Patent ID: 7330296

Claim:
A synchronous detector that detects timing of scanning by an optical scanner, the optical scanner has a light source that emits a light beam, a deflecting unit that deflects the light beam, a scanning optical element that focuses the light beam deflected by the deflecting unit onto a surface to be scanned, comprising: a synchronous optical element that focus the light beam deflected by the deflecting unit onto a photoreceiver, wherein the synchronous optical element satisfies a relationship fm<fd, where fm is a composite focal length of the scanning optical element in a main scanning direction of the scanning optical element, and fd is a composite focal length of the synchronous optical element in a main scanning direction of the synchronous optical element, wherein the synchronous detector comprises a plurality of the synchronous optical elements and a plurality of photoreceivers; the synchronous optical elements have negative power in the main scanning direction of the synchronous optical element.