Patent ID: 7679634

Claim:
An image forming apparatus, comprising: a light emission source configured to output a plurality of optical beams in accordance with image data; a rotary polygon mirror configured to receive the plurality of optical beams at different mirror points with different mirror positions and to deflect the beams into a plurality of scanning optical beams in given directions to cyclically scan on a plurality of predetermined scanning lines in a main scanning direction, said plurality of scanning optical beams being all scanning beams used to form an image corresponding to the image data; a plurality of image carrying members configured to rotate in a sub-scanning direction line by line and to respectively receive the plurality of scanning optical beams to form a plurality of primary separate color images; a single synchronous detection sensor configured to receive the plurality of scanning optical beams reflected from oppositely positioned mirror surfaces of the rotary polygon mirror; and an instruction mechanism configured to instruct the light emission source to light on and off at timings determined based on a plurality of detection signals from the single synchronous detection sensor.