Blackbody

A blackbody absorbs all incident electromagnetic radiation. In thermal equilibrium it emits the universal Planck law spectrum determined solely by its temperature.

Table of contents
- Absorptivity Blackbody
- Luminosity of a spherical blackbody Blackbody

Absorptivity ( $α$ )

The fraction of incident radiant power absorbed under stated spectral and angular conditions. At thermal equilibrium, Kirchhoff's law of thermal radiation equates matching directional spectral absorptivity and emissivity. A blackbody has both equal to one.

Luminosity of a spherical blackbody

An isotropic blackbody surface has outward flux

F = π \int B_{ν} d ν = σ T^{4}

. A sphere of radius

R

therefore has luminosity

L = 4 π R^{2} σ T^{4} .

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