Turbidimetry is a technique based on the transmittance spectra of the light passing through the media containing small particles. It permits measuring the size distribution of particles for size in the range of sub-micrometer or micrometer. But the inversion problem is one of the most important obstacles for its applications. Based on the nonnegative least square method, we have developed a stable and rapid algorithm and a measurement system permitting temporal acquisition (in milliseconds) and realizing in-line measurement. To ensure its performance, the sensitivity and the stability of the system have been examined in different stages: the light source, the spectrometer, and the variation of the media concentration according to the optics configuration. By the measurements in the laboratory and that of the wet steam in a turbine we show that such a system permits measuring very precisely the variation of the volume fraction of the particle or the wetness of wet steam.