Dual-band infrared optical systems require complex designs to meet their optical and thermal requirements. This complexity involves an increase in the number of elements and, typically, a greater variety of substrate materials. Many of the elements are lenses, therefore requiring the maintenance of coating thickness uniformity and durability over curved surfaces. Coating thickness uniformity can be predicted using commercially available modeling software that takes into consideration the locations of sources, monitors, substrates and any motion of the parts during deposition. The coating chamber configuration can then be optimized for thickness uniformity. Coating environmental durability is more difficult to predict, however. It is commonly accepted that there is a correlation between the coating deposition angle, the porosity of the coating and its durability. The durability of a coating deposited on flat witnesses is not necessarily representative of that of the coating deposited over the surface of a curved lens. Therefore, the deposition angle distribution must also be taken into consideration when optimizing for coating uniformity. We discuss the optimization of a coating chamber configuration for the challenging geometry of a 6” diameter lens with a 6” radius of curvature. The configuration is optimized for both thickness uniformity and deposition angle distribution. Coating uniformity and durability are measured on a monolithic lens substrate as well as on witnesses arrayed in surrogate tooling simulating the lens surface. The coating durability on the witnesses and full lens are analyzed with respect to modelled uniformity and deposition angle distributions.
Chris Piazzo, Alexis Weckel, Karen Hendrix, and Mark Kozlowski, "Spectral performance and durability of dual-band infrared antireflection coatings on 3rd Gen lens substrates," Proc. SPIE 10627, Advanced Optics for Defense Applications: UV through LWIR III, 106270I (Presented at SPIE Defense + Security: April 16, 2018; Published: 8 May 2018); https://doi.org/10.1117/12.2307042.
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