Abstract: A device measures, by diffraction, the sizes of substantially spherical particles, in particular opaque drops. This device includes a lens (46) and prisms (49, 50, 51) for receiving the light diffracted by the particles (12), lighted by a light beam, for splitting this diffracted light into annular concentric areas and for concentrating the various parts of the diffracted light, corresponding to these areas, into separated points (24, 26, 28). The light intensifies in these points are detected and the sizes of particles are determined on the basis of these light intensities. The angular aperture of the concentric areas is adjusted according to a pattern which optimizes the discrimination of particle sizes.