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124 TECHNOLOGY / LIGHT SOURCES NOT GETTING NOTICED


As part of the EuroLED 2013 conference in Birmingham this June, Patrick van der Meulen, Application Support Manager at Xicato, presented a talk considering colour inconsistency in LED sources.


When something looks great it often gets noticed, like works of art or cars or landscapes. Isn’t it odd that with lighting, the opposite is true? Think about it, you know great lighting when you don’t notice it and you know bad lighting when you do notice it. Of course “bad” is a terribly biased statement, lighting professionals have very good lists that address all that goes into designing and executing good lighting. Nonetheless, perhaps the most noticeable example of bad lighting to a lay person or professional is inconsistent colour. It’s almost impossible to not see it. Research to date has been clear that colour quality is the key element of customer satisfaction and that consistency is a key factor. Colour inconsistency could be the result of mismatched lamps, mixing technologies, or, perhaps most likely, colour shift. As the adoption of LED lighting picks up pace and purpose built luminaires become the preferred solution, colour shift looms large as an issue that must be addressed. For much of the last decade the lighting industry has focused on greater LED efficacy and lumen maintenance. Important to be sure but lumen maintenance is of little value if your installation looks like a parade of Smarties after a year or two (Figure 1). Unfortunately, unlike lumen maintenance


Figure 2: Colour change over time of white light sources can vary dramatically.


Figure 1: It’s hard not to notice the colour change here!


and TM-21, there are no standards for acceptable colour change over time. There are four key challenges in predicting colour change over time: a) every manufacturer’s LEDs behave differently, b) materials are affected by time and environment, c) manufacturer’s change their products over very short periods of time, and d) manufacturers typically have very little colour change data. If Figure 2, from the U.S. Department of Energy, doesn’t give you pause then you’ve got nerves of steel. As is clearly described by the Lighting Research Center, MacAdam Ellipses provide a way for us to measure and evaluate difference or change in colour. In general, it’s preferred to have each measured light point within a 3 MacAdam Ellipse area. As light points move further apart


and out of a common area, difference in colour become quite noticeable. A common mistake is to confuse correlated colour temperature, CCT, with colour of the light that we see. If we look at a series of MacAdam ellipses in the context of a CIE(xy) colour space at 3000K CCT. We can see clearly that light considered to nominally be at 3000K can appear white, if it’s on the blackbody locus but will tend to tint toward green or pink as it moves off the locus (Figure 3). Within a 3 MacAdam ellipse area, the colour of the light will vary and moving beyond 3-steps, the colour differential becomes severe. This is important because if the light source is initial anywhere within a 3-step space and over time can move 3 or more steps, then the light source could end up in a 6-step area or worse. LED manufacturers are at different stages of understanding how their products change over time. A survey of datasheets will show that some make no report of colour change, some give a single number and other provide specifications for both initial colour differences and changes over time. Xicato has proposed and is using a new metric “C” which represents the steps of colour shift over time in much the same way “L” is used for lumen maintenance. For example, C3 50,000 hours would mean that colour shift will be 3 steps or less after 50,000 hours. If the initial colour point is known, then it’s relatively straightforward to gauge how the light might appear over time. Achieving colour maintenance over time is a total system issue and there are several contributing factors including phosphors, lenses, reflectors and coatings. However, the bulk of the focus remains on the LED. Whether a white LED solution is used or a remote phosphor module like Xicato’s XSM,


the temperature relationship between the phosphor and the diode has proven to have the most significant affect. Keeping the phosphor cool slows the phosphor degradation which in turn stabilizes colour. Thermal design then becomes critical for both the light source and the luminaire. At the light source level, modules can have a tremendous advantage. Using Xicato’s design as an example, the phosphor temperature can be lowered by 30°C or more. Colour consistency and maintenance over time is an area of real concern for all lighting professionals as it directly affects quality of light and end-customer satisfaction. Fortunately, more is being learned and


Figure 3: Each ring represents 1 MacAdam ellipse or 1-step of colour change from 3000k.


measurement tools are improving. Today there are five key factors to be considered when evaluating for colour maintenance: 1. Materials – proper selection of materials in the system will reduce degradation. 2. Design – optimising for long-term performance, specifically with respect to temperature is essential for good colour maintenance 3. Specifications – light source and luminaire manufacturers need to provide both initial colour specifications and colour specifications over time. 4. Testing – testing for lumen maintenance is well established and testing for colour maintenance needs to be expanded, refined and reported 5. Warranty – It’s now possible for light source manufacturers to offer colour consistency warranties. This indicates increased confidence in light source behavior that should be strongly considered. www.xicato.com


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