Plastic optical element and optical scanner and imaging forming device including the same

Abstract

A plastic optical element for an optical system of an optical scanner includes a plurality of optical effective portions through which a plurality of light beams transmit, respectively, formed on at least one of an incidence surface and an exit surface in a sub scan direction, and an optical ineffective portion formed between neighboring optical effective portions not to allow the light beams to transmit therethrough, and including an area in which a local contraction occurs at a time of resin molding.

What is claimed is: 
     
         1 . A plastic optical element for an optical system of an optical scanner comprising:
 a plurality of optical effective portions through which a plurality of light beams transmit, respectively, formed on at least one of an incidence surface and an exit surface in a sub scan direction; and   an optical ineffective portion formed between neighboring optical effective portions not to allow the light beams to transmit therethrough, and including an area in which a local contraction occurs at a time of resin molding.   
     
     
         2 . The plastic optical element according to  claim 1 , wherein
 the optical ineffective portion is set to have a length in the sub scan direction to satisfy the following relation:
     Y> 0.3( a*b )/( a+b )−1.1
 
   
       where Y is the length, a is a thickness of the plastic optical element in the sub scan direction, and b is a thickness of the plastic optical element along an optical axis. 
     
     
         3 . The plastic optical element according to  claim 1 , wherein
 the optical ineffective portion is formed by surface processing to have a reflectivity lower than that of the optical effective portions.   
     
     
         4 . The plastic optical element according to  claim 1 , further comprising
 a concave portion on at least either of two edge surfaces in the sub scan direction.   
     
     
         5 . The plastic optical element according to  claim 1 , wherein
 the plastic optical element is an fθ lens.   
     
     
         6 . An optical scanner comprising:
 a plurality of light sources;   an optical deflector to deflect a plurality of light beams emitted from the light sources; and   an optical system including the plastic optical element according to  claim 1 , to guide the light beams to scan surfaces, respectively.   
     
     
         7 . An image forming device comprising:
 a plurality of photoreceptors; and   the optical scanner according to  claim 6 , to scan and expose surfaces of the photoreceptors.