Long-wavelength cutoff filters of a new type

A method for the design of cutoff filters is proposed that is based on the use of light at nonnormal angles of incidence and on the use of coating materials with a large dispersion of the optical constants. The optical constants are presented of several films made from indium tin oxide--a material that satisfies this requirement. The filtering is due to absorption, critical angle effects, or both. The theory of this type of filter is described, and the calculated performance of two long-wavelength cutoff filters based on the use of indium tin oxide films is given. The angular performance and the producibility of one of these filters are examined. Some applications for these filters are considered, and the feasibility of producing similar filters for other spectral regions is discussed.