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.

This process is independent of the wavelength of the incident radiation and hence pyroelectric sensors have a flat response over a very wide spectral range. The limiting feature on the spectral range is the window material used in the manufacture of the sensor housing. By using different windows materials it is possible to detect radiation at different frequencies.

D* is a figure of merit value which is defined as the rms signal to noise ratio in a 1 Hz bandwidth per unit rms incident radiant power per square root of the sensor area. D* may be defined in response to a black body source as D*(t,f,1) where t is the temperature of the reference black body, f is the modulation frequency in Hz and 1 represents unity bandwidth. The units for D* are cmHz^1/2W^-1.

Noise Equivalent Power (NEP) is the rms value of the incident chopped radiant power necessary to produce an rms electrical signal equal to the rms electrical noise. The rms electrical noise refers to the value calculated for unit square root bandwidth, VHz^-1/2. The units for NEP are WHz^-1/2.