|
|
| Emissivity |
The measurement of the apparent temperature of a radiating hot object using non-contact methods is usually carried out with a radiation thermometer (also known as an optical pyrometer) with traceable blackbody calibrations. However, objects in the real world usually do not emit energy as efficiently as the blackbody.
Therefore, there must be a conversion that corrects this apparent temperature to a true temperature, which is higher. This correction requires knowing the emissivity of the radiating surface.
Emissivity is defined as the fraction of power emitted by the object compared to that emitted by an ideal blackbody at the same temperature.
Pyrometers are calibrated using blackbodies. The emissivity of a laboratory blackbody is, by design, close to unity. This means that no correction is needed when using a pyrometer to read the temperature of other blackbodies. A typical hot target, however, is not a blackbody and its emissivity is less than 1, so that a correction is needed. The magnitude of this correction is illustrated in Figure 1 for pyrometers operating at short wavelengths. As you can see, the correction becomes greater as the wavelength used for measurement becomes longer.
 Figure 1. Emissivity corrections for a pyrometer operating at short wavelengths. Common longer wavelength pyrometers are much worse.
From the graph it is clear that large errors may be present unless a correction is made for surfaces with emissivities less than unity. The errors become smaller for shorter wavelengths, with a dependence that is closely proportional to wavelength so long as we are on the short side of the peak of the blackbody curve. This approximate linearity is illustrated in Figure 2.
 Figure 2. Emissivity correction vs. measurement wavelength |
|
|
