Optical Pyrometers and Emissivity
The Short Version
Optical pyrometers are calibrated in laboratories using blackbodies. Blackbody furnaces have an emissivity as close to 1.00 as possible. Since targets other than blackbodies have lower emissivities, readings will not be absolutely correct unless emissivity is accounted for. Temperature readings taken with short wavelength pyrometers such as disappearing filament optical pyrometers generally have smaller emissivity related errors.
Is It Important
Outside of the laboratory, most operators pay little attention to emissivity.
Actually, in most industrial operations, operators will set the emissivity adjustment
on their optical pyrometers to 1.00 and develop history and practice in their process
as a way to control quality. If you think about it, from about 1917 to recently, the
most popular optical pyrometers in the world were the Leeds and Northrup disappearing
filament optical pyrometers, which normally had no emissivity adjustment on them at all.
The only exception was an optional single adjustment for an emissivity of 0.40 used for
molten ferrous-based metals. Of course, the Leeds and Northrup optical pyrometers operated
at a short wavelength of 0.65 microns. Emissivity is still an issue at short wavelengths
like 0.65 microns, but much less so than at longer wavelengths like 1.0 microns. Temperature
measurement errors due to emissivity will be far greater at 1.0 microns where many infrared
thermometers operate. Perhaps the greatest danger then, is to switch to a pyrometer of a
different wavelength; especially after years of history and data are in place developed with
the other pyrometer. That is, emissivity is wavelength dependent; pyrometers that work at
different wavelength see different emissivities of the same target. That means, the operator
would need to select different emissivities for pyrometers that operate at different wavelengths.
A major issue with adjustable emissivity is that few people can agree on what the correct
emissivity of a material is. In fact, the emissivity of a material is different at different
temperatures, wavelengths and surface conditions. Left to their own devices, operators can
therefore get the temperature reading they want by using their own interpretation and using
the emissivity setting that he or she thinks is correct. It is no wonder then, that the most
repeatable results come from either using an emissivity of 1.0 or having everyone in the
facility using an agreed upon emissivity.
Different targets at identical temperatures may exhibit different brightnesses. The
property that describes the brightness of a target at a given temperature is called the
emissivity of the target. Targets with high values of emissivity will appear brighter
than targets at the same temperature but with low emissivity values. Emissivity values
are assigned relative to the theoretically perfect emitter—the so-called blackbody emitter.
[The perfect emitter is called a blackbody because a perfect emitter must also be a perfect
absorber and it would appear black at room temperature.] The perfect emitter is assigned the
emissivity value of unity [1.00]. All real targets have emissivity values less than 1.00
although this value may be approached quite closely by a small hole in a large isothermal
cavity—the so-called blackbody furnace.
If an emissivity of 1.00 is chosen to calculate the temperature of a target from the
filament current, then the temperature displayed is the brightness temperature. Unless the
emissivity of the target approaches 1.00, the true target temperature will exceed the brightness
temperature. To compensate for deviations of target emissivity from unity, other choices
are made available within the DFP2000 pyrometer software. For instance, molten iron often
has an emissivity near 0.40 in value. If the instrument will be used to measure molten iron
temperatures and true temperature [not brightness temperature] is the quantity to be reported,
then the value of 0.40 is frequently selected for emissivity