Patent ID: 7570428

Claim:
A multilayer minus filter for reflecting a light having at least one reflection wavelength λ 0 and transmitting a light having a wavelength other than the reflection wavelength λ 0 , the filter comprising: a transparent substrate; and a repetition layer formed on a surface of the substrate including a plurality of high refractive index layers having an average optical thickness H per layer and a plurality of low refractive index layers having an average optical thickness L per layer, the repetition layer having an alternative stack of high and low refractive index layers, and satisfying the following formulas: 1.1>( H+L )/λ 0 >0.9 (1) 2> H/L >0.5 (2) wherein the repetition layer is formed by an ion assisted process technology, and wherein a reflection band utilized with respect to a light incident obliquely on the surface of the filter, the reflection band appearing in reflection wavelength λ 0 with respect to a light incident vertically on the surface of the filter, utilizes a reflection band with respect to a light incident obliquely on the surface of the filter formed to be shifted on a short wavelength side.