How to Light

How to Light: ASK THE DOCTOR: The Colour Rendering Index

Dr Gareth Jones is here to answer your technical questions. This month, he discusses the colour rendering index.

Q: What is a colour-rendering index and why does it matter?

The colour-rendering index or CRI of a light source is a measure of its ability to render an object’s colours ‘naturally’ compared with a familiar reference source (either daylight or an incandescent light).

The colour-rendering index is a set of ratios that provides a quantitative indication of a light source’s ability to reproduce the colours of various objects faithfully compared with an ideal or natural light source, such as the D65 ‘daylight’ illuminant (used to express the spectrum of light corresponding to open air daylight under clear skies in Western or Northern Europe).

The index comprises a set of 14 ratios as defined by CIE (15 with Asian skin tone also included as defined by JIS) that represent agreed test colour samples. The Ra value is the average of the first eight sample ratios, which provides an approximation of the light source colour quality as a single value.

Different light sources can have widely different CRI values. Under the worst types of light source for colour rendering, such as the low-pressure sodium lamp, most colours are not rendered well at all, resulting in a poor illumination quality. Gaps in the spectra of a light source will lead to poor colour rendering. For example, shining orange low-pressure sodium light on a green apple will make the apple look grey in appearance because the light source doesn’t provide any green light for the apple to reflect. Thus, the importance of CRI is obvious to ensure that the objects or people under illumination appear in their full range of colours.

But CRI is a blunt instrument – it’s a single value used to sum up how a light source renders a whole range of colours. There is much debate about the use of the CRI scheme for new light sources such as LEDs. New schemes such as the colour quality scales and colour gamut are being explored to represent better the wider range of light sources available today. Lighting is, however, a complex set of applications. High-CRI lighting that evenly spreads the colour rendition may not be the best light source for every application, and accenting brighter saturated colours might be a better approach.

A retail example

An interesting example of this would be the vivid emphasis of certain colours in a retail environment – making red meat look more red or green apples stand out. In this case, the tuning of the lighting characteristics to ensure high contrast in the red or green colour may createa reduction in the measured CRI value of the light source.

This is because the CRI value is determined by how closely the light source renders colour compared to a daylight source.

In this retail example, a lower CRI but plenty of red or green in the spectrum would be desirable. But when the same light source is used for general lighting – which demands good rendering of a wide range of colours – then this may result in the lighting creating an unnatural appearance such as the accentuation of skin tone or skin blemishes on people.

Lighting is a complex area and the spectrum of the light source as well as the overall light level in which it is being viewed requires great expertise to ensure good illumination. Thus, the value of the UK’s lighting design community should not be underestimated as we enter the era of solid-state lighting. Light sources of all shapes, sizes and spectral content can be created by many companies with varying experience of lighting. Having light sources appropriately tested in accredited laboratories is also important to ensure that the results accurately represent how they will perform.

Gareth Jones is the CEO of LUX-TSI laboratories, based in the UK and Malaysia.