Abstract: A granulometry device embodying the invention comprises a measuring probe and a micro-computer. The micro-computer provides digital filtering on a first discrete distribution function that is calculated by statistical inversion from a plurality of radiation measurements supplied by the probe and a theoretical radiation coefficient matrix thereby determining, after an iterative calculation, an optimum relation which should be satisfied by the nodes defining a discrete distribution function so as to offer a maximum correlation with uncertainty measurements. All the discrete distribution functions satisfying this relation are then calculated by statistical inversions. The mirco-computer provides an optimum statistical evaluation of the distribution of particles in a gas or liquid fluid.