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Start to look into LED lighting and quickly it...(Electronics Weekly (UK) Via Acquire Media NewsEdge) Start to look into LED lighting and quickly it becomes obvious that one type of LED will not suit all situations. After a while you become sophisticated enough to take colour-rendering index (CRI) and colour temperature into account, and even then you are only about halfway down the check-list. A little colour science There is no scientific definition of white light because, like beauty, white is in the eye of the beholder. However, everyone agrees that daylight is white, or at least a type of white, and daylight comes from the sun - a ball of gas with a surface temperature about 6,500 Kelvin (K). "Sunlight is pretty close to black-body radiation," Dr James Nobbs of the University of Leeds tells Electronics Weekly. "It has hydrogen and helium in the outer layers, and these have absorption bands, so it is not quite perfect." Nobbs is head of colour physics at a unique institution: Leeds' department of colour science, which was established in the 1880s to support textile dying - a regional speciality at the time. Black-body radiation is the electromagnetic radiation that is emitted by a perfectly non-reflective object - hence 'black' - at any particular temperature. At low temperature it has only long wavelengths, to which decreasingly short wavelengths are added as its temperature rises. A horseshoe in a furnace gets red-hot, then white-hot as yellow green and blue are added to the red in its spectrum. If you could get it to 6,500K, it would emit something like sunlight. Daylight is not quite sunlight, but a mixture of sunlight and the blue of the sky, said Nobbs. It has a continuous spectrum across the 750nm (deep red) to 380nm (violet) that we see in a rainbow. And then the brain gets involved Humans do not have constant colour sensitivity across this band. Instead the eye has four different types of sensors. Three have overlapping sensitivity bands centred on red green and blue. The fourth is for night vision and plays no part in colour vision. Its sensitivity peaks close to 500nm and its output is perceived only as shades of grey. Although the colours of the rainbow have two ends, the brain happily wraps these into a perceived loop. "Red meets blue psychologically," says Nobbs, "there is no jump, it goes red-purple-blue." All the time that things were viewed in broad band light - the sun, candles or hot filaments - there was no argument over colours as they look the same, if a little more vividly by sun than by candle. Then came fluorescent and arc-based light sources, which do not have broad spectrum emissions. Things could change their apparent colour when transferred from filament light to fluorescent light, and so some way of quantifying the way a lamp rendered colours was needed. Enter colour rendering index Over the first half of the 20th Century, scientists worked on ways to quantify this mixture of spectral physics and human perception, an effort which culminated in international agreement on the colour-rendering index - CRI. It involves the appearance of eight colour samples, chosen to be roughly evenly spaced around the perceived loop of rainbow colours, none of them particularly vivid or washed-out. "They tend to be the same intensity, which is depth of colour," says Nobbs. CRI can be evaluated visually or, as each of the eight colours has its spectrum defined exactly, calculated from that information and the spectrum of the light source-under-test. In practice, it is always calculated. The visual method involves viewing the eight colour samples under daylight, then viewing them by the source-under-test. From this work, eight 'special' colour rendering indices are declared from the differences in appearance of each sample under two types of light. The average of the special indices is the CRI of the artificial light source. A slight complication is the colour temperature of the measurement. And then colour temperature It is from the black-body radiation mentioned earlier that the concept of colour temperature comes. If we could get our horseshoe to 2,700K - the temperature of a light bulb filament - it would be emitting a recognisable white light dominated by red, which is called 'warm white'. By the time it got to 6,500K, emission would have a lot more blue in it - a colour called 'cool white'. Somewhere in between, is 'neutral white'. "A 40W bulb is 2,800K, a candle is 1,900K, and a sunset is around 2,000K. People genuinely feel more comfortable under these warmer colours," says Nobbs. CRI is actually measured by first adjusting the colour temperature of the reference 'sunlight' to the effective colour temperature of the source under test. This colour temperature is established by shining both the reference and the source-under-test on a white surface, then adjusting the reference colour temperature so that it looks most like the test source. If this was not done, a perfect source under test would not get a perfect score (100) unless it happened to be the same colour temperature as the reference. CRI is a good technique, with some limitations. "If you have dyes and pigments with very broad characteristics, there is no problem," says Nobbs. "If you create colour by a narrow band of absorption or reflection, that is where the problems arise." Modern vivid dyes have narrow spectrums, and sources such as 'tri-phosphor' fluorescent tube have bands in their spectrums that are narrower than CRI can cope with. For example, says Nobbs, 'tri-phosphor' fluorescent tube is much more efficient that the broadband type it replaced - 83lm/W compared with 65lm/W. It has three narrowband emitters - red green and blue - whose spectrums are carefully positioned to give a CRI of 93 - a very high figure as 'artificial daylight' sources have a CRI of 94. But some garments that are green in daylight are brown under tri-phosphor tubes. "So high CRI can fool," says Nobbs. "If you take traditional fluorescent warm white, it has a CRI of about 54, yet is perfectly acceptable in the domestic environment." There are better colour systems being looked into by organisation all over the world, but none of them have international agreement yet. When the calculated metric is plotted against people's perception of colour rendering, better systems produce less scattered plots compared with simple CRI. However, differences are not huge, and no single index number is ever going to encapsulate everything about colour rendering. Rise of the white LED Almost all white lighting LEDs are based on a blue-emitting die with a phosphor layer on top than converts some of that blue to broadband amber. The result appears white, even though the spectrum has a narrow blue spike and a broad hump stretching from blue to red. Details differ, but generally cool white LEDs have less phosphor in the way so they are more power efficient at the expense of CRI. Warm white LEDs have more phosphor, so more loss and less efficiency, but a higher CRI because there is less blue spike and more broad hump. Warm white LEDs also tend to have phosphors which stretch into deep red. And here comes the first restriction of CRI with respect to white LEDs: none of the eight standard colour samples include deep red. Adding R9 As well as R1-R8 there are seven other Munsell colour samples that are generally measured by automatic spectral test equipment: R9-R15. These are not included in CRI, but one of them, R9, has a lot of deep red in it. "R9 came up when LEDs were used for medical applications because people needed to see the blood," Rudi Hechfellner director of applications LED maker Lumileds tells Electronics Weekly. And it is not just medical folk who are interested. "A good example is grocery. If you are lighting up beef with poor R9, it looks grey and not so attractive. Ketchup and pasta sauce are the same," says Paul Scheidt, marketing manager at LED maker Cree. "There are some other useful additions: R13 is a [caucasion] flesh tone." Lumileds, Cree, and other lighting LED makers can produce R9 figures for interested parties, but do not specify them because they would have to test to that figure and where would it end? Testing all of the 100 samples in the Munsell colour circle? Instead, LED makers publish spectrums for their products, giving customers burdened with tricky applications all the data needed to make their own calculations. For interior domestic lighting deep red rendering may be less important, but there is a preference for lower colour temperatures (extended red spectrums) of around 2,700K in Europe and North America, and cooler 5,000K white in Asia and parts of southern Europe. All lighting LED makers produce a range of colour temperatures, with varying CRIs depending on the technique used to set the colour temperature. In general, power efficiency - which is the main reason LEDs are being introduced for lighting - is good in all LEDs, but falls with reducing colour temperature: from 100-120lm/W for the best cool whites to 70-90lm/W for the best warm whites. These figures will rise as the technology improves. Avoid over-specifying For the uninitiated, the default option can be to pick a high CRI LED over one with an acceptable CRI. Street lights are an example. High-pressure sodium streetlights, the creamy-yellow ones, have a CRI around 25, an efficiency of 50-140lm/W, and are perfectly acceptable. LEDs can replace these at the same efficiency and offer far better colour rendering. However, municipalities are tending to choose lamps with a CRI of 80 when a CRI of 60 would still be an improvement, be better than much domestic lighting, and save 15% power compared to the 80CRI LEDs. And so? The take-away message from all three experts: Nobbs, Hechfellner and Scheidt, is that there are better systems than CRI, but none of them is dramatically better than CRI in most circumstances. And that any system which produces a single number is going to be flawed. The devil is in the detail and you need to understand the limitation of the technology of interest - a blue spike, amber hump and variable red in the case of white lighting LEDs; or spectral gaps with tri-phosphor fluorescents - and use metrics applicable to your source and your application. Avoid over-specifying warm colour temperature or CRI because you will pay for it in power. Pick the colour temperature that your customer desires, work out the CRI that your customer needs, and then check for special spectral requirements. (c) 2011 Reed Business Information - UK. All Rights Reserved. |