1. In an image analyzer for counting objects in a field, a shading corrector adapted for measuring the relative amount of chromophore in a scanned object comprising means to supply a stored video background signal, an attenuator for receiving and attenuating said signal, computer means for controlling said attenuator, means to supply an object-modulated video signal, comparator means for comparing the attenuated signal and the video signal to produce a comparator output signal, and area counter means to receive said output signal and develop a count proportional to the time-width of the scan across the chromophore to represent the area of the chromophore, said count being fed to said computer means.