Patent ID: 8018652

Claim:
An optical filter including a substrate having a plurality of layers of materials stacked there upon, each of the layers formed from one or both of: a first material having a first index of refraction; and, a second material having a second index of refraction being less than the first index of refraction; wherein the plurality of layers of materials includes a first layer and a second layer each and a third layer formed from the first material being stacked in between the first layer and the second layer; wherein the optical thickness of each of said first and said second layers is greater than the optical thickness of said third layer; wherein the first layer and second layers are each formed from an inhomogeneous mixture of said first material and said second material, wherein the optical thickness of said first layer and of said second layer each differ in magnitude from the value 2Q by less than Q/2, and the optical thickness of said third layer differs in magnitude from the value Q by less than Q/2, where Q is the thickness of a given said layer traversed by substantially one quarter of a wavelength of optical radiation of a common reference wavelength at which said optical thicknesses are determined, and wherein all variations in the index of refraction of the first and second layer increase that index of refraction as the depth of the respective layer increases from regions thereof remote from said third layer to regions thereof proximate the third layer.