Image density control method and color image forming apparatus

Claim 1

A color balance control method for use in an image forming system in which a full color image is formed by superposing toner images decomposed into three colors of yellow, magenta and cyan by means of an electrophotographic method, comprising the steps of: correcting a supplement of toner in response to an output of a toner density sensor in a developing device; forming unfixed toner images onto a photosensitive body for each color of the three colors, the unfixed toner images corresponding to each of a medium density reference image and a low density reference image, said low density reference image corresponding to a background potential of a photosensitive member, and said unfixed toner images corresponding to said low density reference image being influenced by a fluctuation of the background potential of a photosensitive member; sensing the unfixed toner images for each color by means of a photoelectric sensor for sensing a deposited quantity of toner; correcting exposure of scanning light onto an original document on the basis of a ratio of a sensed value of the medium density reference image to a sensed value of the low density reference image thereby to balance a medium density portion of color decomposed images for each color; correcting developing bias voltage on the basis of the sensed value of the low density reference image thereby to balance a low density portion of the color decomposed images for each color; and correcting a reference value to be used in a correction for the supplement of toner on the basis of the sensed results of the sensing step.