Patent ID: 7830407

Claim:
An image forming apparatus comprising: a frame formed with a resin; a polygon mirror disposed at the frame, and having a rotational shaft and a deflection surface rotatable about the rotational shaft; a plurality of laser generators disposed at the frame, each of the plurality of laser generators being configured to output a laser beam to the deflection surface; a plurality of photosensitive drums, each of which has a photosensitive surface, the laser beam deflected by the deflection surface being irradiated onto the photosensitive surface to scan the photosensitive surface over a scan part in a main scanning direction; a first sensor disposed inside and mounted on the frame, and located at a first position to detect the laser beam deflected by the deflection surface for one photosensitive drum; a second sensor disposed inside and mounted on the frame, and located at a second position to detect the laser beam deflected by the deflection surface for another photosensitive drum, the second position being different from the first position, wherein the first sensor and the second sensor are provided along a direction orthogonal to the main scanning direction, the first position being a first side of the polygon mirror, the second position being a second side of the polygon mirror opposite to the first side with respect to the polygon mirror, the first position and the second position being asymmetric with respect to the polygon mirror; a storage unit storing correspondence information indicating shift of the scan part from a reference scan part in relation to time difference between detections of the laser beam by the first sensor and the laser beam by the second sensor, the correspondence information varying with temperature variation in the frame, the shift of the scan part being caused by change of positions of the first and second sensors caused by the temperature variation; a measuring unit configured to measure the time difference between detections of the laser beam by the first sensor and the laser beam by the second sensor; a calculating unit configured to calculate, without detecting temperature of the frame, the shift of the scan part corresponding to the temperature variation in the frame, based on both of the time difference measured by the measuring unit and the correspondence information; and a controller configured to control the plurality of laser generators to output laser beams based on the shift of the scan part calculated by the calculating unit.