Changes in the position of best focus over temperature are a major source of contrast degradation in the long-wave infrared. The prime sources of this focus shift are the difference between thermal expansion coefficients of lens material and housing material, and the change in refractive index over temperature ∂n/∂T. These parameters, combined with the limited depth of focus when using lenses for uncooled detectors, can rapidly degrade performance with changing temperature. Firstorder paraxial calculations to model these changes are discussed, with a demonstration of its application to single-element imaging systems. The model is then expanded to include two-element systems where both elements are made of the same optical material, or the more general case where different materials are combined. It is shown how a chalcogenide glasses are well suited for athermalization, and how a combination of material choice and optical prescription can lead to an improved passive optical athermalization scheme, i.e. stable performance over temperature with no moving components. The limits of the used model are discussed and examples given for various focal lengths.
J. Verplancke, N. Schuster, and J. W. Franks, "Using material advances in chalcogenide glasses to improve imaging lenses in the 8-14 µm waveband," Proc. SPIE 10181, Advanced Optics for Defense Applications: UV through LWIR II, 101810E (Presented at SPIE Defense + Security: April 09, 2017; Published: 11 May 2017); https://doi.org/10.1117/12.2261995.
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