Electrophotographic density control

In an electrophotographic copying apparatus which at least comprises a photosensitive member having a photoreceptor surface thereon while being rotatable in one direction;

• a corona charging means for charging said photoreceptor surface therewith;
• a light radiation means for illuminating a surface of an original to be copied therewith;
• an optical arrangement for introducing an imagewise light to an exposing station positioned at a predetermined position along and apart from a circumferential path of said photoreceptive member;
• and an erasing means for erasing the residual electrical charge on said photoreceptor surface therewith while being positioned relatively prior to the position of said corona charging means with respect to the direction of rotation of said photoreceptive member so that said residual electrical charge is electrostatically erased before the successive electrical charge is to be imparted with said electrically charging means, a photoreceptor charge control arrangement to be employed for said electrophotographic copying apparatus which comprises;
• (a) a supplementary exposing means for projecting a predetermined amount of light onto a predetermined circumferential portion of said photoreceptor surface having been already electrically charged with said corona charging means, said supplementary exposing means being positioned between said position of said corona charging means and said position of said exposing station while being along and apart from said circumferential surface of said photoreceptive member, said predetermined amount of light being equivalent to a specific amount of light radiated from said light radiation means for substantially reproducing the original portion having the image density of the minimum reproducible level for a respective copying process to be concerned;
• (b) a surface potential detecting means for detecting the surface potential of said predetermined circumferential portion of said photoreceptor having been already exposed with said supplementary exposing means, said surface potential detecting means being placed at said exposing station and being disposed immediately before said position of said exposing station with respect to said direction of rotation;
• and
• (c) a control circuitry means capable of controlling said amount of light to be given by said light radiation means in response to the variation of surface potential detected by said surface potential detecting means.