Replacing CRI: new ways to specify lighting-colour impacts

The way in which lighting designers specify the colour-rendering properties of white light – and white light’s ability to alter humans’ perception of colour – is about to change, thanks to the development of tunable white light and a near-revolutionary new way of specifying its colour impacts, as described by the US Illuminating Engineering Society (IES) in its Technical Memorandum-30 (TM-30).

On the way out is the colour-rendering index (CRI); established in 1964, CRI categorises white light only in terms of its warmness or coolness. The alternative method being proposed in TM-30 uses two metrics to describe a light source’s colour-rendering properties: fidelity, which resembles CRI in many respects, and saturation, considered through an all-new metric called ‘gamut’ that indicates how saturated an illuminated colour appears to be.

While progress is being made on both, the fidelity metric is closer to finalisation than the gamut metric. Optimal finalisation will result in an American national standard via the IES, and (hopefully) an international standard via the CIE (Commission Internationale de l’Eclairage).

The subject was the focus of a National Lighting Bureau panel discussion held at the NLB’s recent Annual Lighting Forum. Panelists included IES Industry Relations Manager Mark Lien; Greg Yeutter of lighting manufacturer LUXTECH; and lighting designer Randy Burkett.

Lien warned that fully adopting the working metrics in TM-30 now would be premature, because near-term change is inevitable. He commented that the IES developed and issued TM-30 solely as an initial test process for lighting experts to evaluate and improve.

The panelists noted the research conducted at Pacific Northwest National Laboratory that indicates people prefer saturated colours to higher-fidelity rendering, a finding reinforced anecdotally by sales of Kodak’s consumer films: users preferred films that created more saturated colours, making autumn leaves look warmer and skies appear as a deeper blue.

The panel discussion was very interesting. It is online free of charge here.

The National Lighting Bureau is an independent not-for-profit foundation that has served as a trusted source of information about lighting since 1976. The Bureau provides its services to the public free of charge, thanks to the funding of sponsors including professional societies, trade associations, labor unions, manufacturers, and agencies of the US government.


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