Pyroelectric detectors are thermal detectors. That is, they produce a signal in response to a change in their temperature.
Below a temperature Tc known as the Curie point, ferroelectric materials such as TGS or Lithium Tantalate, exhibit a large spontaneous electrical polarisation. If the temperature of such a material is altered, for example, by incident radiation, the polarisation changes. This change in polarisation may be observed as an electrical signal if electrodes are placed on opposite faces of a thin slice of the material to form a capacitor. When the polarisation changes, the charges induced in the electrodes can be made to produce a voltage across the slice if a the external impedance is comparatively high. The sensor will only produce an electrical output signal when the temperature changes; that is, when the level of incident radiation changes.