Quote: Originally posted by annaandherdad  | | The link above on pyrometry talks about measuring the brightness at more than one frequency, and how you can get the temperature from that if you
assume the emissivity is the same at both frequencies. It also talks about how in practice the emissivity is not always the same. I don't see how
you can get really accurate measurements, no matter how many frequencies you sample, unless you have information about the emissivity.
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It's a curve-matching problem. The theory about how black-body radiation arises does not depend on the
particulars of atomic processes. Rather, it's arises from the equipartition of radiation in a closed system. See Planck's Law.
It's not a curve matching problem if you don't know what the curve is. The Planck radiation law applies to a black body, but a surface radiating out
into space is not a black body. Instead the Planck formula for the intensity is multiplied by the emissivity, which in general is a function of
temperature, frequency, and the type of surface. So the actual curve, emissivity times Planck formula, is not known unless you know the emissivity.
That was my point.
Of note is that the derivation only applies when you have equilibrium conditions in a closed environment. As soon as you point a pyrometer at it,
you're no longer in radiation equilibrium, because the instrument is at lower temperature than the black-body being measure. In this situation, you
get energy pushed into the electronic excitations of the materials present. The excitations will be atomic, molecular, or solid state depending upon
the state of matter, but there will always be something.
It's not just pointing a pyrometer at it. It's the fact that even without the pyrometer the body does not radiate as a black body, as I just
explained. If you have closed cavity whose walls are at a constant temperature, then you do get black body radiation, but the radiation received,
coming from a part of the wall, is the sum of the radiated and reflected radiation. That *sum* follows the Planck law, not the radiated part by
itself.
In the laboratory, you get more accurate measurements by minimizing the amount of radiation leaking out of the black body. Also, you maximize the
amount of insulation, so that the equilibrium condition is closer. Thus practical measurements in a foundry are more difficult, because you have
neither high insulation nor a small radiation aperture.
I don't know the historical details of the original measurements. I'm sure it's well-documented, though. |
