a thickness of said lower dielectric layer is set to not less than ??/(9n) and not more than ??/(3n) where n is a refractive index of said dielectric layer with respect to a wavelength ?? of not less than 380 nm and not more than 430 nm, and a thickness of said upper dielectric layer is set to not less than ??/(5n) and not more than ??/(2.5n). 5.