Abstract: Alphanumerical characters and other figures traced on an oscilloscope screen consist of elemental image components defined by code words which are called forth from a computer memory to actuate a function generator whose analog output is inscribed in a mosaic of a storage tube for orthogonal readout in the form of video pulses controlling the oscilloscope beam. A light pen placed against an illuminated spot of the oscilloscope screen picks up a light pulse which causes the inscription, on the same mosaic, of the code word giving rise to the image component whose trace is touched by the light pen. Different voltage levels of the code bits and of video pulses registered in superposed relationship on the storage mosaic enable their separation in a decoder upstream of the oscilloscope; the code word retrieved from the decoder is used to extinguish, intensify or otherwise mark the selected component within the displayed image.