Analysis on NETD of Thermal Infrared Imaging Spectrometer

In order to suppress the noise of thermal infrared imaging spectrometer and improve its detection sensitivity, relationship between noise equivalent temperature difference (NETD) and various noise factors was analyzed. Noise of the thermal infrared imaging spectrometer mainly includes the stray light noise defined as nonimaging light scattered by opto-mechanical components' surfaces and generated by nonworking orders of the grating, the background radiation noise from the opto-mechanical components, and the detector noise. According to the physical mechanism of NETD, the expression containing noise factors of NETD was deduced, and the noise analysis model of the thermal infrared imaging spectrometer was established. An Offner-type imaging spectrometer was taken as an example, whose spectral range is from 8 to 12.5 mu m, F number is 2.7, number or spatial channels is 1024, number of spectral channels is 45. And a HgCdTe detector with a D* of 4 x 10(10) cm Hz(1/2) W-1 is used. Relationship between the NETD of this Offner-type spectrometer and its main influencing factors, including cryogenic temperature and optical properties of inwall surfaces of the spectrometer's mechanical elements, was analyzed. When these parameters are in different value, the proportion of each noise and the main factor affecting NETD are different. Targeted noise suppression methods were proposed for different noise. Usually, background radiation noise is suppressed by cooling the opto-mechanical components and brighten the inwall surfaces. But stray light noise would increase when the inwall surfaces is brightened. Change of NETD with the cryogenic temperature was compared between the inwall surfaces blackened or brightened. When the cryogenic temperature was 90 K, NETD was 0.88 K in both blackened and brightened inwall surfaces case. When the cryogenic temperature was 70 K, NETD was 0.1 K in blackened inwall surfaces case, and 0.52 K in brightened inwall surfaces case. The conclusions have guiding significance for the determination of cryogenic temperature and the inwall surfaces' optical properties of thermal infrared imaging spectrometer.