What is emissivity?

What is emissivity?

All materials absorb, reflect and emit radiant energy, and emissivity is the measurement of how well the surface of a particular material emits heat in the form of infrared energy.

How does emissivity work?

The measurement for emissivity is expressed in numeric terms between zero and one. It represents the ratio between the energy radiated from the material’s surface and the energy radiated from another material referred to as “a blackbody”.

A blackbody absorbs thermal radiation and emits it so well it’s the standard to which all other surfaces are compared, in terms of emissivity.

The blackbody’s emissivity rating is the highest possible, at 1. At the other end of the scale, the more reflective the surface, the lower its emissivity – so a perfect reflector would have a rating of zero.

Shiny polished brass, for example, has an emissivity value of about 0.03.  Brick, on the other hand, with its duller and much less reflective surface, has a higher emissivity value of approximately 0.86.

Bear in mind, though, that in reality there’s no such thing as a material with a perfect zero rating or a material with a perfect one rating – they’re both theoretical and serve to define each end of the emissivity scale.

In real terms, though, the images here are of a Leslie Cube – a stainless steel cube which is filled with hot water to demonstrate how the emissivity of different surfaces can differ, even if they’re all at the exact same temperature.

A thermal image of a Leslie Cube filled with hot water.

A thermal image of a Leslie Cube filled with hot water.

As materials with a low emissivity are more reflective, they reflect more ambient radiation than emitted radiation. Because of this, when viewed through a thermal imaging camera, materials warmer than their environment will appear cooler than they should, and when cooler than their environment they will appear warmer.

In this instance, to measure the true temperature of the cube a thermographer would have to adjust the camera emissivity setting to match the emissivity value for a specific surface finish (such as the tape) and measure that surface’s temperature value.

How is emissivity used?

One good example of the practical use of emissivity would be a high-emissivity solar water heating system.  Another would be the foil blanket wrapped around a runner at the end of a marathon.

The low emissivity surface on the inside of the blanket means that it reflects most of the heat generated by the runner’s body, reflecting it straight back to warm up the runner.

If that inside surface had higher emissivity it would absorb the heat, but then instead of sending it back where it needs to go, it would emit some of the heat on the inside where it’s needed… and emit the rest of that heat on the outside surface, where it’s not.

When it comes to thermal imaging of any kind, you need to be aware of emissivity values to make sure your temperature measurements and other related calculations are accurate.

There are ways of estimating those values, either with lookup tables or by comparing the surface in question with an object with a known emissivity value.

Once you have those values, you can then adjust your thermal imaging camera to create the most appropriate thermographic image for your analysis of the situation.

Thermal Imaging Training Courses

But it’s not just a case of looking at a strangely-coloured image and instantly coming to a conclusion:  the hallmarks of a good professional thermographer are careful thought about the situation, correct setup of the equipment to produce an image containing the appropriate information, accurate analysis of that information and the ability to effectively communicate that analysis to the client.

If you’re interested in a career in any kind of thermal imaging, you’ll need the right kind of training to gain the right qualifications and certifications.

And after taking any of the many specialist thermal imaging courses available from the iRed Academy, you’ll know how exactly to approach a problem situation, how to set up your thermal imaging equipment to produce the right kind of image, how to analyse the image produced and how to explain that analysis in order to provide your client with a solution to their problem.

Talk to us about the kind of thermography career you’re interested in, and we’ll help get you started, get trained, and get qualified.

2018-09-17T10:12:57+00:00August, 2018|Knowledge Base|

About the Author:

iRed is the UK’s largest provider of thermal imaging courses and specialist drone operator training. In operation since 2002, before thermal imaging had been accepted as a mainstream inspection tool, iRed has always delivered industry-leading training that’s regularly updated to meet current commercial needs.
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