Patent ID: 8066381

Claim:
A polarizing beam splitter comprising: first and second prisms each having first and second end surfaces serving as an input surface or an output surface of a light beam and an opposed surface that is one of a plurality of side surfaces; and at least a first parallel flat layer, of a plurality of parallel flat layers, a second parallel flat layer, of the plurality of parallel flat layers, and a third parallel flat layer, of the plurality of parallel flat layers, that are disposed in parallel between the opposed surfaces of the first and second prisms, wherein the third parallel flat layer is disposed in the front of the first parallel flat layer or in the back of the second parallel flat layer in the optical path; wherein the first parallel flat layer polarizes and separates an input beam, and wherein the refractive index of a base material of the first and second prisms is Np, the resultant refractive index of the parallel flat layers other than the second flat layer is Na, and the refractive index of the second flat layer is Nb, and (Na−Np)×(Nb−Np)<0 is satisfied.