Page 462 - The Mechatronics Handbook

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series for average temperature, while the current devices are connected in series for minimum and parallel
for average. In addition to such simple applications of constant current or voltage sources based upon
temperature, there are a wide variety of novel circuits to derive almost any function imaginable as a basis
of temperature measurement.
Although there is a very small area on the silicon chip of the IC, which is temperature sensitive, it is
convenient to regard the entire chip, its case, and the bonded lead wires as the sensor. This increase in
thermal mass lowers the time response of the device to several seconds. Self-heating and heat transfer
through the leads are also of concern and limit the applicability of these devices in critical measurements.

Noncontact Thermometers
All of the previously discussed temperature monitoring systems implied that the sensor of whatever type
is in physical contact with the object being monitored, or in some special cases is the actual object being
measured. Often times it is impractical to make this physical connection and noncontact modes of
temperature measurement have been developed to overcome this objection. Almost all of these techniques
require that the infrared emissions from the surface of the object be measured, but in a few special cases
other surface optical properties such as reflectance can be exploited to determine the temperature
remotely.

IR Emission Thermometers
Any object above absolute zero emits electromagnetic radiation whose spectrum is related to its surface
temperature and surface emissivity. By characterizing the spectrum, the temperature of the object can be
determined directly and absolutely. The microwave background of the universe at 3 K, and the temperature-
dependent color of stars are extreme examples of this phenomena. Temperature can still be determined
from the emitted surface without using a spectrometer. If two bodies are allowed to come into thermal
equilibrium with each other and the temperature of one body is known, the temperature of the other is
also known. This is the basis of all previously discussed temperature-measuring devices assuming conduc-
tion as the principle means of heat transfer. This can be extended to noncontact thermometers since
radiation heat transfer is also a valid means of two bodies coming into thermal equilibrium. Many IR
thermometers are based upon this phenomenon.
In its simplest form, an IR thermometer would consist of a temperature sensor for monitoring the
temperature of an isolated object called the detector, and this detector would only be subject to radiative
heat transfer with the surface whose temperature is to be measured. This would work assuming that both
the surface and detector behave as black bodies, that there is no heat loss from the detector to the
surroundings, and that the field of view of the detector is restricted to the object under measurement
and otherwise totally unobstructed. Each one of these assumptions has to be considered when going
from the ideal case to a real IR thermometer.
The concept of a black body is an idealization where all radiant energy is completely absorbed by the
surface. Under this assumption, the radiant energy is a function only of the temperature of the surface.
The only alternatives to being absorbed by the surface are to be reflected by the surface or transmitted
through the material. The emissivity, which describes the deviation of a real surface from a black body,
is then just one minus its reflectance minus its transmittance. If the emissivity is less than one but
independent of wavelength, then it is a gray body. Few real materials are either black bodies or gray
bodies, thus emissivity corrections must be made which will often be a function of the temperature being
measured. If the surface behaves like a gray body over a limited range of wavelengths, the intensity at a
few wavelengths in this range can be measured to estimate the entire spectral shape. It is fortunate that
many real objects are close to gray over a narrow range of wavelengths around 750 nm and the spectral
shape as a function of temperature for gray objects in the temperature range of 500–3000°C is well
enough behaved at these wavelengths that the spectral shape, and thus the temperature, can be measured
with just two points. The emissivity of a surface can be determined in conjunction with taking its

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