Patent ID: 8348473

Claim:
An illumination optical system comprising: a prism that is disposed on an incident optical axis of light from a light source; and a lens that is disposed between the light source and an incident surface of the prism or between an emitting surface of the prism and an illuminated object, wherein the prism has two reflection surfaces, and at least one of the reflection surfaces is a transmission/reflection surface that serves for reflection and transmission, wherein a refractive index n of the prism with respect to a d-line satisfies 1.6≦n≦2.2, wherein the prism and the lens are arranged in this order from the light source side, wherein a beam from the light source passes through the prism via a transmission surface, the transmission/reflection surface, the reflection surface, and the transmission/reflection surface, in this order, and wherein, when the lens is a positive lens, a shape factor SF 1 of the lens satisfies 0.5≦SF 1 ≦1.25, and when the lens is a negative lens, a shape factor SF 2 of the lens satisfies 0.6≦SF 2 ≦1.25, where SF 1 and SF 2 are defined by (R 1 +R 2 )/(R 1 −R 2 ), R 1 is a radius of curvature of an object side surface of the lens, and R 2 is a radius of curvature of a light-source side surface of the lens.