Effect of atmospheric attenuation on temperature measurements made using IR scanning systems

The atmosphere attenuates IR radiation in certain frequency bands even at distances as short as 1 m. Within the 3-5-microm range used by many IR thermographic systems, H20 and CO2 absorb a finite fraction of the source radiation. To achieve reliable quantitative IR thermography, it is necessary to correct the received signal for this attenuation. This paper develops a simple model and presents numerical calculations of the attenuation expected at a few meters distance for one typical thermographic imaging system. (The extension to other equipment could easily be done by substituting different numerical data for the detector response.) The attenuation factors due to CO2 and H20 are 6 and 8%, respectively, at a 10-m range. A wide variety of target temperature and ambient humidity conditions were examined; representative curves selected from this output are presented. Because of the importance of precise IR measurements for industrial applications, the effect of varying CO2 concentrations was also studied.