Converter film technology for homogeneous white light

Abstract

Rafael C. Jordan, Jorg Bauer, and Hermann Oppermann Fraunhofer Institut fur Zuverlassigkeit und Mikrointegration (IZM) Gustav-Meyer-Allee 25, D-13355 Berlin, Germany Email: jordan@izm.fhg.de, Phone: +49(30)46403-219, Fax: +49(30)46403-271 1. Introduction An important issue for white ultra high power LEDs is the generation of a homogeneous light with high efficiency and a good color rendering index. Different from hot light sources LEDs do not emit the whole range of visible wavelengths. Only a certain wavelength with a limite d full width at half maximum is emitted. Therefore a combination of wavelengths must be used to satisfy the human eye for white light. The CIE chromaticity diagram (Fig. 1) show s, that several combinations of wavelengths let the brain realize white light. Already the combination of two wavelengths (e.g. cyan and red or blue and yellow) let us think, that the source is white, if this wavelengths hit our receptors. This is completely different, if the light is illuminating an object. The reflection spectra of this object, which is crucial for our color f eeling about this object, can not be stimulated in the whole ra nge. For example a red stop sign, which is absorbing all wavelength excepting red, will absorb the blue and yellow light from our "white" light source and due to the missing red, the sign seems to be dark grey or black.