The interaction of light with matter can be broken down into a hierarchy of scattering processes: (1) interaction of light with electrons; (2) interaction of light with particles composed of many molecules; and (3) interaction of light with many particles. As the light wave interacts with electrons in the medium, the light slows by a fraction equal to the index of refraction of the material. This change in speed results in refraction of the light as it travels from one medium to another, and is also responsible for ``specular'' or Fresnel reflection. These processes are familiar in windows, mirrors, and lenses, but also underlie tissue optics and internal dosimetry.
The interaction of light with particles is composed of multiple internal reflections and redirections of the incident light. The resulting scattering profile (angular distribution) is described by complicated formulae, which can only be calculated for geometrically simple shapes. Detailed mathematical descriptions are not available for amorphous biologic shapes, but a general understanding of how scattering changes with wavelength and particle size is possible by examining the scattering profiles for spheres of various sizes. In biologic media, scattering is typically highly forward-directed (anisotropic) for visible wavelengths. Flow cytometry is one application in which single scattering phenomena in cells has been used.
With a few notable exceptions (e.g., cornea) light propagation in biologic materials thicker than tens of microns is typically characterized by multiple scattering events. These multiple interactions wash out any detailed structure associated with individual scattering events and therefore permit radical simplification of the scattering process. Scattering in optically thick media can be characterized by two parameters (the scattering coefficient and the scattering anisotropy), and is often further simplified to a single parameter called the effective scattering coefficient. The effective scattering coefficient is proportional to the density of scattering particles. Experiments show that the effective scattering coefficient of tissue does not vary dramatically with wavelength.
Multiple scattering dramatically affects propagation of light, but when the effective scattering coefficient is known, the changes caused by scattering may be corrected and quantitative absorption, fluorescence, or other spectroscopic measurements are possible. Several methods for accurately determining both scattering and absorption in tissue using reflected and transmitted light are available. Other less accurate methods based solely on reflected light allow non-invasive determination of both absorption and scattering. Such methods include radial or angular measurements of reflected light, time resolved measurements, and coherent backscattering techniques.
In this school, the goals are to consider the physical basis for optical scattering and its role in affecting photobiologic response and tissue optical transport. In addition, practical laboratory methods for the quantitative measurement and consideration of scattering in turbid tissue samples will be presented.}, }