In this chapter, optical models of tissues with basic single and low-step scattering, with ordered and randomly distributed scatterers are analyzed. Three types of eye tissues with various structure and turbidity such as cornea, healthy or cataract lens, and sciera are presented as examples. Basic principles of transmission, reflection, and scattering spectra formation are discussed. Examples of measurements of the Mueller matrix elements for diagnostics and monitoring of biological objects are presented.
3.1 Optical models of eye tissues
3.1.1 Eye tissue structure
Healthy tissues of the anterior human eye chamber (see Fig. 3.1) (e.g., the cornea and lens) are highly transparent for visible light because of their ordered structure and the absence of strongly absorbing chromophors. Scattering is an important feature of light propagation in eye tissues. The size of the scatterers and the distance between them are smaller than or comparable to the wavelength of visible light, and the relative refractive index of the scatterers is equally small (soft particles). Typical eye tissue models are long round dielectric cylinders (corneal and scierai collagen fibers) or spherical particles (lens protein structures) having a refractive index ns; they are randomly (or quasi-orderly) (sclera, opaque lens) or regularly (transparent cornea and lens) distributed in the isotropic base matter with a refractive index n0 < ns Light scattering analysis in eye tissue is often possible using a single scattering model owing to the small scattering cross section (soft particles).
Let us first consider the structure of the cornea and the sciera in more detail to demonstrate tissues with different size distributions and spatial ordering of scatterers. The cornea is the frontal section of the eye's fibrous capsule; its diameter is about 10 mm. The sciera is a turbid opaque tissue that covers nearly 80% of the eye and serves as a protective membrane to provide, along with the cornea, for counteraction against internal and external forces and to retain eye shape. Both tissues are composed of collagen fibrils immersed in a ground substance. The fibrils have a shape similar to that of a cylinder. They are packed in bundles like lamellae. Within each lamella, all of the fibers are nearly parallel with each other and with the lamella plane.
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