PIR - Passive InfraRed sensors - standard burglar alarm and motion sensing light sensors - have a lens that performs like a flies eye. smooth outside. rows of Fresnel ( Frah - nell ) lenses inside. when an IR heat source moves past, the lenses generate a series of pulses. enough pulses in a short interval causes the sensor to flip a switch momentarily. if they did not incorporate the lenses and pulse counter, the sensor would trip continuously when it saw your neighbors car radiator
PIRs are useless at this time of year in the American southwest. the outside air temperature is body heat temperature. moving past the lenses does not generate pulses
if the lens is yellowed and crumbling, it can't transfer enough IR. the sensor is electronically active but optically inert.