134 TECHNOLOGY / LED
In the third part of his series on colour tunability in LED fixtures, Dr Geoff Archenhold explains the different methods for creating variable CCT luminaires and suggests some questions specifiers can ask to ensure they get the right product.
SPECIFYING VARIABLE CCT LIGHTING FIXTURES – PART 3
In the previous articles on colour tuning fix- tures I introduced how colour and CCT were defined and calculated along with poten- tial errors, but all of this science and long equations don’t help the lighting designer to specify variable CCT lighting fixtures. So this article will aid lighting designers in understanding the different methods of creating variable CCT fixtures and give them the heads-up on various questions to ask lighting manufacturers. Firstly, I would like to say Happy New Year to all of you in the lighting design com- munity and what a great year it will be too, with lots of new technologies making an entrance into the mainstream and ever increasing efficiency of LED-based products helping to make our industry ever more environmentally friendly. Of course it’s not all going to be plain sailing in 2013 as eco- nomic winds still blow against the building and construction sector in the UK and many parts of Europe, and the battle of differen- tiating quality from poor quality products is even more challenging. Secondly, I was surprised that one of my roundup predictions was confirmed so early in 2013 with Cree recently announcing the
System Type
Multiple white emitters
Single rmitter with multiple white emitters
Multiple
single colour emitters
Single emitter with multiple colours
Variants
Ease of System design
Easy Easy
Medium to
Complex Complex
Medium to
Complex Table 1 A basic overview of each variable CCT system solution type. No No Yes Yes
production of both the XM-L2 single-die LED and the four-die XLamp MK-R LED. The new XM-L2 LED (figure 1) is a 5 x 5mm package that delivers up to 186 lumens-per-watt at 350mA, 25°C and 5000K with up to an incredible 1198 lumens at 3A of drive cur- rent. The second generation products offer 10% greater lumen output at a 5700K CCT and 17% better lumen output at 3000K. The products are binned at 85°C junction tem- perature and 700mA drive current, and at 3000K deliver 224 lumens and 112 lm/W. The Cree XLamp MK-R was launched only two years after Cree broke the 200lm/W R&D efficacy barrier in their Labs. The MK-R is a single 7mm x 7mm package which holds up to 4 LED die to deliver up to 200 lumens-per-watt (at 1W, 25°C). The 6mm optical source is able to deliver up to 1769 lumens at 15W, 85°C. Characterised at 85°C, the MK-R component is available in 2700K to 7000K colour temperatures and offers minimum CRI options of 70, 80 and 90 (at selected colour temperatures). The MK-R has a low thermal resistance of just 1.7ºC/ W and has a maximum forward current of 1.25A with a maximum junction tempera- ture allowed up to 150ºC. Having such a low
Suitable for Large Area
Yes No No
Suitable for narrow beam angle
No Yes No Driver
complexity - Cost
Relatively easy – 2 channels
Relatively easy – 2 channels
High – number of colour channels = driver channels
High – number of colour channels = driver channels
High – number of colour channels = driver channels
Cost
2x LED + 2 channel driver
2x LED + 2 channel driver
High High High
Figure 1 The Cree XM-L2 single die high power LED
thermal resistance means that with good thermal management design the relative luminous flux will only reduce to 80% even at a junction temperature of 150ºC. I have always stated that R&D lab break- throughs take about two years to filter through into production products, so based on announcements in 2012 we should see 250lm/W LED emitters by 2015 and between 150 and 200lm/W fixtures by 2015 (depend- ing on application, of course!)
How will manufacturers create variable CCT fixtures? The majority of variable CCT fixtures will most likely be created by one of the
Dimming & Intensity capability
Depends on driver and feedback
Depends on driver and feedback
Complex as feedback is essential
Complex as feedback is essential
Complex as feedback is essential
Fixture to Fixture
consistency
Depends on driver and feedback
Depends on driver and feedback
Can be good depending on control system
Can be good depending on control system
Can be good depending on control system
Spectral Quality / Efficacy
Very good / Very good
Very good / Very good
Good / Very Good
Good / Good
Good / Good
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