  Let $\mathbb{I\!D}=[0,\mathrm{L}]\subset\mathbb{R}^{+}$ be a slab geometry
with boundaries $z=0$ and $z=\mathrm{L}$. A laser beam with a flat incident
intensity $\psi_{o}$ enters the slab along the z-axis or unit vector
$\widehat{\mathbf{e}}_{3}$ at $z=0$. The slab contains matter with an
absorption coefficient of $\mathsf{A}$ with respect to the wavelength. If
$\mathsf{A}$ is constant and homogenous then the beam decays as Beer's law
$\psi(z,\widehat{\mathbf{e}}_{3})=\psi_{o}\exp(-\mathsf{A} z)$. If the
absorption coefficient is randomly fluctuating in space as
$\mathbf{A}(z)=\mathsf{A}(1+\alpha \mathbf{G}(z))$--where $\alpha>0$ determines
the magnitude of the fluctuations, and the Gaussian random function has
expectation $\mathbb{E}\lbrace \mathbf{G}(z)\rbrace =0$ and a binary
correlation
$\mathbb{E}\lbrace\mathbf{G}(z_{1})\otimes\mathbf{G}(z_{2})\rbrace=\phi(z_{1},z_{2};\xi)={\mathsf{C}}\exp(-|z_{1}-z_{2}|^{2}\xi^{-2})$
for all $(z_{1},z_{2})\in\mathbb{I\!D}$ with correlation length $\xi$--then the
beam propagation and attentuation within the medium is described by the
stochastic differential equation \begin{equation}
d\widehat{{\psi}(z,\widehat{\mathbf{e}}_{3})}=-\mathsf{A}\widehat{\psi(z,\mathbf{e}_{3})}dz-\alpha\mathsf{A}\widehat{\psi(z,\mathbf{e}_{3})}\mathbf{G}(z)dz
\end{equation} The stochastically averaged solution is derived via a Van
Kampen-type cluster expansion, truncated at 2nd order for Gaussianality, giving
a modified Beer's law \begin{equation}
\mathbb{I}(z,\widehat{\mathbf{e}}_{3})=\mathbb{E}\big\lbrace\widehat{\psi(z,\widehat{\mathbf{e}}_{3})}\big\rbrace=\psi_{o}\exp(-\mathsf{A}z)\exp\bigg(\frac{1}{4}\alpha^{2}\mathsf{A}^{2}{\mathsf{C}}\xi\bigg[\exp(-z^{2}/\xi^{2})\bigg(\sqrt{\pi}z
Erf(\tfrac{z}{\xi})\exp\bigg(\frac{z^{2}}{\xi^{2}}\bigg)+\xi\bigg)-\xi\bigg]\bigg)
\end{equation} The deterministic Beer's law is recovered as $\alpha\rightarrow
0 $.
