SpectralEmissivity & Emittance

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Modifying a Surface To a Known Emissivity for Temperature Measurement

One of the techniques to deal with emissivity problems taught to most beginning Infrared Thermographers and many using so-called spot radiation thermometers or IR Thermometers, is to modify the surface with unknown spectral emissivity to one with a known emissivity. While much of that information seems to get lost in “How-To” books and notes, the Infraspection Institute publishes Tips of the Week on their IR/INFO website, open and free to the public.

They have graciously given us permission to reprint some of their tips, especially those that deal with handling some of problems and solutions for dealing with emissivity. Here, verbatim, is their Tip of September 29, 2003 “Modifying a Surface for Temperature Measurement”, with permission.

“Unknown emittance values are often the greatest error source when taking infrared temperature measurements. This error source can be eliminated by modifying a target with a material having a known E value.

“Some of the modifying materials that thermographers commonly use include flat-finish spray paint, PVC electrical tape, masking tape, and spray deodorants containing powder.

“Prior to modifying any surface:

  • Make sure that it is safe to contact the subject equipment.
  • Obtain permission to modify the surface from the end user.
  • Ascertain that the selected modifying material will not melt, catch fire or emit toxic fumes when heated.

“Once you have determined it is safe to modify a surface, proceed as follows:

1. Place radiometer at desired location and distance from target. Aim and focus.

2. Measure and compensate for Reflected Temperature.

3. Apply a surface modifying material having a known E value on target making certain that material is in full contact with target and there are no air pockets. Modifying material should be larger than radiometer’s spot measurement size for the chosen distance from the target.

4. Enter E value of modifying material into radiometer’s E setting.

5. Measure temperature of modifying material once it has reached thermal equilibrium with target.

6. For greater accuracy, repeat measurement three times and average the results.

“For more information on the above technique, refer to the Infraspection Institute Guideline for Measuring and Compensating for Reflected Temperature, Emittance and Transmittance available from Infraspection Institute.

Radiometric Temperature: Concepts and Solutions

A downloadable (PDF Format) “Application Note” from the Santa Barbara Infrared website. It explains the relationship between emitted thermal radiation, reflected thermal radiation, emissivity and the wavelength region used by a measuring device. It provides several informative examples with figures and graphs.

It reads in part:

“.. a 40°C blackbody in a 40°C room would require no correction. But a 40°C blackbody in a 25°C room would have a radiometric temperature of less than 40°C….
Note that this error will be wavelength dependent. …the reflected energy will be a different fraction of the total flux in the 3-5? band than in the 8-12? band.”