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TECHNOLOGY / LED 115
In his last article, Dr. Geoff Archenhold introduced the concepts of human colour perception, spectral power density, luminosity and CIE functions. In part two he covers the definitions of CCT and CRI and also addresses some issues that future vCCT fixture manufacturers will need to take into account to achieve replicable CCT luminaires.
COLOUR TUNABILITY – PART 2
The number of LED emitters being intro- duced into the market by LED manufactur- ers to enable variable CCT products are set to explode over the next twelve months and a raft of new vCCT LED solutions will find their way to lighting designers’ doorsteps. Therefore, it is ever more important for designers to understand how the various systems will stack up in application and what issues they need to be aware of. However, I shall continue with the descrip- tion of Colour Rendering Index (CRI), Cor- related Colour Temperature (CCT) and the two methods used to create colours.
Colour Rendering Index (CRI) Colour Rendering Index is a quantitative measure of the ability of a light source to reproduce the colours of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in colour-critical applications such as photography, retail, surgical and cinematography. The CIE developed CRI to indicate how co- lours appear under different light sources, particularly fluorescent and high-intensity discharge lamps, and to best correspond them to a human’s perception of colour quality. Because most objects are not a single colour but a combination of several, light sources lacking in certain colours can change the apparent colour of an object which is also known as colour shift. Specifically, CRI measures on a scale of 0 to 100 how a light source shifts the location of eight specified pastel colours compared to the same colours lit by a light source of the same CCT. A CRI of 100, or perfect colour rendering, means the light source renders the eight colours exactly how the reference light source renders them. A light source with a CRI of 80 or higher is considered by the lighting industry to provide excellent colour rendering for most indoor applica- tions. The CRI values of typical light sources are shown in table 1. The original method of CRI representa- tion has been found to be an inaccurate, unreliable predictor of colour preference of solid-state lighting products such as LEDs or OLEDs, which emit different light Spectral
Power Densities (SPDs) than fluorescent or HID lamps, and can result in lower or even negative CRI values. For instance, some LED products with a CRI as low as 25 can produce white light that actually make object colours appear more vivid. The CRI metric only evaluates colour rendering as it ignores other aspects of colour quality, such as chromatic discrimination and observer preferences. Recently a new method, Colour Quality Scale (CQS), for providing colour quality has been proposed which overcomes is- sues found with the CRI measurement but unfortunately this has not replaced CRI as the common method of colour quality as yet. CRI suffers from some of the following issues: • Calculating the arithmetic mean of the errors diminishes the contribution of any single large deviation. Two light sources with similar CRI may perform significantly differently if one has a particularly low CRI in a spectral band that is important for the application. • The CRI is equally weighted, whereas humans favour certain errors over others. A colour can be more saturated or less satu- rated without a change in the numerical value, while in general a saturated colour is experienced as being more attractive. • A negative CRI is difficult to interpret. • The CRI cannot be calculated for light sources that do not have a CCT (ie; non white light). • Eight samples are not enough since manufacturers can optimise the emission spectra of their lamps to reproduce them faithfully, but otherwise perform poorly. • The samples used to calculate the CRI are not saturated enough to pose difficulty for reproduction. • CRI merely measures the faithfulness of any illuminant to an ideal source with the same CCT, but the ideal source itself may not render colours well if it has an extreme colour temperature, due to a lack of energy at either short or long wavelengths (i.e., it may be excessively blue or red). • CRI is discontinuous at 5000K, because the chromaticity of the reference moves from the Planckian locus to the CIE
daylight locus.
The CRI may be theoretically derived from the SPD of the illuminant. However care should be taken to use a wavelength sampling resolution fine enough to capture spikes in the SPD of the light source. In order to improve the CRI metric an enhancement was suggested in 1995 by the CIE that extended the number of test colour samples to fourteen where the last six samples provide supplementary informa- tion about the colour rendering properties of the light source; the first four for high saturation, and the last two as representa- tives of well-known objects to provide a CRI value known as R14. Figure 1 shows the typical output of two white phosphor converted LEDs across the R14 values with one LED (in blue) having a much higher CRI and a positive R9 value showing a much improved SPD in the red colour wavelength region.
Light source
Low-pressure sodium (LPS/SOX)
High-pressure sodium (HPS/SON)
Coated mercury- vapour
Halophosphate warm- white fluorescent
Halophosphate cool- white fluorescent
Tri-phosphor warm- white fluorescent
Halophosphate cool- daylight fluorescent
“White” SON Quartz metal halide
Tri-phosphor cool- white fluorescent
Incandescent/halogen bulb
CCT (K)
1800
Clear mercury-vapour 6410 2100
3600 2940 4230 2940 6340
2700 4200 4080
Ceramic metal halide 5400 3200
CRI ~5
17 24
49 51 64 73 76
82 85 89
96 100
Table 1: Various CCT and CRI values for traditional light sources.
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