The main parameters of a pyroelectric detector are noise-equivalent power (NEP), responsively and frequency response. The NEP for a given source energy, signal rate of change and bandwidth express the lower limit for which the detector is useful, since signals below this limit will be below the noise level. For a typical detector for a source at a color temperature of 500K, at a frequency of 10 Hz and 1 Hz bandwidth, the NEP figure of 10*-9W is quoted for a lithium tantalite pyroelectric material.