Colour rendering from LED lighting

Specifying white LEDs can be far more complicated than you think…

Specifying white LEDs can be far more complicated than you think...

Make sure you bookmark this article by our Technology Editor Steve Bush about LED lighting, and particularly the role of colour.

He writes:

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 get 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, everybody 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.

“Sunlight is pretty close to black-body radiation,” Dr James Nobbs of the University of Leeds told 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 get 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 slightly yellow-ish sunlight and the blue of the sky, said Nobbs.

It has a continuous spectrum across the 750nm (deep red) to 380nm (violet) 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 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.

“Read meets blue psychologically,” said Nobbs, “there is no jump, it goes red-purple-blue.”

Based on human perception of all viewable colours, in the early 1900s American painter Professor Albert Munsell developed the colour classification system that is still used today.

He spotted that any particular colour has three characteristics:

Hue – the colour of the rainbow

Lightness – how grey and dark the colour is

Intensity (chroma) – how washed out the colour is. Pastel shades have low intensity.

Munsell arranged vivid rainbow colours around the edge of a circle, red meeting purple, and put white in the middle circle’s centre.

Then along lines from the centre to the vivid hues he put all of the intensities of each hue – so there is a circle of pastel shades quite near the middle.

Lastly, turning the circle into a cylinder, he put black above the central white and all greys along a vertical line between white and black. And above every hue/intensity point on the circle, its various lightnesses fading to black.

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 vivid by sun than by candle.

Then came fluorescent and arc-based light sources which do not have broad spectrum emissions.

Things could change apparent colour when transferred from filament light to fluorescent light, and so some way of quantifying the way a lamp rendered colours was needed.

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