Suppression of the Self-Radiation Stray Light of Long-Wave Thermal Infrared Imaging Spectrometers

Abstract

For a thermal infrared hyperspectral imaging system, there are characteristics of low signal energy and severe self-thermal radiation. The thermal radiation will influence its imaging quality so much so that it cannot work properly. Therefore, effective methods to suppress self-thermal radiation stray light must be adopted. Based on Offner relay, a long-wave infrared imaging spectrometer with wavelength range of 8–12.5 μm and F number of 2.7 is designed and analyzed. Through setting up its opto-mechanical model, two methods to suppress its self-thermal radiation are introduced and compared. The first method uses a relay lens behind the Offner spectroscopic imaging system to form a real exit pupil, which is matched with the cold stop of focal plane array detector. As a result, its cold shield efficiency is up to 100%. And, the self-thermal radiation outside its field of view can be suppressed. For the second method, its slit and opto-mechanical parts after the slit are all cooled to reduce the background temperature and to lower the self-thermal radiation. Comparing the analysis results of both methods, the effectiveness with the whole cooling opto-mechanical parts to suppress the thermal radiation is much better than the first method. When the temperature of Dewar is 50 K in the first method, the magnitude of self-thermal radiation irradiance at image plane is two orders of magnitude higher than that of signal energy under normal temperature. Therefore, the signal is completely submerged in self-thermal radiation stray light. When the cooling temperature of the whole opto-mechanical parts is 90 K in the second method, self-thermal radiation irradiance at image plane is only 3% of that of signal energy. Therefore, the whole cooling opto-mechanical parts to suppress the self-thermal radiation is much better for the thermal infrared hyperspectral imager.