Skiatron

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.

[1] 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.

[2][3][4][5][6] Being naturally cathodochromic,[7] potassium chloride did not require any special processing or treatment to become a CRT target material.

The skiatron was mounted below a translucent plotting table surface and brightly lit with mercury arc stage lights.

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.