2.1 Approach Followed in this Book
The collection of analytical solutions reported in this book and the related software in the enclosed CD-ROM concern propagation of light through diffusive media. Diffusive media are turbid media where propagation occurs in the diffusive regime, i.e., propagation is dominated by multiple scattering, and photons undergo many scattering events before being detected. The solutions describe how the energy propagates through turbid media in which light-matter interaction can be modeled with elastic scattering and absorption. Scattering interaction deflects photons along new directions of propagation, but the energy, and thus the wavelength and the frequency, of scattered photons remains unchanged. Absorption interaction causes the disappearance of photons. Therefore, the wavelength of the radiation that outlives absorption and propagates through the medium remains unchanged. The turbid media considered in this book are random media, i.e., media where the amplitude and phase of the waves propagating through such media fluctuate randomly in time and space.
The simple model used in this book to represent light-matter interaction is a crude assumption that avoids getting inside the actual complexity of the phenomena of light-matter interaction, and it has the advantage of leading to relatively simple analytical solutions for photon migration valid for many real media of everyday life. Light-matter interaction is characterized by several phenomena that will be ignored in this book since they are not going to affect the results of our investigations. Some light-matter interaction phenomena are mentioned below.
Absorption and scattering are the main phenomena of light-matter interaction. Absorption is a phenomenon related to the absorption bands of molecules. When an electron within a molecule is raised by photon absorption to an excited state, the relaxation to the ground state can occur following a non-radiative decay process with the emission of heat orâand a radiative process with the emission of a photon at a different wavelength (luminescence effects). The non-radiative processes precede andâor compete with the luminescence processes.