We’ve spent the past several weeks discussing heat transfer. Radiation is a special mode of transfer because that is the mode by which energy is transferred from surfaces to our imagers.
All of what we see in the thermal image is nearly always based on the amount of radiation emitted from or reflected by the surface. Most of the time, it is quite easy to distinguish strong or specular reflections because, like a mirror, they move when we move – relative to the surface.
But we are often interested in measuring the temperatures of the objects we are viewing. When we measure radiometric temperatures, we are really only carefully quantifying the radiation that makes up the thermal image. The imagers are calibrated to correlate a temperature with a certain level of radiation intensity. If a surface emitted perfectly, we could easily know its temperature based on how much radiation it emitted. The hotter it is, the more it radiates. But no surface emits perfectly!
From opaque (non-transparent) surfaces we see a combination of emitted and reflected. Because the reflected radiation is not related to the surface temperature, we must tell the imager to disregard that portion of what it sees. We do that by correcting for emissivity (E); reflection (R) then is 1-E. We must also correct for the temperature being reflected. With these two pieces of correction information, the imager’s processor can determine an accurate radiometric temperature.
The correction, however, is not as accurate as we’d like it to be when the surface has an emissivity of less than 0.6 (approximately) or when the reflected temperature is extremely different that the surface temperature. This is true of all imaging systems especially when they are used in the real world versus a laboratory where all variables can be controlled. Clearly the proviso about emissivity means you cannot measure temperatures of nearly all bare metals—a fact that is, unfortunately, not widely reported in our industry.
When you need a high level of accuracy, especially on low emissivity surfaces, and where safety allows, simply apply a small piece of electrician’s tape firmly to the surface. Set the emissivity to 0.95 and the background correction to the temperature of whatever would be reflected if the tape were a mirror. You should be able to achieve an accuracy of +/-2C or 2% of the measurement.
I would urge you to try some simple experiments in the quiet of your office or kitchen until you are comfortable making the corrections and achieving a high level of accuracy.
Try and it out and let us know how it goes in the comments section.
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John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner