Abstract
The generation of white light by Light Emitting Diodes (LED) is presented. Unlike thermal emitters (e.g. incandescent bulbs) or UV excited fluorescent light-sources (e.g. compact fluorescent bulbs), the emission is in the visible range only, allowing high conversion efficiencies. We detail the losses in electrical to white light conversion, give an insight on development challenges to estimate the efficacy increase in the following years. The emitted spectrum can be tuned to adapt the spectral sensitivity of human eye to maximize the efficacy of solid state light sources. As efficacy and light quality is generally a trade-off, we review recent light quality metrics to introduce optimization processes in white light generation.
Zusammenfassung
Die Erzeugung von weißem Licht durch Licht emittierende Dioden (LED) wird in diesem Artikel behandelt. Anders als bei thermischen Strahlern (z.B. Glühbirnen) oder durch UV angeregte fluoreszierende Lichtquellen (z.B. kompakte Fluoreszenzlampen) ist die Emission von LED Lichtquellen, basierend auf blauen LEDs, fast ausschließlich im sichtbaren Bereich. Dies erlaubt die sehr hohen Effizienzen des weißen LED Lichts. Die Details der Verlustmechanismen vom elektrischen Strom bis zum emittierten Licht sind dargestellt, um Einblick in die Möglichkeiten und Herausforderungen der weiteren Entwicklungen darzulegen und um die Effizienzentwicklung für die folgenden Jahre abzuschätzen. Das von LED emittierte Spektrum kann angepasst werden auf spektrale Empfindlichkeit des menschlichen Auges, um maximale Effizienzen der LED zu erreichen. Da aber Effizienz und Lichtqualität in den meisten Fällen sich gegenseitig negativ beeinflussen (z.B. Na-Niederdrucklampen mit gelbem, sehr effizientem Licht oder Glühbirnen mit sehr hochwertigem, aber ineffizientem Licht) wurden die neuesten Qualitätskriterien für weißes Licht analysiert, um einen optimalen Prozess der Erzeugung weißen Lichts mit LEDs abzuleiten.
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Notes
Photopic luminosity function: interestingly the maximum of the curve concurs with the maximum of the 5500 K blackbody radiation, and the reflectance maximum of the green leaf. At lower light intensities the function is lifted towards lower wavelengths (mesopic and scotopic functions).
Integrating the product of the V(λ) and SPD [W], and multiplying with 683 [lm/W].
In the LED lighting industry display backlighting, signage, automotive and outdoor lighting also play an important role, and can be very specific, however, the main features of developments overlap with general lighting.
Switching Mode Power Supplies are generally used for the good conversion efficiencies and compact size.
The phenomenon is called Stokes shift.
Total Internal Reflection (TIR) where two different optical density materials meet, like silicone matrix, or remote phosphor and air.
The material needs to be stable in time, with temperature and humidity, should have an excitation spectrum at the blue wavelengths, and should not be toxic or too expensive.
The same approximation is done for the narrow band theoretical spectrum, 60 % losses are taken for the red die.
A Technical Committee on Colour Rendition by White Light Sources of International Commission on illumination (CIE) was established on this topic (CIE TC1-69).
GE Reveal bulbs are preferred on the US markets that gives a saturated appearance of colours.
One example is that in Asia higher colour temperature bulbs are sold for general lighting, than in Europe or USA.
Different illumination is selected in a fashion-shop window, an aquarium, a meat-shelf, a dining table, a film studio or a press. Additionally, it can happen that a colour composition harmonize under one source and clash, when illuminated by an other.
See the Kruithof curve: Kruithof (1941).
Ohno (2006) and the widespread sales of the Reveal brand bulbs.
Recommendation of CIE TC1-33 (1999).
“It is possible for a light source to perform well on non-saturated samples while poorly on saturated ones. The reverse was found not to be the case“ (Davis and Ohno, 2010).
„Evidence suggests that increases in object chroma, as long as they are not excessive, are not detrimental to color quality and may even be beneficial” (Davis and Ohno, 2010).
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Bakk, I.P., Pachler, P. White light by LED—mechanism of generation and evaluation of quality. Elektrotech. Inftech. (2013). https://doi.org/10.1007/s00502-012-0104-2
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DOI: https://doi.org/10.1007/s00502-012-0104-2