Colour Rendering Index
CRI is a measure of how well light sources render the colours of objects, materials, and skin tones. The test procedure involves comparing the appearance of eight colour samples under the light in question and a reference light source. The general colour rendering index (Ra) scale goes up to 100, and the average differences measured between the two light sources are subtracted from 100 to get the CRI. So small average differences will result in a higher score, while larger differences give a lower number. Of all the colours possible, only these eight are measured. The samples used are pastels, not saturated colours.
If the lamp to
be tested has a correlated colour temperature (CCT) of less than 5000K, the reference source is a black body radiator (an incandescent lamp). By definition, a tungsten lamp will have an Ra of 100. For higher CCT sources, the reference is a specifically defined spectrum of daylight. Therefore, light sources that mimic incandescent light or daylight for the eight colour samples are, by definition, the ones that will score highest on the CRI. For general
illumination, an Ra of 80 is considered the minimum acceptable, while for hazardous jobs, or tasks which rely upon precise colour matching, an Ra of 95+ may be
necessary. Source: CIE
The Colour Quality Scale will provide a more level playing field with which to compare the performance of LEDs, according to some experts
increased chroma – making objects appear more vivid. ‘CQS gives a better score to light sources that increase chroma than it does to those that reduce it,’ says Raynham. ‘This means 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 mean square (RMS) summation process is used, so a single poor score makes a much bigger difference to the overall score.’ Another factor CQS takes into account
is that when a scene is lit with a different lamp, the eye adapts to the white point of the new source. The Colour Measurement Committee’s Chromatic Adaptation Transform of 2000 (CMCCAT2000) is used in the CQS computation, rather than the somewhat discredited mathematical technique called a von Kries transform, used for CRI. While LEDs have been the catalyst for
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an alternative, the new metric is generally ‘a much more robust measure of colour quality than the old CRI system,’ according to Raynham. ‘Overall CQS will provide a more meaningful description of the colour rendering of any type of lamp,’ agrees Yeo. ‘It will provide a more level playing field with which to compare the performance of LEDs with more traditional lamp technologies.’ CJ
Lamp type
Low pressure sodium Quartz metal halide
Incandescent (tungsten halogen) White (blue + phosphor) LED*
Tri-phosphor warm white fluorescent *Lumileds Luxeon Rebel ANSI 3500K
Colour rendering for different lamps CCT (Kelvin) 1,800 4,200 3,200 3,500 2,940
CRI (Ra, CIE 13.3) ~5 85
100 85 73
Source: Wikipedia June 2012 CIBSE Journal 35
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