Understanding Emissivity and Reflected Temperature

What should I enter for my emissivity and reflected temperature values?

First, ask yourself how are you using your thermal imager? Is it to identify thermal patterns or measure temperatures? If you are simply trying to locate thermal patterns such as missing insulation or air leakage in a building (what is called qualitative thermography) measuring an exact temperature really isn’t necessary. In these situations, leave the values at their default settings (typically 0.95 for emissivity and 68F/20C for reflected temperature) and go for it.

If you need to measure an exact temperature of a motor or a bearing (what is called quantitative thermography) then correctly setting emissivity and reflected temperature is a must to get the most accurate reading. Doing so, however, should only be attempted by properly trained and certified thermographers… those who are the most qualified to measure temperatures with an infrared camera.

What Is the Best Approach Until Then?

We recommend using the following basic guideline for taking simple measurements. Know there are many other factors you will need to consider which are covered thoroughly in all ITC training classes. Until then, to get started, let’s first define emissivity and reflected temperature.

What is Emissivity?

Emissivity is how efficiently an object radiates heat. It’s defined as the ratio of infrared energy emitted by the object, compared to that emitted by an ideal blackbody, if both are at the same temperature. It is represented as either a percent or a decimal.

Surfaces exhibit emissivity values ranging anywhere from 0.01 to 0.99. A highly polished metallic surface such as copper or aluminum are often below 0.10 and are practically an infrared mirror. Heavily oxidized metallic surfaces will have a much higher emissivity (0.6 or greater depending on the surface condition and the amount of oxidation). Most flat-finish paints are around 0.90 (in long-wave infrared) while human skin and water are about 0.98.

What is Reflected Temperature?

Reflected temperature (also known as background temperature or Treflected) is any thermal radiation originating from other objects that reflects off the target you are measuring. To properly obtain an accurate surface temperature reading with thermal imaging, this value (along with that of emissivity) must be quantified and programmed into the camera’s “Object Parameters” (or corresponding software if processing a saved image). This is used so that the software can compensate for, and ignore, the effects of this radiation as it does not relate to the actual surface temperature of the object you are measuring.

For higher emissivity objects, reflected temperature has less influence. For lower emissivity objects, however, it’s a critical factor that *must be* understood carefully! As emissivity decreases, what you are measuring (and seeing thermally) is coming more from the surfaces of surrounding objects (including the camera and operator), not the target you are inspecting.

How to Take a Basic Temperature Measurement

The easiest way to get an accurate measurement is to modify the surface with a material that has a high, known, and consistent emissivity value. Standard electrical tape, with its emissivity of .95, is one such item that works well for this purpose.

If the surface is safe to touch, simply adhere a piece of electrical tape to that object and set the camera’s emissivity value to 0.95. Next set the reflected temperature to an appropriate value for the environment. A stable, room temperature environment will provide the best results. If you’re still unsure what that value might be, I highly recommend a minimum of Level I Certification training to understand this concept further.

Next, measure the temperature of the tape with the camera’s spot meter or other measurement tool, being sure that the spot meter’s circle (called the reticle) is filled completely by the target.

IMPORTANT SAFETY NOTE: This “tape method” should *only be utilised* on stationary surfaces that can be accessed safely and are okay to touch. If attempting this type of measurement on an electrical component, the circuit *must* first be shut off and locked-out/tagged-out before proceeding.

What About Measuring Emissivity and Reflected Temperature on Other Surface Types?

For other surface types, or if temperature measurement accuracy on a variety of other objects is important for you and your inspection program, a minimum of a Level I Thermography Certification Training course is required. Level I will teach you how to correctly adjust emissivity and reflected temperature on a variety of other components including those that are electrically energized or are difficult to access. Attendees will learn about the proper procedures needed to evaluate both to ensure thermographers are getting the most accurate temperature readings with their thermal imager.

To learn more about these certification classes, as well as upcoming training dates and locations, visit the Infrared Training Center website: https://www.infraredtraining.com

Credit to Original Author:

Matt Schwoegler - ITC Instructor & Course Developer - Infrared Training Center at FLIR Systems - www.infraredtraining.com

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