138 TECHNOLOGY / LED
one could use a thermistor to measure LED temperature, an ambient light sensor to measure light intensity, a colour sensor to measure LED colour output and current sen- sors to measurement current through LEDs accurately. One or more of these variables would be used to feedback into a control system to regulate the CCT of the fixture to enable an accurate solution. Obviously, such a control system is more complex and requires additional components but in reality this type of solution is the only one if you require high quality, accurate and consistent CCT values between fixtures in a lighting scheme. Caution should be taken that even with closed loop feedback control systems er- rors still occur so, for example, even if an optical colour sensor is deployed within a system potential errors will still be present, such as: • The colour sensor only detects a portion of the light of an LED group due to location. • The colour sensor sensitivity curve drifts and / or changes with temperature. • The sensor’s deviation from linearity changes with temperature. • A deviation between the sensor and the eye sensitivity curve. • Limitation of the analog to digital conver- sion (ADC) resolution. Of course, the number of feedback mecha- nisms employed and the quality/resolution of the feedback signals determine the overall accuracy of a lighting system, but this needs to be traded off against system complexity and cost.
The following questions (see box out) are not exhaustive as I haven’t covered aspects, such as what patents and IPR do you have to enable you to sell variable CCT fixtures etc, but it gives you a clear steer as to the quality of the solutions you will specify and avoid post-installation problems such as co- lour shimmer, which was highlighted to me by several lighting designers during Light + Building last year. If you have any further questions on this matter or any LED lighting technology, I am always at your disposal.
g.archenhold@
mondiale.co.uk
Dr. Geoff Archenhold is an active investor in LED driver and fixture manufacturers and a lighting energy consultant.
Parts 1 and 2 of this variable CCT series of articles are available to read online at
www.mondoarc.com.
10
questions to ask when specifying variable CCT fixtures.
Q.1 Is the control system open-loop or closed-loop? If open loop I would avoid if possible. If closed loop then ask the supplier what they measure, e.g. temperature, output colour etc, and how many times per second the measurements occur. The faster the control system the better the performance is when dimming, for example.
Q.2 What is the CCT and CRI colour ac- curacy of the control system? This enables you to determine how good the control system is and the errors in the system.
Q.3 What is the CCT and CRI error be- tween a group of 20 similar fixtures This enables you to determine if you have CCT and CRI variation issues between fixtures. It doesn’t necessarily fit that Q.2 means that consistency between fixtures is the same accuracy and hence why you must ask Q.3.
Q.4 What current control method do you use (e.g. PWM, PAM or DC)? If the answer is PWM, PAM or other pulsing control methods then you need to follow on with additional questions below.
Q.5 If the control system uses a pulsing technique, what is the pulse frequency? You know the ripple current will be 100% if pulsing, but the frequency is important as it determines the current resolution, which will highlights how good the dimming perfor- mance is. If the frequency is low then you may see stroboscopic effects, but more importantly you will see a colour shimmering effect across a scheme, especially if the pulsing is not synchronised between driver channels and fixtures in the installation. The higher the frequency the better, ie 1000-5000 Hz would be good.
Q.6 What is the current and feedback resolutions used in the system as this determines the accuracy of individual fixtures and the performance during dim- ming? The higher the current resolution and feed- back resolution the better the accuracy. In reality it would be very good to have 16-bit resolution. However, in practice most systems will be 8-bit or 10-bit at most. A word of warning: because a 10-bit system actually needs 16-bits to store ie; 8-bits + 2 bits so some manufacturers will state they
use 16-bits but in reality it is only 10-bits although they use 16-bits to store the 10-bit number! It is important to have as high a feedback resolution as possible because this defines the accuracy of the overall system.
Q.7 What control protocol do you use to control the CCT between fixtures? This is important as you don’t really want to tie yourself into proprietary manufacturer standards if you can help it. There are several standards that are now available that can help control variable CCT lighting systems, including DALI colour and RDM, so opt for one of these if you can.
Q.8 How is the fixture control system af- fected by ambient light or other lighting systems? This is very important because some control systems and fixtures will start to act incon- sistently if ambient light affects the control system and light sensor feedback. A really good test of this would be to point one or more variable CCT lighting fixtures di- rectly at each other and see if they become unstable, or to point one of the fixtures out- side through a window and see if it remains stable with no colour shifts or flickering.
Q.9 Can you show me the CCT and CRI graphs against dimming level and manufacturing batch variation between fixtures? This will show you how the variable CCT sys- tem actually performs when dimmed down. It will also highlight the variation between fixtures within the manufacturing process.
Q.10 What type of variable CCT system do you employ? This will enable you to determine the type of issues you may see as highlighted within this article. When specifying variable CCT systems you should always ask for demon- strations of four similar fixtures to be done at the same time. The reason being that if just one fixture is used for a demonstration, the human eye isn’t very good at seeing co- lour or intensity inconsistencies and so even an open loop system could be demonstrated changing its CCT and would look good. However, if you have multiple variable CCT fixtures your eye will be able to detect subtle variations in colour and intensity (when dimmed to below 50%) quite easily and so you can better judge how the system performs when multiple fixtures are used together within open plan spaces.
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