Thermal imagers have radiation detectors that are

Chapter 12, Problem 12.29

(choose chapter or problem)

Thermal imagers have radiation detectors that are sen-sitive to a spectral region and provide white- black orcolor images with shading to indicate relative tempera-ture differences in the scene. The imagers, which haveappearances similar to a video camcorder, have numer-ous applications, such as for equipment maintenance toidentify overheated motors or electrical transformersand for fire-fighting service to determine the directionof fire spread and to aid search and rescue for victims.The most common operating spectral regions are 3 to 5?m and 8 to 14?m. The selection of a particularregion typically depends on the temperature of thescene, although the atmospheric conditions (watervapor, smoke, etc.) may also be important.(a) Determine the band emission fractions for each ofthe spectral regions, 3 to 5?m and 8 to 14?m, fortemperatures of 300 and 900 K.(b) Using the Tools/Radiation/Band Emission Factorfeature within IHT, calculate and plot the bandemission factors for each of the spectral regionsfor the temperature range 300 to 1000 K. Identifythe temperatures at which the fractions are a maxi-mum. What conclusions can you draw from thisgraph concerning the choice of an imager for anapplication?(c) The noise-equivalent temperature (NET) is aspecification of the imager that indicates the mini-mum temperature change that can be resolved in the image scene. Consider imagers operating at the maximum-fraction temperatures identified inpart (b). For each of these conditions, determinethe sensitivity (%) required of the radiation detec-tor in order to provide a NET of 5C. Explain thesignificance of your results. Note:The sensitivity(% units) can be defined as the difference in the band emission fractions for two temperatures dif-fering by the NET, divided by the band emissionfraction at one of the temperatures.