Method for making optical element, and image forming apparatus

A method for injection molding an imaging optical element that is to be used in an optical scanning apparatus, in which a light ray does not pass over a meridional line including an optical axis of the imaging optical element, the method comprising:

• an initial molding step in which when the imaging optical element is injection-molded, molding conditions are set such that a constant shape error is stably formed on optical functional surfaces of the imaging optical element;
• an optical measurement step in which the imaging optical element formed by the initial molding step is placed in an evaluation apparatus in which components of the evaluation apparatus are arranged in a same manner as components of the optical scanning apparatus during use, and an amount of defocus in an optical axis direction, an amount of wavefront aberration, and an amount of scanning line curvature at a plurality of image heights are measured on an image plane;
• a shape approximation step in which shapes of all of the optical functional surfaces of the imaging optical element are measured, and a curved surface model is determined such that a sagittal line curvature and the sagittal line inclination of the imaging optical element are closest to a measurement result, at positions where light rays heading for a plurality of image heights pass through the optical functional surfaces;
• an evaluation step in which the amount of defocus in the optical axis direction, the amount of scanning line curvature, and the amount of wavefront aberration are evaluated in an optical simulation using the curved surface model based on aspherical coefficients of the optical functional surfaces obtained in the shape approximation step;
• a defocus sensitivity calculating step in which ray trace is performed using an optical model with the aspherical coefficients of the optical functional surfaces changed, an amount of change in the amount of defocus in the optical axis direction at the plurality of image heights are obtained, and a sensitivity of the amount of defocus to a change of each of the aspherical coefficients is calculated;
• an inclination sensitivity calculating step in which ray trace is performed using the optical model with the aspherical coefficients of the optical functional surfaces changed, the amount of scanning line curvature and the amount of wavefront aberration at the plurality of image heights are obtained, and sensitivities of the amount of scanning line curvature and the amount of wavefront aberration to the change of each of the aspherical coefficients are calculated;
• a first redesign step in which the shapes of at least one of the optical functional surfaces are newly redesigned on the basis of the sensitivity of the amount of defocus so that they agree with a difference between the amount of defocus obtained in the evaluation step and a result of calculation of the amount of change in the amount of defocus obtained in the defocus sensitivity calculating step;
• a second redesign step in which the shapes of at least one of the optical functional surfaces are newly redesigned on the basis of the sensitivity of the amount of scanning line curvature so that they agree with a difference between the amount of scanning line curvature obtained in the evaluation step and a result of calculation of the amount of scanning line curvature obtained in the inclination sensitivity calculating step and so as not to deteriorate the amount of wavefront aberration;
• a correction step in which, in the optical functional surfaces obtained in the first and second redesign steps, reflecting a difference between the shapes measured in the shape approximation step and the first and second redesign steps and a shape of a designed value, a shape of a cavity surface of a molding member is corrected and processed;
• and
• an actual molding step in which molding is performed using the molding member obtained in the correction step.