System for measuring the optical density of both the background and the

Abstract

The optical densities of both the background and the image areas of an original are measured so as to obtain a control signal useful in photocopying the original, by a system which includes a detector for generating a measuring signal representing optical densities detected on the original; a detector for deriving from the measuring signal a peak signal level representing the background density; first and second detection circuits for producing respective output signals representing numbers of measuring signal portions detected at first and second reference signal levels, which levels differ from each other and each from the peak signal level; a control rendering the reference signal levels dependent in magnitude on the derived peak or background signal level, and preferably proportional to it; and a comparison circuit for deriving from the output signals a control signal indicating in relation to the background density the relative amounts on the original of image information items that have optical densities detectable at the reference signal levels. In this way, for instance, weak image information differing little in optical density from the background is measurable on an original in relation to both the background and relatively strong image information so that a control signal to aid copying of all the information can be generated.