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#1
07-30-2012, 09:38 AM
 Zep Tepi Guest Join Date: Jan 2012
What Colour radiates heat most efficiently

We all know that black absorbs heat better than white or the lighter colours, but which colour, if any, radiates heat the best ?
#2
07-30-2012, 09:48 AM
 Philster Guest Join Date: Aug 2000
Darker colors get hotter than lighter colors, because they absorb more light and heat up from the extra energy, but this would not affect that rate at which they radiate that heat.
#3
07-30-2012, 10:08 AM
 CalMeacham Guest Join Date: May 2000
There's not going to be a unique answer to your question, because there is no unique spectral makeup to the colors we see -- the apparently same color red can be made up of an infinite number of combinations of spectra, ranging from a sharp lin at one wavelength to a pair of spectral lines all the way to a coninuum of wavelengths that "add up" to the same red.

In fact, since a lot of EM radiation os in the infrared and ultraviolet, you can get essentially the same efficiency from teo different colors that act as perfect blackbodies in the invisible part of the spectrum.
#4
07-30-2012, 10:33 AM
 Francis Vaughan Guest Join Date: Sep 2009
At any given wavelength the absorption and radiation are symmetric - good at adsorbing, equally good at radiating. What confuses matters is when the energy comes in at one wavelength and leaves at another.

The answer to the OP is simple. Black. Make an object black at the wavelengths you want it to be good at radiating at, and it will be as good as it is possible to be. Now what colour it appears to be in the visible wavelengths matters not so much - so long as this isn't the wavelength of interest. In general if something has any colour it must be reflecting light of that colour, and thus it is not efficient at radiating that colour (where the colour is the spectral mix of wavelengths it reflects.) If you are worried about objects that are at ordinary terrestrial temperatures the wavelengths you are worried about are in the far infra red. There is not a great correlation between visual colours and reflectance at those wavelengths. So there is no answer to the OP if colour is defined as a human visual colour. But if you define colour as the reflectance spectra of the object in the IR, the answer is black. Always.

Last edited by Francis Vaughan; 07-30-2012 at 10:35 AM.
#5
07-30-2012, 10:42 AM
 DelightfulExperiment Guest Join Date: Jul 2012
Colour is a visible issue- mostly a matter of reflection vs. absorption of visible light - and doesn't necessarily have anything to do with heat radiation (which is usually invisible unless something is very hot).
#6
07-30-2012, 11:30 AM
 scr4 Guest Join Date: Aug 1999
As others said - a surface that looks dark in the infrared wavelengths will radiate heat most efficiently. This parameter is called emissivity; the higher the number, the better it radiates heat, with 1.0 being a perfect blackbody. You can find tables listing emissivity for different materials (example).

Most everyday objects have fairly high emissivity. (Note, for example, that black paint has only 11% better emissivity than white paint.) The major exception is shiny metals, but only if it's clean, not oxidized, and not coated.

Last edited by scr4; 07-30-2012 at 11:32 AM.
#7
07-30-2012, 10:00 PM
 Napier Charter Member Join Date: Jan 2001 Location: Mid Atlantic, USA Posts: 7,183
If something is black because it has carbon black in it, as very many manufactured black finishes do, then it will have an emissivity near one. But some black things such as magic marker ink are not based on carbon black, and may not be so good. Human skin is also nearly one, regardless of its visible color.
#8
07-31-2012, 02:41 AM
 scr4 Guest Join Date: Aug 1999
Quote:
 Originally Posted by DelightfulExperiment Colour is a visible issue- mostly a matter of reflection vs. absorption of visible light - and doesn't necessarily have anything to do with heat radiation (which is usually invisible unless something is very hot).
For a given wavelength, the ability to absorb radiation is equal to the ability to radiate. So if a particular surface is 50% efficient at absorbing infrared (i.e. absorbs 50% of incident infrared radiation), then it will be 50% efficient at emitting infrared (i.e. it will emit 50% as much radiation as a perfect blackbody).

If that weren't the case, the object would absorb more (or less) radiation than it emits, and eventually end up in an equilibrium temperature that is hotter (or cooler) than the surroundings. Which would be a violation of the laws of thermodynamics.

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