> FOCUS BY CRAIG DILOUIE
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-wave-length 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
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
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
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 tem-
perature choice is a critical design decision.
Above: A suitable application
for tunable white lighting is in
classrooms. The system used in
the room shown above offers
four settings. In “calm” mode,
the lights are set at a standard
intensity level and warm shade
of white light, oriented toward
calming an excited class.