Patent ID: 7177078

Claim:
A projection type display apparatus, comprising: a light source for emitting an illumination light, an image display device, an illumination optical system for irradiating the illumination light from said light source to said image display device, and a projection optical system for projecting an image formed by said image display device; wherein said illumination optical system comprises a first fly-eye lens and a second fly-eye lens arranged at each other's focal points, a first lens group, and a second lens group arranged over a mirror for bending an optical path and having a same focal length as that of said first lens group but a different back focus; and wherein said illumination optical system is useable in both a predetermined first state and a second state satisfying condition formulas below. e 1=−(Ø−Ø11−Ø12)/(Ø11×Ø12) (1) e 2=−(Ø−Ø21−Ø22)/(Ø21×Ø22) (2) Ø11=(1 −fb 1×Ø)/ e 1 (3) Ø21=(1 −fb 2×Ø)/ e 2 (4) 0.95 <e 1 /e 2<1.05 (5) 1.1×(1 −e 1×Ø)/Ø< fb<f× 0.9 (6) Here, Ø indicates a refracting power of an entire relay lens system (inverse number of focal length f), Ø11 indicates a refracting power of a first lens group in the first state, Ø12 indicates a refracting power of the second lens group in the first state, Ø21 indicates a refracting power of the first lens group in the second state, Ø22 indicates a refracting power of the second lens group in the second state, e 1 indicates a paraxial inter-group distance between the first lens group and the second lens group in the first state, e 2 indicates a paraxial inter-group distance between the first lens group and the second lens group in the second state, fb 1 indicates a paraxial back-focus of the relay lens system receiving an infinite light flux in the first state and fb 2 indicates a paraxial back focus of the relay lens system receiving an infinite light flux in the second state, respectively.