This piece was originally published in the March/April 2020 issue of electroindustry.
by Kees Teunissen, Senior Scientist, Light and Vision, Signify Research
For many decades, light sources have been specified by their correlated color temperature (CCT) and general color rendering index (CRI)1. With the adoption of light-emitting diodes (LEDs) for general lighting purposes, these two specification items, alone, are insufficient to fully characterize the appearance of object colors in a lit environment.
The CRI value indicates only the average difference in color appearance for eight color rendering index test-color samples (TCS) between a light source and a mathematical reference illuminant,1 but it does not disclose the direction of the color shifts. Depending on the exact emission spectrum of the light source, colors may appear more (or less) saturated (colorful) under the light source than under the reference illuminant. Therefore, the appearance of object colors under two light sources with identical CRI values can still be different. To address this issue, it is important to specify changes in colorfulness in addition to color fidelity (aka CRI).
Beyond Color Fidelity
In Dec 2018, The Global Lighting Association (GLA)2 published a document, endorsed by NEMA, with specification items to capture saturation effects.3 It includes a color gamut index, Ga, which represents an average change in colorfulness for eight CRI (test color samples) TCS. It is derived from the area formed by these TCS in color space. If the area for the test source is larger than for the reference illuminant, Ga will be >100 (colors will appear more colorful), and <100 when the area for the reference illuminant is larger. The range of possible Ga values increases with decreasing CRI (Ra) values (see figure). Consequently requiring high CRI values limits variations in Ga. Many studies have demonstrated the importance of red rendering, which is somewhat compromised to increase energy efficiency. Red rendering is already partly captured with the red color rendering index, R9, but again this index indicates only a difference without the direction. The GLA publication3, and the freely available accompanying calculation tool2, also includes chroma indices for all TCS to indicate the direction of the color shifts, where C9 represents the red chroma index. Indices Ga and C9 are based on the CIE 13.31 framework and therefore fully compatible with CRI.
Preference for the rendering of object colors depends on the task, expectations, application, culture, and person and can therefore not simply be captured with one average index number. Knowledge of the intended application and expectations is essential to optimize the individual index values. The new indices, Ga and C9, supplement the existing ones, Ra and R9, and provide a more complete description of the color rendition properties of white-light sources. ei
- CIE 3-1995, “Method of Measuring and Specifying Colour Rendering Properties of Light Sources,” Vienna, Austria: CIE
- “Application of CIE 3-1995 with Associated CRI-based Colour Rendition Properties,” December 2018, https://www.globallightingassociation.org/library