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 limited 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) shows, 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 feeling about this object, can not be stimulated in the whole range. 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.

Date Published: 26 September 2007
PDF: 10 pages
Proc. SPIE 6797, Manufacturing LEDs for Lighting and Displays, 67970L (26 September 2007);

Published in SPIE Proceedings Vol. 6797:
Manufacturing LEDs for Lighting and Displays
Thomas P. Pearsall, Editor(s)