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Lighting : April 2010 Whos Who of Lighting
44 LIGHTING MAGAZINE | April/May 2010 TECHNICAL FEATURE LEDs have been used for several years in the generation of white light. The main advantages for LED solutions are extremely long life and high efficiency. LEDs deliver monochromatic light in physically determined wavelengths and as such, this creates different possibilities to achieve white light. These are: - Phosphor conversion white LED - Colour mixed RGB (A) white light sources, mixing red green and blue light to white light - Hybrid of the above two methods All of the above methods to produce white light using LEDs have benefits and disadvantages. In summary it can be said that the methods are either limited by efficiency (lm/W) or colour rendering index (CRI). To overcome these limitations special solutions for “hybrid modules” can be applied. Such hybrid LED solutions can achieve efficiency beyond 70 lm/W and have a CRI of 90+ for warm white CCT. This paper will discuss quality criteria for white light and the basic principles of LED based white light generation. The advantages and disadvantages of these differing methods, given the specific needs for various applications such as shop or museum lighting are also discussed. Furthermore the potential for a “hybrid” method based solution for new applications and its features (i.e. variable CCT) will be discussed in detail. Introduction LEDs have been used for several years for the generation of white light. The main advantages are extremely long lifetime and, potentially, high efficiency. LEDs are often used in applications where maintenance is difficult, space is limited and energy efficiency is important. However, white LEDs from various suppliers and producers often do not match the quality criteria for general illumination applications. For a long time the LED industry has concentrated on increasing efficiency, rather than addressing the critical, quality aspects of white light. Quality criteria of light efficiency The efficiency of a light source is determined by the conversion of electrical power into electromagnetic radiation and the spectral distribution of the electro magnetic radiation. The visible range for humans is around 400 - 700nm, this range we typically call light. Radiation outside of this visible range is, from a lighting point of view, wasted. Colour rendering The colour impression of a non luminous object is determined by its absorption or reflection of light. An observer will perceive the reflected part. If an object is illuminated by light, comprising all visible wavelengths, all wavelengths can be reflected by the object. Thus, all colours can be reflected and represented by the object. White light comprising all visible spectral parts, therefore has a colour rendering index (CRI) of 100. Sunlight or thermal radiators like incandescent lamps or low voltage halogen lamps have a CRI of 100. This is a reason for the wide usage of those light sources. White fidelity In physics the black body model is used to describe the characteristics of a thermal radiator like an incandescent lamp. The colour temperature of a light source is determined by comparing its chromaticity with that of an ideal black body radiator. The temperature – usually measured in Kelvin (K) – at which the heated black body radiator matches the colour of the light source, is that source’s correlated colour temperature (CCT). For a black body source, it is directly related to Planck’s law and Wien’s displacement law. In colour theory, the Planckian locus is generally the path that the colour of a black body would take in a particular colour space as the black body temperature changes. Only light with a spectral distribution close to the Panckian locus, is perceived by human as white light. Even very small deviations from the black body curve are detected by the human eye. uV-/ir free The radiation of incandescent lamps, fluorescent lamps and discharge lamps includes parts of the ultraviolet and infrared spectral range. These spectral parts can have various harmful effects to humans or objects. UV radiation can bleach objects or materials; infrared radiation can heat up the illuminated objects and reduce their durability. LED light does not have high energetic UV or long wavelength IR radiation in its spectra. WHITE LEDS WITH INCREASED EFFICIENCY AND VARIABLE CCT Martin Hartmann* * Global Senior Product Manger LED, TridonicAtco, 6850 Dornbirn, Austria: paper presented at IESANZ Convention, Queenstown NZ, October 2008. ➤ IN COLOUR THEORY, THE PLANCKIAN LOCUS IS GENERALLY THE PATH THAT THE COLOUR OF A BLACK BODY WOULD TAKE IN A PARTICULAR COLOUR SPACE AS THE BLACK BODY TEMPERATURE CHANGES.
Whos Who of Lighting 2009
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