Photographic copying machine with exchangeable format masks

Abstract

When the size of the frame of a format mask is changed, an automatic frame-area measurement is performed, so that the exposure-duration control system of the copying machine be informed of the size of the format-mask frame to be employed. The machine is provided with photo-detectors which receive light through a transparent original for exposure-duration control purposes. At least one photodetector is used for the format-size measurement, the photodetector signal being transmitted through a controllable-gain amplifier to the exposure-duration control circuit. When an automatic frame-area measurement is commanded, all filters and shutter elements move out of the copying-light path and a calibration filter moves in so that the light incident on the photodetector does not greatly exceed that transmitted by a typical original. With no original to be copied present, but only the calibration filter, a counter stepwise varies the gain of the controllable-gain amplifier until the output signal of the latter reaches a predetermined value, whereupon the counter is arrested and the thusly established gain maintained during subsequent normal exposure-duration control performed during actual copying of originals.

Dependencies are inferred from the claim text.