# Optical Characterization of Homogeneous and Heterogeneous Intralipid-Based Samples
## 2 Light Propagation in Turbid Media  
Light propagation in a turbid medium can be described with a low number of well-defined parameters. To describe the absorption process, a single parameter is needed. This is the absorption coefficient (\(\mu_{a}\)), which has the dimension of the units of reciprocal distance and gives information on the probability of the occurrence of an absorption event. Scattering processes are more complex than absorption to be described, mainly because once a scattering event has occurred, the spatial position of the scattered photon should be provided. Information on the probability of the occurrence of a scattering event is given by the scattering coefficient (\(\mu_{s}\)), which is homologous to \(\mu_{a}\). As for the matter of the spatial distribution of the scattered photons, the scattering phase function \(p(\theta)\), that is, the relative number of scattered photons as a function of scattering angle, has to be known to fully describe the process. Usually, the full scattering pattern is not known and an anisotropy factor (g, dimensionless) is given. The proper definition of the anisotropy factor is the following  
\[g=\int_{0}^{\pi}p(\theta)\cos\theta 2\pi\sin\theta d\theta \tag{1}\]  
The \(g\) factor is a measure of the amount of forwarding direction retained after a single scattering event. When a photon is scattered by a particle and its trajectory is deflected by a deflection angle \(\theta\), the component of the new trajectory which is aligned in the forward direction behaves like \(\cos(\theta)\). There is an average deflection angle, and the mean value of \(\cos(\theta)\) is defined as the anisotropy. From the knowledge of the scattering and the anisotropy coefficients, it is possible to define the reduced scattering coefficient, \(\mu_{s}\)' = \(\mu_{s}(1-g)\), which gives information on the probability to have a forward scattered photon. As previously mentioned, the propagation of light in turbid media is dominated by the scattering interaction between a photon and a particle (this term indicating every scattering structure of the sample) which deflects the photon along a new direction of propagation. Scattering techniques are mainly based on signals coming from many scattering structures (particles or nonuniformities) and, thus, provide quantitative measurements of the average structural and dynamical properties of materials. The choice of the scattering technique for analyzing the specific material is mostly set by the characteristic length scales of the structures being probed. Scattering techniques generally work best when the wavelength of the radiation is comparable to the size of the structures that scatter the radiation. For light scattering, the wavelength is about 500 nm, making it ideal for probing soft materials such as tissues, colloids, emulsions, polymer solutions, and surfactant solutions, all of which have structures on length scales from a few hundred to thousands of nanometers. Valuable information on the shape, the size, and the average weight of the scatterers, and on their mutual interactions can be obtained from the SLS technique. Insight into the scatterers' dynamics can be gained by analyzing the fluctuations of the scattered intensity that is the essence of the DLS technique. Transillumination of turbid media with continuum and time-resolved techniques can be used to map the optical parameters, including the reduced scattering and absorption coefficients that characterize the investigated medium, giving average information about the interaction of light with the heterogeneities of the medium itself.