Pyroelectric crystals have a rare asymmetry due to their single polar axis. This causes their polarisation to change with temperature. This so-called pyroelectric effect is used in sensor technology. For this, a thin pyroelectric crystal is coated perpendicular to the polar axis with electrodes. On the upper electrode of the crystal, an absorbing layer (black layer) is applied. When this layer interacts with infrared radiation, the pyroelectric layer heats up and surface charge arises. If the radiation is switched off, a charge of the opposite polarity originates. However, the charge is very low. Before the finite internal resistance of the crystal can equalise the charges, extremely low-noise and low leakage current field-effect transistors (JFET) or operational amplifier (OpAmp) convert the charges into a signal voltage. Thermopiles, too, belong to the group of thermal detectors, however, the measuring effect is less significant. While pyroelectric infrared detectors show a good signal/noise ratio up to modulation frequencies of 4 kHz, e.g. in FTIR spectrometers, thermopiles produce good results up to modulation frequencies of specific Hertz only.