Chapters 8 OPTICAL PYROMETRY

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Chapters 8 OPTICAL PYROMETRY
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try adding minus one to the denominator

Max Planck (1900)
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8.1 Historical Resume Gustav Kirchoff, in
1860, defined a black body as a surface that
neither reflects nor passes radiation 1. He
suggested that such a surface could be realized
by heating a hollow enclosure and observing the
radiation from a small times access hole (e.g. a
cylindrical hole of a depth from five to eight
times its diameter that radiates from one end
closely approximates black body). Kirchoff
defined the emissivity E of a non-black body as
the ratio of its radiant intensity to that of a
similar black body at the same.
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Henri LeChatelier introduced, in 1892, the first
practical optical pyrometer. It included an oil
lamp as the reference light source, a red glass
filter to limit the wavelength interval, and an
iris diaphragm to achieve a brightness match
between the light source and the test body.
Wilhelm Wien, in 1896, derived his law for the
distribution of energy in the emission spectrum
of a black body as
(8.1)
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Where J represents intensity of radiation
emitted by a black body at temperature T, and
wavelength ?per unit wavelength interval, per
unit time, per unit solid angle, per unit area.
Max Planck, to remedy deviations that
appeared between (8.1) and the experimental facts
at high values of ?T suggested, in 1900, the
Mathematical expression to describe the
distribution of radiation among the various
wavelengths that is, he simply added a -1 to the
denominator of Wiens equation .
(8.2)
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However , in attempting to explain the
significance of -1 in the denominator , Planck
developed the quantum theory ( where in he
postulated that electromagnetic waves can exist
only in the form of certain discrete packages or
a quanta ). Subsequently he received the Nobel
Prize for his work. The numerical values and the
units, both in the International System (SI) and
in the U.S. Customary System, for all the
quantities involved in(8.2) are given in Table
8.1. An example illustrates the use of (8.2).
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Example 1. Find the intensity of radiation
emitted by a black body at 2200oR at a wavelength
of 3µin both SI and U.S. Customary units, and
check. U. S. Customary

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That the SI answer checks the U. S. answer can
readily be seen by applying several conversion
constants .That is
In 1927 the International Temperature Scale (ITS)
was defined. Temperatures above the gold point
(then 10630C) were to be given by Wiens
equation (8.1) used in conjunction with a
disappearing filament optical pyrometer.
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In 1948 the ITS was redefined so that
temperatures above the gold point were to be
given by Plancks equation (8.2) and a
disappearing filament optical pyrometer. This
definition of the higher temperatures continues
to the present (see Chapter 4). Note that in
practice it is the spectral radiance of the test
body (at its temperature) relative to that of a
black body at the gold point that defines its
temperature on the IPTS according to (4.4).
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8.2 Principles of optical Pyrometry An optical
pyrometer 2, 3 consists basically of an
optical system and a power supply. The optical
system includes a microscope, a calibrated lamp
and a narrow band Wave filter, all arranged so
that the test body and the standard light source
can be viewed simultaneously. The power supply
provides an adjustable current to the lamp
filament (see Figures 8.1 and 8.2)
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Optical pyrometry is based on the fact that
the spectral radiance from an incandescent body
is a function of its temperature 4,5. For
black body radiation, the well-known curves of
Planks equation describe the energy distribution
as a function of temperature and wavelength
(Figure 8.3). If a non-black body is being
viewed ,however, its emissivity ,which is a
function of wavelength and temperature, must be
taken into consideration ( Figure 8.4 ). In
general, to obtain the temperature of a test
body, the intensity of its radiation at a
particular wavelength is compared with that of a
standard light source.
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Red Filter The very narrow band of
wavelengths required for the comparison just
noted is established in part of the use of a red
filter in the optical system, and in part by the
observers eye. A red filter exhibits a
sharp cutoff at ?0.63µ(where 1µ0.001mm) This
means that below about 1400?, the intensity of
radiation transmitted by a red filter is too low
to give adequate visibility of the test body and
the standard filament. On the other hand, a
red filter exhibits a high transmission for
?gt0.65µ (see Figure 8.5), so that it is the
diminishing sensitivity of the eye that provides
the necessary cutoff at the high end of the band.

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The particular wavelength that is effective in
optical pyrometry is usually taken as 0.65µ
Brightness Temperature Several steps are
involved in determining temperature by an optical
pyrometer 6, 7. First, the brightness of the
test body is matched against the brightness of
the filament of a calibrated lamp at the
effective wavelength of 0.655µ. Because the
image is nearly monochromatic red, no color
difference is seen between the lamp filament and
the test body, and thus the filament seems to
disappear against the background of the target.
Of course, matching should be recognized as a
null balance procedure.
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It is the optical equivalent of a Wheatstone
bridge balance to determine resistance
measurements or a potentiometric balance for
voltage measurements. Second, the filament
current necessary for the brightness match must
be measured. For the highest accuracy , this is
best achieved by a potentiometric measurement
across a fixed resistor in series with the lamp
filament . Third , the filament current
measurement , obtained at the match condition ,
must be translated into brightness temperature by
means of a predetermined calibration relationship
( see Sections 8.3 ) .

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This calibration is predicated on the
existence of a lamp with a stable , reproducible
characteristic with temperature and time.
Finally , brightness temperature must be
converted to actual temperature through applying
the emissivity of the test body ( see Tables 8.2,
8.3 , and 8.4 as well as the examples that follow
) .
The brightness temperature is defined as
that temperature at which a blackbody would emit
the same radiant flux as the test body.
This is the temperature as observed with an
optical pyrometer. For non-black bodies the
brightness temperature is always less than the
actual temperature. Thus according to Table 8.2 a
black body(e 1)
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would appear 1 . 1 times as bright as
carbon(e0.9) 2.3 times as bright as tungsten
(e0.43) and 3.3 times as bright as
platinum(e0.3)
when all are at the same temperature . To
say it another way , the actual temperatures of
these materials would be 2000oF for the black
body , 2016? for the carbon , 2137? for the
tungsten , and 2198? for the platinum , when all
indicate a brightness temperature of 2000? (
according to Tables 8.2 , 8.3 , and 8.4 ) .
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From Wiens law , (8.1) , the relationship
between the actual temperature (T) and the
brightness temperature (TB) can be approximated
in terms of the pyrometers effective wavelength
of radiation(?) the second radiation constant
(c2) , and the target emissivity(e)
(8.3)
If the transmission (t) of the viewing
effective emissivity (e) should be used system in
( 8.3 ) is not unity , then the in place of the
source
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Of course, if el and tl the actual
temperature will equal the brightness
temperature, since, according to (8.3), ln 10.
Equation 8.3, with (8.4) factored in, is solved
graphically in Figure 8.7, and tabularly in
Tables 8.3 and 8.4. Several examples will
illustrate the use of these graphs and tables.

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Example 2 A target brightness temperature
of 1600K is measured with an optical pyrometer
having an effective wavelength of 0.655 µ
At this wavelength the effective emittance of
the target is determined to be 0.6. Estimate and
check the true target temperature in degrees
Celsius and in degrees Fahrenheit.
Graphical Solution According to Figure
8.7a, at TB1600K, and at e 0.6,?TT-TB62?,
Thus TTB?T1662K Tabular Solution
According to Table 8.4 , at 1327? , And at e
0.6,?T62?,

Tabular Solution According to Table 8.4 , at
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Graphical Solution According to Figure 8.7a, at
TB1600K, and at e 0.6,which checks the
graphical solution. According to Tables
8.3 ,at TB2421? and at e0.6 ?T112?.
Thus T24211122533? Or T1662K

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which checks the graphical solution. Numerical
Solution For small temperature differences, by
Wiens law (8.1)

which once more provides a close check to the
graphical and tabular solutions.
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Example 2. The clean surface of liquid
nickel, when viewed through an optical pyrometer
having the conventional effective Wavelength
0.655µ yields a brightness temperature of 2600?.
Estimate and check the true nickel temperature.
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TB 2600?,
Tabular Solution According to Table 8.3, at
and with an effective emissivilty estimated to be
0.37, according to Table 8.1,
257?.
Thus T T 2857?.
According to Table 8.4 , at
TB 1427?, and e0.37
by double interpolation
143?
Thus T14271431570? 2858?
which is consistent with the Fahrenheit solution.
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Graphical Solution According to Figures 8.7b , at
TB 1700K, and at?T 142? e0.37
Thus T14271421569?2856?
which check the tabular solutions. Numerical
Solution According to (8.5),
which provides a fair check to the graphical
and tubular solutions. Since ?t is large in this
of example, the approximation (8.3) is not as
reliable as for smaller ?t.
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Brightness temperature thus depends on the
sensitivity of the eye to differences in
brightness, and on a knowledge of the mean
effective wavelength of the radiation being
viewed. It does not depend on the distance
between the test body and the optical pyrometer,
however. We must be sure that the radiation
observed is that being emitted by the test body
rather than reflected radiation, since there is
no relationship between the temperature of a
surface and the radiation it reflects.
Also, smoke or fumes between the optical
pyrometer and the target must be avoided as must
dust or other deposits on the lenses, screens ,
and lamp windows .

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Pyrometer Lamp Of all the elements in the
optical pyrometer, the pyrometer lamp is the most
important, since it provides the reference
standard for all radiance measurements. The
most stable lamps consist of a pure tungsten
filament enclosed in an evacuated glass tube.
The vacuum is necessary to minimize convection
and conduction heat transfer effects.
The tungsten is always annealed before
calibration and, once calibrated, can be used
typically for 200 hours before recalibration is
required .
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Although theoretically the optical pyrometer
has no upper temperature limit, practically, for
long term stability, the lamp filament cannot be
operated above a certain current or brightness.
This limit corresponds approximately to 1350?.
Absorbing Glass Filter Glass filters ,
which absorb some of the radiation being viewed ,
are used when temperatures higher than 1350? are
being measured in order to reduce the apparent
brightness of the test body to values that the
filament can be made to match . Thus
it is not necessary to operate the standard lamp
filament at as high a temperature as would
normally be called for by the brightness of the
test body.
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Such practice adds to the stability and life
of the filament. The absorbing glass filters are
inserted between the objective lens and the
pyrometer lamp as shown in Figures 8.1 and 8.2.
Thus a black body at a temperature above
1350? appears the same through the absorbing
glass as another black body would appear in the
same pyrometer without a screen at a temperature
below 1350?. In addition, use of the proper
glass screen allows closer color matching between
the pyrometer lamp and the attenuated source, and
this leads to more precise brightness matching.
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Black Body The emissivity of most materials
is such a variable quantity and so strongly
dependent on surface conditions 10 that it
becomes almost mandatory to sight on the test
body in a black body furnace. In lieu of
laboratory-type testing in the highly favorable
conditions of a black body furnace, one can often
approximate a black body in field-type
applications For example, if the surface
temperature of an incandescent material in a test
rig is required, a small hole can be drilled
directly into the surface for sighting purposes.
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The hole depth should be about five times its
diameter, as previously mentioned. Regardless
of the emissivity of the hole walls, the multiple
reflections inside the cavity makes the hole
radiate approximately as a black body.
Temperatures based on such techniques are almost
certain to be more valid than temperatures based
on flat surface sightings, as corrected by
estimated surface emissivities. In any case,
whenever black body conditions prevail, if the
brightness match is achieved, and if the
pyrometer is properly calibrated, then
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Hot Gas Measurements The radiation
characteristics of hot gases are not like those
of hot solids in that gases do not emit a
continuous spectrum of radiation. Instead, the
emissivity of a gas exhibits a rapid variation
with wavelength . That is gases which radiate or
emit strongly only at certain characteristic
wavelengths, correspond to absorption principles
of radiation pyrometry lines (see entirely Figure
8.8). It is clear that different from those
just de- scribed are called for in the
temperature measurement of gases , and the
appropriate literature should be consulted for
further information 11 .
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At least three types of calibration are to be
distinguished in optical pyrometry. Primary
This type of calibration is done only at the
National Bureau of Standards 2, 12. The
filament current required to balance the standard
lamp brightness against pure gold held at the
gold point temperature in a black body furnace
constitutes the basic point in the calibration.
Higher temperature points are determined by a
complex method that is detailed in NBS Monograph
41, and is based on the use of tungsten strip
lamps and sectored disks. Representative
uncertainties in achieving the IPTS by optical
pyrometry are4? at the gold point, 60? at
2000? and 40? at 4000?.
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In this type of calibration 3, 4 the
output of a primary pyrometer(i.e., one
calibrated at the NBS) is compared with that of a
secondary pyrometer (i.e., one to be calibrated)
when the pyrometers are sighted alternately on a
tungsten strip lamp operated at different
brightnesses. Such lamps are highly
reproducible sources of radiant energy and can be
calibrated with respect to brightness
temperatures from 800 to 2300?, with accuracies
only slightly less than would be obtained
according to the IPTS . Note that in this
method the source need not be a black body so
long as the pyrometers are optically similar.
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Industrial Two secondary optical pyrometers
can be intercompared periodically by sighting
them alternately on the same source. Note
that here the source need not be a black body and
the comparison pyrometers need not have primary
calibrations. The method is most useful
for indicating the stability of the pyrometers,
and thus can indicate the need for a more basic
calibration. 8.4 The Two-Color Pyrometer The
accuracy of a temperature determination by the
single-color optical pyrometer just discussed is
based on black body furnace sightings or on known
emissivities.
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A two-color pyrometer, on the other band, is
used in an attempt to avoid the need for
emissivity corrections. The principle of
operation is that energy radiated at one color
increases with temperature at a different rate
from that at another color 13, 14.
The ratio of radiances at two different
effective wavelengths is used to deduce the
temperature. The two-color temperature will
equal the actual temperature whenever the
emissivity at the two wavelengths is the same.
Unfortunately this is seldom true.
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All that can be said is that when the emissivity
does not change rapidly with wavelength, the
two-color temperature may be closer to the actual
temperature than the single-color brightness
temperature. If the emissivity change with
wavelength is large, however, the converse is
true. Kostkowski 13 of the NBS indicates that,
in any case, the two-color pyrometer is less
precise than the single-color optical pyrometer,
Typically, when both were sighted on a black
body, the optical came within 2? of the known
temperature, whereas the two-color pyrometer was
on by 30?.
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8.5 Automatic optical Pyrometer Since 1956 the
automatic optical pyrometer has dominated the
field of accurate high-temperature measurement.
In such an instrument, the pyrometer lamp
current is adjusted automatically and
continuously by detector that views alternately
the target and the pyrometer lamp. Thus a
brightness-temperature balance is achieved by the
automatic optical pyrometer which, in principle,
has considerably greater sensitivity and
precision than a manually adjusted pyrometer that
depends on the subjective judgment of the human
eye (see Figure 8.9) 3, 9, 15.
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Figures 8.9 Operational diagrams of (a)
manually-adjusted optical pyrometer, and (b)
automatic optical pyrometer (after Leeds and
Northrup).
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Accuracies obtainable with commercial
automatic optical pyrometers are on the order of
7? for 1500-2250?, and of 12? from 2250-3200?.
The operation of an automatic optical
pyrometer can best be understood by referring to
Figure 8.10. A revolving mirror alternately scans
the target and the standard lamp filament at high
speed. The optical system thus projects
alternately an image of the target and of the
lamp filament onto the photo multiplier tube.
One technique for determining brightness
temperature is to adjust the reference lamp
current until a null intensity is sensed in the
photo multiplier output current.
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Another is to maintain the constant lamp
current and determine brightness temperature by
sensing an off-balance meter reading. In
either case, a direct reading of brightness
temperature is forthcoming, and hence the
designation automatic optical pyrometer 8.