Electrostatic copying machine

摘要

An exposure unit radiates a light image of an original document onto a charged photoconductive member to form an electrostatic image through localized photoconduction. A developing unit applies a powdered developing substance comprising carrier particles and toner particles to the photoconductive member to develop the electrostatic image and produce a toner image which is transferred and fixed to a copy sheet to produce a permanent reproduction of the original document. The tone density, or the ratio of toner particles to carrier particles in the developing substance is electromagnetically measured, and the developing ability of the developing substance is opticaly measured. The developing substance is replaced when the difference between the toner density and developing ability exceeds a predetermined value. Prior to exposure with the light image a reference electrostatic image is formed on the photoconductive member and the electrostatic potential thereof is measured. The intensity of the light image of the original document and/or a developing bias voltage are regulated in accordance with the sensed electrostatic potential. The reference image may be of a reference surface having a predetermined optical density or a portion of the original document produced by reverse optical scanning.