In the old days, colour rendering was easy. Incandescent and halogen sources rendered colours effortlessly well. But newer sources such as fluorescent, metal halide and most recently LEDs, find colour more difficult.
In retail lighting, halogen remains the gold standard for colour quality. Specifiers discussing LED products still refer back to halogen as the ideal.
But do we compare to halogen because it’s truly unbeatable, or just because it’s there?
The latest colour technologies are abandoning halogen as a reference and beating their own path.
The influence of LEDs on colour quality in lighting has been a mixed bag.
In the early days of LEDs, colour rendering was very much a trade-off with luminous efficacy: you got more bang for your LED buck if you compromised on colour. The light from LEDs is essentially blue,, and requires a phosphor to be added to turn it white. But this also cuts the light output, so makers of cheap LEDs scrimp on the phosphor to get a bit more light out at the expense of colour quality. That’s why bad quality LEDs are always too cool and blueish in colour – never too warm.
Things have improved a lot since then, but even for good-quality LED products, colour rendering on the 0-100 CRI scale around the 80 mark. To get the 90+ figures of halogen, you still pay quite a premium. This fact, coupled with the dreadful colour quality of some early LEDs (and indeed, some of those still on sale, especially in the consumer market) has cemented in many buyers’ minds the idea that going LED means compromising on colour.
If you strayed from the black body locus, could you produce lighting effects that might be useful?”
But there’s another side to the LED story that’s only just starting to emerge.
By tuning the precise combination of chips and phosphors, LED technology offers the opportunity to hone and control the colour of light like never before. And as scientists experiment with colour, they’re finding that LEDs can do exciting things
that halogen never could.
LEDs put to the test
In 2013, a group of lighting professionals and members of the public gathered at a research centre in Amsterdam.
They had been invited as part of a study commissioned by LED module maker Xicato, known for its Artist Series of light sources, which use a phosphor mounted separately from a set of LEDs (rather than sitting directly on top), to achieve faithful colour rendering and consistency.
Participants were asked to compare various identical displays of objects – a book, a tablecloth, a bowl of fruit – under different lights. They picked the ones they liked best and tried to describe why.
Some of the displays just looked brighter, cleaner, more… vibrant.
The study was a test of Xicato’s new Vibrancy range of modules, which are tuned to bring out certain colours – particularly blues, purples and pinks – more vividly than traditional light sources. Xicato developed it by casting aside the received wisdom about lighting and colour.
Mimicking the black body
Incandescent sources like fire, the sun or incandescent lamps, glow in different colours at different temperatures (hence the term ‘colour temperature’). This range of colours is known as the black body locus, and most artificial light sources have tried to stick closely to it.
Xicato wondered why. If you strayed from the black body locus, could you produce lighting effects that might be useful?
It’s not as if incandescent sources are perfect. For instance, a lamp with a warm colour temperature will give a nice cosy feel and will tend to make reds and oranges look vivid. But it can also make whites look yellowish. A cooler coloured lamp gives you nice ‘clean’ whites, but dulls the reds.
LED may once have been responsible for some crimes against colour rendering, but now it is showing its potential to do new things with colour”
With LEDs, it doesn’t have to be this way. By choosing just the right combination of chips and phosphors, you can tune the light any way you like, and get effects you couldn’t achieve before.
Xicato accepts that its Vibrancy range won’t be suitable for all applications – the light can have the effect of making certain colours look too vivid, and a little unnatural. But for a lot of retailers, the Vibrancy range offers a powerful new tool to make displays really stand out.
Xicato isn’t the only manufacturer cutting loose from the black body curve and tweaking its light sources.
Philips CrispWhite is a similar technology, now available in spotlights such as the ProAir. Philips says the product offers the best of both worlds, making ‘whites appear more white and colours seem more vibrant’, with ‘clean white light and deep rich colour’.
Philips says: ‘Retailers generally want high-quality light sources that illuminate fabrics in the truest way possible, making colours appear rich and intense while keeping whites bright. A warm colour temperature is often preferred, but a drawback has been that whites can appear yellow.
‘CrispWhite has been developed as a special solution for fashion environments to bring out in the best way both colours and whites… You can now get the perfect combination of warm and cool in a single light source.’
Another way to alter the colour of LEDs is to use a completely different kind of LED chip. Last year Shuji Nakamura was awarded the Nobel physics prize for his invention of the blue LED in 1993. But while the world is still coming round to his invention, Nakamura has moved on. He’s now with US LED lighting firm Soraa, which makes products based not on blue LEDs but on violet ones. These are made from a different combination of chemicals from conventional blue LEDs, and are combined with a special mix of phosphors to make white.
The result is lamps with exceptionally high colour rendering indices – 93 according to lab tests conducted for Lux by Photometric & Optical Testing, and that includes a score of 91 for the rendering of the saturated red colour known as R9, which LED lamps typically render badly.
The colour quality of Soraa’s lamps is so good that the company protests that the traditional measures of colour quality simply don’t do its products justice.
Verbatim has a range of lamps with similar qualities, using a violet chip and a mix of red, green and blue phosphors. This has allowed it to produce LED candle lamps with colour temperatures as warm as 1800K, mimicking the warm orangey glow of a dimmed incandescent.
Time to ditch CRI?
So with LED opening up new colour possibilities that hadn’t been thought of before, has the colour rendering index outlived its usefulness? Some manufacturers think so.
CRI is a bit of a blunt instrument, and it’s causing frustration among manufacturers who believe their products offer better quality than CRI suggests. It’s an important metric, but you have to remember that it’s just an average of a selection of colours. A good CRI doesn’t necessarily mean that the light will render any particular colour faithfully.
More recently the Colour Quality Scale, developed by researchers at the National Institute of Standards and Technology in the US, has emerged as a competitor to CRI. But you’re unlikely to find a CQS figure on a product datasheet – it’s a long way from being a standard metric.
LED may once have been responsible for some crimes against colour rendering, but now it has grown up, and stepped out of the shadow of its predecessors, and showing us its potential to do new things with colour that we never thought of before.