A process to detect the entry of a target into a zone, such process wherein it has the following stages: an illuminating beam is projected in the direction of the target by at least one source, the beam reflected on the target after its projection by one or more sources is received by at least two detectors placed in proximity to the zone and whose fields of detection incorporate a common part, each detector being synchronized to at least one of the sources, each travel distance separating the source and each detector via the target is computed or else only one of these distances is computed and then associated with a target presence criterion supplied by another detector, a target detection signal is delivered when the distance or distances thus computed, as well as qany presence criterion or criteria supplied, allow a point located geometrically in the zone under surveillance to be defined.