Photocopier exposure control

Claim 1

In a photocopier including a light source, a diffuser reflector for reflecting light during an exposure cycle from the source uniformly onto a predetermined exposure area and means for moving a light-transmissive photosensitive film and an original document in intimate contact therewith through the predetermined exposure area at a constant velocity, the original document having image and non-image areas providing a differential reflectivity, the photosensitive film being between the source of exposure light and the original document and the photosensitive film being differentially desensitized according to the differential reflectivity of the original document, an automatic exposure control comprising: means for collecting at a plurality of predetermined spaced positions continuously during the exposure cycle samples of light representative of the sum of the intensity of the light from the source and the intensity of the light reflected from the original document at the exposure area, each said sample representing said sum over a portion of the exposure area, means for averaging the intensities of a said plurality of samples of light, means for simultaneously transmitting samples of light from said means for collecting to said means for averaging, and means responsive to said means for averaging for controlling the output from the light source to maintain the sum of the intensity of the light from the source and the intensity of the light reflected from the original document a predetermined constant at the exposure area, said means for controlling including a semi-conductor switching device connected in series with the light source and a signal triggering circuit for said device, said triggering circuit being connected to said means for averaging for control thereby.