This work presents a spectral measurement method coupled with an optical model of microalgal cells to retrieve the cell concentration and mean diameter of non diluted culture samples from a ballistic transmission measurement on the [400-750nm] spectral range. The AlgaSim simulation program was implemented to simulate the extinction cross sections of microalgal cells represented as spheres with concentric layers of different biological materials, and described by a set of physical and chemical parameters. The measurement system is composed of one integrating sphere specially designed to perform a sound measurement of the ballistic transmission, from which the spectral extinction coefficient of the sample algal suspension can be extracted. This study shows that from the measured extinction coefficient, by inverting AlgaSim simulation model, the concentration and mean diameter of two different monospecific microalgal cultures could be screened at different growth stages.