Patent ID: 8797622

Claim:
A scanning optical apparatus comprising: a light source; a deflector configured to deflect a beam of light from the light source in a main scanning direction; an illumination optical system disposed between the light source and the deflector and configured to convert the beam of light emitted from the light source into a beam of light slightly converging in the main scanning direction and focused near the deflector in a sub-scanning direction; and a scan lens configured to cause the beam of light deflected by the deflector to be focused into a dot-like image on a target surface to be scanned, wherein the illumination optical system has a first diffractive power if φdM in the main scanning direction and a second diffractive power φdS in the sub-scanning direction, the first and second diffractive powers being different from each other, and has a first refractive power φnM in the main scanning direction and a second refractive power φnS in the sub-scanning direction, the first and second refractive powers being different from each other, wherein the illumination optical system has a focal length fi [mm] in the main scanning direction, which satisfies: 10≦fi≦30; wherein a ratio mM/mS of a lateral magnification mM in the main scanning direction to a lateral magnification mS in the sub-scanning direction, of an entire optical system which includes the illumination optical system and the scan lens, satisfies: mM/mS≧ 1.2; wherein a holding member provided to retain a distance between the light source and the illumination optical system has a coefficient Z of linear expansion which satisfies: 3.0×10 −5 ≦Z≦ 9.5×10 −5 ; wherein a ratio φnM/φdM of the first refractive power φnM in the main scanning direction to the first diffractive power φdM in the main scanning direction satisfies: g 2( fi )≦φ nM/φdM≦g 1( fi ) where A(Z)=(3.532×10 7 )Z 2 +3023Z+0.7010, B(Z)=(5.719×10 7 )Z 2 +4169Z+0.7678, C(Z)=(1.727×10 7 )Z 2 +3244Z+0.4217, D(Z)=(1.373×10 8 )Z 2 +3232Z+1.224, g1(fi)=fi{D(Z)−B(Z)}/20−0.5D(Z)+1.5B(Z), g2(fi)=fi{C(Z)−D(Z)}/20−0.5C(Z)+1.5A(Z); and wherein a ratio φnS/φdS of the second refractive power φnS in the sub-scanning direction to the second diffractive power φdS in the sub-scanning direction satisfies: φ nS/φdS< 1.3.