Method for illuminating particles for the purpose of forming their images

A method for illuminating particles for the purpose of forming their images, in which method a coherent light beam is injected into the core of a monomode optical fiber at one end of said fiber, the other end of this optical fiber illuminates a field optic that focuses the light beam emerging from the optical fiber onto the objective optic of a camera, defining a measurement region in which said particles lie, and said particles are illuminated by the light beam emanating from said core for allowing the camera to form images thereof, wherein the peak power of said injected coherent light beam is chosen so that, on the one hand, the particles are correctly illuminated so that the camera can form their image, and, on the other hand, the spectrum of the illuminating light beam includes, in addition to the wavelength of the laser beam, lines of shifted wavelengths generated by Raman conversion in the core in order to remove the background noise and the moiré effect from the images.