Infrared to visible light conversion

Over the last half century various techniques and devices have bean developed to give a visual display of infrared signals and images. In the present article a relatively recent development is described in which conversion from infrared to visible is accomplished in a single luminescent material. Most luminescent solids convert ultra violet or more energetic radiations into visible light, the so-called Stokes type process but the new ‘anti-Stokes’ phosphors add up the infrared photon energies in twos or threes, or more, and emit larger energy photons of visible light. The quantum conditions for this to happen are discussed and the advantages of using combinations of rare earth impurities in crystalline solid hosts explained. Low crystal symmetry overcomes the parity rule restriction and use of different rare earth ions for infrared absorption from those giving the conversion process lifts the efficiency of the conversion. Experimental methods of probing the luminescence kinetics in such systems are described, e.g. looking for non-linear dependence of emission intensities on infrared intensities, measuring life times of the intermediate and final excited states of converter ions and studying the effects of phosphor composition and variations with temperature. Common conversion efficiencies at ordinary temperatures are in the 1 or 2 per cent range but 30 per cent efficiency has been achieved in some cases by operating at liquid nitrogen temperatures. Prospects for future improvements are discussed including the possibility of looking at long to short wavelength conversion in other parts of the spectrum.