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.

The Details

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