Abstract: The method consists in causing objects to pass in front of a background having a distinctly different brightness, in aligning a collimator by means of a sighting telescope in a direction of observation such that the path of the objects intersects a line defining the direction of observation of the collimator, in eliminating light from all directions other than the direction of observation, in detecting the variation of light intensity corresponding to the passage of the object by means of an electrical signal which is delivered by a photoreceiver and is a function of the luminous flux picked up by the photo-receiver and in differentiating the signal with respect to time by means of an electronic circuit so as to obtain an output signal which is a function of the variations of luminous flux on the photoreceiver.