How to use closed-loop LED tuning for variable white colour balance.

September 04, 2018 // By Tom Griffiths
Tom Griffiths, of AMS, describes the difference between the three models of colour tuning control in use with lighting today. In more detail is describes how sensor-enabled closed-loop control and the optical design considerations which need to be taken into account when adding closed-loop control components to a lighting fixture.

With the decline in the euro-per-kilolumen price of LEDs, the economics of LED lighting have turned in favour of white colour tuning in new commercial and high-end residential lighting installations.

For instance, a typical office-lighting troffer can have a selling price between €150 and €300; the linear strip of LEDs in it will generally contribute less than €4 to the bill-of-materials (BoM) cost. This huge differential between the cost of the main component and the price the user pays for the fixture gives the manufacturer scope to add features and value, and thereby resist pricing pressure from low-cost, and potentially lower quality, competitors.

Colour tuning is a particularly attractive way to differentiate an otherwise standard lighting fixture. The term colour tuning generally denotes the ability to adjust the white correlated colour temperature (CCT) of a fixture’s light output. This would typically be bounded by very cool-white at a CCT of up to 6500K and very warm-white with a CCT of 2200K. A typical tunable white luminaire would employ less extreme tuning in a range from an incandescent-like 2700K to the 5000K favoured for office lighting. In the example of the commercial troffer, this requires dual linear strings of LEDs at CCT values of ≤2700K and ≥5000K, and a sophisticated control circuit.

Even if this raises the troffer’s manufactured cost for LEDs and electronics to €25, it is still a small fraction of the selling price. And the colour tuning capability allows the fixture manufacturer to market the light under the banner of ‘human-centric lighting’ (HCL). HCL is aimed at enabling the lights to cover CCT ranges which can help support the human body’s natural circadian rhythm.

For example, a tunable white system could be configured to produce a cooler (blue-white) light in the middle daylight hours, gradually warming the colour temperature in the approach to dusk to give warm (yellow-white) light, potentially mitigating some of the blue light influence at night (see Figure 1). Studies show that such adjustment of the colour of artificial light produces hormonal responses in humans which promote productivity and alertness in daylight hours, and which assist relaxation and sleep in the night.

Fig. 1: HCL systems adjust lighting from cooler to warmer colour temperatures towards dusk.

In other words, HCL offers the promise of increased user comfort and potentially productivity as well as health-supporting benefits compared to fluorescent or other non-tunable lights. These attractive benefits can help to sustain the premium pricing of many of today’s commercial and residential lighting products.

Next: Three methods


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