Introduction to Light Scattering by Biological Objects

The practices of scattering and absorbing electromagnetic radiation enjoy wide use in various fields of science and technology that aim to study the structure and properties of inhomogeneous media. The theory and practice of light-scattering methods is now a fairly well-developed field, owing to the methods’ profound importance for applications such as atmospheric and oceanic optics, radio-wave propagation and radio communication, physical chemistry of solutions and colloids,7 materials science and chemical technology, biophysics, and laser biomedicine. The theoretical models, experimental measurement procedures, and data interpretation methods have been developed by experts in various disciplines (from astrophysics to laser ophthalmology); therefore, there are certain traditions and terminological barriers that hinder effective interactions among various research groups. To illustrate, for experts in atmospheric optics and astrophysics, the ideology of radiation transfer theory (RTT) is natural, and using language related to the apparatus of correlation functions and structural scattering factor is more habitual when interpreting data by small-angle x-ray scattering or neutron scattering. Another example is the composite medium technology whose basic concepts are effective dielectric permeability and effective refractive index. In colloid optics, the model of scattering via an isolated particle is most popular; this model is described either in terms of Maxwell’s rigorous electromagnetic theory or on the basis of various approximations. Despite terminological and other differences, the basis of many methods using the scattering of neutrons, x-radiation, light, or radio waves proves to be sufficiently versatile. With respect to the scattering of electromagnetic waves of various frequencies, this versatility is probably explained by the common classical basis, Maxwell’s electromagnetic theory, which is applied with physical models of scattering media. Even in the case of particle scattering due to potentials associated with this or another inhomogeneity of the medium, the general theoretical interpretation of the scattering (e.g., in terms of T-matrix formalism) may be exactly the same as that in the case of electromagnetics. In view of the great diversity and structural complexity of biological systems, the development of adequate optical models of the scattering and absorption of light is often the most complex step of a study. These models include virtually all sections of dispersion media optics: (1) simple singlescattering approximation; (2) incoherent multiple scattering, described by the radiation transfer equation (RTE); and (3) multiple-wave scattering in condensed systems of electrodynamically interacting scatterers and inhomogeneities. Quite plainly, such a broad range of problems rules out the possibility of a more or less detailed treatment of technical details. Therefore, the material presented in this chapter includes only certain elements of the theoretical apparatus used in the above-mentioned sections of scattering media optics; otherwise, it includes references to the relevant literature.