Field flattener for flattening a bent image surface

摘要

A field flattener having a function of flattening a bent image surface on which an image is formed through an optical system, is composed of a material of which a refractive index is equal to or larger than 2. A sectional configuration of the field flattener cut in an optical-axis direction containing the optical axis is formed to take a stepped shape toward its peripheral edge from the optical axis. An axial height of the stepped portion configuring the surface in the optical-axis direction in this stepped shape is set so as to equal to or larger than twice a wavelength of beams to be used.