4AP10

This cathode ray tube was the first of its kind to use dark trace technology.  Referred to as a 'Skiatron', this CRT was used in WWII radar systems to allow daytime viewing. The screen was coated with a #10 phosphor (referred to as scotophor) made from potassium chloride. Normally grey when not powered, the screen turns a dark magenta when bombarded by electrons. Using magnetic deflection, an unusually fine spot can be produced. Therefore, a dark image can be viewed on a white background. The image can then be projected onto a screen by means of a reflection-optics system using the Schmidt principle. Much care needed when operating,  as the anode voltage rating is 9000 volts. This early first generation CRT has problems concerning the clearing of the screen. Areas of constant bombardment started to darken,  and measures had to be taken periodically to "clean" the trace. Using low level bombardment and elevated temperatures over 20 minute time intervals, the screen could be rejuvenated.