Scanning optical apparatus

A scanning optical apparatus comprising:

• a light source configured to emit a light beam;
• a light deflector configured to deflect the light beam from the light source in a main scanning direction;
• an incident optical system disposed between the light source and the light deflector and configured to render the light beam from the light source nearly parallel in the main scanning direction and to converge the light beam in a sub-scanning direction to bring the light beam to a focus in proximity to the light deflector;
• and a scanning lens configured to focus the light beam deflected by the light deflector onto a target surface to form spot-like images, wherein the incident optical system comprises one or more lenses which provide at least one refracting surface and at least one diffraction surface, and wherein φnS/φdS<0 is satisfied, where φnS is a refractive power in the sub-scanning direction and φdS is a diffraction power in the sub-scanning direction;
• wherein φnM/φdM satisfies the following inequality:
• 3×10−5α2+0.0043α.+0.5525(φnM/φdM(4×10−5α2+0.0051α.+0.735
• where φnM is a refractive power in the main scanning direction in the incident optical system, φdM is a diffraction power in the main scanning direction in the incident optical system, and α is a coefficient of linear thermal expansion of a retaining member for retaining a distance between the light source and the incident optical system.