(also dark-trace tube), a cathode-ray tube whose screen darkens wherever an electron beam impinges, changing to one of the colors in the violet-brown region of the visible spectrum. The degree of darkening depends on the energy of the electrons and on the current density. The screen of a skiatron consists of a colorless crystalline layer of sodalite or a hal-ide of an alkali metal, which is applied to a thin mica sheet or to the bottom of the tube’s bulb. Since the trace on the screen can persist for a very long time—up to several days or even months—a skiatron is provided with means for erasing the written information rapidly (in a few seconds), usually by warming the screen for a short time.

The bombardment of sodalite or of halides of alkali metals by electrons causes light-absorption centers to form in the material; these centers are responsible for the color change peculiar to the substance. When a crystal is illuminated, intensive light absorption occurs in these centers and the crystals, which were originally transparent, acquire a color complementary to the color of the absorbed light. For example, after absorbing light in the yellow-green portion of the spectrum, KCI crystals become violet; after absorbing blue light, KBr crystals become brown.

Skiatrons are used to obtain radar images for subsequent projection onto a large screen. In this case, the skiatron screen is illuminated with a bright light source.