Emissivity refers to an object’s ability to radiate infrared (IR) energy. Because infrared instruments measure radiant energy, it’s imperative for a thermographer to understand Emissivity and how it can vary. All objects above 0 Kelvin radiate IR energy. The amount of energy that is radiated is dependent upon an object’s temperature and emittance. Increases in temperature and/or emittance will increase the amount of infrared energy radiated.
Although many equate emissivity to values published in emittance tables, Emissivity (E) is a dynamic characteristic that’s influenced by several factors. Among them:
♦ Wavelength. For most objects, Emissivity varies with wavelength.
♦ Object Temperature. Changes in object temperature cause changes in Emissivity
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- For clean metals, E increases with temperature rise.
- For dielectrics, E decreases with temperature rise.
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♦ Viewing Angle. Imaging at angles other than perpendicular causes changes in Emissivity.
♦ Target Geometry. Target shape affects Emissivity. Compared to a flat surface,
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- Concave shape increases E.
- Convex shape decreases E.
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♦ Surface Condition. Surface roughness, texture, or condition (dirt, oxidation, or paint)
can significantly affect Emissivity.
Although thermographers frequently obtain emittance values from published tables, this practice can introduce significant temperature-measurement errors since emittance tables cannot account for several of the above factors. Because of this, calculating emittance with one’s thermal imager will help to ensure measurement accuracy.
A simple procedure for calculating emittance may be found in the Standard for Measuring and Compensating for Emittance Using Infrared Imaging Radiometers. To order a copy of this or other standards, visit the Infraspection Online Store, or call Infraspection Institute at 609-239-4788.TRR
EDITOR’S NOTE:
This Article Is Based On The Jan. 10, 2022
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ABOUT THE AUTHOR
Jim Seffrin, CMRP, has been an Infraspection Institute Certified Infrared Thermographer since 1984. A co-founder of Jersey Infrared Consultants and a practicing thermographer with 36+ years of experience as an infrared consultant, he was appointed Director of Infraspection Institute in April 2000. For information on a wide range of topics related to infrared thermography and associated training and certification, visit IRINFO.org and/or infraspection.com, or email [email protected].
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