Basic life cycle of a Monte Carlo photon packet. The photon packet is generated by a source and emitted in a randomly sampled direction. It is then propagated through the grid structure until a predetermined randomly sampled optical depth has been reached. Upon absorption, a randomly sampled fraction of photon packets can undergo re-emission or scattering, which will alter the properties of the photon packet. The life cycle of a photon packet is terminated when the photon packet is absorbed and not re-emitted or scattered, or when it leaves the boundaries of the system.