Temperature Measurement Theory and Application
and Presenter: John Merchant, Sales Manager, Mikron Instrument
Infrared thermometers for non-contact temperature measurement
are highly developed sensors which have wide-spread application
in industrial processing and research. This paper describes, in
non-mathematical terms, the theory upon which the measurement
technology is based, and how this is used to deal with the variety
of application parameters which confront the intending user.
An infrared thermometer measures temperature by detecting the
infrared energy emitted by all materials which are at temperatures
above absolute zero, (0°Kelvin). The most basic design consists
of a lens to focus the infrared (IR) energy on to a detector,
which converts the energy to an electrical signal that can be
displayed in units of temperature after being compensated for
ambient temperature variation. This configuration facilitates
temperature measurement from a distance without contact with the
object to be measured. As such, the infrared thermometer is useful
for measuring temperature under circumstances where thermocouples
or other probe type sensors cannot be used or do not produce accurate
data for a variety of reasons. Some typical circumstances are
where the object to be measured is moving; where the object is
surrounded by an EM field, as in induction heating; where the
object is contained in a vacuum or other controlled atmosphere;
or in applications where a fast response is required.
for an infrared thermometer (IRT), have existed since at least
the late nineteenth century, and various concepts by Fˇry were
featured by Charles A. Darling (1) in his book "Pyrometry," published
in 1911. However it was not until the 1930's that the technology
was available to turn these concepts into practical measuring
instruments. Since that time there has been considerable evolution
in the design and a large amount of measurement and application
expertise has accrued. At the present time, the technique is well
accepted and is widely used in industry and in research.
previously stated IR energy is emitted by all materials above
0°K. Infrared radiation is part of the Electromagnetic Spectrum
and occupies frequencies between visible light and radio waves.
The IR part of the spectrum spans wavelengths from 0.7 micrometers
to 1000 micrometers (microns). Figure 1. Within this wave band,
only frequencies of 0.7 microns to 20 microns are used for practical,
everyday temperature measurement. This is because the IR detectors
currently available to industry are not sensitive enough to detect
the very small amounts of energy available at wavelengths beyond
IR radiation is not visible to the human eye, it is helpful to
imagine it as being visible when dealing with the principles of
measurement and when considering applications, because in many
respects it behaves in the same way as visible light. IR energy
travels in straight lines from the source and can be reflected
and absorbed by material surfaces in its path. In the case of
most solid objects which are opaque to the human eye, part of
the IR energy striking the object's surface will be absorbed and
part will be reflected. Of the energy absorbed by the object,
a proportion will be re-emitted and part will be reflected internally.
This will also apply to materials which are transparent to the
eye, such as glass, gases and thin, clear plastics, but in addition,
some of the IR energy will also pass through the object. The foregoing
is illustrated in Figure 2. These phenomena collectively contribute
to what is referred to as the Emissivity of the object or material.
which do not reflect or transmit any IR energy are know as Blackbodies
and are not known to exist naturally. However, for the purpose
of theoretical calculation, a true blackbody is given a value
of 1.0. The closest approximation to a blackbody emissivity of
1.0, which can be achieved in real life is an IR opaque, spherical
cavity with a small tubular entry as shown in Figure 3. The inner
surface of such a sphere will have an emissivity of 0.998.
kinds of materials and gases have different emissivities, and
will therefore emit IR at different intensities for a given temperature.
The emissivity of a material or gas is a function of its molecular
structure and surface characteristics. It is not generally a function
of color unless the source of the color is a radically different
substance to the main body of material. A practical example of
this is metallic paints which incorporate significant amounts
of aluminum. Most paints have the same emissivity irrespective
of color, but aluminum has a very different emissivity which will
therefore modify the emissivity of metallized paints.
as is the case with visible light, the more highly polished some
surfaces are, the more IR energy the surface will reflect. The
surface characteristics of a material will therefore also influence
its emissivity. In temperature measurement this is most significant
in the case of infrared opaque materials which have an inherently
low emissivity. Thus a highly polished piece of stainless steel
will have a much lower emissivity than the same piece with a rough,
machined surface. This is because the grooves created by the machining
prevent as much of the IR energy from being reflected. In addition
to molecular structure and surface condition, a third factor affecting
the apparent emissivity of a material or gas is the wavelength
sensitivity of the sensor, known as the sensor's spectral response.
As stated earlier, only IR wavelengths between 0.7 microns and
20 microns are used for practical temperature measurement. Within
this overall band, individual sensors may operate in only a narrow
part of the band, such as 0.78 to 1.06, or 4.8 to 5.2 microns,
for reasons which will be explained later.
Basis for IR Temperature Measurement
The formulas upon which infrared temperature measurement is based
are old, established and well proven. It is unlikely that most
IRT users will need to make use of the formulas, but a knowledge
of them will provide an appreciation of the interdependency of
certain variables, and serve to clarify the foregoing text. The
important formulas are as follows:
Kirchoff's Law When an object is at thermal equilibrium, the amount
of absorption will equal the amount of emission.
Stephan Boltzmann Law The hotter an object becomes the more infrared
energy it emits.
Wien's Displacement Law The wavelength at which the maximum amount
of energy is emitted becomes shorter as the temperature increases.
Planck's Equation Describes the relationship between spectral
emissivity, temperature and radiant energy.
Thermometer Design and Construction
basic infrared thermometer (IRT) design, comprises a lens to collect
the energy emitted by the target; a detector to convert the energy
to an electrical signal; an emissivity adjustment to match the
IRT calibration to the emitting characteristics of the object
being measured; and an ambient temperature compensation circuit
to ensure that temperature variations within the IRT, due to ambient
changes, are not transferred to the final output. For many years,
the majority of commercially available IRT's followed this concept.
They were extremely limited in application, and in retrospect
did not measure satisfactorily in most circumstances, though they
were very durable and were adequate for the standards of the time.
Such a concept is illustrated in Figure 4.
modern IRT is founded on this concept, but is more technologically
sophisticated to widen the scope of its application. The major
differences are found in the use of a greater variety of detectors;
selective filtering of the IR signal; linearization and amplification
of the detector output; and provision of standard, final outputs
such as 4-20mA, 0-10Vdc, etc. Figure 5 shows a schematic representation
of a typical contemporary IRT. Probably the most important advance
in infrared thermometry has been the introduction of selective
filtering of the incoming IR signal, which has been made possible
by the availability of more sensitive detectors and more stable
signal amplifiers. Whereas the early IRT's required a broad spectral
band of IR to obtain a workable detector output, modern IRT's
routinely have spectral responses of only 1 micron. The need to
have selected and narrow spectral responses arises because it
is often necessary to either see through some form of atmospheric
or other interference in the sight path, or in fact to obtain
a measurement of a gas or other substance which is transparent
to a broad band of IR energy.
common examples of selective spectral responses are 8-14 microns,
which avoids interference from atmospheric moisture over long
path measurements; 7.9 microns which is used for the measurement
of some thin film plastics; and 3.86 microns which avoids interference
from CO2 and H2O vapor in flames and combustion gases. The choice
between a shorter, or longer wavelength spectral response is also
dictated by the temperature range because, as Planck's Equation
shows, the peak energy shifts towards shorter wavelengths as the
temperature increases. The graph in Figure 6 illustrates this
phenomenon. Applications which do not demand selective filtering
for the above stated reasons may often benefit from a narrow spectral
response as close to 0.7 microns as possible. This is because
the effective emissivity of a material is highest at shorter wavelengths
and the accuracy of sensors with narrow spectral responses is
less affected by changes in target surface emissivity.
will be apparent from the foregoing information that emissivity
is a very important factor in infrared temperature measurement.
Unless the emissivity of the material being measured is known,
and incorporated into the measurement, it is unlikely that accurate
data will be obtained. There are two methods for obtaining the
emissivity of a material:
by referring to published tables and b) by comparing the IRT measurement
with a simultaneous measurement obtained by a thermocouple or
resistance thermometer and adjusting the emissivity setting until
the IRT reads the same. Fortunately, the published data available
from the IRT manufacturers and some research organizations is
extensive, so it is seldom necessary to experiment. As a rule
of thumb, most opaque, non-metallic materials have a high and
stable emissivity in the 0.85 to 9.0 range; and most un-oxidized,
metallic materials have a low to medium emissivity from 0.2 to
0.5, with the exception of gold, silver and aluminum which have
emissivities in the order of 0.02 to 0.04 and are, as a result,
very difficult to measure with an IRT. While it is almost always
possible to establish the emissivity of the basic material being
measured, a complication arises in the case of materials which
have emissivities that change with temperature such as most metals,
and other materials such as silicon and high purity, single crystal
ceramics. Some applications which exhibit this phenomena can be
solved using the two color, ratio method.
Given that emissivity plays such a vital role in obtaining accurate
temperature data from infrared thermometers, it is not surprising
that attempts have been made to design sensors which would measure
independently of this variable. The best known and most commonly
applied of these designs is the Two Color-Ratio Thermometer. This
technique is not dissimilar to the infrared thermometers described
so far, but measures the ratio of infrared energy emitted from
the material at two wavelengths, rather than the absolute energy
at one wavelength or wave band. The use of the word "color" in
this context is somewhat outdated, but nevertheless has not been
superseded. It originates in the old practice of relating visible
color to temperature, hence "color temperature."
basis for the effectiveness of two-color thermometry is that any
changes in either the emitting property of the material surface
being measured, or in the sight path between the sensor and the
material, will be "seen" identically by the two detectors, and
thus the ratio and therefore the sensor output will not change
as a result. Figure 7 shows a schematic representation of a simplified
the ratio method will, under prescribed circumstances, avoid inaccuracies
resulting from changing or unknown emissivity, obscuration in
the sight path and the measurement of objects which do not fill
the field of view, it is very useful for solving some difficult
application problems. Among these are the rapid induction heating
of metals, cement kiln burning zone temperature and measurements
through windows which become progressively obscured, such as vacuum
melting of metals. It should be noted however, that these dynamic
changes must be "seen" identically by the sensor at the two wavelengths
used for the ratio, and this is not always the case. The emissivity
of all materials does not change equally at two different wavelengths.
Those materials that do are called "Greybodies." The ones that
do not are called "Non-Greybodies." Not all forms of sight path
obscuration attenuate the ratio wavelengths equally either. The
predominance of particulates in the sight path which are the same
micron size as one of the wavelengths being used will obviously
unbalance the ratio. Phenomena which are non-dynamic in nature,
such as the "non-greybodyness" of a material, can be dealt with
by biassing the ratio, an adjustment referred to as "Slope." However,
the appropriate slope setting must generally be arrived at experimentally.
Despite these limitations, the ratio method works well in a number
of well established applications, and in others is the best, if
not the most preferred solution.
Infrared thermometry is a mature but dynamic technology that has
gained the respect of many industries and institutions. It is
an indispensable technique for many temperature measurement applications,
and the preferred method for some others. When the technology
is adequately understood by the user, and all the relevant application
parameters are properly considered, a successful application will
usually result, providing the equipment is carefully installed.
Careful installation means ensuring that the sensor is operated
within its specified environmental limits, and that adequate measures
are taken to keep the optics clean and free from obstructions.
A factor in the selection process, when choosing a manufacturer,
should be the availability of protective and installation accessories,
and also the extent to which these accessories allow rapid removal
and replacement of the sensor for maintenance. If these guidelines
are followed, the modern infrared thermometer will operate more
reliably than thermocouples or resistance thermometers in many
1. Darling, Charles R.; "Pyrometry. A Practical Treatise on the
Measurement of High Temperatures." Published by E.&F.N. Spon Ltd.
Reproduced with permission of MIKRON INSTRUMENT COMPANY, INC.