Abstract: A light pen directed toward an area of a fluorescent screen controls a pulse generator, which causes said area to be brightened and also causes a defined target mark to be displayed. The coordinates of the target mark may be ascertained from the instantaneous deflecting voltages which control the electron beam and may be indicated or stored or supplied to a computer for evaluation. In such computer, the coordinates of a plurality of target marks may be used to ascertain the length of curves and contours or the areas of regions which have been circumscribed with the light pen. Various possible uses and methods of evaluation are described as well as equipment for carrying out the method.