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Blog #035 – Your THERMOGRAPHY COMFORT LEVEL – Black body verses gray body verses non-gray body

As I have mentioned before, if you are going to use your thermal imaging camera in a professional manner it will be important for you to go through a training course, such as the one I teach in Sacramento.  It’s called ATT (Applied Thermography Training) and covers the physics of infrared, the principles of thermodynamics and psychrometry and the application of those principles to moisture investigations and building envelope investigations.    In thermography courses you will be exposed to words like “black body”, “gray body” and “non-gray body”.  If someone was curious as to whether you have attended a thermography training course they might just ask you what the difference is in a black body, a gray body, and a non-gray body.  Do you feel comfortable in explaining it?  In my opinion there is a minimal amount of practical use of this knowledge but, again, I think a professional should know as much as he/she can about the technology they are using.  Customers can usually tell if you know what you are talking about and their “confidence level” is effected by how much you know about the subject.  So, let’s make sure we are “comfortable” with these words.

A “black body” is referring to an object whose surface absorbs and emits 100% infrared radiation.  It has an emissivity of 1.0.  There is no reflectivity or transmissivity.  Can you think of an example of such a surface?  No!  There are no surfaces that you will be looking at that have an emissivity of 1.0.  A black body is a hypothetical radiation source.  It is useful in that we use it as a reference for describing the emissivity and reflectivity of real life surfaces, surfaces that have less than emissivities of 1.0.

A “gray body” refers to those surfaces that have both emissivity and reflectivity.  And it has nothing to do with the color gray.  Such surfaces do not have transmissivity.  Almost everything you point your camera at would fall under this category:  paint, sheetrock, wood, metal, brick, carpet, soil, human skin, etc.

Taken with Fluke infrared camera.  Example of gray body.

Taken with Fluke infrared camera. Example of gray body.

Taken with Fluke Ti32 thermal imaging camera.  Example of gray body.

Taken with Fluke Ti32 thermal imaging camera. Example of gray body.

A “non-gray body” refers to those surfaces that have all three components of radiosity:  emissivity, reflectivity and transmissivity.  The only surface that I can think of that you will run into in building diagnostics and that would fall into this category would be thin plastic.  Notice the images below.  In both cases the infrared radiation from the people on the other side of the thin plastic is coming through the material.  That’s called transmissivity.  So, we have emissivity, reflectivity and transmissivity.  The thin plastic is therefore called a “non-gray” body (regardless of its color). 

Taken with Fluke infrared camera.  Example of non-gray body.

Taken with Fluke infrared camera. Example of non-gray body.

 

Taken with Fluke IR camera.  Example of non-gray body.

Taken with Fluke IR camera. Example of non-gray body.

Is there practical value in all of this?  This little piece of information about gray bodies and non-gray bodies serves to remind us that every surface we look at with an infrared camera has at least two components of radiosity: emissivity and reflectivity.  And it reminds us that most everything is a gray body.  Rarely are we concerned with transmissivity.  However, gray bodies differ greatly from each other relative to the percentage of emissivity and reflectivity.   Although paint (0.95 ε) and unoxidised aluminum (0.03 ε) are both “gray bodies” they give significantly different results in accuracy of temperature.   As you know, the higher the emissivity the more accurate the temperature reading from your infrared camera!
Black body verses Gray body verses Non-gray body.

Black body verses Gray body verses Non-gray body.

How’s your comfort level in thermography?
(The above comments represent my opinion).

Rod Hoff / Restoration Consultants Inc
Thermography Instructor / IR camera sales
3284 Ramos Circle, Sacramento CA 95827
toll free 888-617-3266 ext 301
fax 916-736-1134
rhoff@restcon.com

Provider of Fluke TiS, TiR, TiR1, Ti, Ti25, TiR27, Ti27, TiR29, Ti29, TiR32, Ti32, TiR3, and TiR4 infrared cameras.

See DEMO www.moistureview.com/demo.html

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About infrarod

Rod Hoff is a Thermographer and instructor with Restoration Consultants, Inc. He teaches a two-day IR class in moisture and building envelope investigations. A graduate from Florida State University, with a degree in education, he received his formal training in Thermography from Snell Infrared and Restoration Consultants, Inc.
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