FAQ - Colour Rendering Index and LED Lighting
Background
The apparent colour of objects depends not only on the reflectance of the object (i.e. what proportion of radiant energy it reflects) but also the spectral content of the light incident upon it. A consequence of this is that the same object can appear to be a different colour, depending on the light illuminating it. White light with different spectral power distributions can have the same Correlated Colour Temperature (CCT) while rendering various coloured objects differently. LED lights in particular can have different spectral power distributions from product to product, all vastly different to that of traditional sources such as daylight and incandescents, making their effect on room appearance difficult to anticipate at the time of purchase. CCT alone is therefore not enough to ensure the colour quality of a light source.
Figure 1. Appearance of colour sample under different light sources1
Colour Rendering Index
Colour Rendering Index (CRI) is a measure of how accurately colours are rendered under a test light source in comparison to their apparent chromaticity under a reference source with the same CCT. It is a reference-based metric developed by the International Commission on Illumination (CIE) that utilises eight standard colour samples and six special colour samples as part of the test-method. Generally ranging between 0 and 100, a CRI of 100 indicates colour rendering in a manner identical to the reference, where the reference is blackbody radiation for CCTs below 5000K1.
According to the US Department of Energy, a CRI in the 70s is acceptable for interior applications, a CRI in the 80s is good, and a CRI in the 90s is excellent.
CRI and LEDs
While CRI is generally indicative of colour fidelity, the metric's applicability to the performance of LED light sources is limited; the CIE have stated in their standard CIE 177:2007 that "CRI is generally not applicable to predict the colour rendering rank order of a set of light sources when white LED light sources are involved in this set"2. As such, CRI can at best be regarded as a rough guideline for the colour rendering quality of an LED light source.
References
1 http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/led-color-characteristics-factsheet.pdf
Solid-State Lighting Technology Fact Sheet: LED Color Characteristics
2CIE 177:2007 Colour Rendering of White LED Light Sources
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