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About fifteen years ago I read in a book (Physics by James Walker 3rd edition) that dark brown color reflects IR better than any other color and consequently weather heat since our bodies get most heat as IR and therefore wearing very dark brown clothes (almost black) helps keep the body cool. He went so far as to show picture of Saharan Bedouins walking between sand dunes and dressed in very dark clothes on a sunny day as proof that they knew it was the perfect color through the generations. Since I spent most of my life in the hotest spots inthe planet, ever since redding that I've been wearing dark brown clothes most of the time despite the fact that all people around me wear white clothes and drive white cars and dispit government media warnings and advice to wear white clothes and dispit the fact that I always felt uncomfortably warmer in brown clothes and the fact that since middle ages women in the midle east wear dark colors due to a famous verse of a poet flirting with his lover when he saw her in black clothes in Baghdad I couldn't trust my senses and general knowledge nor the public media more than a university physics book I picked from the university library. Today I was thinking of using crystals to split a green leaser but I found out that the result will be in the IR range and then I did a small experiment with an IR remote and phone camera to see which color is appropriate to observe reflected IR.
The first picture is IR on a brown smooth door and the second is on a white wall (due to curfew I couldn't get colored cards of the same material)

brown door

white wall

Edit:
The brown door has a glossy surface layer that reflects light more than the wall so the first image should have been even more darker.The upper bright spot in the first picture shoud be neglected because it appears with visible light as well and its a result of the glossy coating. only the lower dimmer region similarin shape to the one in the lower picture should be considered.

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  • $\begingroup$ Im not sure of the book edition, I searched online and the cover I remember matches the cover of the 3rd edition on Amazon $\endgroup$
    – Amr Berag
    Jun 25 at 21:16
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    $\begingroup$ I think it might a false logic to correlate reflected visible wavelengths (let alone perceived colour which isn't even an accurate indicator) with IR reflection. For example, the answer to the obvious question "what kind of colour reflects the most blue wavelengths?" is blue. You would never ask "what kind of colour that isn't blue reflects the most blue wavelengths"? It doesn't make a lot of sense. White might reflect more visible light so at least it should be cooler from visible wavelengths, but whether or not that outweighs the heat contribution from other wavelengths is another story. $\endgroup$
    – DKNguyen
    Jun 25 at 21:38
  • $\begingroup$ @DKNguyen Do you mean Dr. Walker is wrong? Is IR reflection more correlated with the type of material than with color but also notice that black or darkness is not a color, in his book if I remember correct he said very dark brown that it looks almost black or something like this. $\endgroup$
    – Amr Berag
    Jun 25 at 22:04
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    $\begingroup$ You're missing the point. My point is that if you're interested in the how much infrared a material reflects, just use how much infrared that material reflects instead of some other wavelength just because you can detect that wavelength with your eyes. You don't try and determine how much blue something reflects by looking at how much green it reflects. A wavelength of infrared is its own colour, just one you can't see. And in your specific window examples, you're ignoring how most materials suddely absorb UV but not blue, or how plastics are suddenly transparent to NIR but not red. $\endgroup$
    – DKNguyen
    Jun 26 at 3:21
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    $\begingroup$ Appearing dark to the human eye doesn't mean much. It's just because your eyes can't see it. The material could be absorbing or reflecting those invisible wavelengths for all you know and in either case it would appear dark to your eye. Visible and near IR are the dominant emissions from the sun. So if something is dark, you don't really know what it could be reflecting or absorbing outside the visible. But if it's white, you know for sure it's reflecting the visible wavelengths (and again you don't know what it is doing outside the visible). $\endgroup$
    – DKNguyen
    Jun 26 at 4:43
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IR is an extremely broad wavelength range (from near infrared, 800 nm to far infrared up to 1000000 nm). What you deal with when you experiment with a TV remote and a phone camera is near infrared (800-1000 nm). The material properties in that range are still pretty comparable to the optical range (although we don't see "infrared colors"). With a near-infrared filtering camera you will be surprised to see that many colored objects appear white, though. This is especially striking for many black colored objects. The reason is probably simply that the designed objects are not "optimized" for the NIR range because nobody would see those "colors" anyway, and wouldn't pay for it.

However, this has nothing to do with far infrared heat radiation (or is rather only a tiny part of it) which starts at about 3000 nm up to 50000 nm. Absorption spectra in that range are totally unrelated to the near infrared. The latter is still dominated by electronic transitions (similar to the colors we see) inside molecules, while the former is dominated by the vibrations (or phonons, quantum-mechanically) between the molecules.

That is the reason why you cannot tell anything about the thermal radiation behavior (especially reflectivity, or "FIR color") of clothes by referring to their various visible colors. It's totally different physics, so to say. The only notable exception are metals: they reflect well in the optical and NIR range, and so do they in the FIR range. This is the reason why it is very difficult to determine the temperature of metal objects with a thermal camera (you might just see the reflections of nearby objects, e.g. yourself, and hence, their temperatures instead of the true temperature of the metal part).

What does have an indirect thermal effect, though, is the absorption of visible light and NIR and the subsequent conversion into heat radiation (much larger wavelength) due the temperature of the object being raised. Black clothes absorb the light of the sun very effectively and raise your temperature a lot while you are in the sun, as opposed to white clothes, which reflect most of the sunlight, and hence, allow less to be converted into heat.

I would claim that what you have read in your book is a very popular science myth, set up to explain the confusing fact that people in the middle-east or africa wear black clothes, where this is actually probably just an accidental cultural mannerism, rather than a kind of "intuitive science by wise ancient grey-bearded sufi shamans".

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  • $\begingroup$ People wearing dark clothing then wasn't an accidental cultural thing. Heat does not just come onto us from the sun. It also comes off our warm bodies. When all that body heat hits the white clothing we might be wearing, it also gets reflected right back into our bodies. So when we wear white, we are actually heating our bodies. This means that the best color to keep cool, is black. Black does absorb all wavelengths coming from the sun, but black also absorbs energy leaving our bodies, instead of reflecting it back into our bodies. $\endgroup$
    – joseph h
    Jun 25 at 22:59
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    $\begingroup$ +1. By the way I'm from the African Sahara as well and our people wear mostly white clothes even funeral and mourning costumes have been strictly white for thousands of years. $\endgroup$
    – Amr Berag
    Jun 25 at 23:12
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    $\begingroup$ @josephh That book is really good especially for newcomers. I remember randomly picking it based on its cover when the main references weren't available and getting full marks in physics in my first year depending only on it after miserably failing my first test.But it completely avoids calcus and uses only algebra. $\endgroup$
    – Amr Berag
    Jun 25 at 23:59
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    $\begingroup$ Note you don't just care about the visible colour but also the invisible colour. vislble-black IR-white clothes might end up being better than visible-white IR-black clothes, or they might not $\endgroup$
    – user253751
    Jun 26 at 11:23
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    $\begingroup$ @josephh: this is basically true, and it is probably the rationale behind the recommendation to wear black clothes in hot climate. But this assumes that 1) visibly black clothes correlate with being also highly absorptive in the far infrared (while white clothes are the opposite), making them also efficient in emitting body heat to the environment instead of reflecting body heat back to us; 2) the positive effect of radiating body heat to the environment surmounts the negative effect of absorbing sunlight in the visible and infrared. I seriously doubt both points... $\endgroup$
    – oliver
    Jun 26 at 11:54

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