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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

        • For clean metals, E increases with temperature rise.
        • For dielectrics, E decreases with temperature rise.

♦  Viewing Angle. Imaging at angles other than perpendicular causes changes in Emissivity.

♦  Target Geometry. Target shape affects Emissivity. Compared to a flat surface,

        • Concave shape increases E.
        • Convex shape decreases E.

♦  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




This Article Is Based On The Jan. 10, 2022
“Tip of the Week”
From The IRINFO.org Website




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 jim@infraspection.com.



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