absorption spectrum of a black object

Discussion in 'Physics' started by praondevou, Jul 16, 2011.

  1. praondevou

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    Reading about solar heat collectors I was asking myself, what's the absorption spectrum of black paint. Usually these collectors are painted in black, because black, as we all know, does absorb visible light.

    As one can see in this link: http://en.wikipedia.org/wiki/File:Solar_Spectrum.png
    there is a lot of energy in the infrared spectrum too. Is this also being absorbed by black paint? I know that in practice the collector is often behind a plastic a glass cover, so infrared wouldn't probably pass anyway, but I want to know if there is any information on the absorption spectrum of a black object. (because I couldn't find any)

    Thanks
     
  2. Kermit2

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    http://en.wikipedia.org/wiki/Black_body

    Covers the theory of something black which absorbs radiation

    Something black behind glass or plastic WILL receive infrared radiation and WILL absorb most of it.(unless the glass is specially treated to reduce the passage of infrared)
     
  3. praondevou

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    Sorry, that's not completely true. For exemple you can't see a soldering irons tip on a thermal image camera through common glass.
     
  4. Kermit2

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    WHAT freq specifically are you concerned with?

    Sit in a car with the windows rolled up on a summer day and try to tell me infrared rays do not pass through glass...

    :rolleyes:
     
  5. someonesdad

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    Without any data, I'd tentatively agree with Kermit2's statements -- often, substances like paints will have emissivities in the 0.9 to 0.95 range, which mean they're fairly good approximations to a blackbody. However, when it comes to optical properties of things, I'm from Missouri -- so I'd want to see some measurement data. I'd imagine it would depend on the composition of the paint, particle sizes, etc. I have a spectroradiometer, so I could make some measurements, but my instrument only goes to 850 nm, which is only in the near IR. But no doubt the folks interested in solar energy have researched this topic to death, so the task would be to find their data.
     
  6. Wendy

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    And then there is the fact just because something looks black in our narrow range of vision doesn't mean it is across the board.
     
  7. praondevou

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    Ok I looked it up in Wikipedia "A black body is an idealized physical body that absorbs all incident electromagnetic radiation."

    I wasn't talking about an idealized object. I was talking about common real world black matte objects, objects that almost do not reflect nothing of for humans visible light.

    I agree that it probably depends on the type of paint and/or surface characteristics of how much IR is being absorbed or reflected.

    The car example doesn't serve as an explanation. How do I know that it is not the visible light that passed through the glass, was absorbed, therefore heating up objects which in turn emit IR radiation which cannot pass back through the glass? http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/grnhse.html#c2
    The truth may be somewhere in the middle. I did some tests with glass and a thermal imaging camera and also with a temp-gun, there is definitely a difference with or without glass if I point them to a heat source.

    But there are contradictive informations on this on the internet and this also wasn't my question. :)

    Question was: Will a black (real world) object absorb more/any infrared radiation from the sun than a non-black object? I'm good in finding things on the net but this one I cannot come to a conclusion.
     
  8. Kermit2

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    I also did some net searching

    IR will pass through glass

    IR will NOT pass through glass

    Both are true depending on the wavelength of the IR. The transistion seems to be somewhere around 6000 to 10000 nm. Shorter wave lengths(NIR) pass with no problem, and upon striking a surface are absorbed. When the object gains heat from this process it REradiates infrared, but this infrared radiation is a much longer wavelength and cannot pass through glass, therefore the heat builds up behind the glass.*greenhouse effect*

    Your IR thermometer uses Long wavelength IR and therefore glass will block it from reading temperatures.

    surface mounted thermocouples could be used to calculate the ratio of absorbed radiation vs reflected using the blackbody formulas. The problem is that outdoor weather is extremely variable and IR is even more variable due to moisture and dust in the air, which absorbs it much more than it does visible light. Getting an accurate answer seems like a very difficult proposal, and probably explains why such data is not found even with a lengthy net search.
     
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  9. praondevou

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    LOL. :D Thanks for the info. Very interesting subject, I guess I'll have to investigate this further.

    The background is actually a practical one. If I had to choose a paint for a heat collector, I'd like to choose one that apart from visible light also absorbs some of the IR. Now I know that the type of cover obviously also plays a major role...
     
  10. Kermit2

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    Actually glass is a GOOD cover for use in solar heating schemes.

    The IR spectrum which will not pass through glass, will also not pass through the atmosphere, so only the near infrared light from the sun reaches the ground. Glass is transparent to NIR so there is no problem with using it for greenhouses and such. The longer wavelengths that won't pass through glass never make it to the ground, so it is irrelevant that glass won't pass it.

    Glass also block a majority of the UV radiation. Mostly the UV-B and UV-C, with some of the low energy(less dangerous) UV-A able to pass through.
     
  11. praondevou

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    not so sure about that :confused: I had my temp gun pointed towards the sun. There is a big difference if I do the same through the windows glass... I don't know if that comparison would be valid, though
     
  12. shortbus

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    Years ago, in the beginning of the solar heating rage (in the mid 1970's) the color that was found to be the best for actual efficiency in heating was a flat greenish blue. Think the color of weathered copper. It absorbed solar heat and did not reflect it the best, even better than black.

    While black absorbs solar light, it is also the best emitter of heat. Think about it, car radiators where aways black until the shift to aluminum from brass. Think about the heat sinks for electronics, the most effective ones are black.

    If black was the most effective at absorbing light/heat plants would be black instead of green. Evolution could have made them any color but green won out. one of the reasons swimming pools are greenish blue is to absorb sun light and heat to warm the water.

    Here is an excerpt from wiki-
    In physics, a black body is a perfect absorber of light, but, by a thermodynamic rule, it is also the best emitter. Thus, the best radiative cooling, out of sunlight, is by using black paint, though it is important that it be black (a nearly perfect absorber) in the infrared as well. From; http://en.wikipedia.org/wiki/Black
     
  13. Kermit2

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  14. praondevou

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    I should have known it. There is no absolute answer to my question.

    1. I found various graphs confirming that part of the NIR radiation hits the earth surface.
    2. when it comes to common glass it obviously also lets pass NIR but not FIR.
    3. same with black paint, they can have completely different reflectance spectra
    4. the car example is still bothering me, but now I guess it works like this: visible and NIR passes through glass and heats objects up. It is then emitted by the objects, only with a longer wavelength, one that cannot pass the glass barrier.

    reflectance spectra of two different paints. (one of them absorbs visible light as well as infrared.
    [​IMG]
    Radiation arriving on the earths surface
    [​IMG]

    and last but not least downgoing and upgoing radiaton on earth
    [​IMG]

    Thanks for all your feedback
     
  15. someonesdad

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  16. Wendy

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    I remember arguing with my Dad the reverse effect is true. The interior of a car at night is slightly colder than ambient, which (to me) explains why the windows will frost up even though the temperature is above freezing.
     
  17. russ_hensel

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    A back object with the sun shinning on it will cool down if it is hotter than the sun ( surface about 6000 degrees ). Actually I think it is equilibrum ( on earth ) at less than 1000 degrees.
     
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