Laser beam printer with optical system for preventing ghost images

Abstract

The present invention provides a laser beam printer which comprises a beam source for emitting a beam corresponding to an image signal, a photosensitive member for forming an electrostatic latent image thereon with the beam, a polygon mirror for reflecting the beam emitted by the beam source to scan the photosensitive member with the beam, an image forming lens provided between the polygon mirror and the photosensitive member in the optical path of the mirror for focusing the reflected beam from the polygon mirror on the surface of the photosensitive member, a box accommodating the beam source, the polygon mirror and the image forming lens therein and having a window for passing therethrough the beam from the image forming lens toward the photosensitive member, an optical filter provided over the opening of the window, and light blocking means disposed in an optical path along which a portion of the beam reflected from one of the mirror faces of the polygon mirror travels toward the image forming lens again after the beam portion reversely travels through the image forming lens upon reflection at the surface of the filter and is reflected from another mirror face adjacent to said one mirror face. This arrangement precludes the ghost image to be otherwise formed by the beam portion reflected from the filter surface.