Image density control apparatus for copying machines

Claim 1

In a copying machine having a mechanism for projecting a predetermined density pattern on a photoconductive surface through an image projecting optical system, developing the resulting pattern image, detecting the density of the developed image on the surface of the photoconductive surface by density sensor means and controlling the density of the image to be formed on the photoconductive surface in accordance with the detected density, and a magnification control system for varying the magnification of the image to be projected on the photoconductive surface by controlling the image projecting optical system, an image density control apparatus comprising: a reference pattern so positioned as to be projectable on the photoconductive surface by the optical system and having its size so determined that even when a minimum copy magnification is set by the magnification control system, the projected image has a predetermined area at least required for detecting the density, means for limiting an area of charge bearing portion of the photoconductive surface produced by projection of the pattern to a constant area, said means being so positioned to limit the area posterior to charging the photoconductive surface and prior to developing thereof, and a control means for controlling the area limiting means in relation with the timing of projection of the reference pattern on the photoconductive surface with the operation of the copying machine, whereby the area of charge bearing portion of the photoconductive surface which is to be developed and to be detected is rendered definite irrespective of the copy magnification.