Optical pyrometers (whether visual or photoelectric) are
normally calibrated in terms of blackbody radiation and, when sighted on an
unenclosed or freely-radiating surface, measure an apparent or spectral
radiance temperature, i.e. the temperature of a blackbody having the same
spectral radiance. The spectral radiance temperature Tr
for a wavelength λ is related to the true
temperature T of the radiating body by the equation (1/T)
− (1Tr) = (λ log
ε(λ))/c2
where ε(λ) is the spectral
emittance of the body and c2 is the second radiation constant
in the Planck equation, equal to 0.014 388 mK.
The emissivity of a material is a function of its
surface shape and texture, its temperature and the wavelength. The figures in
the table below refer to a smooth, polished surface and are given only as a
guide to the values that might be encountered in practice. Much more detailed
information on metals, alloys and non-metallic solids is to be found in
Touloukian and De Witt (1972).
Material |
Emissivity |
T/K |
|
Wavelength |
|
0.65 μm |
1.0 μm |
5.0 μm |
|
Aluminium . . . . . . . . . . . . . . . . |
(0.1 at 2 μm) |
0.05 |
500 |
|
Aluminium
oxide . . . . . . . . . . . . . . |
0.1 |
0.06 |
0.39 |
1 200 |
|
(recrystallized
alumina) . . . . . . . . . . . |
0.15 |
0.07 |
0.43 |
1 400 |
|
|
0.25 |
0.1 |
0.46 |
1 600 |
|
|
0.4 |
0.2 |
0.6 |
1 800 |
|
Beryllia† . . . . . . . . . . . . . . . . |
0.5 |
0.35 |
0.8 |
1 100 |
|
Chromium . . . . . . . . . . . . . . . . |
0.35 |
— |
— |
1 550 |
|
Cobalt . . . . . . . . . . . . . . . . |
0.35 |
0.25 |
— |
1 300 |
|
Gold . . . . . . . . . . . . . . . . |
0.15 |
0.05 |
0.03 |
1 000 |
|
Hafnium . . . . . . . . . . . . . . . . |
0.45 |
— |
— |
1 200 |
|
Iridium . . . . . . . . . . . . . . . . |
0.3 |
0.23 |
0.1 |
1 500 |
|
Iron . . . . . . . . . . . . . . . . |
0.35 |
0.3 |
0.15 |
1 400 |
|
Stainless steel
. . . . . . . . . . . . . .
|
0.33 |
0.3 |
0.2 |
1 200 |
|
,, ,, (oxidized) . . . . . . . . . . |
0.8 |
0.8 |
0.7 |
1 200 |
|
Kanthal A
(oxidized) . . . . . . . . . . . |
0.85 |
0.85 |
0.75 |
1 300 |
|
Molybdenum . . . . . . . . . . . . . |
0.4 |
0.3 |
0.15 |
2 000 |
|
Magnesia†. . . . . . . . . . . . . . . . |
0.25 |
0.2 |
0.37 |
1 400 |
|
Nickel . . . . . . . . . . . . . . . . |
0.45 |
0.35 |
0.15 |
1 100 |
|
Nickel
(oxidized) . . . . . . . . . . . . . |
0.88 |
0.84 |
0.75 |
1 300 |
|
Nickel/20%
chromium
. . . . . . . . . . . |
0.4 |
— |
— |
1 200 |
|
,, ,, (oxidized) . . . . . . . . |
0.9 |
(0.85) |
(0.8) |
1 200 |
|
Niobium . . . . . . . . . . . . . . . |
0.4 |
0.32 |
0.2 |
2 000 |
|
Osmium . . . . . . . . . . . . . . . |
0.43 |
— |
— |
1 500 |
|
Palladium
. . . . . . . . . . . . . . . |
0.35 |
— |
— |
1 500 |
|
Platinum . . . . . . . . . . . . . . . |
0.35 |
0.25 |
0.08 |
1 200 |
|
Platinum 13%
rhodium . . . . . . . . . . . |
0.28 |
— |
— |
1 100 |
|
Rhenium . . . . . . . . . . . . . . . |
0.4 |
0.35 |
0.2 |
1 400 |
|
Rhodium . . . . . . . . . . . . . . . |
0.2 |
— |
— |
1 400 |
|
Ruthenium . . . . . . . . . . . . . . . |
0.34 |
— |
— |
1 500 |
|
Silicon . . . . . . . . . . . . . . . . |
0.4 |
0.25 |
0.25 |
1 500 |
|
Silver . . . . . . . . . . . . . . . . |
0.05 |
0.05 |
0.05 |
1 000 |
|
Tantalum . . . . . . . . . . . . . . . |
0.41 |
0.3 |
0.18 |
2 200 |
|
Thoria† . . . . . . . . . . . . . . . |
0.4 |
0.35 |
0.37 |
1 400 |
|
Titanium . . . . . . . . . . . . . . . |
0.6 |
0.3 |
0.18 |
1 000 |
|
Tungsten . . . . . . . . . . . . . . . |
0.43 |
0.38 |
0.12 |
2 400 |
|
Uranium . . . . . . . . . . . . . . . |
0.3 |
— |
— |
1 100 |
|
Vanadium . . . . . . . . . . . . . . . |
— |
0.65 |
0.28 |
800 |
|
Zirconium . . . . . . . . . . . . . . . |
0.5 |
0.45 |
0.3 |
1 600 |
|
Zirconia† . . . . . . . . . . . . . . . |
0.4 |
0.2 |
0.45 |
1 400 |
|
|
|
|
|
|
Y. S. Touloukian and D. De Witt (eds) (1972)
Thermophysical Properties of Matter, vols 7, 8 and 9, Plenum Press.
