Page 294 - Instrumentation Reference Book 3E

278 Temperature measurement

14.6 Measurement techniques
radiation thermometers

14.6.1 Introduction
As was mentioned in Section 14.1, thermal energy
may be transferred from one body to another by
radiation as well as by conduction. The amount
of thermal energy or heat leaving a body by A
radiation and the wavelength of that radiation
are functions of the temperature of the body.
This dependence on temperature of the char-
acteristics of radiation is used as the basis of
temperature measurement by radiation therm-
ometers. Radiation thermometers are also known
as “radiation pyrometers.”

14.6.1.1 Blackbody radiation
An ideal blackbody is one that at all temperatures
will absorb all radiation falling on it without
reflecting any whatever in the direction of inci-
dence. The absorptive power of the surface, being
the proportion of incident radiation absorbed,
will be unity. Most surfaces do not absorb all
incident radiation but reflect a portion of it. That
is, they have an absorptive power of less than
unity.
A blackbody is also a perfect radiator. It will
radiate more radiation than a body with an
absorptive power of less than unity. The emissive
power is called the “emissivity” of a surface. The
emissivity is the ratio of the radiation emitted at a Figure 14.42 (a) Blackbody radiator, (b) absorption of
given temperature compared to the radiation ray of radiation by blackbody radiator.
from a perfect blackbody at the same tempera-
ture.
The total emissivity of a body is the emissive be at a uniform temperature. To show that the
power over the whole band of thermal radiation orifice B behaves as a blackbody, consider the ray
wavelengths and is represented by E~. When only of radiation C entering the chamber through B.
a small band of wavelengths is considered the The ray will suffer many reflections on the inside
term “spectral emissivity” is used. and a subscript walls of the enclosure before it emerges at B.
is added defining the wavelength band, e.g., ~1.5 Provided the walls of the chamber are not per-
indicates the emissivity at 1.5 pm wavelength. fectly reflecting the total energy of the radiation
The emissivity of surfaces is not usually the will have been absorbed by the many reflections
same over all wavelengths of the spectrum. In before the ray can emerge. The orifice is then
general the emissivity of metals is greater at totally absorbing all radiation that enters it. It is
shorter wavelengths and the emissivity of oxides a blackbody.
and refractory materials is greater at longer wave- To show that the orifice must also radiate as a
lengths. Some materials may have a very low blackbody first consider a body in a radiant flux
emissivity at a particular wavelength band and at any single wavelength. If that body did not
higher emissivities at shorter and longer wave- radiate energy at that wavelength as fast as it
length. For instance, glass has an emissivity of absorbed it, it would rapidly get warmer than its
almost zero at 0.65 pm. environment. In practice a body will be at ther-
mal equilibrium with its surroundings so it must
Realization of a blackbody radiator A blackbody be radiating energy as it receives it.
radiator is achieved in practice by an enclosure, Therefore the emissivity E of a body must
A in Figure 14.42, having a relatively small equal its absorbance a, The orifice B which is a
orifice B from which blackbody radiation is blackbody absorber must also be a blackbody
emitted. The inside walls of the enclosure must radiator.