Patent ID: 8009186

Claim:
An image forming apparatus comprising: a plurality of light source devices; a rotary polygonal mirror configured to scanningly deflect, with different deflecting surfaces, a plurality of light beams emitted from said plurality of light source devices; a plurality of imaging optical systems provided in relation to the plurality of light beams, respectively, scanningly deflected by said different deflecting surfaces and configured to image the plurality of light beams upon different photosensitive members, respectively; and a synchronism detecting system configured to detect a synchronizing light beam for determining a writing start timing of an image upon each of said plurality of photosensitive members, with respect to each of the plurality of light beams scanningly deflected by said different deflecting surfaces of said rotary polygonal mirror; wherein each deflecting surface of said rotary polygonal mirror has a chamfer, wherein said synchronism detecting system detects a light beam emitted from a light source device, among said plurality of light source devices, other than a light source device which emits a light beam for forming an image on a photosensitive member for a black color, wherein the light source device which emits a synchronizing light beam to be detected by said synchronism detecting system is configured to emit light at the timing as a light beam is incident on the chamfer so as to perform the synchronism detection through said synchronism detecting system, and wherein said light source device which emits a light beam for forming an image on the photosensitive member for the black color, among said plurality of light source devices, does not emit light at the timing as the light beam is incident on the chamfer.