Originally Posted by
Eddie
While a given object may have a high contrast in visible light, it may have no contrast at all in IIR, or vice-versa. This effect is especially common in the desert at sunrise and sunset where the temperature of the terrain and the temperature of vehicles crosses for a short period, rendering IIR missiles useless. Equally if it's really hot, the terrain and vehicles can be totally without contrast during the day in the high sun hours.
You get the exact same issue with CCD sensors at night, of course, and also if the object is well camouflaged you can find that there is insufficient contrast for the sensor to track it.