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 be measured.
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.
A second basic type of detector relies on the interaction of a photon with an electron, resulting
in an electric current. In a photomultiplier tube, the emitted electrons are accelerated and used to
create an amplified current, which is measured. Photovoltaic cells may be employed as radiation
detectors. The photovoltaic effect results from the generation of a potential across a positive–
negative junction in a semiconductor when it is subject to a flux of photons. Electron-hole pairs are
formed if the incident photon has an energy level of sufficient magnitude. This process results
in the direct conversion of radiation into electrical energy, in high sensitivity, and in a fast response
time when used as a detector. In general, the photon detectors tend to be spectrally selective, so
that the relative sensitivity of the detector tends to change with the wavelength of the measured
radiation.
Many considerations enter into the choice of a detector for radiative measurements. If time
response is important, photon detectors are significantly faster than thermopile or thermistor
detectors, and therefore have a much wider frequency response. Photodetectors saturate, while
thermopile sensors may slowly change their characteristics over time. Some instruments have
variations of sensitivity with the incident angle of the incoming radiation; this factor may be
important for solar insolation measurements. Other considerations include wavelength sensitivity,
cost, and allowable operating temperatures.