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Light Transport in Tissue Dimensional quantities Tissue is assumed to be a random turbid medium, with variations in the optical properties small enough to prevent localized absorption. In other words, tissue is considered to have volumetric scattering and absorption properties rather than being composed of discrete scattering and absorption centers distributed in a non-scattering, non-absorbing medium. The advantage to the distributed scattering center approach is that for perfect spheres the phase function is known, however there is little similarity between perfect spheres and tissue. Volumetric absorption (or scattering) is obtained by multiplying an absorption (or scattering) cross section with the density of absorbers (or scatterers) [31]. This is how the absorption coefficients $\mu_a$ and scattering coefficient $\mu_s$ are defined. The scattering and absorption coefficient are typically measured in inverse millimeters and the reciprocal of these coefficients is the average distance that light will travel before being scattered or absorbed, respectively. The thickness of the slab is denoted by d. In addition to the thickness, light propagation through a slab is determined by three parameters: the absorption and scattering coefficients and the phase function. The phase function is the fraction of light scattered into the direction of the unit vector $\hat\mathbf{s}'$ by light incident from the direction of the unit vector $\hat\mathbf{s}$. The phase function is discussed in detail below. The radiance is $L(\mathbf{r},\hat\mathbf{s})$; the position is denoted by the vector r and the radiance is specified by the direction of the unit vector $\hat\mathbf{s}$. The radiance has units of energy per area per solid angle (Watts sr-1 cm2). Sometimes this is called ``specific intensity'' or just ``intensity.'' The fluence $\varphi(\mathbf{r})$ is the total radiance at a point r. The fluence is obtained by integrating the radiance over all angles. The product of the fluence and the absorption coefficient equals the heat source: the amount of energy deposited in a unit volume of tissue.

S. A. Prahl."Light Transport in Tissue," PhD thesis, University of Texas at Austin, 1988.