Pyrosensors

In the past pyrosensors were used as detectors for
non-contact temperature measurement. The basic principle
behind this was the fact, that a black, solid body of temperature
T emits electromagnetic radiation with a energy distribution, that
depends only on the temperature of the body. The wavelength
where the energy distribution shows a maximum, is given by
lmax (unit μm)= 2899 / T(unit K). For a human body of
temperature 37°C the emitted radiation has a maximum at
wavelength 9.4 μm.
In the last years it became very popular to use pyrosensors to
detect the presence (or more precise the movement) of a human
body. These sensors consist of two or four pyroelectric elements
in a hermetic housing with a silicon window which is transparent
in the range of 5 to 10 μm. The pyroelectric elements show
ferroelectric behaviour and hence are characterized by some
permanent electrical polarization on the surface of the elements. However, the polarization is not detectable because the surface charges are compensated by some charged ions in the environment of the surface. If infrared radiation hits the pyroelectric elements, their temperature increase by a very small amount. Due to the thermal expansion, the crystal structure has to rearrange and hence the polarization on the surface of the elements changes. This change in polarization can be detected by electrodes on top and bottom side of the pyroelectric elements. The electrical charge necessary to compensate the change in electrical polarization is detected and amplified by an internal FET. Each temperature change of the elements leads to a short spike in output signal of the FET which is used to detect the presence of a human body. The changes in polarization are very small and therefore it is necessary that the changes are fast enough in order to be detectable by the FET. As a consequence, only moving infrared emitting sources can be detected. In order to cancel false signals due to temperature change of the environment there are at least two pyroelectric elements
connected in series with antiparallel polarization. For movement sensors usually a Fresnel lens (sometimes also a mirror) is used to focus the infrared radiation onto the detecting elements of the pyrosensor.