where u ν is the spectral energy density of the radiation field. The three parameters A 21, B 21 and B 12, known as the Einstein coefficients, are associated with the photon frequency ν produced by the transition between two energy levels (states). As a result, each line in a spectrum has its own set of associated coefficients. When the atoms and the radiation field are in equilibrium, the radiance will be given by Planck's law and, by the principle of detailed balance, the sum of these rates must be zero: 0 = A 21 n 2 + B 21 n 2 4 π π c B ν ν ( T ) − − B 12 n 1 4 π π c B ν ν ( T )