Color separation optical system and image projection apparatus

A color separation optical system comprising:

• a dichroic surface which transmits light from a light source in a first wavelength band and reflects light therefrom in a wavelength band longer than the first wavelength band;
• and
• a polarization beam splitting surface on which the light transmitted through the dichroic surface impinges,
• wherein, when linearly polarized light impinging on the dichroic surface at an incident angle smaller than 45 degrees and then impinging on the polarization beam splitting surface at an incident angle larger than 45 degrees in an optical path for the light in the first wavelength band is referred to as first incident-angle light, a wavelength which is longer than a wavelength at the maximum transmittance of the dichroic surface for the first incident-angle light and at which the transmittance of the dichroic surface for the first incident-angle light is 10% is referred to as a wavelength Wa, and a range of wavelengths shorter than the wavelength Wa is referred to as W,
• the polarization beam splitting surface has a characteristic in which its transmittance for the first incident-angle light in the wavelength range W is equal to or lower than 5%.