Patent ID: 7719759

Claim:
An imaging optical system, comprising: a first reflector, a second reflector, and a third reflector arranged on an optical path of incident ray, wherein the three reflectors are arranged so as not to obscure the incident ray; and light beams reflected by the three reflectors form an image plane, and further wherein one of the first reflector and the third reflector is a convex mirror and the other is a concave mirror; and the three reflectors are positioned with respect to each-other such that, in terms of a central chief ray, which is defined as a chief ray of light beams forming an image at a center of the image plane, vertexes of a triangular bipyramid are defined by an appropriate point on the central chief ray that is incident to the first reflection surface, a reflection point of the central chief ray on the first reflection surface, a reflection point of the central chief ray on the second reflection surface, a reflection point of the central chief ray on the third reflection surface, and an image forming point of the central chief ray; and a plane containing three points composed of the reflection point of the central chief ray on the first reflection surface, the reflection point of the central chief ray on the second reflection surface, and the reflection point of the central chief ray on the third reflection surface coincide with a bonding plane between two triangular pyramids that form the triangular bipyramid; and further wherein, in terms of a reference plane containing three points that are the reflection point of the central chief ray on the first reflection surface, the reflection point of the central chief ray on the second reflection surface, and the reflection point of the central chief ray on the third reflection surface, when an angle formed by the central chief ray incident to the first reflector and the reference plane is represented as θ and an angle formed by the central chief ray reflected by the third reflector and the reference plane is represented as θ′, the following condition is satisfied: 0.5<θ′/θ<2.0.