In a group of such atoms, if the number of atoms in the excited state is given by N 2, the rate at which stimulated emission occurs is given by ∂ ∂ N 2 ∂ ∂ t = − − ∂ ∂ N 1 ∂ ∂ t = − − B 21 ρ ρ ( ν ν ) N 2 where the proportionality constant B 21 is known as the Einstein B coefficient for that particular transition, and ρ (ν) is the radiation density of the incident field at frequency ν. The rate of emission is thus proportional to the number of atoms in the excited state N 2, and to the density of incident photons.