The main advantage of the microphysical approach to radiative transfer is that it establishes a direct link between the MMEs and the RTE via a sequence of nine unambiguously defined and physically realizable assumptions and approximations summarized in Section 19.13. By keeping assumptions 1, 2, and 4–8 from Section 19.13, but relaxing approximations 3 and 9, one can extend the micro-physical approach and establish a similar direct link between the MMEs and the effect of WL of electromagnetic waves by a sparse DRM. Specifically, one can supplement the computation of the ladder component of the dyadic correlation function with the computation of the so-called “cyclical” component. The latter is caused by pairs of multi-particle sequences exemplified by Plates 18.1e and 18.1h. As we have seen in Section 18.4, the sum of the ladder and cyclical components can be expected to provide a better representation of optical observables at certain points located in the far zone of the particulate medium.

It is important to recognize that WL is not an independent physical phenomenon. It is implicitly contained in the exact solution of the MMEs (Section 18.4) but “falls through the cracks” when one resorts to the ladder approximation in order to simplify the computation. Therefore, one may characterize WL as the difference between the exact solution of the MMEs for a sparse DRM and the ladder approximation, although this characterization may still not be fully accurate since it neglects the existence of multi-particle sequences that go through a particle more than once.