What is a pyroelectric infrared sensor?

A pyroelectric infrared (PIR) sensor is an electronic device that measures infrared (IR) light radiating from objects in its field of view. These sensors work based on the principle of pyroelectricity, which is the ability of certain materials to generate a temporary voltage when they are heated or cooled. Essentially, all objects with a temperature above absolute zero emit heat energy in the form of radiation, mostly in the infrared spectrum. When this infrared radiation strikes the pyroelectric material within the sensor, it causes a change in the material’s temperature.

This temperature change alters the spontaneous polarization of the pyroelectric crystal, resulting in a charge accumulating on the crystal’s surfaces. This charge is typically very small and requires amplification. Therefore, PIR sensors usually incorporate a field-effect transistor (FET) integrated into the sensor package to act as an impedance converter and amplifier. The sensor is designed to detect *changes* in infrared radiation rather than static levels. This makes them exceptionally well-suited for applications like motion detection, as a moving person or animal creates a fluctuating IR signature in the sensor’s view.

The sensor element itself is often housed behind a special lens, typically a Fresnel lens, which focuses the incoming infrared radiation onto the sensor element and divides the field of view into detection zones. Movement across these zones creates the necessary temperature fluctuations for the sensor to trigger. The core phenomenon enabling this is the pyroelectric effect, a property intrinsic to certain non-centrosymmetric crystalline materials.