To measure high particle concentrations, an optical fiber bundle sensor has been designed, based on the backscattering of light by particles in suspension. The sensor response depends on-the concentration and also on the size and optical properties of particles. as shown by the results with silica particles. With this sensor, the measurement range of volume concentration is between 10(-3) and 0.4, for a particle size distribution between 1 and 180 mu m. Historically, the optical fiber sensor has been developed for Cemagref to measure solid transport during torrential swelling. It is also used in different applications such as sedimentation and circulating fluidized beds. In CFB application, Monte Carlo calculation has been developed to predict the sensor response because the calibration is impossible.