Patent ID: 7499072

Claim:
A color image forming apparatus for forming a color image on a recording medium, comprising: a plurality of image carriers for a plurality of colors; a plurality of polygon motors for driving a plurality of polygon mirrors; a light beam drive circuit for emitting and driving a light beam in response to an image signal for a given color to irradiate the light beam onto one of the polygon mirrors so that the light beam scans and exposes one of the image carriers; an index sensor provided on a scanning path of the light beam for receiving the light beam and outputting a main scanning reference signal for the given color; a signal source for generating a reference clock signal; a pseudo index forming circuit connected to the signal source for generating a pseudo main scanning reference signal having a fixed cycle, by dividing frequency of the reference clock signal generated from the signal source; a polygon drive clock generation circuit connected to the signal source for generating a drive clock signal to drive a polygon motor for the given color, by dividing frequency of the reference clock signal generated from the signal source; a controller for adjusting the drive clock signal so that a phase difference between the pseudo main scanning reference signal and the main scanning reference signal which has been outputted from the index sensor for the given color is equal to a prescribed phase difference; and a signal forming device which forms a plurality of pseudo main scanning reference signals; wherein the controller controls image formation on a predetermined surface of the recording medium according to the main scanning reference signal and the plurality of pseudo main scanning reference signals, and wherein the signal forming device forms a first pseudo main scanning reference signal having a first cycle and a second pseudo main scanning reference signal having a second cycle different from the first cycle from among the plurality of pseudo main scanning reference signals, and the controller selects one of the first and second pseudo main scanning reference signals, and generates an image formation start trigger signal based on the selected pseudo main scanning reference signal.