Electrophotographic copying apparatus

摘要

A photoconductive drum is electrostatically charged and radiated with a light image of an original document to produce an electrostatic image. A toner substance is applied to the drum to produce a toner image which is transferred and fixed to a copy sheet. A bias electrode is provided adjacent to the drum during the toner development stage to which is applied a bias voltage selected to prevent application of toner to white or background areas of the electrostatic image. The bias voltage is produced as a function of the sensed electrostatic potential of the electrostatic image and is automatically limited to prevent image degradation and loss of density. To reproduce photographs or low contrast documents such as diazo copies, the bias voltage is limited to a lower value to increase the image density and prevent dark areas from appearing washed out. The light image intensity is increased for reproduction of documents having colored backgrounds so that the backgrounds will reproduce white.