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An Emittance Greater Than 1.0 ?
Tip written by: Infraspection Institute

According to the laws of physics, only a perfect blackbody may have Emittance of 1.0. Although the E value of real objects must be less than 1.0, some radiometers allow entry of E values exceeding 1.0. The following describes how these radiometers achieve the impossible.

Emittance is a measure of how well an object radiates energy when compared to a blackbody at the same wavelength and temperature. Emittance for any object is measured on a scale between 0 and 1.0. Since blackbodies (E=1.0) exist only in theory, real world objects will have E values of less than 1.0. The E value of an object can never exceed 1.0.

Assuming that most objects are opaque (T=0), they must be somewhat reflective. When making an infrared temperature measurement, this reflected energy represents an error source. To overcome errors due to reflections, quality radiometers have inputs for reflected temperature. By measuring reflected temperature and entering this value into the radiometer’s computer, this error source is compensated for in the radiometer’s software.

Less sophisticated radiometers often lack inputs for reflected temperature. To compensate for this, these radiometers allow the user to exceed E values of 1.0. Although this overcompensation may allow the user to match a desired reference temperature, it can lead to significant errors. For infrared temperature measurement, the best solution is to use quality radiometric equipment and eliminate or avoid reflections whenever possible.

Emittance and proper imager operation are two of the many topics covered in the Level I Infraspection Institute Certified Infrared Thermographer® training course. For information on our open enrollment or Distance Learning courses, please visit us online at www.infraspection.com or call us at 609-239-4788

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