Abstract :
The Skiatron, or dark-trace tube, was developed to meet a radar requirement for a large-screen ¿black on white¿ picture with a persistence of several seconds. The present Skiatron consists of a magnetically focussed and deflected cathode-ray tube, with a screen consisting of a translucent micro-crystalline layer of potassium chloride. This screen is obtained by evaporation of the material in vacuum on to the tube face. An intensity-modulated scanning electron beam produces a picture by causing darkening in the areas bombarded. The picture is episcopi-cally projected using external illumination from mercury-vapour lamps. Screen characteristics have been measured by a photo-electric cell and a recording galvanometer. It is shown that the intensity of coloration increases with excitation until a saturation value is reached. Coloration produced by a pulse of excitation decays in several seconds and is referred to as ¿transient,¿ while that produced by a series of pulse excitations, applied at intervals, may take several hours to decay and is therefore known as ¿persistent.¿ Increase of temperature reduces contrast but increases the decay rate. Decay rate also increases with illumination, from zero in the dark to a saturation value at about 5 000 ft candles. The overall efficiency of the screen increases with beam voltage. The decay rate of the persistent colour can be greatly increased by periodic electron bombardment at low intensity. The presence of impurities also increases the decay rate, but the effect is not permanent. Application of basic coloration theory to the Skiatron indicates that the persistent colour is due to ionic diffusion. The Skiatron is fairly successful for radar work but is unsuitable in its present form for television.
