Emissivity of a flat surface varies with zenith angle

AI Thread Summary
The discussion centers on the emissivity of a flat surface and its relationship with the zenith angle, expressed by the equation e=E*cos(theta). It is argued that varying emissivity leads to different emitted radiation levels, suggesting the surface acts as an anisotropic source. However, a counterpoint is made that the surface remains isotropic due to the cosine law, which only affects the projected area rather than the radiation uniformity. The key takeaway is that while emissivity changes with angle, it does not necessarily imply anisotropic radiation. The debate highlights the complexity of radiation behavior in relation to surface emissivity.
Callisto
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If the emissivity of a flat surface varies with zenith angle according to

e=E*cos(theta)
where E is the emissivity at zenith.
Would this surface radiate isotropically?

I think that because the emissivity varies then the emitted radiation varies accordingly so the energy measured from any fixed point and at any angle would not be the same so therefore the surface would be an anisotropic source of radiation.

Does this sound like a fair argument?
anybody care to correct me?

Callisto
 
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Callisto said:
If the emissivity of a flat surface varies with zenith angle according to

e=E*cos(theta)
where E is the emissivity at zenith.
Would this surface radiate isotropically?

I think that because the emissivity varies then the emitted radiation varies accordingly so the energy measured from any fixed point and at any angle would not be the same so therefore the surface would be an anisotropic source of radiation.

Does this sound like a fair argument?
anybody care to correct me?

Callisto


No, the surface is an isotropic radiator. The cos law means the projected
area of the surface is being reduced. Any deviation from the cos variation
means it's not an isotropic radiator.
 
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