Multilayer optical interference filter with wideband spectral transmission region and reduced ripple

Claim 1

A multiple layer optical interference filter with a wideband spectral transmission region with reduced ripple structure and which is designed to reflect radiation of longer wave lengths, comprising a carrier having an alternating system of layers of alternatingly high and low refractivity, said layers being of an optical thickness which satisfies the relation nidi cos Alpha i lambda o/4 (where ni denotes the index of refraction of the layer substance, di the thickness of the layer, lambda o the wavelength of the reflection adjacent the long wave side to the transmission region of interest, and Alpha i the angle which is formed by the light ray passing through the filter and the normal to the layer plane of the layer in question), said system of layers comprising a light transparent equalizing layer adjacent to the outer of said low reflectivity layers of such refractive index (n) and such thickness (d) that undesired secondary maxima of reflection are reduced, and wherein the thickness (d) of the equalizing layer is equal to d 3 lambda /8 . 1/n cos Alpha , where lambda denotes the mean wave length of the transmission region of interest to be smoothed, and Alpha the angle which the light ray in the equalizing layer forms with the mean wave length; and wherein the index of refraction (n) of the equalizing layer is larger than the index of refraction of the low refractivity outer layer adjacent to said equalizing layer.