Focal-plane electronic image sensors that would not saturate when exposed to intense illumination have been proposed. These sensors could be used to acquire accurate, scientific-quality data on images (including spectral images), even when the images contain both very bright and very dark areas. The proposed sensors would not contain automatic gain-control (AGC) circuitry, yet their dynamic ranges would exceed those of sensors with AGC. Unlike AGC circuitry, the circuitry of the proposed sensors would not change noise levels and would not compress image data.
The proposed sensors would be of the active-pixel-sensor (APS) type. The designs of these sensors would exploit the possibility of making APS circuitry operate on each pixel individually during acquisition of image data. In particular, the amount of photocharge accumulated in each pixel during each exposure would be monitored, and the pixel would be either reset or turned off when the charge increased beyond a preset threshold.
In one suggested implementation, a comparator and a counter would be added to the readout circuit of each pixel. When the amount of charge reached the preset threshold during an exposure, the pixel would be reset and the count incremented by one. At the end of the exposure, the total readout signal charge for each pixel would be the sum of (1) the number of resets ×the threshold charge plus (2) the charge accumulated since the last reset. In the presence of bright light, the repeated resets would prevent saturation. In the presence of dim light, the photocharge would be allowed to grow for as long a time as necessary during an exposure.
In another suggested implementation, the readout circuitry for each pixel would include a sample-and-hold circuit. In this case, the pixel value would be captured whenever the sampled photocharge exceeded a preset threshold and the integration time counted separately. In either implementation, it may be possible to obtain satisfactory performance while using an analog-to-digital converter of fewer bits than would ordinarily be needed for a given dynamic range.
This work was done by Gregory Bearman, Bedabrata Pain, and Robert Stirbl of Caltech for NASA's Jet Propulsion Laboratory.
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