135
CCT 2700K
Item
Forward Voltage *5 Luminous Flux *6
Chromacity coordinates *7 General Colour Rendering Index *8
Forward Voltage *5 Luminous Flux *6
5700K
Chromacity coordinates *7 General Colour Rendering Index *8
Forward Voltage *5 Luminous Flux *6
3800K
Chromacity coordinates *7 General Colour Rendering Index *8
Table 2 The electro-optical characteristics of the Tiger Zenigata.
following methods: 1. Multiple single white emitters. 2. Single emitter with multiple white phos- phors.
3. Multiple single colour emitters (includ- ing white).
4. Single emitter with multiple colour emitters (including white).
5. Variations of the above themes.
In addition each variant of the vCCT fixture can be controlled and maintained using: • Open loop feedback (OLF) • Closed-loop feedback (CLF)
Each of the above variations obviously have advantages and disadvantages which I will try and highlight in as simplistic way as pos- sible through table 1 (opposite).
Multiple single white emitters This solution is relatively easy to imple- ment as it uses individual LEDs to create a variable CCT output by, for example, having two controllable channels: one channel with a certain colour temperature, say 2700K, and the second channel contain- ing LEDs with a CCT of, say, 5700K. The fixture can then have its CCT changed by controlling the relative intensity amplitude of each channel (or banks of LED CCTs) independently. For example, if Channel 1
Figure 2 Sharp’s new variable CCT Tiger Zenigata
was switched on to full but Channel 2 was switched off, the fixture would provide a CCT output of 2700K. However if Channel 1 was switched off and Channel 2 was fully on, the fixture would output a CCT of 5700K and any ratio of output in between would create a CCT between 2700K and 5700K, thus making the fixture variable CCT. This technique is ideal when large areas are to be illuminated, such as for linear strips or 600mm x 600mm panels. However the main issue in the past has been that the cost of placing twice as many LEDs within a fixture has been cost prohibitive, especially when they are only ever used at 50% overall power input. The system cost issue will be mitigated as the rapid cost reduction of LEDs continues to accelerate and the LED costs become a smaller cost in the overall system. A further potential disadvantage of this technique has been changes in CRI as the CCT is changed from its lowest to high- est CCT settings due to different phosphor types of each channel. Again, this effect has recently been overcome with the avail- ability of high CRI phosphors >80 at higher CCT values >5000K. The advantage of this technique is that only two channels are required to operate and so the driver electronics are much easier and lower cost to implement within the system. As two white LEDs are used in each channel, the power spectral graphs are continuous, so the quality of light spectrum is very good compared to discrete colours. In addition, the efficacy will also be quite high and es- sentially the efficacy is dependent on the efficacy of the types of white LEDs used in both channels. As the efficacy gap between phosphor converted Warm and Cool white LEDs closes it means that a system based on this technique would have a relatively constant efficacy across any CCT value. The fixture to fixture CCT consistency can
Symbol Condition MIN. TYP. MAX. Unit.
33.0 1600 - -
90
33.0 1740 - - -
33.0 1800 - -
90
(36.5) (1840) (0.458) (0.410) (94)
(37.5) (2000) (0.330) (0.344) (90)
(35.5) (2070) (0.395) (0.382) (93)
40.5 - - - -
40.5 - - - -
40.5 - - - -
- - -
- - -
- - -
Figure 3 Tiger Zenigata showing the CCT according to the overall current through channels.
be managed by appropriate binning strate- gies for each of the two channels and good current regulation tolerance between each channel of the LED driver. The LED driver doesn’t require significantly high bit resolu- tions in order to change the CCT or intensity compared to other techniques.
Single Emitter with multiple white emitters Such a solution hasn’t really been widely available but it would have the majority of the advantages and disadvantages of the previous system type. Recently, Sharp has launched the Tiger Zenigata LED array (figure 2) which combines two independent channels at 2700K (96 LEDs of 12 series by 8 parallel) and 5700K (72 LED of 12 series by 8 parallel) to offer a cost effective LED solution for variable CCT fixtures. The Tiger Zenigata electro-optical char- acteristics at a case temperature of 90ºC is shown in table 2 and highlights the high CRI of 90, even at the 5700K CCT range. The 3800K characteristics is based on a combination of both the 2700K and 5700K channels. Importantly, Sharp has characterised this type of variable CCT solution by show- ing how the CCT can be controlled using an open loop solution just by setting each channel forward current accordingly as shown in figure 3. Here, the summation of current through both the 2700K and 5700K LED channels equal 700mA and shows that the CCT is fairly linear. One of the disadvantages of using open loop control is the fact that the luminous flux will vary across the CCT range as shown by the data in figure 4. Although the output only varies by approximately 10% and is therefore unseen when at high light output levels, there is still a fluctuation of output as the CCT is changed and this may be more
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