Thermal infrared systems often show cosmetically undesirable image anomalies which are not seen in visible optics. These effects include narcissus, scan noise, beam wander, ghosting, and shading. The effects are similar to what we generally think of as ghost images, and the resulting imagery can vary from slight brightness variations over the format to sharp bright or dark areas. While the mechanisms differ, all of these effects are due to the detector seeing more (or less) thermal energy over the field of view or through scan than dictated by the scene energy itself.

As we discussed in the section "Cold Stop Efficiency" earlier in this chapter, one of the most important methods for evaluating the properties of thermal infrared systems is to put your eye "figuratively" at the detector and look forward (into the exit pupil) and ask yourself "what do you see." This is sometimes called the detector's eye view. For an IR system with 100% cold stop efficiency you should see a solid angle containing only scene energy, and everything outside this solid angle should be cryogenically cold. If this is the case, then you will indeed be accurately recording or imaging the thermal radiance from the scene. However, if you can see any thermal energy outside the solid angle representing scene energy and inside the cold stop solid angle, this represents extra energy, which will behave in a similar fashion to stray light in visible systems. We will now review the primary...