BRENT PROTZMAN explains the different approaches to color tuning in SSL luminaire design and why LED-based color tuning can help deliver familiar, comfortable light.
As LED lighting and controls become more commonplace in the workplace and the home, the next frontier is color tuning. Incandescent lighting suffered from being inefficient, but it delivered color and warmth that made people feel comfortable. LED lighting has traditionally been associated with monotone, albeit efficient light. But new technologies allow LED color to be adjusted, either in terms of white point, full color, or both, almost infinitely to deliver the perfect color temperature for any application. Meanwhile, solid-state lighting (SSL) technology has evolved in terms of better LEDs and luminaire architectures to deliver high-quality light at the color temperature of choice.
There are three predominant color-tuning techniques, and in this article we'll explain how they differ, and where each is most applicable:
• Dim-to-warm tuning
• White color tuning
• Full color tuning
Dim-to-warm tuning best mimics incandescent light, allowing LED lighting to be used in restaurant and residential settings where people typically expect and desire a familiar, incandescent glow. When an incandescent source is turned on at full output, the filament of the lamp glows white-hot. The color temperature varies from about 2700K to 3000K in correlated color temperature (CCT), depending on the lamp. Dimming naturally cools the filament, moving it back to yellow then red the lower you dim the light. This is generally perceived as a warmer light at lower levels, preferred for intimate settings and producing a calming effect.
This described color shift is familiar and alluring, especially in residences, restaurants, and similar spaces. To mimic the behavior of incandescent lamps, LED light sources that are capable of dim-to-warm dimming will adjust the CCT of the LED light engine in an attempt to emulate the color temperature of an incandescent lamp throughout its dimming curve. Fig. 1 depicts this behavior comparing CCT curves for an incandescent lamp and an LED luminaire that's capable of warm dimming.
Warm-dimming-capable SSL products generally require only one control input. The LED driver translates the control input into the appropriate intensity and color temperature. For screw-based LED lamps with integral drivers, the control input is typically forward- or reverse-phase line-voltage control. For LED luminaires where the LED light engine and driver are separate, the control input can include phase control as well as 0-10V analog control or digital inputs such as Lutron EcoSystem, the digital addressable lighting interface (DALI), or DMX. Using a digital control scheme, the power and control wiring can be run separately, and the fixtures can be grouped and zoned through software without needing to change line-voltage wiring.
FIG. 1. Illustrating dim-to-warm tuning, the CCT curves compare an incandescent lamp relative to a warm-dimming LED fixture.
Be aware that the simulated warm dimming behavior of LED light sources may not exactly match that of existing incandescent lamps and may show variations from one manufacturer to the other. The actual performance depends on the architecture of the LED light engine including the mix of LEDs used, and the operation of the driver electronics including the control of the different-CCT or -color LED strings.
White color tuning
White color tuning is the second major class we will discuss here. Selectively tuning the white CCT implies the flexibility to adjust the color temperature of the fixtures to the color temperature preference of the occupant.
In the past, the choice of color temperature was fixed once the lamp or fixture was selected. If a 3500K-CCT legacy lamp was specified in a socket or luminaire, only 3500K could be delivered, even if the finishes in the space ended up being warmer or cooler than originally specified. Emerging studies on health, comfort, and productivity suggest that being able to change, or tune, the color temperature of a light source to match the needs of the application, event, or occupant preference has significant benefits.
Now solutions are available to adjust the color temperature from, let's say, 2500K to 5000K. Tunable-white applications use LED drivers and light engines in a variety of ways to independently control both the color temperature and the intensity of a fixture. Designers and facility managers never have to settle for a mismatch, even after furnishings are installed or as a result of ongoing changes to space design and usage. In some applications, white color tuning can also be set up to provide warm dimming, while providing additional flexibility when necessary.
Tunable white is a technique primarily achieved with LED luminaires, although more sophisticated LED screw-base lamps, such as those controlled wirelessly, may offer it as a feature. There are two primary types of tunable-white lighting products: fixtures with two different CCT LED light engines and fixtures with separate control inputs for intensity and color.
Fixtures with two different CCT light engines
The first method for achieving tunable white consists of separate control of two different color-temperature LED loads (e.g., 3000K and 5000K). The relative intensity of the two loads determines the resulting color temperature of the system as well as the intensity. The color temperature of the fixture can be tuned within the bounds set by the individual LED color temperatures. This can be achieved with two dimmable LED drivers, each with a separate control input. The control input can be phase control, 0-10V, or digital (Fig. 2).
FIG. 2. White color tuning can be achieved with separately-controllable drivers for light engines of different color temperature, although setting intensity and CCT can be imprecise.
This type of control requires additional effort to either maintain color temperature as light levels change, or to maintain light level as color temperatures change. In either case, advanced programming of custom dimming profiles or scenes is necessary, or the user must tinker with the light level until it matches their desired intensity and color. Some drivers and/or fixtures in the SSL sector have integrated support for configuration of dimming profiles.
Separate intensity and color control inputs
Moving up perhaps in system architecture complexity, some fixtures have separate intensity and color-temperature controls. This type of tunable-white fixture uses one control input for intensity and a separate control input for color temperature. This method greatly reduces the complexity of either the system setup or the user experience. The control inputs for these fixtures are typically 0-10V or digital control.
FIG. 3. The LED fixtures shown are equipped with separate light engines and drivers, and a digital control.
Implementation of this separate-input method requires a driver, which has the capability of dynamically mixing its output, using two or more color channels, to generate the requested intensity and color (Fig. 3). This method greatly reduces the complexity of either the system setup or the user experience with the tradeoff being the more complex driver design.
Full-color tuning is also making its way onto the scene. Originally used almost exclusively in theatrical applications, this type of tuning is now being used to create theatrical effects in commercial and residential spaces. Indeed, manufacturers of products from residential lamps to commercial luminaires have now implemented full-color tuning.
This type of color-changing lighting or color modification is different than tunable white and dim-to-warm because it is not based only on white color temperature. Full-color tuning is meant to produce a color output anywhere within the visible color spectrum, and is most commonly accomplished through the mixing of several base-color LEDs. A common example is red/green/blue or RGB color control. To provide a finer range of color control, additional LEDs are included such as amber (RGBA) or white (RGBW).
Full-color tuning is not necessarily used only where saturated colors are desired. Full-color tuning has the capability of matching the color appearance of a broad range of fixture types including warm dim and white color tuning.
Other architectural uses for full-color tuning include tuning for preferred appearance of retail products such as fruit. A lighting designer might choose to deliver a creative appearance of a mixed-use space. For example, 3500K light might be used at a workstation, a pool of reddish light might fill a corridor, and purplish light might wash a lobby wall. Color tuning can also provide higher-efficacy lighting at night when color rendering is less important. Finally, this method is often used in very dynamic color-changing applications and involves advanced controllers that interpret digital control signals and regulate the current to individual LEDs.
Color tuning is an emerging and exciting trend in the lighting control industry. The evolution of LED technology has spawned tremendous opportunities for designers to create unique and engaging environments. Look for manufacturers with a wide variety of solutions and resources to help specify these solutions, and strong fixture OEM partnerships that enable the specification of controls and luminaires that are tested together to provide the highest-quality dimming and performance.
BRENT PROTZMAN is manager of building science at Lutron Electronics (lutron.com).