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PIR sensors typically contain a plastic lens that is actually made up of multiple Fresnel lenses. These lenses are not clear like glass but are designed to interact with infrared light, bending it towards the sensor. Behind this lens cluster lie the pyroelectric crystals within a metal casing that filters out visible light, ensuring that only infrared radiation affects the sensor.
As a person moves in front of the lens, their emitted infrared heat creates 'hot spots' on the sensor, each moving with the person's motion. As these hot spots traverse across the crystals at different times, they create the necessary temperature imbalance, producing the signal detected by the FET.