Analysis And Measurement Of Internal Stray Radiation Of Thermal Infrared Imaging Spectrometer

Thermal infrared remote sensing instrument can detect the thermal radiation and quantitative information of the target in dozens to hundreds of different spectral bands.It can distinguish the tiny difference of various materials,work effectively all day and has many unique applications that are different from other spectrum.However,when working in room temperature,the instrument suffers from the internal stray radiation generated by the thermal infrared spectrometer,which occupies the dynamic range,limits the integral time and the signal-to-noise ratio.Moreover,the internal stray radiation varies a lot with temperature,which will seriously affects the quantification.It has become an obstacle that restricts the performance improvement of thermal infrared imaging system,which needs to be analyzed,measured and suppressed.In this paper,we introduced the current state of the thermal infrared imaging spectrometer study,and found out the internal stray radiation issue remains to be tackled due to the complex internal mechanical structure,large surface area,variety optical elements and unstable application environment.Then,the detailed optical and mechanical design of a thermal infrared imaging spectrometer based on the Offner structure with convex grating was presented.The analysis method,evaluation index,measurement and suppression method of the internal stray radiation of the thermal infrared spectrometer were studied in depth.The main results are as follows:1)For the common methods of internal stray radiation simulation,such as theoretical calculation,forward tracking simulation and reverse tracking simulation,there exist model over-simplified and simulation data insufficient issues that will cause a certain amount of error.In this paper,two new methods are proposed,i.e.,the independent component surface source method and the micro element segmentation method,in which the amount of tracing light has been greatly increased thus enhance the simulation calculation.These methods turn out to be more effectively and accurately when determine the position of the source of internal stray radiation.2)Aiming at the lacking of effective index for internal stray radiation evaluation,the equivalent blackbody coefficient of internal stray radiation was proposed.Based on the parameter settings of typical thermal infrared detector,four kinds of typical spectrometers were designed with different grating structure,i.e.,plane grating,prism grating prism,Dyson concave grating and Offner convex grating.The internal stray radiation of the four typical optical structures were evaluated with the equivalent blackbody coefficient according to the corresponding simulation model established in this paper.3)Aiming at the lacking of experimental method to measure the internal stray radiation of spectrometer,the model of internal stray radiation transmits through system to detector pixel and that of black body signal radiation transmits through cold diaphragm to detector pixel were both analyzed.On this basis,two new measurement methods were proposed,one is based on the radiometric calibration of detector and spectrometer,the another is on the calibration of thermal infrared spectrometer by temperature variation.The results at different integration time,different spectral channels of these two methods were compared respectively,and they are proved to be universal.In addition,the comparison of the gray value at different spectrometer temperatures of these two methods verified their correctness.4)The transmission process of internal stray radiation was analyzed,and the suppression methods were proposed.Different suppression measures,such as,the overall temperature control,the local temperature control,the local surface emissivity optimization,the data compensation and the narrow-band or linear gradient filter,are utilized.The suppression effect of the above methods are simulated and predicted.Moreover,the effectiveness of the local surface emissivity optimization method and data compensation method are verified by experiments.