Polarizing beam splitter, polarization conversion element using the same, and image projection apparatus

A polarization conversion element comprising:

• a medium;
• and
• a polarization splitting film formed of at least two thin-film layers having different refractive indices,
• wherein the medium and the at least two film layers are arranged in order from a light incidence side,
• wherein the following conditional expressions are satisfied
• 38°<sin−1(sin(θc)*nH/nb)<52°,
• 100 nm<ndL<350 nm,
• 100 nm<ndH/cos(θc)<200 nm, and
• θc=cos−1(√(nH2−nL2)/nH),
• where nb denotes a refractive index of the medium,
• ndH denotes an average value of optical thicknesses nH×dH of the thin-film layers having a refractive index nH where nH is a refractive index and dH is a thickness of a thin-film layer having the highest refractive index out of the thin-film layers,
• ndL denotes an average value of optical thicknesses nL×dL of the thin-film layers having a refractive index nL where nL is a refractive index and dL is a thickness of a thin-film layer having the lowest refractive index out of the thin-film layers, and
• the average values ndH and ndL are obtained by adding the optical thicknesses of all thin-film layers excluding a layer adjacent to the medium and dividing the sum thereof by the number of thin-film layers.

An image projection apparatus comprising:

• a medium;
• and
• a polarization splitting film formed of at least two thin-film layers having different refractive indices,
• wherein the medium and the at least two film layers are arranged in order from a light incidence side,
• wherein the following conditional expressions are satisfied
• 38°<sin−1(sin(θc)*nH/nb)<52°,
• 100 nm<ndL<350 nm,
• 100 nm<ndH/cos(θc)<200 nm, and
• θc=cos−1(√(nH2−nL2)/nH),
• where nb denotes a refractive index of the medium,
• ndH denotes an average value of optical thicknesses nH×dH of the thin-film layers having a refractive index nH where nH is a refractive index and dH is a thickness of a thin-film layer having the highest refractive index out of the thin-film layers,
• ndL denotes an average value of optical thicknesses nL×dL of the thin-film layers having a refractive index nL where nL is a refractive index and dL is a thickness of a thin-film layer having the lowest refractive index out of the thin-film layers, and
• the average values ndH and ndL are obtained by adding the optical thicknesses of all thin-film layers excluding a layer adjacent to the medium and dividing the sum thereof by the number of thin-film layers.