In-Dewar Optical Design Analysis for Cooled Hyperspectral Imaging

Abstract

Long-wave infrared (LWIR) hyperspectral imaging (HSI) systems enable the ability to probe the LWIR spectral band to identify specific materials; offering additional situational awareness compared to a passive LWIR imager. The HSI camera breaks the spectral band into hundreds of narrow sub-bands to accurately distinguish unique spectral signatures of differing materials within the field of view (FOV) of the camera. This separation of the waveband results in small amounts of signal focusing on individual pixels of the focal plane array (FPA). Due to the emissive nature of LWIR radiation, both the spectrometer and focal plane need to be cryogenically cooled, minimizing the impact of noise and self-emission from the optics. This introduces a large amount of thermal mass in the cold space which results in a long cool down time, or a larger cooler, compared to an equivalent cooled broadband LWIR system. The resulting increase in size, weight, and power (SWaP), as well as cooldown time, can have adverse impacts to the operational capability of the platform the system is mounted on. To minimize this impact, the design form of the in-dewar optics is typically an all-refractive solution, using multiple lens elements to minimize the space claim. Visible hyperspectral imagers typically use reflective solutions to maximize the SNR of the system. However, this leads to a larger amount of space claim than a refractive configuration which is why it is typically not pursued for the LWIR. The purpose of this analysis was to perform an apples-to-apples comparison between a LWIR refractive and reflective solution, which ultimately demonstrated that the reflective solution does not provide any significant benefit in the LWIR.