Abstract: A circuit for correcting a raster input scanner or raster output scanner is described. During calibration, a calibration strip is scanned, the output of the light detector is compared to a reference voltage, and the difference is saved. This is a closed loop so that the difference value will settle at a value such that the detector output will accurately match the reference value before the final difference value is stored. In one mode, the correction is applied to one dynode of the PMT light detector, which varies the gain without varying the bandwidth of the circuit. The difference voltage is analog, but is stored in a digital memory through the use of A to D and D to A converters.