resetting each pixel to a preselected voltage during a reset time period;    initiating an integration time period; and    immediately terminating said integration time period such that substantially zero photogenerated charge is accumulated.    
     
     
         12 . The method of  claim 11  wherein the step of resetting each pixel to a preselected voltage during a reset time period comprises the step of resetting each pixel to a dark level.  
     
     
         13 . The method of  claim 11  wherein the step of resetting each pixel to a preselected voltage during a reset time period comprises the step of resetting each pixel to a level other than a dark level.