Scotophor

A scotophor is a material showing reversible darkening and bleaching when subjected to certain types of radiation. The name means dark bearer, in contrast to phosphor, which means light bearer. Scotophors show tenebrescence (reversible ) and darken when subjected to an intense radiation such as sunlight. Minerals showing such behavior include hackmanite sodalite, spodumene and tugtupite. Some purealkali halides also show such behavior.

Scotophors can be sensitive to light, particle radiation (e.g. electron beam – see cathodochromism), X-rays, or other stimuli. The induced absorption bands in the material, caused by F-centers created by electron bombardment, can be returned to their non-absorbing state, usually by light and/or heating.

Scotophors sensitive to electron beam radiation can be used instead of phosphors in cathode ray tubes, for creating a light absorbing instead of light emitting image. Such displays are viewable in bright light and the image is persistent, until erased.

The image would be retained until erased by flooding the scotophor with a high-intensity infrared light or by electro-thermal heating. Using conventional deflection and raster formation circuity, a bi-level image could be created on the membrane and retained even when power was removed from the CRT.

In Germany, scotophor tubes were developed by Telefunken as blauschrift-röhre ("dark-trace tube"). The heating mechanism was a layer of mica with transparent thin film of tungsten. When the image was to be erased, current was applied to the tungsten layer; even very dark images could be erased in 5–10 seconds.

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