Rare-earth doped crystals of calcium and strontium fluoride
fluoresced in the visible region of the spectrum when irradiated by infra-red radiation.
The rare earth ions absorb radiation at one wavelength and when they were irradiated by a
second infra-red wavelength they emitted visible light. By flooding the crystal with a
local source of infra-red, visible radiation was emitted when the crystal was irradiated
by infra-red radiation from the scene. With this mechanism image conversion from infra-red
to visible was obtained without intermediate electronics.