Floating-Gate Adaptation for Focal-Plane Online Nonuniformity Correction

Stochastic adaptive algorithms are investigated for online correction of spatial nonuniformity in random-access addressable imaging systems. The adaptive architecture is implemented in analog VLSI, integrated with the photosensors on the focal plane. Random sequences of address locations selected with controlled statistics are used to adaptively equalize the intensity distribution at variable spatial scales. Through a logarithm transformation of system variables, adaptive gain correction is achieved through offset correction in the log-domain. This idea is particularly attractive for compact implementation using translinear floating-gate MOS circuits. Furthermore, the same architecture and random addressing provide for oversampled binary encoding of the image with equalized intensity histogram. The techniques apply to a variety of solid-state imagers, such as artificial retinas, active pixel sensors, and IR sensor arrays. Experimental results confirm gain correction and histogram equalization in a 64 64 pixel adaptive array integrated on a 2.2-mm 2.25-mm chip in 1.2m CMOS technology.