Patent ID: 7298550

Claim:
A dichroic mirror comprising: a transparent substrate; and a dielectric multilayer film formed by alternately laminating high-refractive-index layers and low-refractive-index layers on the transparent substrate; wherein the dielectric multilayer film includes from 50 to 150 alternately laminated high-refractive-index layers and low-refractive-index layers each having an optical film thickness from 1.5λ 0 /4 to 2.5λ 0 /4, where λ 0 is a design wavelengths; the dichroic mirror having a reflection band located at wavelengths shorter than the design wavelength, and the dichroic mirror having a transmitting band located at wavelengths longer than the design wavelength, wherein a wavelength gap exists between the reflection band and the transmitting band; the average reflectance in the reflection band is 0.1% or less; the transmittance in the transmitting band existing in a different wavelength band from the reflection band is 80% or greater; and the wavelength gap between the reflection band and the transmitting band is a region of the transmission characteristic of the dichroic mirror where the transmittance exceeds 0.1% and does not exceed 80%, and wherein the wavelength gap is 13 nm or less.