136 TECHNOLOGY / LED
tion. The Brilliant Mix solution utilises two off-white (white LEDs which have a known blue and green tint to their output) plus a Red/Amber or Blue LED to enable full CCT and CRI control. The advantage of the new Brilliant Mix concept is that it can provide high efficacy output (>100 lm/W) and high CRI (>90) across CCT tuning points from 2700K to 5000K using only 3 driver chan- nels.
Figure 4 Tiger Zenigata luminous flux output according to CCT.
noticeable when the fixture is dimmed to low intensity outputs. In order to ensure such issues are taken out of production systems it is possible for the driver and control system to include either a colour or intensity feedback to ensure the CCT and luminous flux curves are linear accordingly. The quality of the output spectrum is guar- anteed because the system uses phosphor converted white LEDs as shown in figure 5 which outlines the continuous 2700K, 3800K and 5700K CCT spectrums.
Multiple single colour emitters These types of systems can be as simple as using single Red, Green, Blue and Amber/ White LED emitters to produce a white colour tuneable system by adding their emission spectrum together. This approach offers white light sources with potentially very high luminous ef- ficiency. Theoretically dichromatic (two colour) white-light sources are most ef- ficient, offering an efficacy of >440lm/W. The CRI of dichromatic sources is low but can be improved dramatically by increas- ing the number of primary-colour LEDs for a white source. However, sources with a greater number of primary-colour LEDs re- quire more complex LED drivers and control systems and usually have a lower luminous efficacy. Trichromatic (three colour) LED- based white-light sources achieve a good balance between CRI and efficacy with CRI exceeding 85 and efficacy exceeding 300lm/W if the optimum combination of wavelengths is achieved. One disadvantage of such solutions occur due to different LED die materials used within the lighting system and the output and efficiency of the LEDs change with temperature. An interesting paper by Chhajed et al in 2005 demonstrated the variation of LED spectral output and the CCT within a three LED system as shown in figures 6 and 7 respectively. They observed that the chro- maticity point shifts, CRI decreases (84 to
Figure 5 Tiger Zenigata power spectral density graphs for 2700K, 3800K and 5700K CCTs.
72), colour temperature increases (6500K to 7200K) and the efficacy decreases (319 to 297lm/W) as the junction temperature of the LEDs increases from 20 to 80°C. A serious disadvantage of three colour systems occur with changes in CRI as the CCT values change particularly for warm- white light with a low blue component, this method becomes more inefficient (mixing proportion for 2700K: 43%R, 55%G, 2%B). The CCT variation issues become acutely apparent when one then tries to dim the intensity down from 100% to say 1% as the blue content is already low at 2% when the light is at 100% on, so to dim down requires very high resolution current control for the blue channel, which provides certain challenges. In reality, most variable CCT systems will not be able to obtain high reso- lution output stages because they use PWM type topologies and as the resolution of the PWM increases the PWM flicker frequency decreases, which is not good for visual qual- ity or human health. One way of overcoming this issue is to use DC or analogue current methods so there is reduced flicker. In order to improve the spectral quality it is possible to add further LED channels of dif- ferent colours, e.g. white or amber LEDs to the system. Adding further LED wavelengths into the light engine will improve CCT and CRI of the system but impacts on the system efficacy and of course makes the control systems more costly and complex. A novel solution available in the market to- day is the Osram Brilliant Mix LED combina-
Single emitter with multiple colour emit- ters (including white) This type of variable CCT solution is very similar to the previous system type. How- ever, it is much more used for narrow beam angle fixtures such as stage or theatre projection lighting systems because the different LEDs are close together within a single emitter. Therefore, it has similar ad- vantages and disadvantages but in addition the thermal variations are more complex because the power density and thus the thermal density will be much higher.
Open Loop feedback control systems The advantage of open look control systems are that no feedback is necessary for the system to operate, so the driver systems is much less complex. As discussed, the issue with these types of systems that there is no (or little) CCT white point stability as there is no feedback to enable the driver system to automatically compensate for variations in LED current(s), LED junction and ambi- ent temperatures, LED colour binning and external lighting conditions. Therefore, it is recommended that if you want to specify variable CCT lighting fixtures you will need to avoid any system based on open loop feedback control as colour accuracy and consistency between fixtures will not be guaranteed.
Closed Loop feedback control systems A closed loop control system is one where the control system self-compensates ac- cording to certain variables which are fedback from the systems outputs. In the case of a variable CCT system for example
Figure 6 Spectral variations of a three LED (RGB) white light system with junction temperature (from Chhajed et al. 2005).
Figure 7 Chromaticity/CCT variations of a three LED (RGB) white light system with junction temperature (from Chhajed et al. 2005).
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