Light beam scanner with focus-varying mechanism

Claim 1

A light beam scanner for scanning a surface in a main scanning direction with a light beam emitted from a light beam source while said surface is being continuously fed in an auxiliary scanning direction at a constant speed, said light beam scanner comprising: (i) a rotating polygonal mirror for reflecting and deflecting the light beam toward said surface; (ii) an optical system for focusing the light beam from the light beam source to fall on said rotating polygonal mirror as a linear image normal to the axis of rotation thereof where said linear image moves along each facet of the polygonal mirror; (iii) a focus-varying mechanism disposed between said light beam source and said rotating polygonal mirror for adjusting the focus position of said light beam as it falls on said rotating polygonal mirror; (iv) control means for controlling said focus-varying mechanism in synchronism with the rotation of said rotating polygonal mirror to maintain said focused linear image on said rotating polygonal mirror; and (v) an anamorphic optical system disposed in a light beam path following said rotating polygonal mirror for focusing the light beam, which has fallen as a linear image on said rotating polygonal mirror, as a spot image on said surface being scanned.