Patent ID: 6128144
Filing Date: 2000-10-03
Classification: G02B

Abstract:
An image-forming optical system for forming an object image,said image-forming optical system comprising a prism member,said prism member having a plurality of reflecting surfaces arranged so that an optical path of light rays is bent to intersect itself at least once in said prism member and then the light rays exit from said prism member to an image plane side of said image-forming optical system,wherein said prism member is formed by a unitary medium free from an optical surface within said unitary medium,wherein at least one of said plurality of reflecting surfaces is a rotationally asymmetric aspherical surface that satisfies at least one of the following conditions (1-1), (2-1) and (3-1) to correct aberrations due to decentration: ##EQU5## wherein FX, FXn, FY and FYn are focal lengths defined such that when a light ray emanating from a center of an object point and passing through a center of a pupil to reach a center of an image is defined as an axial principal ray, and a Y-axis is taken in a decentration plane of the surface, and further an X-axis is taken in a direction perpendicularly intersecting the Y-axis, and further an axis constituting an orthogonal coordinate system in combination with the X- and Y-axes is defined as a Z-axis, the axial principal ray and a light ray which is parallel to the axial principal ray at a slight distance d in the X-axis direction are made to enter said optical system from an entrance side thereof, and a sine of an angle formed between said two rays as projected on an XZ-plane at an exit side of said optical system is denoted by NA'X, and further a value obtained by dividing said distance d by said NA'X is defined as a focal length FX in the X-axis direction of said optical system, and FXn is a focal length in the X-axis direction of that portion of said rotationally asymmetric aspherical surface on which the axial principal ray strikes, and further the axial principal ray and a light ray which is parallel to the axial principal ray at a slight distance d in the Y-axis direction are made to enter said optical system from the entrance side thereof, and a sine of an angle formed between said two rays in a YZ-plane at the exit side of said optical system is denoted by NA'Y, and further a value obtained by dividing said distance d by said NA'Y is defined as a focal length FY in the Y-axis direction of said optical system, and FYn is a focal length in the Y-axis direction of that portion of said rotationally asymmetric aspherical surface on which the axial principal ray strikes.