On the speckle pattern of a light field in a dispersion medium illuminated by a laser beam

A simple analytic model is proposed for analysis of the speckle pattern of laser radiation in a dispersion medium by the example of a biological tissue. It is shown that three components — unscattered, diffraction, and diffusion, can be distinguished in the radiation flux propagating through a turbid medium. The widths of their angular spectra differ by a few orders of magnitude. Taking into account the interrelation between the theory of light-field coherence in a scattering medium and the theory of radiation transfer, the expressions are obtained for the average sizes of speckle spots of three types formed by these components. These sizes are of the order of millimetre, micrometre, and fractions of micrometre. As laser radiation penetrates into a tissue, first large spots are observed, then middle-size spots dominate, and after that — very small spots appear. Other parameters of the speckle pattern are also presented.