Ageing effects in cryogenically-cooled infrared filter radiometers and their implications to the performance of space instrumentation

E Theocharous, N P Fox, G J Hawkins

The 2004 Conference on Characterization and Radiometric Calibration for Remote Sensing, Space Dynamics Laboratory, Utah State University Research Foundation, 23-26 August (2004)

Abstract

The gradual reduction in the spectral responsivity of infrared detectors cooled to 77 K in some well-defined wavelength regions has been reported. These regions coincide with the absorption bands of solid ice, so the origin of these bands was assigned to the deposition of a thin film of solid ice on the surface of the detector. In this presentation we will report the observation of drifts in the responsivity of cryogenically-cooled infrared filter radiometers which have very strong wavelength dependence.

The origin of these variations was investigated and was shown to arise due to a thin film of ice formed on the multi-layer bandpass filter used to define the spectral response of the filter radiometer. The thin layer of ice interacts with the characteristics of the filter (which itself consists of a number of thin dielectric layers) and modifies the filter spectral transmission thus modifying the response of the filter radiometer of which the filter is part of. These observations have significant implications on a number of applications in which cryogenically cooled filter radiometers are used because the authors have identified the presence of ice in all cryogenically-cooled detector dewars studied so far. These observations have significant implications on a number of applications, including space instruments, which use cooled infrared transmitting filters for Earth observation. Debris from the spacecraft engines is known to accumulate on cold surfaces of instruments carried on board. The deposition of this debris on cold filters can modify the spectral response of the instruments, which use these filters to define a spectral response. The changes introduced by these effects are spectral so the calibration of the instrument using a warm and a cold blackbody, commonly used in on-board calibrations of space instruments, will not be effective.