Photon escape probabilities; Concepts and clarifications

The concept of the escape probability of photons created within an optically thick (scattering and/or absorbing) medium is well established as a useful tool for qualitative and quantitative analysis of photon transfer. However there exist some ambiguities of notation and definition which can present problems in applications of the concept. This review relates a number of different treatments or analyses involving photon escape probabilities, introducing a common notation in order to clarify the topic.