An apparatus for the instantaneous measurement of the relative light intensity was constructed by Brechtel (1962). An analogical apparatus was built, from the same point of view, but on another principle. Namely two selenium photo-cells, connected in opposition, produce anti-currents in a closed circuit. The differential current is compensated or reduced to zero by a linear potentiometer. The position of this potentiometer can be related to a value for the relative ratio between the light intensity above and below the forest canopy. The neutral density filters, which are necessary for the used photo-cells, can enlarge the reading scale, if they are chosen judiciously. Some positive measurements were done in a forest stand. Generally the apparatus will give good results and will be easily used for the characterization of light condition in forest stands.