This thesis describes the design, implementation and experimental evaluation of a digital imaging based system for the on-line, non-intrusive, measurement of size distribution and volumetric concentration of solids, in a dilute pneumatic suspension. Inferential establishment of mass flow rate, derived from the combination of solids concentration and electrostatic cross-correlated velocity information, is also covered. A review of methodologies and technologies for the optical measurement of the parameters in question is presented, together with their advantages and shortcomings. Both the fundamental validity and industrial applicability of each technique are discussed. Based upon this review an imaging arrangement based upon the novel application of perpendicular laser sheet illumination is proposed. The strategy is intended to avoid depth of field issues experienced with diffuse illumination and circumvent particle movement issues experienced in conventional laser sheet work. Novel processing algorithms are developed and the feasibility of particle shape quantification is investigated. A static interrogation system with front illumination is used to allow the basic limits of measurement to be established. Subsequently, a full on-line prototype instrument is produced and practical concerns are discussed and addressed. Throughout the work emphasis is placed on practical methodologies and cost-effective implementations that would be applicable in an industrial environment. The results of extensive experimental evaluation of the system, based on laboratory testing and industrial trials using a pulverised coal/biomass mixture, are presented and discussed. Comparisons between off-line and on-line results are presented and reference data, recorded with an industry standard laser diffraction instrument, are considered. The performance of the system is discussed and suggestions given for the future development of the methodology.