Shading control for photocopier - has aperture and exposure control to correct for variation in photo-transistor material response

An automatic contrast control for a photocopier has adjustable apertures in the scanner to modulate the light intensity w.r.t. time for successive copies. The adjustment is such that a constant contrast is obtained, with a constant density. The apertures are linked to a pivot driven by a bimetallic strip or spiral. This provides an exposure time control, with longer down times between copies correspondingly corrected for. The adjustment takes into account the variation in response of the sensitive layer on the process drum, and the recovery period for the layer.