# I How to calculate the energy emitted through IR waves

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1. Apr 1, 2016

### amukher

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. Apr 1, 2016

### 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.

3. Apr 1, 2016

### Vagn

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.

4. Apr 1, 2016

### drvrm

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
where
L(1)
the spectral radiance in frequency of that surface;
L(2)
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>

5. Apr 1, 2016

### amukher

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.

6. Apr 1, 2016

### Vagn

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