The results of singular approach usage in the tasks of description and classification of appearance of optical anisotropy of different types of phase-inhomogeneous biological layers (surfacescattering, optically thin and optically thick) have been presented. The characteristic values of the fourth Stokes vector parameter (S₄ = 0 — linear polarization — (L-state); S₄ = ±1 — circular polarization — (±C-state)) have been chosen as the main analytical tool describing polarization-singular states. The value of S₄ has been determined by the value of phase shift between the orthogonal components of amplitude in the point of biological layer laser image and therefore is azimuthally stable. Hence, statistic moments of the first to the fourth orders characterizing the distribution of the amount of characteristic values S₄ = 0; S₄ = ±1 have been used for definition and differentiation of optical properties of different types of biological layers — surface scattering, optically thin and optically thick human skin.