Explore and control LED-based tunable-white lighting
Two-color sources can enable tunable-white light, explains ISHITA GOSWAMI , but more colors can provide a broader tunable range, better quality light, and granular intensity control.
Solid-state lighting (SSL), leveraging white LEDs, has disrupted markets for traditional lighting products for some time now. With the transition to this energy-saving lighting technology, vendors have been able to offer cool, neutral, and warm white shades that have been quickly understood and accepted by consumers and professional buyers. But LED sources also afford luminaire manufacturers the ability to offer intelligent products that can be tuned to a desired CCT at any time by the user of a space. Let’s discuss why the time is right for tunable LED-based products and how such products can be designed for optimum tuning range, light quality, and intensity control; and subsequently how such a product would be controlled.
Indeed, warmer white lighting, at lower correlated color temperatures or CCTs, is known to help establish a comforting or relaxing environment, which many people welcome early in the morning or in an evening setting. Cool or neutral lighting at higher color temperatures, on the other hand, can have an invigorating effect and therefore is often preferred in contexts such as industrial workplaces, offices, or kitchens to help enhance concentration and maximize human productivity. But SSL can offer flexibility beyond choosing a CCT based on a specific application and accepted lighting practices.
With LEDs affording the opportunity to experiment with light in practical situations, human responses to varying light specifications are becoming more widely understood. Industry and academia are engaged in exploring, demonstrating, and validating the benefits of human-centric lighting (HCL) in applications such as hospitals, retail stores, schools, and offices. Consumers are ready for
tunable white lighting, opening the way for groups such as architects, interior designers, lighting specifiers, and facility managers to use their knowledge of the effects of different white shades to inf luence human moods and behavior and so establish even better environments for working, living, healing, buying, and spending leisure time.
Differences in tunable white sources White LEDs are fabricated either by phosphor conversion of blue or near-ultraviolet emission, or by mixing light from multiple red, green, and blue (RGB) monochromatic emitters. A combination of these two methods is also sometimes used. Adjusting the phosphor-coating composition or color mixing causes the characteristic of the white light to vary. A tunable-white light source is characterized by how many colors or whites are used to achieve the final CCT. There are two, three, five, and even potentially seven color sources that can be mixed for tunable-white developments today, but the actual implementation depends on ease of use, quality, and cost.
The cool, neutral, and warm shades mentioned earlier are referred to as fixed or static CCT white lighting. Typical CCT ranges for warm, neutral, and cool white are 2700–3000K, 4000–5000K, and 5000–6500K, respectively. Together, these CCT ranges define a continuum of tunable-white CCTs that would be perceived as being white.
In a few tunable-white light sources using three or more colors, these white CCTs lie along the daylight locus (DL) and black body locus (BBL) that traverse the long-established CIE (International Commission on Illumination) color space and those sources provide a higher-quality white as defined by CRI, the R9 CRI red sample, and the relatively new TM-30 color-fidelity metric published in
2015 by the Illuminating Engineering Society of America (IES;
http://bit.ly/2bBubZM).
Fig. 1, for example, charts the CRI and R9 values of a three-color Luxi Tune source over a broad range of CCTs. Other tunable-white color sources using a two-color cool-white and warm-white averaging effect are limited in the range over which they can deliver high CRI and high color fidelity.
Importance of path Each tunable-white solution has a predefined path or tuning profile. With three or more colors, it is possible to track the BBL as mentioned earlier and independently dim the intensity of the resulting white light. Two-color sources, on the other hand, follow a straight line over a limited tuning range, and the resulting f lux also has limitations due to the averaging effect of cool white and
warm white needed to strike the right CCT, which has to be compensated with adding more LEDs in the tuning mix.
Both options are currently considered suitable for creating tunable-white LED light engines or modules. However, it is generally accepted that a high-quality white should have no more than a two standard deviation color matching (SDCM) variation along the tuning path. A two-color solution cannot meet this expectation over the entire range of 2700K–6500K CCTs. It is also recognized that true white may lie above, on, or below the BBL, depending on the observer. This f lexibility of a tuning curve that is offset above or below the BBL by design is not possible with a two-color solution. Based on research and customer feedback, LED Engin has established a path for its own tunable-white LuxiTune products, which is within 2 SDCM below the BBL over the 2100K–4300K range and gradually transitions toward the daylight locus from 4300K–6500K.