Patent ID: 8520281

Claim:
A scanning optical apparatus comprising: a plurality of individually modulated light sources; a first optical element configured to convert light emitted from each of the plurality of light sources into a beam of light; a second optical element configured to convert beams of light having passed through the first optical element into linear images extending in a main scanning direction; a polygon mirror configured to spin at high speed and to deflect the beams of light having passed through the second optical element in the main scanning direction; and a third optical element configured to convert the beams of light having been deflected by the polygon mirror into spot-like images and focus the spot-like images on a target surface to be scanned, wherein the third optical element has a pair of lens surfaces; wherein each lens surface of the third optical element is aspheric in a main scanning plane, a curvature thereof in a sub-scanning plane changes continuously and symmetrically along the main scanning direction from a point on an optical axis toward both outer ends of the lens surface, and a shape of the lens surface is symmetrical with respect to the sub-scanning plane passing through the optical axis; and wherein the third optical element is configured such that a first optical axis defined as an optical axis of an incident-side lens surface of the third optical element is inclined in the main scanning plane with respect to a normal line extending from a scanning center on the target surface and an intersection point between the first optical axis and the incident-side lens surface is shifted with respect to the normal line, and that a second optical axis defined as an optical axis of an emission-side lens surface is inclined in the main scanning plane with respect to the first optical axis and an intersection point between the second optical axis and the emission-side lens surface is shifted with respect to the first optical axis, so as to satisfy the following formula: Max [P sub ( y )]−Min[ P sub ( y )]<0.1 *DP where y is an image height on the target surface in the main scanning direction; DP is a dot pitch on the target surface in the sub-scanning direction; and P sub (y) is a pitch of adjacent images from the plurality of light sources formed on the target surface in the sub-scanning direction, if the image height is y.