Abstract: An optical scanning device is provided of the kind having optical elements which are rotated and/or oscillated to scan incident light across a detector array. By arranging for successive scans of a plurality of rows of detector elements to overlap, each line of picture information of a scene under surveillance is scanned at least twice during a complete monitoring of the scene, the corresponding detector array output signals being summed to assemble a plurality of summed signals in which there is coherent addition of picture information and incoherent addition of noise. The summed signals are processed into television format for display.