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Black Body
In sci.astro message ,
Sat, 3 Dec 2016 16:48:40, Mike Dworetsky om posted: A planet or moon or asteroid is a reasonable approximation to a BB if it does not have an atmosphere, and if it does, the overall emission of energy will balance the absorption of energy if you take into account all the sources of opacity and reflection (albedo) at different wavelengths. Aside : the later part of that may not apply to technological planets. Earth absorbs, AIUI, about 120 PW of insolation; energy consumption is nearing 20 TW. If, after magicking away the anthropogenic greenhouse effect, the world population is raised to the US consumption level by, say cheap fusion power, multiply that consumption by 100, getting 2 PW, meaning another degree or so of global warming. Now cheapen the fusion ... So astronomers' usage of the term is at variance with that used by physicists. We would say that if the Moon were a truly black body it would be visible only as an occulter of more distant objects. A physicists' black body absorbs all incident radiation (over the frequency range of interest) and radiates in accordance with Planck's law. The Sun is almost that, in the visual; it absorbs all incoming light and radiates almost in accordance with Planck. The Fraunhofer lines are a minor, albeit interesting, deviation; they are narrow, and all the energy that they absorb will be re-radiated. Such a black body will absorb all energy that it receives, and will heat up unless the input energy is removed by passive or active cooling. -- (c) John Stockton, other side of London. Mail Web - FAQish topics, acronyms, and links. |
#12
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That is clear. |
#13
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So, let's assume that we have the ability to generate a constant radiation in that enclosure. Not just one source of radiation, but as much as needed. Let's also assume that the size of those sources is less than photon. (So technically, they won't interfere with the bouncing process). In this case, do you estimate that we could get a constant BB signature from that aggregated radiation? Last edited by David Levy : December 5th 16 at 03:50 PM. |
#14
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Black Body
Dne 05/12/2016 v 15:40 David Levy napsal(a):
'Poutnik[_5_ Wrote: The is no sense ion closing it. Neither the incoming rafiation would bounce forever, but would be absorbed quickly. However, if it had been absorbed quickly, then by the time that we will try to measure it, we might find that its amplitude is already zero. So, let's assume that we have the ability to generate a constant radiation in that enclosure. Not just one source of radiation, but as much as needed. Let's also assume that the size of those sources is less than photon. (So technically, they won't interfere with the bouncing process). In this case, do you estimate that we could get a constant BB signature from that aggregated radiation? I am not sure what you try to achieve. BB radiation is radiation belonging to radiative equilibrium maintained at given temperature. At given temperature, all radiation is absorbed by BB, and at the same time the same radiaton is emitted by BB, to maintain zero net energy flow of the equilibrium. If black body and generara body are at radiative equilibrium, the portion that is not absorbed, is not emitted, so incoming and outgoing flows are the same as for BB. -- Poutnik ( The Pilgrim, Der Wanderer ) A wise man guards words he says, as they say about him more, than he says about the subject. |
#15
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Black Body
In article id,
Dr J R Stockton writes: So astronomers' usage of the term is at variance with that used by physicists. Not at all, though sometimes people are careless and write "black body" when they mean "gray body." Also, astronomers may be willing to accept looser approximations than physicists are. We would say that if the Moon were a truly black body it would be visible only as an occulter of more distant objects. It would still be a strong emitter in the infrared, of course. The Moon's geometric albedo is 0.12, which means it absorbs and emits with an efficiency of 88% in visible light. Whether "black body" is a good approximation of that or not depends on context. A cheap laboratory blackbody has (from memory) an emissivity around 99.5%. The Sun is almost [a blackbody], in the visual; For some value of "almost," I suppose it is. A quick glance at Allen's gives a brightness temperature of the continuum -- ignoring spectral regions affected by absorption lines -- of 6125 K at 440 nm and 5940 K at 550 nm. At longer wavelengths, the opacity tends to decrease with wavelength reaching a minimum at about 1.6 microns. That minimum is important for measuring photometric redshifts of galaxies. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#16
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This explanation is clear. However, I would like to know if there is any theoretical option to maintain a constant Black Body radiation in an insulated enclosure without the need to enter the radiation through a tiny hole or any sort of cavity. The radiation should be generated internally (with as many sources as/if needed). So, what is needed (theoretically) in order to maintain this goal? Is it possible to active a constant BB radiation in an insulated enclosure (assuming that there is hypothetical generator for internal radiation)? |
#17
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Black Body
On 06/12/2016 12:42, David Levy wrote:
'Poutnik[_5_ Wrote: ;1329242'] I am not sure what you try to achieve. BB radiation is radiation belonging to radiative equilibrium maintained at given temperature. At given temperature, all radiation is absorbed by BB, and at the same time the same radiaton is emitted by BB, to maintain zero net energy flow of the equilibrium. If black body and generara body are at radiative equilibrium, the portion that is not absorbed, is not emitted, so incoming and outgoing flows are the same as for BB. Thanks This explanation is clear. However, I would like to know if there is any theoretical option to maintain a constant Black Body radiation in an insulated enclosure without the need to enter the radiation through a tiny hole or any sort of cavity. The purpose of the small hole and the large internal area is to allow there to be a reservoir of the black body radiation at the internal characteristic temperature of the nominal black body source which isn't too badly perturbed by losing energy by radiation out of the small hole. It is a practical limitation rather than anything else. The radiation should be generated internally (with as many sources as/if needed). So, what is needed (theoretically) in order to maintain this goal? For what sort of temperature? Everything radiates thermal energy but there is a world of difference between a black body at 300K and one at 3000K (it starts to get difficult to find stuff that doesn't melt!). Is it possible to active a constant BB radiation in an insulated enclosure (assuming that there is hypothetical generator for internal radiation)? To a first approximation for thermal radiation at ambient Earth temperatures everything that isn't a shiny metallic surface or designer mirror finish is a pretty good approximation to a black body at 300K. I don't see any reason why in principle you couldn't take a series of solid state LEDs each with 50nm bandwidth and construct a moderately good approximation to BB radiation over a fair range of wavelengths. It gets harder at the UV short wave end and at unpopular IR wavelengths. You really need to describe what you are trying to do. Making things that visually look blacker than black usually involves taking a pigment that is already pretty black and adding structures to it that mean that every incident light ray has to bounce multiple times before it can escape. Nanostubes in a grid much like the foam cones used in anechoic chambers. http://nerdist.com/the-worlds-blacke...orn-by-snakes/ -- Regards, Martin Brown |
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Thanks for your great explanation.
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We only discuss on a principle issue. Quote:
I would like to understand the following: Let's use one cube of insulated enclosure, and set inside only one LED. Let's assume that the ambient temp is 0K. After operating the LED, I assume that we should get some radiation above 0K with black body signature. Now, let's set two similar cubes (with one LED in each one) next to each other and eliminate the shared wall between them. Hence, we should still get an insulated enclosure which is double in its size and has two LEDs. So, does it mean that this insulated enclosure should also have a black body radiation? If yes, then let's add more and more cubes. Therefore, is there any limit in the Number of cubes which we can set one to each other and still get black body radiation? If there is no limit, then theoretically, we can set infinite number of cubes next to each other with just one LED in each one, and get some sort of infinite insulated enclosure bar. Do you expect that the internal radiation in that bar will also have a black body signature? |
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Black Body
Dne 08/12/2016 v 15:29 David Levy napsal(a):
I would like to understand the following: Let's use one cube of insulated enclosure, and set inside only one LED. Let's assume that the ambient temp is 0K. Note that temperature 0 K is not achievable by finite number of operations, as the consequence of the 3rd law of thermodynamics. After operating the LED, I assume that we should get some radiation above 0K with black body signature. Sure, as the wall gets heated. by LEd ( I hope for a power LED ). But as LED is involved. be aware of BB radiation is radiation at radiative thermodynamic equilibrium. Now, let's set two similar cubes (with one LED in each one) next to each other and eliminate the shared wall between them. Hence, we should still get an insulated enclosure which is double in its size and has two LEDs. Yes, it is clear. So, does it mean that this insulated enclosure should also have a black body radiation? Do the 2 LEDs, seeing twice volume, stop emitting energy ? If not, they heat the environment as well. But as 0 K is not achiveable, there is ALWAYS some thermal radiation present. If yes, then let's add more and more cubes. Therefore, is there any limit in the Number of cubes which we can set one to each other and still get black body radiation? BB radiation does not care. It is always present. If there is a heat source increasing temperature, it gets increased as well, proportionally to T^4. If there is no limit, then theoretically, we can set infinite number of cubes next to each other with just one LED in each one, and get some sort of infinite insulated enclosure bar. In fact, you need not any LED. Do you expect that the internal radiation in that bar will also have a black body signature? Sure. There is no cavity with theraml radiation absent. If cavity opening is negligible, the radiation can be considered as BB. Note that if multiple cavities are connected, so there is option of radiative equilibrium between them, the requirement of the negligible opening is not valid any more. -- Poutnik ( The Pilgrim, Der Wanderer ) A wise man guards words he says, as they say about him more, than he says about the subject. |
#20
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Black Body
Dne 08/12/2016 v 21:57 Poutnik napsal(a):
Dne 08/12/2016 v 15:29 David Levy napsal(a): .. After operating the LED, I assume that we should get some radiation above 0K with black body signature. Sure, as the wall gets heated. by LEd ( I hope for a power LED ). But as LED is involved. be aware of BB radiation is radiation at radiative thermodynamic equilibrium. What means that the LED would keep the enclosure from the reaching radiative equilibrium. That means e.g. the blue light intensity in case of blue or white LED would be MUCH higher than the equilibrium intensity for given temperature. -- Poutnik ( The Pilgrim, Der Wanderer ) A wise man guards words he says, as they say about him more, than he says about the subject. |
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