Why lighting the colour red is so difficult – and how to get it right

You don’t get far in lighting without coming across the concept of colour rendering. Lighting 101 teaches us that some lights show colours better than others – this is measured on a scale from 0-100 giving every lighting product a colour rendering index or CRI. Simple.

But if it’s that easy, why does it so often go wrong – especially with LEDs? Why is Britain full of washed-out shop displays, alien autopsy cafes and harshly lit hospitals?


Trade-off
The first reason is that colour rendering tends to be a trade-off with luminous efficacy: you can get more bang for your buck if you’re willing to compromise on colour.

Another big part of the problem has to do with the colour red. But why is red in particular a headache? Well, first of all because reds are everywhere: there’s a lot of red in the colour of human skin, of meat, fruit and veg, clothes, the Lux logo… we could go on.

Butchers, bakers, surgeons and fashion retailers are among those who need their lights to render red really well. This is gauged using a special measurement called R9, and specifiers will pay a premium for lights with high R9 values, to bring out that red so the steaks look juicy, the loaves look fresh and the clothes look sexy.

Incandescent and halogen sources rendered reds effortlessly well. Other light sources – including fluorescent, metal halide, and most recently LEDs – can struggle. LEDs are, after all, blue blooded – most of them make white by combining blue light with a yellow phosphor.

Although it may not be obvious to the naked eye, this method of creating white light gives an uneven distribution across the colour spectrum – with strong blues but weak reds.

The result is that LED lights, which everyone knows are taking over the world, typically aren’t great at reds. For a lot of applications, it’s not the end of the world, but it means specifiers need their wits about them.

Using more or better phosphors is one solution, and things are improving, but manufacturers still face a dilemma between colour quality and light output, which is why cheap or very high output LED products often don’t show reds well.


Colour and clients
Xicato is one company that knows colour rendering inside out. Its Artist Series range of modules, which use a phosphor mounted separately from a set of LEDs, rather than directly on top, achieve CRIs in the mid to high 90s, and the R9 value is just as high. 

Vice president Roger Sexton says, ‘It’s not that R9 is more important than the other R numbers, but it has come to the fore because it’s so hard to achieve.’

‘I believe it’s one reason why CFLs never really became mainstream, because they made carpets or curtains look a bit green, or your skin didn’t look healthy. In homes or in hospitality, you really should have a high R9.’

Healthcare is another area where lighting designers are particularly keen for R9 to be rendered well, because doctors must be able to assess skin tone and tissue colour to make good decisions and correct diagnoses.

‘LEDs have got better, the industry has got better,’ says Sexton. ‘But it’s always a trade-off – that’s just physics.’

He remains confident that Xicato’s technology, with its remote phosphors to avoid colour shift and improvements in efficacy, will keep giving it a competitive advantage.

Mats Nordin, Verbatim’s LED sales manager for EMEA, says colour rendering is often not even considered by clients, because they just assume they’ll get the same quality from LED as they did from halogen or incandescent. But people are learning.

‘I can see change in the market,’ says Nordin. ‘They’re getting more discerning. We’re getting more questions on it.’

Not surprisingly, there are also a lot of LED manufacturers who would rather you didn’t pay too much attention to R9. And, conveniently for them, the standard method of measuring colour rendering turns a bit of a blind eye to reds.

The colour rendering index is getting on a bit – it’s nearly half a century old, having been developed in the 1960s in response to the rise of fluorescent and other gas discharge sources as alternatives to incandescent.

And unfortunately, the standard palette of eight colours (known as R1 to R8) used to measure CRI doesn’t include red – the closest is a light fuchsia and what you might call a baby pink (see page 88 for more detail on the science). Other colours including saturated red, yellow, green and blue are optional extras, numbered R9 to R14, rather than forming part of standard CRI calculations.

But if a product renders some colours well, it probably renders others well too, right? Not necessarily. Because a CRI measurement is an average of the colours R1 to R8, a seemingly good CRI figure might just mean that a light source renders certain colours really well – while ruining others. And with an LED product, it’s likely to be red that loses out. A CRI of 80 has become an industry benchmark for many applications, but that headline figure doesn’t tell you what variation lies beneath – the R9 value could be painfully low.

So, even if a specifier has looked beyond the headline lumen output figure and found out the CRI, that’s only part of the story. What’s the R9? Manufacturers tend not to quote the figure unless it happens to be particularly flattering.

For these reasons among others, many in the lighting industry feel that CRI is a bit of a blunt instrument.

Verbatim’s latest lamps aim to cover a wider part of the spectrum than is measured by CRI, and Mats Nordin says, ‘It’s difficult for us to explain that the quality of this light is much better than the CRI suggests.’

Xicato’s Sexton says CRI still ‘makes sense’ as a metric, but it needs to be used and understood properly – with attention paid also to the saturated colours not included in the CRI average.

There are alternatives to CRI: a new method that included red was proposed in the 1990s, but the lamp industry said there wasn’t enough evidence to tinker with the existing system.

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, and has gained many supporters, but you’re unlikely to find a CQS figure on a product datasheet – it’s still a long way from being a standard metric.


Don’t take it as red
It would be wrong to suggest that the rise of LEDs is bad news for colour quality. Far from it. New technologies create the potential for high R9 values and all sorts of other exciting colour qualities that weren’t possible before. And as technology improves, the problems of LEDs that don’t do red justice should be alleviated.

But for now, if you want to know how your red shoes are going to look under your new lights, keep your eyes peeled for that R9 figure in the product data – and if it’s not there (which it often isn’t), ask. Otherwise, you may be left feeling blue.