Long-wavelength cutoff filters of a new type

Abstract

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.

Implementation of Long-Wavelength Cut-off Filters Based on Critical Angle

“…1(b). It was shown in our previous paper that there is very little polarization dependence of these curves in the cut-off and rejection regions [1]. Also indicated in this diagram is the expected transmittance of the system when the outer surfaces of the prisms are not provided with antireflection coatings.…”

“…A tolerance analysis of our original ITO-based system indicated that the performance of the designs based on this principle would be very insensitive to film thickness variations but quite sensitive to variations in the optical constants [1]. Published data showed that the optical constants of ITO layers depend critically on the doping level and on the process used for their production.…”

“…In 1999 we described a new type of long-wavelength cut-off filter that was based on thin film interference and critical angles as well as on the use of at least one material with a rapid variation of the refractive index with wavelength [1]. Examples of materials with such behavior are, for example, some transparent conducting coatings and Reststrahlen materials.…”

“…Because of the high-refractive index of the prisms, the best method for combining the two prisms is through optical contacting. A detailed description of the theory of such filters and some numerical results for devices based on indium tin oxide (ITO) and magnesium oxide layers can be found in [1,2].…”

New design of interference band-pass infrared filter

“…Wavelength selective multiple interference filters are widespread in scien− ce and technology [3][4][5]. Their operation principles are well understood and implemented in commercial software [6].…”