Thermal considerations in the design of a long focal length, low f-number, long wave infrared imager

Sensors operating in the 8-12 micron long wave infrared (LWIR) portion of the electromagnetic spectrum have long been used to extend the useful range of operating conditions beyond those of sensor systems operating in the visual spectral band. Infrared systems must cover widely varying fields-of-view (FOV) depending on application, at fast f/numbers compared to systems operating in the visible band. Typical FOVs for LWIR sensors run the gamut from < 1 degree to >50 degrees for large focal planes, necessitating the use of long focal lengths. When the focal length of the optics increases, the sensitivity to defocus caused by thermal effects also increases. Optical materials with useful transmission in the infrared region exhibit larger changes (> 400X) in refractive index with temperature (dN/dT) than optical glass. This in turn introduces larger changes in focus over temperature for infrared systems compared to comparable focal length visual systems. Thermal expansion and contraction of the materials also contribute to changes in system performance and the coefficient of thermal expansion (CTE) is generally larger for infrared materials than for visual band optical glasses. The thermal performance problem is exacerbated with low f-numbers systems. The ability to detect targets having a small temperature difference from ambient is proportional to the light collecting ability of the optics, especially when uncooled detectors are used. It is typical to require f-numbers in the f/1 regime for the LWIR for uncooled applications. Methods have been developed to find optical designs with reduced thermal sensitivity for these applications.