Fig. 1.

Definition of laser fluence rate. The tissue is irradiated by a collimated laser beam of power P incident on tissue area A, i.e., with irradiance P/A (Watt/area). The infinitesimally small volume inside the tissue, here the green sphere at tissue location r (coordinate r not shown), receives a continuous stream of collimated and diffuse photons through its surface (represented by the arrows). The fluence rate is defined as all the incoming laser power divided by the (yellow) cross-sectional area of the sphere (Watt/area)