Spectral Emissivity & Emittance

Measurement of spectral emissivity from 2 micrometers to 15 micrometers
Charles D. Reid and E. D. McAlister
JOSA, Vol. 49, Issue 1, pp. 78- (1959)

Citation
C. D. Reid and E. D. McAlister, “Measurement of spectral emissivity from 2 micrometers to 15 micrometers,” J. Opt. Soc. Am. 49, 78- (1959)
http://www.opticsinfobase.org/abstract.cfm?URI=josa-49-1-78

San Diego CA, USA –Surface Optics’ ET10 measures emissivity values in two most commonly used spectral regions, 3 to 5 and 8 to 12 microns.

Its main application is to produce emissivity values for the infrared cameras.

Advanced IR cameras require the input of an emissivity value for accurate temperature calculations. The emissivity values obtained from tables can be far from real leading to large temperature uncertainties.

The ET10 can be used in the lab or in the field and on small or large objects. With the ET10 one can measure emissivity of any surface in just a few seconds.

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The National Physical Laboratory in The UK offers a spectral emissivity measurment service (downloadable brochure - PDF 284kb) through its Infrared Optical Spectroscopy group to measure the spectral emissivity as needed by customers.

They also offer related calibration measurement services on their webpage as stated below:

“Optical Properties of Materials Measurement Service offers calibrations in the areas of Spectrophotmetry (reflectance and transmittance measurements, colorimetry, measurement of appearance) and Infrared Spectrometric Measurements (reflectance and transmittance measurement)”

On another webpage they repeat much of what’s in the downloadable document above and provide both equations and an informative sketch to illustrate the various measurement parameters involved.

Additionally, they provide a list of related reference documents that support the technologies involved in the services. These are quoted below:

“CLARKE, F.J.J. Measurement of the radiometric properties of materials for building and aerospace applications. Proc. Soc. Photo-Opt. Instrum. Eng., 1980, 234, 40-47.

CLARKE, F J J, and LARKIN, J A. Measurement of total reflectance, transmittance and emissivity over the thermal IR spectrum. Infrared Physics, 1985, 25, 359-367.

CLARKE, F J J, and LARKIN, J A. Emissivity determined from hemispherical reflectance and transmittance throughout the thermal infrared spectrum. High Temp. - High Press., 1985, 17, 89-96.

CLARKE, F J J and LARKIN, J A. Improved techniques for the NPL hemispherical reflectometer. Proc. Soc. Photo-Opt. Instrum. Eng., 1988, 917, 7-14.”

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.“

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.”