LRT Digest 3: New colour metrics and their use
Authors: Boyce PR , Stampfli JR 
Published: May 2019

Executive summary

The two metrics traditionally used to quantify the colour properties of light sources are the correlated colour temperature and the CIE General Colour Rendering Index. With the arrival of LEDs as a major light source questions began to be asked about the merits of both of these metrics.

The question asked about correlated colour temperature was how far should the chromaticity of a light source be allowed to depart from the Planckian locus before the light emitted could no longer be said to be white? A tolerance to such a departure ( Duv) already existed but now gathered much more attention.

The questions asked about the CIE General Colour Rendering Index were more searching. The limitations of the CIE General Colour Rendering Index were explored and, as a result, several alternative approaches to quantifying the colour rendition properties of light sources were proposed. The most comprehensive approach was produced by the Illuminating Engineering Society of North America, first in its Technical Memorandum IES TM-30-15 and more recently in its revision, Technical Memorandum ANSI/IES TM-30-18, which has been accepted as an American Standard. Both these documents describe a system that contains two high-level summary metrics: One for the average fidelity, i.e. how accurately a test light source renders 99 colour samples relative to how they are rendered under a reference illuminant, and the other for the average colourfulness, i.e. the overall increase or decrease in colourfulness of the same colour samples under the same test light source compared to the same reference illuminant. Associated with these overall average metrics are a number of more detailed metrics and graphical presentations. These aim to quantify and illustrate the variations in fidelity and the direction and magnitude of the shifts in chroma and hue around the hue circle.

Compared to the CIE General Colour Rendering Index or the CIE Fidelity Index, a metric published by the CIE in 2017, ANSI/IES TM-30-18 provides a more comprehensive approach to quantifying and understanding the effects of light source spectrum on the perception of colour. Unfortunately, the new colour metrics described in IES TM-30-15 and ANSI/IES TM-30-18 have not yet been accepted by the CIE. Despite this, some light source manufacturers have started to provide information on their products expressed in terms of the ANSI/IES TM-30-18 colour metrics and designers are beginning to request them. The expectation is that, eventually, the ANSI/IES TM-30-18 metrics will be adopted by many countries and authorities, because they provide a much more comprehensive description of the colour properties of a light source than the CIE General Colour Rendering Index. This will be of value to light source manufacturers and lighting designers as well as those who prepare lighting codes and guides. It is expected that in the future the minimum set of data considered acceptable for describing the colour properties of a light source are likely to be the correlated colour temperature and the associated Duv value, the CIE Fidelity Index or the matching ANSI/IES TM-30-18 Fidelity Index, together with the ANSI/IES TM-30-18 Gamut Index and the ANSI/IES TM-30-18 Colour Vector Graphic.