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I How to calculate the energy emitted through IR waves

  1. Apr 1, 2016 #1
    The energy emitted by a body in watts/m2 is = εσT4. In the case of a perfect black body, ε=1. If the body only emits IR light, what should be the value of ε?
  2. jcsd
  3. Apr 1, 2016 #2


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    Staff: Mentor

    If the "body" is a light source not due to thermal radiation, then there is no ε. A CO2 laser will not emit the same power as a remote control, even if they are at the same temperature.
  4. Apr 1, 2016 #3
    The emissivity is material dependent and surface roughness dependent. For example polished metal and foil of the same metal will have different emissivities, as shown here.
  5. Apr 1, 2016 #4
    such bodies exist which only emits a particular radiation - then its perhaps not a black body ; call it a real body or grey body ;
    The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation
    then one can define emissivity with respect to black body as a ratio and it can be from 0 to 1.
    it can be taken as ratio of the following
    Emissivity = L(1) of the body/ L(2) for a black body
    the spectral radiance in frequency of that surface;
    is the spectral radiance in frequency of a black body at the same temperature as that surface;
    For details see<https://en.wikipedia.org/wiki/Emissivity#Spectral_hemispherical_emissivity>
  6. Apr 1, 2016 #5
    Let us say that the atmosphere blocks all visible light wavelengths and allows only IR wavelengths to reach the earth. The earth would then be a source of thermal radiation. To calculate the heat emitted by the earth, I would need the value of ε.
  7. Apr 1, 2016 #6
    The emissivity depends on the body shape (e.g how smooth it is) and the material. There's no easy way to predict them, they are basically empirical values
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