Xerographic copying apparatus

Abstract

Development of a charged photoconductive medium in xerographic copying apparatus is prevented when a properly formed charge image is missing as a result of insufficient imagewise exposure, by a system in which the intensity of the exposing illumination is sensed by a detector such as a photoelectric cell or a phototransistor, and when the illumination is below a level sufficient to form a proper charge image a signal emitted by a detector causes a discharging device such as an A.C. corona or a lamp to discharge the charged medium at a location between the exposure station and the developing station of the apparatus. Means also are provided whereby said signal causes interruption of the feeding of copy material to receive an image, and whereby it causes the execution of a preset copy program to be blocked until the exposing illumination is restored to a proper level of intensity. As a further feature of the invention, the discharging device is a light source which normally is operated at a reduced intensity for effecting an integral exposure of the medium to light after the imagewise exposure, and which is switched to full intensity for discharging the medium when the illumination for imagewise exposure is insufficient.