An IR system is said to be 100% cold stop efficient if the detector can see or record energy only from the scene. What this really means is that with 100% cold stop efficiency, the detector records energy from both the cone of light representing scene energy and from the cryogenically cold thermal baffle, known as a cold stop (remember that being cryogenically cold, there is virtually no energy emitted from the cold stop itself). We have used the example in the previous section of putting your eye figuratively at the detector and looking out toward the front of the system and asking yourself "what do you see?" If, for every pixel on your FPA, you can convince yourself that your eye sees only the solid angle representing the imaging light (scene energy) and also portions of the thermal baffles representing the cold stop, then the system is indeed 100% cold stop efficient.

Note in Fig. 12.3 we have shown a series of small stray-light baffles within the cold stop or cold shield area. Without these stray-light suppression features, there may be stray radiation paths which will cause unwanted radiation to reach the FPA.

In Fig. 12.4 we show on the left a system which is not 100% cold stop efficient and one which is 100% cold stop efficient on the right. Casually looking at these figures shows little difference. The lower sets of figures are enlargements of the areas within the dashed circles...