Evidence is presented which suggests that a class of modified Bessel-function distributions may have special significance in describing the statistics of radiation scattered by media characterized by a wide range of length scales. It is shown that these distributions may be obtained mathematically by applying a limit procedure to the random-walk problem with a variable number of steps. The choice of distribution for the step-number fluctuations is briefly discussed.

• Received 14 December 1977

DOI:https://doi.org/10.1103/PhysRevLett.40.546