Radiative energy flux can be detected in a sensor by two basic techniques. The detector is subject to
radiant energy from the source whose temperature is to be measured. The first technique involves a
thermal detector in which absorbed radiative energy elevates the detector temperature, as shown in
Figure 8.28. These thermal detectors are certainly the oldest sensors for radiation, and the first such
detector can probably be credited to Sir William Herschel, who verified the presence of infrared
radiation using a thermometer and a prism. The equilibrium temperature of the detector is a direct
measure of the amount of radiation absorbed. The resulting rise in temperature must then bemeasured.
Thermopile detectors provide a thermoelectric power resulting from a change in temperature. A
thermistor can also be used as the detector, and results in a change in resistance with temperature.