Patent ID: 8144179

Claim:
An image forming apparatus, comprising: a laser beam source; a plurality of photoconductive members respectively corresponding to a plurality of colors; a modulation drive unit configured to subject image data of each of the colors to pulse width modulation by an image clock having a cycle of a pixel length of the image data, and drive the laser beam source; a single piece of polygon mirror whose reflection surfaces are disposed in a rotation direction thereof with a plurality of different inclination angles respectively corresponding to the plurality of colors, and via a plurality of different optical paths respectively corresponding to the inclination angles, the polygon mirror scanning the photoconductive members with a light coming from the laser beam source sequentially for each of the colors in a main scanning direction; a plurality of beam detectors respectively corresponding to the plurality of photoconductive members, each disposed adjacent to and on an upstream side of the corresponding photoconductive member in the main scanning direction; and an image clock/data switching unit configured to generate the image clock of a frequency varying with the colors for making the same a color-based image magnification defined by an optical path length of each of the optical paths of the colors, and switch at least either the image clocks generated for each of the colors or the image data of each of the colors in synchronization with a detection signal coming from the beam detector, the image clock/data switching unit further configured to, when first, second, third, and fourth colors are switched in a sequential manner: set, at a cycle of twice a switching cycle of each of the colors, first frequency data corresponding to the first color and third frequency data corresponding to the third color, and generate an image clock for the first color and an image clock for the third color using a phase-locked loop based on the first and third frequency data; and set, at a cycle of twice the switching cycle of each of the colors, second frequency data corresponding to the second color and fourth frequency data corresponding to the fourth color, and generate an image clock for the second color and an image clock for the fourth color using the phase-locked loop based on the second and fourth frequency data.