1. Field of the Invention
The present invention relates to a light scanner used in, e.g., a laser beam printer, a laser facsimile or a digital copier, and to an image forming apparatus using the same.
2. Description of the Related Art
Many light scanners used in, e.g., laser beam printers include the following: a semiconductor laser used as a light source; a polygon mirror used as an optical deflector; a first imaging optical system that forms a line image on the optical deflector by focusing a light beam from the light source so as to correct a tilt of the deflection surface of the optical deflector; a second imaging optical system that forms a uniform spot on the surface to be scanned at constant velocity; a scanning start signal detector that detects the light beam scanned by the optical deflector; and a detecting optical system that focuses the light beam from the light source on the scanning start signal detector.
The second imaging optical system of a conventional light scanner includes a plurality of large glass lenses, which are so-called f-θ lenses. However, the f-θ lens is expensive and is difficult to reduce in size. To achieve miniaturization and low cost, a light scanner using a single curved mirror for the second imaging optical system has been proposed recently (see JP 8(1996)-94953 A, JP 11(1999)-30710 A or the like).
The light scanners of those documents are described as a device that allows a light beam from the curved mirror to be guided directly on the image surface. Actually, however, a reflecting mirror has to be placed between the curved mirror and the surface to be scanned (i.e., a photosensitive drum) so as to guide the light beam on the surface to be scanned. This is because the angle of reflection of the light beam from the curved mirror is small. For the light scanner of JP 8(1996)-94953 A, the second imaging optical system requires a long lens for correcting a tilt of the deflection surface of the optical deflector in addition to the curved mirror. For the light scanner of JP 11(1999)-30710 A, the cross section of the curved mirror in the sub-scanning direction is not arc-shaped, but is expressed by a quartic polynomial. Thus, there is difficulty in processing and measuring the curved mirror.