|
LEDs (Light Emitting Diodes) become the most important light sources in traffic signal applications, but their use increases in other areas, e.g. indoor and outdoor signals and information boards, now also in general lighting. They become popular also as instrumental radiation sources. LEDs emit in narrow emission bands, and as every semiconductor, their optical characteristics are temperature dependent. In outdoor applications they are exposed to extreme temperatures, influencing both their absolute intensity and their chromaticity. In the present paper we investigate the spectral power distribution of different types of real LEDs, as well as the change of their spectrum with temperature. The question of measuring these spectra will be re-visited. Instruments tested fall into three categories: high-end double spectrometer, single grating instrument and array type instruments of different quality. These instruments show very different stray light rejection, band-pass, and wavelength accuracy characteristics, leading to non- negligible errors of LED chromaticity if calibration is made with incandescent lamps. Results on the influence of these factors and of spectral bandwidth and sampling rate for measuring modern LEDs will be provided. Chromaticity will be calculated based on above spectral measurements and color differences will be evaluated. This becomes a hot topic with present LED cluster construction, as LEDs mounted in a panel side by side have to agree both in chromaticity and luminous intensity to an extent that is below visual acceptability levels.
|
