
406 OPTICS FOR ENGINEERS
temperature at 300 K, a typical temperature for objects in a natural scene such as a person or
animal at 310 K walking through a forest with temperatures around 300 K. The peak derivative
lies near 8 μm in the far-infrared band, although high values extend to either side by several
micrometers. For hotter objects around 500 K, the peak shifts down toward the mid-infrared
band, although again, the useful spectrum extends some distance in both directions.
Thus for natural scenes, one would prefer a far-infrared camera while for hotter objects
one would choose a mid-infrared one. Because of the overlap, decisions are often made ...