During World War II, radar displays using potassium chloride evaporated on a mica plate as target material were actively developed in England, Germany, the United States, and the Soviet Union. Being naturally cathodochromic, potassium chloride did not require any special processing or treatment to become a CRT target material. When hit by the
electron beam from the back of the CRT, this normally white material turns a magenta color, producing a dark spot or line on the display, which resulted in the term "
dark trace" being applied to these devices. The pattern remains on the display until erased by heating the potassium chloride layer. This physical property is known as
tenebrescence or reversible
photochromism. Skiatrons were used as an early form of
projection television display, particularly in
radar stations during
World War II. The skiatron was mounted below a translucent plotting table surface and brightly lit with
mercury arc stage lights. The image on the surface reflected onto the bottom of the plotting table, using a
spherical mirror and a
Schmidt corrector plate, in the same fashion as an
opaque projector, producing an image of the radar display at a much larger size. In
RAF stations, the surface had a map on it, in
Royal Navy ships it was normally a series of radial lines. Operators viewing the surface would place markers on the projected traces, adding new markers as the traces moved. This produced trails of markers making the path of the targets clear. A variety of methods were used to erase the skiatrons. UK radars used fans to cool the tubes which were being heated by the
stage lighting of the projectors. Simply turning off the fans made the tube begin to warm up, the erasure taking perhaps 10 to 20 seconds. German examples used a thin, transparent layer of
tungsten deposited on the front of the tube, which heated up when current was passed through it. This provided much faster erasing. == Post-war developments ==