New colour metrics proposed for characterising lighting

23 September 2015 News Opto-Electronics

Accurately quantifying the colour rendition characteristics of a light source is a complex problem.

Many aspects of colour rendition, such as colour fidelity, colour discrimination or colour preference, should be simultaneously considered during the design and specification process. At present there is no one metric or measure that can accurately quantify all aspects of colour rendition and/or identify the most desirable light source for every application.

The Illuminating Engineering Society (IES) has now published a technical memorandum – TM-30-15 – which describes a method for evaluating light source colour rendition that takes an objective and statistical approach, quantifying the fidelity (closeness to a reference) and gamut (increase or decrease in chroma) of a light source. The colour metric applies to the characterisation of both LED-based solid-state lighting (SSL) and legacy sources, and has been proposed to the International Commission on Illumination (CIE) for adoption as an industry standard.

Although based on the new colour samples, the new Rf fidelity metric is similar to CRI, while the new Rg gamut metric is based on the Gamut Area Index (GAI). The new method also generates a colour vector graphic that indicates average hue and chroma shifts, and which helps with interpreting the values of Rf and Rg.

TM-30-15 provides equations and direction for calculating Rf and Rg, including the spectral reflectance functions for the 99 CES (colour evaluation samples). It is accompanied by a software tool to aid in calculation and display of the results; access information for the software tool is provided in the publication.

The IES TM-30-15 colour rendition method consolidates and synthesises numerous research efforts that have been ongoing for several years, and was developed by representatives of the manufacturing, specification and research segments of the lighting industry. The technical memorandum is available for purchase from the IES website.

For more information visit www.ies.org

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