Patent ID: 7444090

Claim:
An image forming apparatus comprising: (a) an image carrier; (b) a charger for charging the image carrier; (c) an exposure unit for imagewise exposing the image carrier; (d) a color toner developing unit for developing the image carrier to form a color toner image; (e) a black toner developing unit for developing the image carrier to form a black toner image; (f) a transfer unit for transferring the color toner image and the black toner image, by respectively applying a transfer bias voltage from a power source at each of a transfer section, onto an intermediate transfer member made of a black material; (g) a controller for controlling a transfer output of the transfer unit; (h) an optical density sensor for detecting each of optical densities of color toner patch images formed on the intermediate transfer member; and (i) a toner adhesion amount detector that detects an amount of toner adhered of a black toner patch image formed on the image carrier, wherein the controller forms color toner patch images for yellow, magenta and cyan on the image carrier, transfers the color toner patch images onto the intermediate transfer member while changing the transfer output of the transfer unit, detects the optical density of each of the color toner patch images transferred onto the intermediate transfer member using the optical density sensor, obtains the transfer output for the transfer unit to transfer the color toner images based on the detected optical densities of the color toner patch images, obtains a correlation between the obtained transfer output for color toners and a developing condition of each of the color toner images, forms the black toner patch image on the image carrier, detects the amount of toner adhered on the black toner patch image using the toner adhesion amount detector, and determines the developing condition for the black toner developing unit based on the detected amount of toner adhered and a corresponding exposure potential, and the controller determines the transfer output of the transfer unit to transfer the black toner image, based on the correlation and on the determined developing condition for black toner.