CMOS active pixel sensor having a pixel local exposure control function

摘要

A system (22) and a method for imaging a scene of interest utilize variable exposure periods that have durations based upon detecting a fixed voltage drop in order to determine the scene segment radiance. The rate (16, 18 and 20) of voltage drop corresponds to the degree of scene segment radiance, such that high radiant scene segments yield faster voltage drops than lower radiant scene segments. The variable exposure period is determined within each pixel (25) in a pixel array (24) of the system to gather exposure periods from different segments of the scene being imaged. The measured exposure periods are translated into grayscale information that can be used to generate a composite image having various levels of grayscale that is representative of the imaged scene. Each pixel includes a photo sensor (36), an analog-to-digital converter (38, 40, 68 and 78) and a memory (40) to measure, digitize and store the exposure period. The memory contains a number of memory cells (82 and 84) having a three-transistor configuration that are each connected to a bi-directional bit line (78). The bi-directional bit line functions as both a read bit line and a write bit line. The three-transistor configuration allows for non-destructive read-outs of data stored in the memory cells.