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Let's describe the phenomenon these phenomenons  in a physical point of view. The optical intensity $I_0$ of the laser beam is defined as $I_0=\frac{1}{S}\frac{d E}{dt}$, where $E$ is the beam energy and $S$ the beam surface. When emitted on a medium surface, the laser beam is absorbed with an exponential decay along medium depth $z$:  \begin{equation}