All objects with a temperature above absolute zeroemit heat energy in the form of
radiation. Usually this radiation isn't visible to the human eyebecause it radiates at infrared
wavelengths, but it can be detected by electronic devices designed for such a purpose.
The term passive in this instance refers to the fact that PIR devices do not
generate or radiate energy for detection purposes. They work entirely by detecting infrared
radiation emitted by or reflected from objects. They do not detect or measure "heat".
