When there is thermodynamic equilibrium at temperature T, the cavity radiation from the walls has that unique universal value, so that I ν, Y (T Y) = B ν (T). Further, one may define the emissivity ε ν, X (T X) of the material of the body X just so that at thermodynamic equilibrium at temperature T X = T, one has I ν, X (T X) = I ν, X (T) = ε ν, X (T) B ν (T).