Color synthesis optical element, projection-type display device using same, and method for controlling display thereof

A color synthesis optical element comprising:

• an exit surface;
• first to third incident surfaces;
• and
• first and second films that are provided such that the film surfaces intersect each other and that selectively reflect or transmit incident light according to wavelength of said incident light;
• wherein:
• said first film transmits, of visible light of a first polarization, at least light of a specific wavelength band and reflects, of visible light of a second polarization whose polarization state differs from that of said first polarization, at least light of said specific wavelength band;
• said second film transmits, of visible light of said first polarization, at least light of said specific wavelength band and transmits, of visible light of said second polarization, at least light of said specific wavelength band;
• cutoff wavelengths of said first and second films with respect to said second polarization are set within a range of bands other than the wavelength bands of red, green and blue that are the three primary colors of light;
• and
• light of said specific wavelength band of said first polarization and light of said specific wavelength of said second polarization are entered from different incident surfaces of said first to third incident surfaces and are exited by way of said first and second films from said exit surface.