The color appearance of light sources can have a major impact on how people perceive spaces. In high-end retail, hospitality and similar applications, color temperature choice is a critical design decision.
Traditionally, the choice of color was more or less static after installation. With conventional lamps, the owner could install new lamps, color filters or independently controlled layers of warm and cool lamps. In the large majority of applications, however, this is not economical.
With LED lighting, color output can be tuned to virtually any perceivable color, including any shade of white light. This allows manufacturers, designers or users to adjust color output automatically or manually based on preference or other inputs.
The ability to adjust color output can play a supporting role in circadian lighting schemes. While visual acuity is most responsive to medium-wavelength light (around 555 nanometers, or “green”), circadian regulation is most responsive to short-wavelength light (460 nanometers, or “blue”). Research suggests spectrum can increase circadian response by a factor of two, though quantity of light (particularly on vertical planes) may be more important, and duration and timing of light exposure are also factors.
While circadian lighting is still a pioneering trend, color output control offers immediate practical application. Possibilities include the ability to produce an ideal appearance for a space or objects, imitate the color appearance of a traditional source, and create a custom source. Additional capabilities include calibrating CCT across installed luminaires and maintaining it over time; blending electric lighting with the daylight cycle; and accommodating space changes such as new finishes, furnishings and displays.
With color tuning, we are adjusting color output, but what does that mean?
A series of energy wavelengths make up the visible light spectrum. Each wavelength corresponds with a perceived color. The primary colors are red, green and blue. Add them together and they form white light.
As our understanding of color perception developed, the International Commission on Illumination (CIE) published the Color Space chart in 1931. It graphs perceived colors as the ratio of red, green and blue.
In lighting, a related metric is correlated color temperature (CCT), which describes the hue of a light source compared to an idealized blackbody radiator. CCT is rated based on corresponding Kelvin (K) temperatures. A light source with a high CCT (4,000+ K) has a “cool” or bluish-white hue; medium CCT (3,500–4,000K) has a “neutral” white hue; and low CCT (less than 3,000K) has a “warm” or orange-white hue. These values are plotted on the CIE Color Space as a curve called the blackbody locus.
In lighting design, CCT is an important choice along with color-rendering metrics, which measure color fidelity. Warm light sources bring out flesh tones and warmer colors, while cooler sources enrich blues and greens. Regardless of CCT, white light has a natural appearance if its color output lies along the blackbody locus. If it deviates significantly from the locus, it can appear either greenish or pinkish in tint. ANSI Standard C78.377 designates maximum deviation recommended for LED general lighting, which provides the basis of LED binning and ensuring good color quality.
The above understanding of color—spectrum, hue and tint—forms the basis of color-tuning LED technology.
Color-tunable product types
With LED technology, manufacturers have the ability to change spectral output either as a factory setting or in response to a program or external signal. This gives us three distinct type of color-tunable LED lighting.
Full color tunable: These products allow a range of saturated colors as well as white-light CCTs based on RGB plus amber or white LEDs. The manufacturer may add other colors. White light is challenging for these systems, notably in regards to color rendering.
Dim to warm: These products, when dimmed, automatically make CCT warmer to imitate incandescent or halogen lamp dimming. They typically operate at 2,700–3,000K, which warms to as low as 1,800K during dimming.
Tunable white: These products—ordinarily luminaires—emit white light at an adjustable range of CCTs. They typically combine separately controllable arrays of warm-white (typically 2,700K) and cool-white (typically 5,000–6,500K) phosphor-coated LEDs. Relative dimming of these arrays changes CCT while also allowing light-intensity control. The manufacturer may add other colors to provide good color fidelity and, potentially, a broader choice of color. Some tunable white products are also capable of dim-to-warm operation.
The blackbody locus is represented as a curve on the CIE Color Space; significant deviation can result in tint that affects color rendering. The approach of relative dimming between warm and cool LEDs to achieve tunable white lighting results in a linear gamut. In other words, the extremes of warm and cool may be on the blackbody locus, but the shades of white in between may be tinted. This can work well if the CCT adjustment range is small, though care should be taken when mixing these tunable white products with fixed-CCT products, as users may notice a difference in color between them during dimming.
For a wider range, some products offer an area or triangular gamut approach by adding additional color LEDs. The added colors extend the choice of CCTs and allow saturated colors while fleshing out the light’s spectral content, resulting in nonlinear tuning that follows the blackbody locus.
Tunable white lighting is often achieved using an LED driver and a dimmer. The devices may communicate using low-voltage wiring (0–10V DC, DALI, DMX, proprietary) or wireless transmissions (Bluetooth, Wi-Fi, ZigBee, EnOcean, proprietary). Special instructions are needed to dim the different LED color arrays. The user interface sends these instructions to the driver or a lighting controller that talks to the driver.
A basic approach for warm-cool gamut control is a dimming control connected to two dimmable LED drivers each with its own control output. In this case, CCT and intensity are directly related, so it’s important to specify the levels of intensity to gain the desired CCT.
For area gamut control, one control input is dedicated to intensity and another to CCT. In this case, the driver must be able to dynamically mix the output from two-plus primaries. A user interface might include a slider for intensity and another for CCT, which allows CCT to stay constant during dimming. Manual controls include sliders, keypads and other approaches.
While lighting controls enable tunable-white lighting, color control is a new application. Often, a dimming channel is dedicated for CCT control. More sophisticated approaches incorporate multichannel hybrid LED systems with phosphor-coated white LEDs plus color settings. The additional colors fill in gaps or weaknesses in the color spectrum, resulting in very high color fidelity.
Color points can be moved off the blackbody locus to enrich specific colors in finishes, art and merchandise. Spectra can be programmed for circadian lighting schemes. These systems typically use thermal and optical sensing and feedback to calibrate and maintain the same CCT across installed luminaires.
As the market develops, we may see even more capabilities such as very long fades and color-based programming.
Many aspects of application are the same as when applying standard dimming controls. The designer must know who will be using the controls to ensure their location and level of complexity is appropriate for the user. The dimmer should offer an appropriate dimming range, tune intensity or CCT as precisely as needed, provide appropriate transitions or smoothness across the range (both intensity and CCT adjustment), and, if needed, dim to off. The luminaire should respond to the control signal at an appropriate speed. All devices should be compatible, and the dimmer and driver pairing should not produce objectionable flicker.
Take note of the dimmer and driver curves. The dimming curve may be linear, meaning a slider moving halfway down will result in a 50 percent dim level. Or it may be square law, taking into account adaptation by the eye. For example, a 25 percent dim level appears to be about 50 percent of light output, which is predictable using the square law. Matching dimmers and drivers with the same or different dimming curves can produce different effects.
New dimension of control
Traditionally, lighting control was limited to intensity control (dimming) and on/off (switching). LED technology enables color output control as a new capability, which opens up a broad range of new applications. We are just at the beginning of this trend and what it can do, but familiarizing oneself with the technology can open immediate opportunities.
The U.S. Department of Energy’s Solid-State Lighting Program published a CALiPER report and fundamentals guide for color-tunable LED lighting. For more, visit SSL.energy.gov.