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TECHNOLOGY / LED
As part of Enlighten Americas’ ‘Town Hall Meeting’ session, Randy Burkett led a debate on CQS - the new alternative to CRI measurements and perhaps one more appropriate for LED light sources
For many years Colour Rendering Index (CRI) has been used as a metric to describe the impact of a light source on the colour of surfaces. However, with the advent of LEDs people have been questioning if CRI is still a good measure.
The first thing to point out is that no one single number index system is ever going to fully describe how a light source impacts on various colours. It is easy to imagine two lamps with the same CRI one of which is good with red colours but distorts blue co- lours and the other which is good with blues but distorts reds. However, the question re- mains is CRI the best metric to describe the colour quality of lamps? CRI was developed in the 1960s and was designed to give some idea of the performance of the recently developed fluorescent lamps. It has worked reasonably well until the advent of LEDs. The problem with LEDs is that they tend to put a lot of energy into narrow bands in the spectrum and so can end up with quite poor colour properties, however, CRI does not always score such light sources poorly. The Colour Quality Scale (CQS) has been developed at National Institute of Standards and Technology (NIST) by Wendy Davis and Yoshi Ohno with the objective of overcom- ing some of the shortcomings of the existing CRI method whilst still providing a single number to describe performance. CRI and CQS are both calculated by simulating the appearance of a number of colours when they are illuminated with a test light source and with an idealized source of the same colour temperature. The differences in co- lour are assessed and the larger the colour differences the lower the overall score. However, there are a number of differences between CRI and CQS in the way this is done.
The first major change is that instead of using eight pastel colours of the Ra system to compare light sources CQS uses 15 more
Increased chroma Natural image Reduced Chroma
saturated colours.
The use of the additional, more saturated colours ensures that there is much less chance of a small imperfection in colour rendering going undetected. The next change is that when a scene is illuminated with a different light source the eye adapts to the white point of the new source. In the CRI calculation this was allowed for using a mathematical tech- nique called a von Kries transform. The Von Kries chromatic adaptation correction has been shown to perform poorer than other available models, and Colour Measurement Committee’s Chromatic Adaptation Trans- form of 2000 (CMCCAT2000) is used in the CQS system.
The next change is that for CRI the colour difference is calculated in CIE W*U*V* colour space, in the CQS calculation the CIE L*a*b* space is used as it has been shown to be more perceptually uniform than W*U*V* Whilst CRI is purely about how close colours appear to their true colour under a given light source, CQS also considers people’s preference for colour changes that increase chroma and thus make objects look more vivid. The three images above illustrate this. Most people will prefer the image on the left where the chroma has been increased. CQS gives a better score to light sources that increase chroma than to those that reduce it.
When the colour differences with each of the colour samples are summed with CRI
then a simple average is taken, this means that it is still possible for a light source to get a reasonable score even if it has a very poor score on one colour. In CQS a root means squares summation process is used and thus a single poor score makes a much bigger difference to the overall score. There are two further points of difference between CRI and CQS. The first of these is that CQS is scaled so that it always gives a result between 0 and 100, whereas, with CRI it is possible to get negative scores. The other difference is that for light sources with very warm colour temperatures (less than 3,500 K) the overall score is reduce slightly, this is because these warm colour temperatures tend to lead to smaller colour differences and thus make it harder to distinguish colours. This makes relatively little difference for most common light sources, it reduces the score of a standard incandescent filament lamp by about 2%, but it makes a much bigger difference for red lights based on LEDs
These features of CQS make it a much more robust measure of colour quality than the old CRI system. Like all single number met- rics it has limitations in the information it can convey, but it should prove to be much more reliable and useful than CRI.
Based on an information sheet by Peter Raynham, Lighting Education Trust Lecturer at the Faculty of the Built Environment, University College London.
The eight colours used for Ra
Spectra of LED and fluorescent lamps with similar colour appearance. Note that, although the fluorescent has a higher peak than the LED, as the peak is narrower there is a lot less energy close to the peak wavelength
The 15 colours used for CQS
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