LIGHTING LEDS
are involved in this set,’ said a 2007 CIE technical report, Colour Rendering of White LED Light Sources. The problem is that LEDs have a different spectral power distribution (SPD) from traditional sources. To produce white light, phosphor is either added to a blue LED, or red, green and blue LEDs are mixed together. The former, most common, method produces an SPD that combines a distinct blue peak with a broad, secondary peak resulting from the fluorescent emission of the phosphor. The RGB approach features a distribution with three quite distinct spectral peaks. ‘The shape of an LED’s SPD is thought
to explain why Ra values, computed using CIE 13.3, can disagree with visual observation,’ says Robert Yeo, owner of Pro-Lite Technology, which specialises in measuring the colour and brightness of lamps and luminaires. ‘CIE 13.3 computes the general CRI Ra based upon the mean of the first eight special colour rendering indices. These indices are computed from the difference in colour of non-saturated hues when they are illuminated by the test light source compared with the reference illuminant. LEDs, with their characteristic spectral peaks, render saturated colours very well, and can give illumination that we would regard as very appealing compared to more traditional lamps. CIE 13.3 fails to account for this phenomenon.’ Researchers Wendy Davis and Yoshi
Ohno, at the National Institute of Standards and Technology (NIST) in the US, have devised an alternative metric which arguably overcomes the shortcomings of CRI. Like CRI, the colour quality scale (CQS) generates a single number
that describes the colour rendering performance of a lamp. And like CRI, CQS is also calculated by simulating the
Colour Quality Scale: Key differences with CRI l
Rather than the eight pastel colours used by the Ra system to compare light sources, CQS has 15 saturated colours. This means less chance of imperfections in colour rendering going undetected.
l
When a different lamp is used, the eye adapts to the white point of the new source. The Colour Measurement Committee’s more accurate Chromatic Adaptation Transform of 2000 (CMCCAT2000) is used in the CQS system, instead of the Von Kries chromatic adaptation correction used for CRI which performs less well than other available models.
l
CRI focuses on how close hues appear to their true colour under a given light source, while CQS considers people’s preference for colour changes that increase chroma. It gives
34 CIBSE Journal June 2012
a better score to light sources that increase chroma than to those that reduce it, so that a source can get a reasonable score even if it has a poor score on one colour.
l
CQS is scaled so that it always gives a result between 0 and 100, whereas with CRI it is possible to get negative scores.
l
The overall score is reduced slightly for light sources with very warm colour temperatures (less than 3500K). This is because these warm colour temperatures tend to lead to smaller colour differences, making it harder to distinguish colours. While this makes relatively little difference for most common light sources – it reduces the score of a standard incandescent filament lamp by about two per cent – it makes a much bigger difference for red lights based on LEDs.
appearance of a number of colours when they are illuminated with a test light source and with an idealised source of the same colour temperature. The differences in colour are assessed and, the larger the colour differences, the lower the overall score. However, there are a number of differences between the two metrics when it comes to the way this is achieved. One of the key departures is that CQS compares the rendering performance with 15 more highly saturated colours, rather than the eight pastel shades. ‘This ensures that there is much less chance of a small imperfection in colour rendering going undetected,’ says Bartlett researcher Peter Raynham. Another major difference is that, while
CRI is about how close to their true tint and shade under a given light source, CQS accounts for the human vision system preference for light sources that promote
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