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Author Topic: Emissivity, Reflectance, and Transmittance  (Read 5973 times)
Bill Warner
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« on: December 31, 2008, 02:58:32 PM »

Let's talk about emissivity, reflectance, and transmittance of infrared energy.  This gets very technical so bear with me...

To understand emissivity, reflectance, and transmittance, one must also understand the principal "Conservation of Energy"
Infrared radiation obeys many of the laws which apply to visible light.  When infrared energy strikes an object it may be (1) partially reflected; (2) partially absorbed; (3) partially transmitted.
Conservation of Energy means that "The total energy displacement in an object is equal to the amount of energy striking the object.
The mathematical or scientific formula for conservation of energy (for those who want to know) is:
Reflected energy + Emitted energy + Transmitted energy = 1 which equals Total Energy

Now for some definitions...



Transmittance:
Is the ability of a material to transmit infrared energy through it.
Good transmitters are used as lenses for thermal imaging cameras and systems.  The 2 commonly used materials for such lenses are germanium and silicon.

Reflectance:
Is the ability of material to reflect infrared energy. 
Good reflectors appear like their surrounding temperature.  Their apparent temperature is often quite different than their true temperature.
Examples of good reflectors include shiny metals, glazed ceramics, glass, mirrors, and high polished surfaces.  Good reflectors are poor emitters of infrared and have low emissivity ratings.

Emissivity:
Is a common measure of a material's thermal energy absorption and radiation capability. This measure is the material's emissivity rating.
Emissivity scales range from 0.0 (zero) to 1.0 (one) depending on the type of material.  A material that absorbs the total amount of IR energy which strikes it, has an emissivity of 1.0 and is also known as a "black body". (Black bodies are used to calibrate thermal imagers and equipment)
Emissivity and reflectance are opposites when transmission is not a factor.
Objects or materials that are good absorbers are also good emitters of IR energy.  Good emitters radiate IR energy well and appear like their true temperature.
Examples of good emitters include wood, brick, soil, water, and many painted surfaces.

Emissivity ratings are important when measuring true temperatures of object and materials.  Thermal imaging when applied to residential and commercial applications are typically not concerned with true or accurate temperature measurement (in most cases). We are more concerned with relevant temperatures as compared to surroundings. Because there are so many different materials used in home and commercial construction, true temperatures must be averaged as relevant temperatures.  We know that good emitters (good absorbers) include many of the materials used in residential and commercial construction.  Good emitters typically have an emissivity of 0.85 - 0.98.  Residential and commercial thermographers will average the ratings and typically calibrate their imagers at ~0.94

When one needs to become more accurate with temperature measurement, the thermographer will adjust the imager closer to the material's actual emissivity rating.  Emissivity tables for different materials can be found throughout the internet.  A fairly good one relevant to our applications can be found at:
http://www.raytek.com/Raytek/en-r0/IREducation/EmissivityNonMetals.htm

The following images are nearly identical except for the emissivity settings of the camera... notice the temperature measurement differences are quite substantial.  Emissivity and reflectance can play havoc with true temperature measurement.



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« Reply #1 on: December 31, 2008, 03:31:22 PM »

Bill,

Lots of good insight!

Thanks
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« Reply #2 on: January 01, 2009, 11:35:31 PM »

Very interesting Bill, personally I find this very informative, just how these machines function intrigues me to no end, I can read and read about this technology all day long and never get bored.

Thank you very much for sharing your knowledge with us all, and please keep "Teaching" if you would please.

Every little bit of information you post, I learn something new, and I love it.

Dale
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« Reply #3 on: January 02, 2009, 09:24:17 AM »

good stuff Bill...please keep it up...
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« Reply #4 on: January 03, 2009, 09:31:37 PM »

Great info Bill, thank you!

I have a question pertaining to the pics you posted (just to make sure I understand the basics of emissivity ..... the object being metal showed a higher temp at a higher emissivity setting because of its reflective properties ...correct?
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Bill Warner
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« Reply #5 on: January 07, 2009, 04:11:37 PM »

That's generally correct Chris although polished metals (or any low emissivity material for that matter) can reflect cooler temperatures as well. (as can also be seen in the images).  In most applications that we encounter in residential and commercial applications, low emissivity objects will appear cooler than their actual temperature unless there is something hotter in the vicinity.  It is simply a reflection of heat energy whether warm or cool.

poor emitters = poor absorbers = good reflectors

Good emitters = good absorbers = poor reflectors

Polished aluminum has an emissivity of 0.05 (steel is generally in the 0.10 to 0.28 range) so it is a very good reflector making it difficult to measure it's true temperature without additional methods and means.
In this particular study we were more concerned in identifying abnormal heat signatures as the client had an intermittent quality control issue which they believed to be thermally related.  Adjusting emissivity played a key roll in this study but was not necessarily the defining factor.  As stated before, emissivity of materials varies depending on the material properties and surface properties.  Emissivity is also affected by viewing angle (different reflections) and spectral (infrared) wavelength.

Typically in a residential and commercial applications, emissivity is less of a factor as we are typically looking for relative thermal differences to identify moisture, air intrusion, missing or marginal insulation, etc.
These are known as "Qualitative" infrared inspections (as opposed to "Quantitative" inspections).

  • Quantitative inspections are concerned with accurate temperature measurements and require determining the emissivity of components and materials.
  • Qualitative inspections, on the other hand, are simply concerned with relative temperature measurements and do not require the determination of emissivity values.

Since emissivity and reflectance are closely related, a common mistake is incorrectly identifying a reflected temperature as a significant thermal anomaly. If reflectance is suspected, I will change view angle and see if the anomaly moves or changes with me or my new angle.  Common areas where I have observed reflectance (and low emissivity) include: electric service enclosures and components within, glass throughout the home, high polished surfaces such as ceramic tile encountered in bathrooms and kitchens, bathtubs and toilets, pooled water, metal plumbing components and fixtures, other metal components and surfaces throughout the home, some plastics, high gloss painted surfaces, and even paneling on interior walls.

Two common tricks of the trade to achieve more accurate temperature measurement (quantitative) is to apply black (high emissivity) spray paint or common black electrical tape to reduce reflectance and provide a higher emissivity reference surface.  Both of these have an emissivity value ~0.96

Did this help or confuse?
« Last Edit: January 07, 2009, 05:18:34 PM by bwarner » Logged

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« Reply #6 on: January 14, 2009, 10:29:00 PM »

It should be noted that emissivity tables should be considered relative and not precise.  These tables are created under laboratory conditions.  Field and environmental conditions including temperature will have an affect on material emissivity and must therefore be adjusted for when performing quantitative inspections.  There are mathematical formulas for doing so, but I will not divulge into those for this discussion.
Again, most of the time residential and commercial inspectors will be performing qualitative inspections instead.  Due to this fact, it has been recommended that thermal images included in reports of this nature not include the temperature scale bar.  I follow this protocol with my reports.
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Chris Duphily
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« Reply #7 on: January 17, 2009, 12:46:36 PM »

Great information Bill, thanks for the lesson.
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« Reply #8 on: May 28, 2010, 10:53:31 PM »

Bill this has me going I never would have thought of reflectors...
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Bill Warner
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« Reply #9 on: May 28, 2010, 11:15:10 PM »

Yup. Very important.  Reflections encountered in homes have put egg on the face of a few novice or improperly trained ($500 2 day class...) inspectors who didn't understand.  Especially at ceramic tiled floors and walls where a pinless moisture meter may falsely register positive.


I'm glad you're reading through these threads and learning a bit.  There's a bunch of info in this forum from several guys!
« Last Edit: May 28, 2010, 11:16:55 PM by Bill Warner » Logged

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« Reply #10 on: May 28, 2010, 11:17:11 PM »

Bill who offers a good class or schooling to learn this I see this as a good Niche for myself to get into.
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Bill Warner
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« Reply #11 on: May 28, 2010, 11:28:16 PM »

The ones on the home page of this site.  http://www.your-leaking-house.com
Infraspection, ITC, Snell group, United Infrared... they're all good.  Dale has the logos hot linked to their respective sites.
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« Reply #12 on: May 28, 2010, 11:29:09 PM »

Thanks..
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« Reply #13 on: May 29, 2010, 10:54:05 AM »

Most often mis-perceived is that only hot objects "reflect".
In visual light, this is the case. We see light reflections (which is actually heat reflections as light is heat).

However in IR, which does not always measure visible light differences as we see them, cold may overcome the object.

Remember; everything on earth has a temperature. The camera sees the "net temperature".
A cold object behind you (32F) takes away from the objects actual temp (80F)  and the camera registers the "net" temp (56F).

Second; emissivity tables are worthless (they are just a general reference)!
Emissivity adjustment  from camera to camera will be different.
If I determine that my coffee cup emissivity is .86, Bill's camera may measure the same emissivity as .82.

The emissivity setting on your camera is a "correction factor" for YOUR camera!
It is the adjustment you must make on YOUR camera to correct the apparent temperature reading of YOUR camera to the actual temperature of the object.

I can not tell Bill what to set his camera at from measurements I took with my camera.

What is the emissivity of horse hair?
I had to prove this in my ITC  Lvl II field exercise.
What I came up with was not what the ITC rater expected and he called me out on it, but when I provided how I determined my measurement process I passed with a 100%.

Though we seldom need this accuracy in our applications, it is something that "must be understood" so you know the level of accuracy or what may be creating an unexpected reading.

BTW: Thanks for your time and educational information  Bill. Keep up the good work!

Don't forget to discuss the RAT process to adjust the Reflected Apparent Temperature.
These go hand-in-hand.

We may not need to know this stuff if we buy into those cheep-shit cameras from "over yonder" though!!??  ;-)
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« Reply #14 on: May 29, 2010, 11:02:03 AM »

Dave thanks for the information may I ask what camera do you use and where did you learn about IR.
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