Page 305 - Instrumentation Reference Book 3E

Measurement techniques: radiation thermometers 289

Figure 14.57 Appearance of image in optical thermometer.

sivity is proportional to the wavelength of the exact bandpass of photodiodes varies somewhat
radiation used to make the measurement. For from type to type depending on the manufacturing
instance, in the case of oxidized steel at 1000°C process used, but the above figures are typical.
with an emissivity of 0.8 a total radiation therm- Normally the range of wavelengths used is
ometer will have an error in excess of 50 degrees reduced to a narrower bandpass than that
while the optical thermometer reading will be detected by the semiconductor sensor. For
within 20 degrees. However, the optical therm- instance, for general applications above 600°C a
ometer has two major drawbacks. First, it is only narrow bandpass centered on 0.9pm is usually
suitable for spot measurements and requires a used. Wherever possible silicon is to be preferred
skilled operator to use it. Second, it is not capable as it will tolerate higher ambient temperatures
of a quick response and is totally unsuitable for than germanium and in general it has the higher
control purposes. speed of response. Small P-I-N photodiodes can
Photoelectric radiation thermometers are have a frequency response up to several hundred
ideally suited to the short wavelength applica- megahertz while P-N devices more usually have a
tion. Structurally they are essentially identical response of several kilohertz. Like all other
to a total radiation thermometer except that the semiconductor devices the electrical output of
thermal sensor is replaced by a photodiode. photodiodes is temperature-dependent. It is there-
A photodiode is a semiconductor diode, which fore necessary to construct these radiation
may be either a silicon or germanium junction thermometers with thermistors or resistance therm-
diode constructed so that the incident radiation ometers in close proximity to the photodiode to
can reach the junction region of the semiconduc- provide ambient temperature compensation.
tor. In the case of germanium the diode will be a
plain P-N junction; in the case of silicon it may be
either a P-N or P-I-N junction. In service the 14.6.2.5 Choice of spectral wavelength,for
diodes are operated with a voltage applied in the spec@ applications
reverse, i.e., non-conduction, direction. Under It might seem at first sight that apart from optical
these conditions the current carriers, i.e., electrons, radiation thermometers the obvious choice
in the semiconductor do not have sufficient should be to use a total radiation thermometer
energy to cross the energy gap of the junction. so as to capture as much as possible of the radiant
However, under conditions of incident radiation emission from the target to achieve the maximum
some electrons will gain enough energy to cross output signal. However, as already mentioned
the junction. They will acquire this energy by above, except at the lowest temperature ranges,
collision with photons. The energy of photons is there are several reasons for using narrower
inversely proportional to the wavelength. The wavelength bands for measurement.
longest wavelength of photons that will, on
impact, give an electron enough energy to cross
the junction dictates the long wave end of the Effect of radiant mission against wavelength
spectral response of the device. The short wave- One reason relates to the rate at which the radiant
length end of the response band is limited by the emission increases with temperature. An inspec-
transparency of the semiconductor material. The tion of Figure 14.58 will show that the radiant
choice of germanium or silicon photodiodes is emission at 2 pm increases far more rapidly with
dictated by the temperature and therefore the temperature than it does at, say, 6 pm. The rate of
wavelength to be measured. Silicon has a response change of radiant emission with temperature is
of about 1.1 pm to 0.4 pm. The useful bandpass of always greater at shorter wavelengths. It is clear
germanium lies between 2.5pm and 1.Opm. The that the greater this rate of change the more