The intensity of a collimated laser beam transmitted through a highly scattering medium is attenuated considerably due to photon losses caused by absorption and, much more, by scattering. However a small fraction of the collimated laser beam is transmitted through the scattering medium without any scattering event, but he intensity (Ic(x)) of the collimated beam in the depth (x) decreases with an exponential function depending on the sum of the integral absorption (Ã‚Âµa) and scattering (Is) coefficients of the irradiated volume of the collimated laser beam: These "collimated", "geometric" or "ballistic" photons are propagating, according to the laws of the geometric optic, as though travelling through a non scattering medium like in pure water.
K. Doerschel, K. Doerschel,
"High resolution coherent tomography", Proc. SPIE 10311, Medical Optical Tomography: Functional Imaging and Monitoring, 103110K (5 August 1993); doi: 10.1117/12.2283765; https://doi.org/10.1117/12.2283765