An Ultrathin Foil Neutral Density Filter Set for the Infrared

A solution to the need for OD 3 to OD 10 neutral density filters in the infrared (2 to 25 um) with flat wavelength dependence is suggested using submicron thickness polymer films as substrates for multi-layer or co-deposited metallic alloy coatings. Initial studies used Lexan and although they showed feasibility, a stronger and more temperature resistant ultrathin substrate such as polyimide film on the order of 100 nm in thickness will be investigated. Computer modeling based on the matrix method of Abeles as described by Heavens will be used to suggest candidate substrate and/or multilayer coating systems. Novel instrumentation will be used to minimize stresses in the coatings that would adversely affect resulting filter performance. Prototype filters based on the best combination of computer modeled performance and technical tractability will be fabricated and delivered for testing at NIST. The successful outcome will provide reference standards for IR calibration in the photonics industry as well as potential thin foil products for other IR applications, such as detector windows for thermal imaging devices.