Off-axis multiple scattering of a laser beam in turbid media: comparison of theory and experiment

The off-axis forward-scattered radiation from a low energy He-Ne laser, scattered by high density hydrosols, is studied experimentally and theoretically. The validity range of theoretical calculations is determined using four measurements with two optical depths and two detector fields of view. For narrow field of view detectors, experimental and theoretical agreement is good close to the beam axis and decreases with transverse distance; agreement is good at greater distances from the beam axis using an open detector. The small-angle approximation adequately describes multiple scattering close to the beam; calculations are significantly improved by combining the small-angle approximation with diffusion theory.