Challenges in Application of Luminescent Materials, a Tutorial Overview

In this paper, a number of different application fields of inorganic luminescent materials are being discussed. In a tutorial manner, it will be shown how device requirements are being translated into properties the luminescent materials need to have. To this end many different material properties that have a strong influence on the device performance are discussed, such as the shape, the spectral position of the absorptionand emission bands and the decay time of the emission. The light yield under excitation with ionizing radiation, the energy resolution and the occurrence of afterglow are being treated as well. Subsequently, strategies are shown how to optimize these properties. Examples are given for light sources (fluorescent lamps and LEDs), radiation sources used for disinfection purposes and also for devices used in medical imaging and in horticultural applications. 1. LUMINESCENT MATERIALS FOR LEDS The first very successful electrical light source, the incandescent lamp, is around for about 150 years. Their widespread use was a revolution in view of ease of use and the absence of smell. They also played a major role in the electrification of many countries. Incandescent lamps are now gradually being phased out, in view of the rather low light generation efficiency of this lighting concept. The light generation efficiency is usually expressed in terms of lm/W, or in other words the amount of visible light generated divided by the electrical energy used to generate the light. The eye sensitivity for photons is strongly dependent on their wavelength. Incandescent lamps have an efficacy in the order of 10 lm/W. Higher filament temperatures result in higher efficacies, this is being applied in halogen lamps, with efficacies up to some 20 lm/W. The next revolution in general lighting was marked by the invention of the fluorescent lamp. Important inventive steps were disclosed in [1], the first commercially viable product was invented by GE, in 1936 [2]. Initially, this concept was thought to be very suitable for colored advertisement lamps, especially blue light could be generated with much higher efficacy than by filtering blue light out of the light generated by incandescent lamps. Very soon afterwards, the potential of this concept to generate white light became very clear and already before the Second World War, fluorescent lamps were in use. Fluorescent lamps do not generate as much IR radiation as incandescent lamps, for this reason their efficacy is much higher than that of incandescent lamps. In the 1970-ies, after the invention of rare-earth based luminescent materials, a significant further increase in efficacy was realized. Again, the dependence of the eye-sensitivity on the photon wavelength played an important role. Most of the luminescent materials applied in fluorescent lamps before the advent of rare-earth phosphors show broad emission spectra, f.e., spectra with a FWHM of 50–100 nm. To generate white light with a high color rendering index (the ability to reproduce all colors in a natural way), blue, green and red light have to be present in the emission spectrum of the lamps, with maxima at about 450 nm, 550 nm and Received 11 May 2014, Accepted 19 June 2014, Scheduled 23 June 2014 * Corresponding author: Cees Ronda ([email protected]). The author is with the Philips Group Innovation Research, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands.