Image formation apparatus that can maintain appropriately toner density in developing device

摘要

An image formation apparatus for forming a full color image in the four colors of cyan, magenta, yellow and black has a optical ATDC sensor attached in a cyan developing device to control the toner density under optical ATDC. A black developing device has the toner density controlled by AIDC. A standard develop efficiency eta STD of the cyan developing device obtained as a result of the AIDC is compared with a develop efficiency eta of black toner. The toner supply amount of the black developing device is determined according to the comparison result to supply the toner. Thus, an image formation apparatus is provided that can detect at favorable accuracy the toner density of a developing device incorporating a developer of which the toner density cannot be detected using optical ATDC to supply a required amount of toner.