Infrared filters for remote sensing of Earth

What is it about?

This paper describes the design and manufacture of the filters and antireflection coatings used in the HIRDLS instrument. The multilayer design of the filters and coatings, choice of layer materials, and the deposition techniques adopted to ensure adequate layer thickness control is discussed. The spectral assessment of the filters and coatings is carried out using a FTIR spectrometer; some measurement results are presented together with discussion of measurement accuracy and the identification and avoidance of measurement artifacts. The post-deposition processing of the filters by sawing to size, writing of an identification code onto the coatings and the environmental testing of the finished filters are also described.

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Why is it important?

The HIRDLS instrument contains 44 precision narrow-band interference filters, and 2 designs of broadband antireflection coatings applied to Germanium lenses and Zinc Selenide window components. The filters are notable for the combination of their very small size, mechanical precision, and accuracy of spectral placement. The cold focal plane array set of 22 filters operates at 65K. The filters are of bandwidths ranging from 1% to 8% and are located at wavelengths in the range 6 to 18 microns with a wavelength placement accuracy of 0.1% in some channels. The antireflection coatings are notable for both the wide spectral range over which they work, together with operation out to 18 microns without using Thorium Fluoride.

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