A method of flow measuring of a refractive index of particles and of classifying particles in at least two populations of particles present in a fluid by using a light source of short coherence time, of coherence length Lc < 100 μm, used under extinction conditions at a center wavelength (λ) selected as a function of the characteristics of the particles to be analyzed and the range of refractive indices expected for the particles under consideration, the method comprising the steps of: using the light source to form a converging illuminating beam with an aperture angle comprised between 1° and 60°, selected as a function of the characteristics of the particles to be analyzed and the range of refractive indices expected for the particles under consideration at the selected center wavelength (λ) and presenting lighting uniformity better than 10% in a parallelepipedal measuring volume a×b×c, where a×b is the rectangular section of the beam, a being the smallest dimension of a measurement window which is in the direction of travel of the particle, b being the largest dimension of the measurement window which is perpendicular to the direction of travel of the particle, having an aspect ratio a/b of less than 4, and where c is the depth of field defined as being the geometric range about the measurement point for which the power varies by no more than 10%, its value being situated at about 500 μm ± 250 μm; causing the fluid with particles to flow through a measurement orifice; measuring variations in impedance RES as the particles go through the measurement orifice; causing the fluid with particles to flow in the measurement window placed at the outlet from the measurement orifice and illuminated by the beam; measuring extinctions EXT on the axis of the beam as the particles pass through the measurement window, the measurement being performed by means of a device or a detector having a converging reception beam of aperture angle comprising between 1° and 60° selected as a function of the characteristics of the particles to be analyzed and the range of refractive indices expected for the particles under consideration; merging the RES and EXT data to form events therefrom; calculating a relative refractive index IDX for each event from RES and EXT; and performing the classification of all of the events using at least two parameters selected from RES, EXT and IDX. using the light source to form a converging illuminating beam with an aperture angle comprised between 1° and 60°, selected as a function of the characteristics of the particles to be analyzed and the range of refractive indices expected for the particles under consideration at the selected center wavelength (λ) and presenting lighting uniformity better than 10% in a parallelepipedal measuring volume a×b×c, where a×b is the rectangular section of the beam, a being the smallest dimension of a measurement window which is in the direction of travel of the particle, b being the largest dimension of the measurement window which is perpendicular to the direction of travel of the particle, having an aspect ratio a/b of less than 4, and where c is the depth of field defined as being the geometric range about the measurement point for which the power varies by no more than 10%, its value being situated at about 500 μm ± 250 μm; causing the fluid with particles to flow through a measurement orifice; measuring variations in impedance RES as the particles go through the measurement orifice; causing the fluid with particles to flow in the measurement window placed at the outlet from the measurement orifice and illuminated by the beam; measuring extinctions EXT on the axis of the beam as the particles pass through the measurement window, the measurement being performed by means of a device or a detector having a converging reception beam of aperture angle comprising between 1° and 60° selected as a function of the characteristics of the particles to be analyzed and the range of refractive indices expected for the particles under consideration; merging the RES and EXT data to form events therefrom; calculating a relative refractive index IDX for each event from RES and EXT; and performing the classification of all of the events using at least two parameters selected from RES, EXT and IDX.