In solid-state lasers the optical decay limits the storage of energy in active medium.
Also, the term optical decay is used to the effect of quick reduction of luminosity of astrophysical objects [1] In the first approximation, the optical decay can be treated as just spontaneous emission, and its rate is determined with the Einstein Coefficients.
In the idealized case, the evolution of the system is disturbed only by the interaction with the continuum of the modes of the electromagnetic field.
For the narrow spectral lines, the decay is almost exponential; then, the profile of the spectral line is determined by the Fourier transform of the exponential decay of the quantum amplitude of probability that the system is still excited; this profile is Lorentian.
[2] [3] The decay rate can be affected by the distortion of uniform density of states of photons, due, for example, an external cavity [4] or a nanofiber located in vicinity of the atom [5] or by simply placing the system near a dielectric or metallic boundary.