| 摘要 |
A decentered prism optical system has a first surface at a side thereof closer to a pupil. The first surface has a transmitting or reflecting action and also has a wide effective area. The first surface is formed from a rotationally symmetric spherical or aspherical surface. The optical system has at least three surfaces (3, 4 and 5), and the space between these surfaces (3 to 5) is filled with a transparent medium having a refractive index larger than 1.3. A bundle of light rays from an object first passes through a pupil (1) of the optical system (7) along an optical axis (2) and enters the optical system (7) through a first surface (3) having both transmitting and reflecting actions. The incident light rays are reflected toward the pupil (1) by a second surface (4) which is at a side of the optical system (7) remote from the pupil (1) and which has only a reflecting action. The reflected light rays are then reflected away from the pupil (1) by the first surface (3). The reflected light rays pass through a third surface (5) having only a transmitting action, and reach an image plane (6) to form an image thereon. The first surface (3) is a rotationally symmetric surface, e.g. a spherical surface, and the second surface (4) is a rotationally asymmetric surface.
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