A plurality of optically isotropic, parallel dielectric plates of uniform thickness, mutually separated by gaps having a uniform width. The plates are connected by spacers located in the gaps. The gaps are either filled with a gas or evacuated. The plate thickness is on the order of one-half of the wavelength of incident radiation traveling within the medium filling the gaps. Each plate is composed of a transparent material characterized by its index of refraction. The stack of plates should include at least fifteen plates. This abstract is provided to comply with the rules requiring an abstract, and is intended to allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 CFR §1.72(b).