The detector mounts are ready to roll but the it is getting more than a bit cold outside, so here goes the theory of operation and block diagram section.
A simplified view of the long wave IR detector is shown in figure 1. The background radiation is ever present, and the detector is mounted in an isothermal block. As a result, the TO-5 can of the detector also serves to emit background radiation at the same color temperature as the ambient environment.
As such, when a human hand enters the viewing area of the IR detector assembly, the radiated energy from the hand results in an increased output voltage. Figure 2 shown below, shows what happens as the human hand is moved back and forth. Namely the output varies as the hand transcends more of less of the detectors viewing area.