In this chapter, coherent effects that accompany the propagation of laser radiation in tissues and the interaction of laser radiation with cell flows are considered. These effects include diffraction, formation of speckle structures, interference of speckle fields, scattering from moving particles, etc. Principles of quasi-elastic light scattering (QELS) spectroscopy, diffusion wave spectroscopy (DWS), full-field speckle imaging (LASCA), confocal microscopy, optical coherence tomography (OCT), and second-harmonic generation (SHG) imaging are discussed.
4.1 Formation of speckle structures
Speckle structures are produced as a result of interference of a large number of elementary waves with random phases that arise when coherent light is reflected from a rough surface or when coherent light passes through a scattering medium. The speckle phenomenon is a three-dimensional interference effect that exists in all points of space where the reflected or transmitted waves from an optically rough surface or volume intersect. Generally, there are two types of speckles: subjective speckles, which are produced in the image space of an optical system (including an eye), and objective speckles, which are formed in a free space and are usually observed on a screen placed at a certain distance from an object. Since the majority of bioobjects are optically nonuniform, irradiation of such objects with coherent light always gives rise to speckle structures that either distort the results of measurements and, consequently, should be eliminated in some way, or provide new information concerning the structure and the motion of a bioobject and its components. In this tutorial, we will mainly discuss the information aspects of speckle fields.
Figure 4.1 schematically illustrates the principles of the formation and propagation of speckles produced in the regime of transmission and reflection of coherent light in an optically nonuniform media; Fig. 4.2 shows a real speckle pattern formed at He:Ne laser beam transmission through a thin layer of a human epidermal sample. The average size of a speckle in the far-field zone is estimated as where Î» is the wavelength and Ï is the angle of observation.