Abstract
The spectrum of intensity fluctuations of monochromatic light scattered by a suspension of particles undergoing Brownian motion contains a contribution, SN(f), due to fluctuations in the number of illuminated particles in addition to the interference terms. Limiting forms of SN(f) are given for a regular three-dimensional illuminated volume Ωi. For an Ωi with sharp boundaries SN(f) to An0D1/2/2π3/2f3/2 for f >> D/l2, where l is the smallest dimension of Ωi, D is the diffusion constant, n0 is the particle number density, and A is the surface area through which particles may enter or leave Ωi. The intensity fluctuation spectra of laser light scattered by suspensions of polystyrene spheres in water from 5 × 10-4 Hz to 5 × 103 Hz has been measured.