LED and Colour

Making it WHITE

Every lighting development since Edison and Swann, has gone through a number of iterations, as you’d expect. 
First of all, the technology needs to be consolidated and strengthened so that the product does what its meant to do, and carries on doing it in all kinds of conditions. For the LED-as-light-source, that process took from the mid 1950s to the mid 1990s.

Then there is always the period when a new piece of technology appears when we’re sufficiently mesmerised by the fact of something new happening that we don’t notice that - not to put too fine a point on it - its actually a rubbish product. For the LED-as-light-source, that took from the mid 1990s to …. well …. how about today?

Why so critical?

The final test of an LED has to be in the quality of the light that it produces – not its efficiency, nor its longevity, however important those factors may be. No – the singlemost important issue is whether the light is any good. The first generations of architectural LEDs only produced coloured light – there was no WHITE light available. The development of the WHITE LED was seen as the Grail that everyone in the industry sought. And today it seems that we’re in a mid-evolutionary condition where the world is being shared between Neanderthal LEDs that demonstrate very little colour sophistication and a breed of Lux Sapiens that can (almost) decide for itself what tone of WHITE it would like to be.

Let’s talk about WHITE light, because a long battle has been fought to achieve it. In a similar way to the fluorescent tube, the ‘root colour’ of an LED is BLUE. The WHITE light comes from a process of phosphor coating, which acts on the BLUE light and transmits a WHITE light, the tone of the light being based on the make-up of the phosphor coating.

Are you still with me?

The real issue for LED developers and luminaire manufacturers is to ensure colour constancy in the WHITE light from the many millions of LEDs that are produced in the world. A method of measurement had to be agreed, and that method has become known as the MacAdam Ellipse Tolerance. I don’t intend to go into the history of where the MacAdam Ellipse came from but go to Wikipedia: (reference: “MacAdam ellipse”) if you have to.

When you look at two light sources, you can ask yourself ‘Are they both the same colour?. That, simplistically, is what this is all about. The MacAdam Ellipse Tolerance method is a way of describing how efficient the LED source is, and the lower the number, the better the LED colour performance. A common value for LED is a 7-ellipse tolerance. An excellent quality of colour performance would be a 1- or 2-ellipse tolerance.

But while we’re about it, let’s not forget a corresponding issue – it might be an ‘accurate’ WHITE light, but is it a good WHITE light? And what does a good WHITE light mean anyway? Our idea of a good WHITE light is based on nature; its what we see out of the window, or from what we see when we set fire to something (even a Norwegian fish). Those ‘natural’ WHITES have been measured and have been plotted onto colour diagrams, so whenever a new WHITE light comes along we can check it against the reference colours. The accuracy between the two is known as the Colour Rendering Index., where an Index value of 1.0 is perfect – and anything less than 0.8 isn’t worth considering. The latest commercial struggle in the LED market is based on whether your LED is better than 0.9, and there’s no reason why it shouldn’t be – though there are plenty of LED products offering +0.8 which are absolutely fine for general usage.

So there you are: two things to look at when you’re searching for a good LED light source – what is its MacAdam ellipse Tolerance and what is its Colour Rendering Index.