Patent ID: 8102563

Claim:
An image forming apparatus, comprising: an intermediate transfer medium movable in a sub-scanning direction and on which a toner image is transferred; a first photosensor including a first light emitting portion for projecting light upon a first side region of the intermediate transfer medium, and a first light receiving portion for receiving light reflected thereby; a second photosensor including a second light emitting portion for projecting light upon a second side region of the intermediate transfer medium, and a second light receiving portion for receiving light reflected thereby; and a control unit configured to perform calibration of the first photosensor and the second photosensor by controlling an amount of light emitted from the first light emitting portion and the second light emitting portion based on detection of an amount light reflected directly from an unmarked surface of the intermediate transfer medium, wherein, after calibration of the first photosensor and the second photosensor, toner images representing a first concentration mark having varying degree of gray concentrations and a second concentration mark having varying degree of gray concentrations are formed on the image intermediate transfer medium as the intermediate transfer medium is moved in the sub-scanning direction, wherein the first concentration mark and the second concentration mark are separated from each other in a main-scanning direction, and wherein, after calibration of the first photosensor and the second photosensor, toner images representing a first set of registration marks comprising four different colors and a second set of registration marks comprising four different colors are formed on the intermediate transfer medium as the intermediate transfer medium is moved in the sub-scanning direction, wherein the first set of registration marks and the second set of registration marks are separated from each other in a main-scanning direction.