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.