This chapter provides details on calculating signal and noise in thermal imagers. A detectivity model is used to calculate thermal imager contrast threshold function (CTFsys). CTFsys is used in the targeting task performance (TTP) metric to calculate imager resolution.
Thermal imagers sense heat energy with wavelengths between 3 and 12 μm. The 3- to 5-μm band is called midwave infrared (MWIR), and the 8- to 12-μm band is called longwave infrared (LWIR). Figure 9.1 shows typical atmospheric transmission for a 1-km horizontal path. There are three transmission windows from 3 to 4.2 μm, 4.4 to 5 μm, and 8 to 13 μm.
Everything near room temperature radiates in the infrared. The emissivity of natural objects is generally above 70%. Most manmade objects are also highly emissive. Thermal sensors derive their images from small variations in temperature and emissivity within the scene. Typically, the thermal scene is very low contrast.
Figure 9.2 shows the spectral radiant exitance from blackbodies at 300 K and 303 K. The difference between the two curves is also shown. The difference is small; however, a 3-K contrast represents good thermal imaging conditions.