A radiation profile line originates from illuminating a scene by a fan beam of radiation and imaging the thus illuminated scene on a surface of an array of photosensitive elements. A read-out circuit has inputs connected to associated detection elements in the uppermost row. The read-out circuit comprises level detector circuitry to have the output signals of the columns take up either a "1" state or a "0" state. The outputs of the level detector curcuitry, each one of which corresponds to one of the inputs, are connected to a circuit that delivers a digital code representative of the position of the element in the uppermost row which has received the largest amount of light.