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  <h2>Kirchhoff's law</h2>
  Some books say S(T) = B(T) only at LTE. However, if you use the excitation temperature instead of kinetic temperature, S(T_exc) = B(T_exc) still holds even though T_exc != T_kin (pp67-68 Draine), because S = j/k only depends on the material property. In fact, isn't S(T_exc) = B(T_exc) the defining equation for the excitation temperature? Well, the defining equation is level population equals Boltzmann...
  <address>Last update : 12 May 2018 by S. Moon</address>
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