handbook of modern sensors

14.6 Thermal Detectors 437
Section 16.4.1 of Chapter 16) and another similar optical sensor measures ambient
temperature to calculate T .
14.6.5 Active Far-Infrared Sensors
In the active far-infrared (AFIR) sensor, a process of measuring thermal radiation
flux is different from the previously described passive (PIR) detectors. Contrary to a
PIR sensing element, whose temperature depends on both the ambient and object’s
temperatures, the AFIR sensor’s surface is actively controlled by a special circuit to
have a defined temperature T s , which, in most applications, is maintained constant
during an entire measurement process. To control the sensor’s surface temperature,
electric power P is provided by a control (or excitation) circuit (Fig. 14.28A). To
regulate T s , the circuit measures the element’s surface temperature and compares it
with an internal reference. Obviously, the incoming power maintains T s higher than
ambient. In some applications, T s may be selected higher than the highest temperature
of the object; however, in most cases, just several tenths of a degree Celsius above the
ambient is sufficient. Because the element’s temperature is above ambient, the sensing
element loses thermal energy toward its surroundings, rather than passively absorbs
it, as in a PIR detector. Part of the heat loss is in the form of a thermal conduction, part
is a thermal convection, and another part is thermal radiation. That third part is the one
which has to be measured. Unlike the conductive and convective heat transfer, which
is always directed out of the sensing element (because it is warmer than ambient), the
radiative heat transfer may go in either direction, depending on the temperature of
the object. Of course, the radiative flux is governed by the fundamental Eq. (3.138)
of Chapter 3, which is known as the Stefan–Boltzmann law.
(A)
(B)
Fig. 14.28. The AFIR element radiates thermal flux η toward its housing and absorbs flux
b from the object (A); timing diagrams for radiative flux, surface temperature, and supplied
power (B).