Colour temperature and colour rendering of white LEDs
Optical measurement of white LEDs represents a major challenge due to factors such as operating temperature, burn-in phase and ageing, which affect lamps’ emission performance.
These spectral and intensity changes also cause a change in colour appearance. The latter must be analysed and specified properly to ensure good reproducibility of the measurements. Colour appearance has long been described by the concept of colour temperature. This article discusses the concepts of colour temperature and correlated colour temperature.
In the mid-twentieth century, scientists classified the effects of varying types of illumination on perceived colour. The reasoning was that this categorisation would help lighting engineers specify the type of lighting best suited to the application. It is known as the Colour Rendering Index (CRI).
This index assists lighting manufacturers in targeting their products at the various markets. For example, illumination in a photographer’s workroom would require a high CRI. In the case of street lighting, CRI is often sacrificed for the sake of efficacy.
Fig 1: The three steps of the binning process.
LED manufacturers support the use of CRI in LED specifications to allow a direct comparison of their products with conventional lighting. It is useful as it helps the end-user understand the advantages of SSL as a viable alternative for mainstream lighting.
The validity of CRI testing for white light LEDs formed by grouping red, green and blue chips has been questioned. Users report that colour perception under such illumination is more accurate than the low CRI measured would seem to suggest. We will discuss here the application of the CRI in LED lighting.
Solid-state lighting (SSL) sources offer controllability during the manufacturing process of the following properties:
- Spectral power distribution.
- Spatial distribution.
- Colour temperature/colour rendering.
- Temporal modulation (flickering).
- Polarisation properties.
Such “smart” light sources can be adjusted to specific environments and requirements, allowing them to offer many benefits in the fields such as automobiles and transportation, communication, imaging, agriculture and medicine. This variation calls for clear specification and accurate measurement.
Binning of white LEDs
White LEDs are sorted in a process known as binning. It ensures consist light colour and quality. The binning process comprises three steps (see Fig. 1).
- The LEDs are sorted according to their light output or luminous flux.
- They are sorted by the correlated colour temperature (CCT) of the white light they produce.
- They are sorted by the voltage drop across the LED. This voltage drop is known as the forward voltage (VF).
This article discusses the colour temperature and colour-rendering index measurements which are not part of the binning process.
Colour temperature
Why “colour temperature”, Kelvin?
To understandwhy we refer to “colour temperature” and why it is measured in Kelvin, we must must understand the operation of blackbody irradiators.
Blackbodies are objects or matter which absorbs all incident electromagnetic radiation – none is transmitted or reflected. Blackbody radiators are perfect incandescent radiators (see Fig. 2).
Fig. 2: Blackbody radiation.
The spectrum of optical radiation emitted by a blackbody, and therefore the apparent colour, depends on the temperature of the blackbody (Planck’s Law)(see Eqn.1).
(1)
The temperature can be used to describe the colour appearance of a light source – the colour temperature (Kelvin) – because the apparent colour and temperature of blackbodies are linked.
Colour temperature relates to the chromaticity diagramme (see Fig. 4). The blackbody locus (Planckian Locus) represents the chromaticity of a blackbody radiator at different temperatures. For colours based on the blackbody theory, blue occurs at higher temperatures while red occurs at lower, cooler temperatures, contrary to the cultural associations attributed to colours according to which red relates to “hot” while blue represents “cold”.
The mired is a unit of measurement expressing colour temperature (see Fig. 5, Eqn. 2).
Fig. 3: Blackbody radiation.
The mired system is an easy way to deal with the problems of matching the colour temperatures of different sources. It is used widely in colour photography. Gels and filters used for controlling colour temperature have mired values, either plus (+) or minus (-), and indicate the colour shift they effect. This system allows repeatable and predictable control of the colour temperature of the light radiated from the source and ensures consistency, from shot to shot.
Correlated colour temperature
Where the chromaticity of a light source falls exactly on the blackbody locus, the colour appearance of the source can be described by a specific colour temperature (tungsten filament lamps).
An exact match can often not be made between the chromaticity co-ordinates of the source and the blackbody locus. Correlated colour temperature (CCT) is then used to describe the nearest visual match.
Fig. 4: The CIE Chromaticity Diagramme.
CCT is the absolute temperature the blackbody has when it has approximately the same colour appearance as the source. It is also measured in Kelvin. Straight lines of constant CCT on the chromaticity diagramme are used for some common sources. CCT usually does not have anything to do with the surface temperature of the source. Some typical CCTs of sources are given in Table 1.
Table 1: Typical colour temperature of various sources.
Table 2: Colour temperature of fluorescent lamps and LEDs.
Differences in colour temperature during binning
The American National Standards Institute (ANSI) specifies a colour temperature difference during the binning process which correlates with the differences obtained with compact fluorescent lamps. A maximum difference of 350 K is specified, which relates to a seven-step MacAdam ellipse (see Fig. 7).
Reputable LED manufacturers have narrowed their binning selection to a two to four-step MacAdam ellipse, where a two-step Mac Adam ellipse specifies a maximum difference of 120 K.
Colour Rendering Index (CRI)
The effect of a light source on the appearance of coloured objects is described by its colour rendering properties (see Fig. 7).
Fig. 5: Blackbody colours in mired scale.
Fig. 6: Seven-Step MacAdam Ellipses.
CRI applied
Important to consider are both the absolute and possible colour appearance of the object to be lit, as well as possible colour shift under different light sources.
The CIE test-colour method rates lamps using colour rendering indices which represent the degree of the resultant colour shift of test objects under test lamps, compared with the colour of the test object obtained under reference illuminant with the same CCT.
The CIE Special Colour-Rendering Index (Ri) is calculated on the basis of the colour difference ΔE determined in the previous step:
(2)
where:
M = the mired value desired.
T = the colour temperature in Kelvin (K).
This step is repeated for the eight test-colour samples (R1 to R8 are obtained where i = 1 to 8, (see Fig. 8). Ri = 100 means perfect colour rendition for that specific test-colour sample.
Fig. 7: Good vs. poor colour rendering.
The CIE General Colour-Rendering Index (Ra) is the arithmetic mean of the eight Ri values: Ra = (R1 + R2 + R3 + R4 + R5 + R6 + R7 + R8) ÷ 8
The factor 4,6 used in the formula for Ri was selected so that Ra becomes 50 when a standard warm white fluorescent lamp is used as the test source and an incandescent lamp is used as the reference lamp. Some more test-colour samples have been added to improve the results. The measurement time has almost been doubled (see Fig. 9).
Fig: 8. Ra with eight test colours. Note: These colours are of moderate lightness and approximately equally spaced in hue.
Fig. 9: Ra with 15 test colours.
Conclusion
Correlated colour temperature measurements play a very important part during the binning process of white LEDs. The test method used is a well-established procedure based on sound theory and practical experience. Discrepancies are not experienced regularly and satisfactory results are usually obtained. Reputable
LED manufacturers even narrow their binning selection to a two to four-step MacAdam ellipse. It shows that they have confidence in the stability and quality control of their processes and in the selection process during CCT measurements. It is also an indication of the importance of specifying narrow limits for the CCT of their product.
We all agree that it is important that people and objects must look as expected when the lighting engineer designs lighting systems and installations.
Unfortunately, the validity of CRI testing for white light LEDs has been questioned following reports that, in some cases, colour perception of under such illumination is much more accurate than the low corresponding CRI would suggest. The test method for the CRI of white LEDs is still considered and debated by the CIE.
References
[1] Illuminating Engineering Society: IES Lighting Handbook, tenth edition, ISBN # 978-0-87995-241-9
[2] Wyszecki & Stiles, Color science – concepts and methods, quantitive data and formulae, second edition, 2000.
[3] Dr. Thomas Nägele & Richard Distl: Handbook of LED metrology, instrument systems, Version 1.1.
[4] International Commission on Illumination, technical report, “Measurement of LEDs”, 2007.
[5] Wikipedia: http://en.wikipedia.org/wiki/Color_temperature
[6] Luxtra Futuresmart: http://luxtralighting.com/colour-rendering-index.php
[7] Olino Renewable Energy: http://www.olino.org/us/articles/2009/11/30/a-close-look-at-the-color-rendering-index-cri-or-ra
[8] Lamptech: http://www.lamptech.co.uk/Documents/FL%20Colours.htm
[9] Digikey Electronics: http://www.digikey.com/en/articles/techzone/2011/may/led-specs—understanding-the-color-white.
Acknowledgement
This article is based on a paper delivered at the Eleventh IESSA Congress and AGM, 2015, and is published here with permission.
Contact Elsie Coetzee, NMISA, Tel 012 841-3047, emcoetzee@nmisa.org
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