Einstein coefficients, characterise the probabilities of radiative quantum transitions. They were introduced by A. Einstein in 1916 in his treatment of the theory of the emission and absorption of radiation by atoms and molecules on the basis of the idea of photons; in doing so he was the first to put forward the idea that stimulated emission exists. The probabilities of spontaneous emission (see Spontaneous emission), of absorption and of stimulated emission are characterised respectively by the coefficients Aki, Bik and Bki (the indices show the direction of the transition between the upper Εk and the lower Εi energy levels). At the same time Einstein derived Planck's radiation law by considering the thermodynamic equilibrium of matter and radiation, and obtained the relations between the Einstein coefficients. (see Thermal radiation).