Method and apparatus for compensating for sensitivity variations in the

Abstract

A correction circuit processes digitized signals from an image sensor and generates gain correction values to compensate for variations in the output of the sensor. While imaging a gain calibration object, the sensor is operated in a calibration mode in which a plurality of calibration values are generated that pertain to each photosite. The digitized calibration values are transformed into log space for processing by a gain level averaging circuit. The log calibration signals are first subtracted from a reference corresponding to a maximum expected signal value. The difference signals are serially accumulated by means of pair of registers and an adder, and the sum is stored in a gain memory. In a subsequent normal operating mode, the summed signals for each photosite are retrieved from the gain memory and bit-shifted to form an average correction value for each photosite. The correction values are applied to an adder in synchronism with sensor signals from like photosites and added therewith in log space to provide gain compensation.