Reduced scattering coefficient determination by non-contact oblique angle illumination:: methodological considerations

The reduced scattering coefficient, mu(s)', was determined using images on the backscattered light intensity from an oblique angle illuminated turbid media. The distance Delta r between the point of incidence (hot-spot) and the circular symmetric diffuse reflectance centre, is proportional to 1/mu s' (mu(a)<<mu(s)). Previously, Delta r was obtained analyzing a 1D strip aligned with the laser beam. We propose a 2D algorithm that utilizes images with extended dynamic range, achieved by superimposing images with multiple integration times. The algorithm includes compensation for vignetting effects and camera non-linearities. The hot-spot algorithm accounts for the laser beam shape, and minimizes speckle variations by utilising several images. The diffuse centre position is obtained from momentum analysis on multiple thresholded images, using successively decreasing thresholds. The degree of diffuseness is determined based on the circular symmetry of the thresholded images. Both methods were evaluated on 18 optical liquid phantoms with mu(s)epsilon[1.5, 3.0] mm(-1) and mu(a)epsilon[0.01, 0.16] mm(-1). The 2D method displayed a higher linearity with mu(s)' due to more stable hot-spot detection, than the ID method. The anisotropy factor g was obtained by fitting measured and Monte Carlo simulated spatially resolved intensity decays and verified with a laser Doppler flowmetry technique. With an optimal compensation for the mu(a) dependence, the rms error in mu(s)' estimation was 2.9%.