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Black surface absorbs more heat but what if it's completely dark?

  1. Sep 9, 2011 #1
    Hi,

    Computer desktop cases now tend to have a black interior like this one:
    http://www.corsair.com/cases/carbide-series/carbide-series-400r-mid-tower-case.html

    I don't know what is the reason for this black interior finish to be so dominant in most of computer cases. One of the member of a tech forum advanced an explanation that black absorbs better the interior heat which then spreads the heat better outside through convection.

    However, when the case is closed. The interior is dark black anyway regardless of the color of the surface of the metal inside. Would the infrared rays still "see" the difference of color of the interior surface?

    BTW, if you happen to know why modern computer case is black, I would appreciate to know the explanation.

    Thanks in advance.
     
  2. jcsd
  3. Sep 9, 2011 #2

    LJW

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    It's not about "seeing" the color- infrared light isn't a sentient being and doesn't choose where to go. Rather, it's more about the interactions of said light with the surface. Yes, it would still behave normally (same as if there was visible light).
     
  4. Sep 9, 2011 #3
    The long-wave (infrared) albedo is independent of the short-wave (visible light) color. A white object is still white in the dark. It's just that you can't see it the way human eyes are structured.

    As an example of the first statement, human skin varies in its absorption and reflectance of visible light depending on its "color". There is very little variation in albedo in the infrared range.
     
  5. Sep 9, 2011 #4
    For the uninitiated, can you please clarify the implication of " little variation in albedo" ?
     
  6. Sep 10, 2011 #5
    "Implication?" Certainly. What this means is that no matter what color your skin is it absorbs radiant heat (infrared) from your surroundings at essentially the same rate in Joules per unit area.

    In contrast, the color of your skin does affect the amount of visible light that is absorbed by your skin; with albino skin absorbing less light in Joules per unit area than does extremely dark skin.

    The myth that peoples with dark skin are better adapted to work under harsh sunshine and high ambient temperatures is just that--a myth.
     
  7. Sep 10, 2011 #6

    Ken G

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    This may depend on the context of the claim. Note that here we are not always talking about the skin's ability to absorb heat, we can be talking about the skin's ability to shield the interior from UV rays that are not much higher frequency than blue light (you did mention "harsh sunshine" after all). I agree it is a myth they could work better in high ambient temperatures-- that's the heat issue. On the UV protection issue, I would note the purpose behind releasing pigments that "tan" the skin.

    In relation to the original question, your point seems to stress that making the interior black would not have an effect on cooling the computer, as that deals with absorption and emission of light that is quite far into the IR region. That seems a reasonable conclusion, so we don't know if there is any good reason other than style.
     
  8. Sep 10, 2011 #7
    The insides of some computer cases are black.
    Many computer cases are black on the outside.

    Quelle surprise.

    Does it cost more or less to colour one side of a panel black and the other something different?

    Many laptop cases are black on the inside and then lined with reflective foil.

    Draw your own conclusions that any connection to thermal performance is purely coincidental.
     
  9. Sep 10, 2011 #8
    Good point, Ken. On that matter of UV resistance, I have it on good authority (personal testimony of people with dark skin) that although darker skin does offer somewhat greater protection against UV radiation, people with dark skin do get sunburned--often quite seriously.

    I used to teach a course in physiological climatology (the effects of weather and climate on the human body). The issue of the the effects of skin color always generated much discussion. At that time, there were not a lot of refereed publications on the topic. Most of what had be published was the result of studies by the U. S. Army Quartermaster Corp at their research facilities in Natick, Massachusetts. The consensus of opinion was that the advantage of dark skin in resisting UV radiation was real but slight. I readily admit that I have not followed recent research--not having taught the course in many years.
     
  10. Sep 10, 2011 #9

    Ken G

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    No question, it's not like 45 spf or anything!
    I'm mostly going on my own personal experience-- I need a lot more sunscreen when I'm white as ghost in early summer than I do when I'm more tanned late in the summer! Plus, there must be some reason for the pigmentation that correlates with sunny climates, though I'm sure you're right that the danger is more in overestimating its significance than in understating it.
     
  11. Sep 10, 2011 #10
    Yes, I have read further about albedo after klimatos explanation and had understood that the interior color of the computer case has negligible impact on heat dissipation.

    Laptop casing is in plastic. The plastic shell is itself the case so it is not surprising that the interior is the same than the exterior. In my initial question, I was referring to desktop computer cases. On these desktop cases, the exterior and the interior are very often two different materials. Understandably the exterior surface often has a better finish, very often the front face is in plastic. Because of that, I think it would increase significantly the cost to give the interior the same finish.

    As the black steel in the interior only appear in mid and high end cases, I assume this cost extra and not at all for economic reason. One easy verification is that any case below $50 has the "traditional" mat silver metal color.

    What puzzles me was that the "fashion" of black interior of computer cases is so common that I thought there would be another reason than style. Because I suppose there are many other ways to style other than black.
     
  12. Sep 10, 2011 #11

    Integral

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    I think this is being over thought. Why are so many computer cases black? Best answer is because they look good and sell.

    Do you really think that box store computers are engineered to that level? The main thought but into these things is the bottom line... i.e. how much can we make. Engineers, even Chinese engineers, cost money.
     
  13. Sep 11, 2011 #12
    Actually, I can help a little with this.
    I'm an electrical engineer and have some experience with heat management.

    Electrical devices that become substantially hotter than there surroundings make good use of radiation as a means of cooling. Clearly, this isn't the case for the devices that are forced cooled by a fan, but it is for many other devices in the package.

    Black, as we perceive it, isn't essential. Rather, something that's dark in the long infra-red (ie 5-10 microns) performs. This includes any number of paints and coatings in a great variety of "perceived" colors. It also includes anodized finishes on aluminum.

    As to what goes on in a case, we somethimes use the case as a cooling system by making it absorb the infra red and then transfer it to the outer skin. The parts radiate to the case, then, some of the heat is reflected, some reradiated back, and some desposed of, outside the case.

    I hope this is clear and helps.

    Best Regards,

    Mike
     
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