1) Field of the Invention
The present invention relates to an optical scanner used in an image forming apparatus, and more specifically to a synchronous detector for deciding start of writing in the optical scanner.
2) Description of the Related Art
Optical scanners are widely known and used as writing apparatuses in image forming apparatuses such as digital copiers, optical printers, photolithographic apparatuses, and facsimile apparatuses.
With such an optical scanner, for the purpose of detecting a position or a timing to start writing an image on the surface to be scanned, a photoreceiver is disposed at the position to start writing the image on the scanned surface or a position equivalent thereto either directly or via a slit or the like. As a result, at the moment the light beam passes the position in which the photoreceiver has been disposed the photoreceiver outputs a start writing image signal, and writing is started. The arrangement for detecting the position or the timing to start writing is called as a synchronous detector.
For example, the art described in Japanese Patent No. 3236017, Japanese Patent Application Laid-open No. 5-150176, and Japanese Patent Application Laid-open No. 7-281113 relates to a type in which an optical element having positive power is disposed upstream of the photoreceiver, and the art described in Japanese Patent Application Laid-open No. 2001-281571 relates to a type in which an optical element is not disposed upstream of the photoreceiver.
If the synchronous detector detects a wrong timing to start writing, the error is magnified with regard to the shift in the position of the light beam spot on the surface to be scanned, and cause color blurring. In particular, this effect becomes pronounced in the case of a synchronous detector in which an optical element having positive power is disposed upstream of the photoreceiver.