Explore and control LED-based tunable-white lighting (MAGAZINE)

Oct. 21, 2016
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.

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.

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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.

FIG. 1. The graph of CRI and R9 values across a broad range of CCTs is attributable to the three colors used in the LuxiTune white-light source.

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 influence 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). Fig. 1, for example, charts the CRI and R9 values of a three-color LuxiTune 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 flux 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.

FIG. 2. More colors allow a tunable-white light source to deliver a tuning curve that can track the black body and daylight loci over a wide CCT range.

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 flexibility 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. Fig. 2 illustrates this tuning range.

Flexibility in creating ambience

By adjusting in-source color mixing to follow a curve such as that shown in Fig. 2, it is possible to deliver tunable-white light sources that permit smooth adjustment between the limits of extremely cool (high CCT) to extremely warm (low CCT). Some sources also allow the intensity to be dimmed from 100% to as low as 0.5%, at each CCT over the tuning range. This is known as CCT tuning with deep dimming.

FIG. 3. Adjustment of color saturation along iso-CCT lines (blue arrows) allows for more tuning options.

Moreover, natural human responses to phenomena such as sunrise and sunset have conditioned people to expect tones to be cooler when lighting is brightest and to become warmer as lighting is dimmed. Accordingly, retailers or owners of venues such as bars or restaurants often seek to attract customers by presenting a cool and invigorating environment during daytime hours, while using lighting to create a warm glow moving into the evening. This is another version of tunable white known as warm dimming.

Moving forward from the CCT tuning and dimming options that are possible today, one potential next step for tunable-white lighting is to introduce control over color saturation. This is known as Du´v´ tuning as represented in Fig. 3. Du´v´ tuning requires three control handles for CCT, intensity, and saturation. LED Engin has demonstrated Du´v´ tuning within the 7-MacAdam-ellipse rectangles of the ANSI white space along iso-CCT lines, using a DMX controller.

Control requirements and wireless options

The prospect of tunable-white lighting raises questions as to how users can apply settings or adjust the lighting to achieve the effects they desire. In fact, the adoption of tunable-white lighting is tightly linked to ease of use which is in turn determined by how intuitive the controls are. A color-aware user interface is required that allows the user to set flux levels and CCT directly without having to interpret what settings on the controller correspond to actual flux and CCT output.

A suitable control strategy needs to have two handles, capable of controlling CCT and intensity, independently and simultaneously. The basic LuxiTune light engine uses two 0-10V controls; 0-10V has been around in the lighting industry for a while. Although LED Engin has used DMX (both 512A and RDM) successfully for tunable white, DMX is not widely used outside professional or stage lighting markets. DALI (digital addressable lighting interface) is another alternative that is more popular in commercial markets, particularly in Europe. Currently, DALI protocols are available for managing one variable (Device Type 6, or DT6), or two (Device Type 8, or DT8), but DT8 devices cannot yet be certified by DALI.

The options for controlling tunable-white lighting continue to evolve and now include some important developments such as wireless connectivity. One opportunity may be to leverage the rapid pace of progress in the smart-building space. The ZigBee protocol has made some inroads in lighting. LED Engin has tested the tunable-white market with a ZigBee Home Automation (ZHA) enabled tunable-white solution. However, few if any ZHA controllers in the market today are suited for tunable-white applications as they lack the two separate handles for independently controlling intensity and CCT.

Bluetooth Low Energy (BLE) mesh networking promises advantages for tunable-white lighting including an end-to-end solution. Not only is it possible to have two independent control handles for CCT and intensity, the BLE control interface is user friendly, and lights can be controlled by an app on a smart device with touchscreen operation. Secure networks with four levels of access can be set up to control multiple light nodes that extend over large distances without requiring hubs or gateways, and can be controlled from a single terminal with minimal restriction on communication range.

Because all devices on a BLE network can advertise their presence and status, the controlling app can be allowed to access all lighting fixtures and groups of fixtures. This feature would help in commissioning and managing a network of tunable light fixtures in a commercial environment spread across several floors. Moreover, autonomous coordination between nodes and the ability to incorporate input from sensors, would provide the opportunity to implement advanced features such as activating lights in sequence as a person walks along a corridor. LuxiTune with the BLE mesh option can incorporate all these advantages for luminaire developers.

The market for tunable-white lighting may yet be too young for manufacturers to back one wireless option over another. One effective way to give luminaire product developers the flexibility to have multiple connectivity choices is a modular design. You can realize a flexible scenario by having a basic 0-10V driver that works with different modular control boards such as in the tunable-white light engine shown in Fig. 4. This allows a tunable-white solution to be assembled and commissioned with minimal effort by plugging the chosen control module into headers on the basic driver board.

Future of dynamic lighting

FIG. 4. A modular platform enhances flexibility as the options for tunable control continue to evolve.

Lighting designers, luminaire manufacturers, and end users have become familiar with the effects that can be achieved by dynamic white lighting. The market is now ready to accept tunable-white lighting products capable of supporting even more varied effects. Some products have already been successfully realized. Other aspects need to come fully into place, such as the realization that dynamic lighting offers quantifiable benefits for applications in retail, healthcare, hospitality, commercial, and education. Control options for tunable white can be simplified with improvements such as BLE mesh, and intuitive controls are the key to enabling this exciting technology to deliver its full potential.

ISHITA GOSWAMIis a product manager at LED Engin (ledengin.com).