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Abstract

The present invention provides a method of designing a post-objective type optical scanner having an optical deflector composed of cylindrical or spherical reflecting surfaces for deflecting a converged light beam from a condensing lens by reflecting the converged light beam as it rotates, and a compensating lens located in an optical path between the optical deflector and a scanning surface for converging the deflected light beam from the optical deflector to a point on the scanning surface, of which power in the sub-scanning direction varies with a length from the center thereof in the scanning direction. The method is characterized in that a scanning speed(S) defined by a scanning width on a scanning surface per an angle unit of rotation of the optical deflector is selected by exploiting a characteristic that a maximum curvature of field in the scanning direction drops to a minimum value in a certain range predetermined by a length from a point of deflection of the optical deflector to the scanning surface(L) based on a correlation between the scanning speed and the maximum curvature of field in the scanning direction. In the method, offset of the f.multidot..theta. characteristic is maintained to be within .+-.0.5%, and the certain range is expressed as ##EQU1##

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First Claim