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Colour and emissivity
09-05-2012, 08:58 AM
Post: #1
Colour and emissivity
There seems to be a lot of ambiguity when searching on the Internet regarding the relationship between colour and emissivity. I was informed on a IRT training course that colour is completely irrelevant when accounting for the emissivity of a target. However, some text found on the Internet suggests that emissivity and colour are closely related, e.g. white paper has an emissivity of 0.75 and black paper has an emissivity of 0.89. Can anybody clear this confusion up for me??
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09-06-2012, 05:11 AM
Post: #2
RE: Colour and emissivity
"However, some text found on the internet", mmm, what text?

Colour and emissivity are not related!

This often leads to confusion, the main reason for this is because we know that going out into the sun in a black shirt will feel hotter than in a white shirt. So I suppose people wonder why this happens if their emissivity is the same.

Heat transfer through thermal radiation occurs in the visible spectrum in addition to the infrared portion of the spectrum. This means that in the visible portion of the spectrum, black will absorb more of the sunlight than white. However if you were to be in a darkened room, only exposed to infrared radiated energy, you would notice no difference between a black or white shirt.

Colour of materials is sometimes listed in emissivity tables as being different from each other. This can be the case for a number of reasons. Firstly they could be totally different types of paper, photo glossy paper and blotting paper would have a different emissivity. Adding pigment to a material changes the material slightly, and this can cause an emissivity change, so the change can be due to a material change rather than a colour change. For instance, the black paper listed above obviously is not exactly the same material as the white paper, they would both be a mix of slightly different materials and pigments. Also some tables may list the total spectrum emittance, rather than just the LWIR region.

Emissivity really should be measured, and tables should be avoided.

Also, try not to believe everything that you see on the internet. I'm sure if you look hard enough you will find photos of Elvis living happily on Mars. Use caution...
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09-06-2012, 06:14 AM
Post: #3
RE: Colour and emissivity
Dear Bobberry

Many thanks for your reply regarding colour and emissivity. You have certainly helped me to understand the common mis-understanding!
I have been back on the Internet doing some more 'research' and I can see where some organisations have got the facts slightly wrong and this has led to confusion! It seems that some of the more unreliable and perhaps, more confusing information is on University papers and such-like.
I think my confusion simply came from the dreaded emissivity tables that are posted on the Internet. Like you stated in your reply; one paper may differ to another paper, not simply by its colour but also by its material and composition, thereby leading to emissivity differences. I have seen one table where yellow paper has an emissivity of 0.7 and black paper an emissivity of 0.9, but it does not state paper type, etc.
Thanks once again for your prompt and helpful reply.

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09-08-2012, 08:54 AM (This post was last modified: 09-08-2012 04:48 PM by acardoso.)
Post: #4
RE: Colour and emissivity
Resistivity, magnetic permeability and dielectric permittivity fully characterize a material from the electromagnetic point of view according to Maxwell model.
The conjugation of these three properties determines the behavior of a material to electromagnetic waves at a given frequency, and concurrently the line impedance of the material.
Another way to say this is – these three properties say if a material conducts electromagnetic waves in the optical phase or in the electric one.
Reflection and refraction are same phenomena, which occur at the interface between two mediums. If there is no refraction, there is no reflection; all energy is transmitted at the interface, the angle of incidence is equal to the angle of polarization (transmission). This also means the line impedance of the two mediums is equal. That’s when we say impedances are fully adapted.
If the impedances are not adapted, the reflection is partial, and the part of the energy transmitted does a different angle with the interface of the angle of incidence.
There is another case, theoretical, no adaptation at all, where the refraction doesn’t exist because no energy is transmitted, full reflection.
So color, as we percept it, is what we see when a light source have some wave lengths absorbed and others reflected on the surface. Color perception depends of the light source, the relation between the impedances of two mediums and position of the observer. If observer is the same medium, he will see reflected light; otherwise, he will see refracted light.
Infrared thermography “lives” from low reflectivity, or we will not see emission.
In fact, there is a relation between color precept under a certain light source and emissivity at the infrared region, but isn’t a straight through relation. Only with an equation system can be solved.
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