Steady state luminescence radiance expressions for two-level and three-level molecular systems are derived in terms of transition probabilities, source irradiance, emission pathlength, absorption band halfwidth, total molecule concentration, frequencies of the transitions, statistical weights after transitions, and nonradiational activation and deactivation rate constants. Limiting radiance expressions are given for both low (conventional sources) and high (lasers) intensity sources. Saturation irradiances for the two-level and three-level molecular systems are also given. Conclusions concerning the expressions are given. One conclusion, which has special importance, involves the production of similar phosphorescence intensities whether the first triplet electronic state is excited directly or is excited via the first excited singlet with intersystem crossing to the first triplet state.