The disclosure relates to measuring devices that are particularly suited for the purpose of in-situ characterization of particles present in fluid substances or in air using a low-coherence interferometer. Specifically, the characterization includes average size, size distribution, volumetric density, and composition. The low-coherence interferometer utilizes a split band of radiation to illuminate a sample probe and a reference probe then combines the reflected radiation from both probes to determine the photon pathlength distribution of the tested particulate or colloidal containing stream and from this information determine the size characteristics of said stream. The methodology is relevant to possible spatially distributed control of chemical processes such as emulsion polymerization to produce paints, coatings, synthetic rubbers, or crystallization processes in pharmaceuticals, food, and bulk chemicals industries. Another application relates to on-line control of particle size and volumetric density is in combustion for diagnostics. The invention can be used for the characterization of coal particles, dense sprays and solid propellants or any other system, which is too dense for conventional optical measurement techniques. Beside the intrinsic particulate nature of these systems, random index of refraction variations are also created due to turbulence/temperature interactions. The remote optical characterization of systems with high-concentration of suspended solids is also important for water quality control and pollution monitoring.