LIGHTING BY CRAIG DILOUIE
Adjusting CCT has many applications.
For example, in a restaurant, the atmosphere could be changed from a cool CCT
for a business luncheon during the day to
a warm CCT for intimate dining at night.
While increasing CCT can produce some
energy savings, aesthetics is the primary
reason for adjusting CCT.
Similarly, CRI can also be raised and
lowered by adjusting red in a mix of red,
blue, green and either yellow or amber.
At first glance, changing CRI doesn’t
make sense. There are few, if any, applications where a user would want to
reduce color rendering to support visual
needs. However, it makes sense when
considering the energy-saving potential.
There is a tradeoff between CRI and
luminous efficacy (lumens per watt). As
CRI is lowered, efficacy increases, which
can translate to energy cost savings. For
example, changing CRI from 90 to 48
can reduce energy consumption by 23
percent, with possible energy savings of
up to 30 percent.
Typically, when a space is unoccupied, LED lighting can be automatically
reduced either through on/off switching
or dimming. However, in some applications, light levels must be maintained
for safety or other reasons while only
parts of the space are occupied. A good
example is an airport concourse at night.
At that time, the lights must be on and at
full output even though the concourse is
empty or only partially occupied. In this
application, the luminaires in central circulation could be zoned to operate at full
output and normal CRI and CCT, while
CRI is reduced in peripheral areas based
on schedule or occupancy.
Starting in 2005, the Massachusetts
Institute of Technology (MIT) conducted
several studies to explore the potential
of CRI modulation as an energy-saving
strategy. The main research question
was to determine how far CRI could be
reduced before users found it noticeable
A series of visual experiments involving people observing CRI modulation
in booths provided information about
perceptions to different CRI levels and
visual tolerance to reduced CRI. A year
later, MIT conducted a full-scale, mock-up pilot study in an open office and two
private offices at its Media Lab.
Eight experimental LED panels, manufactured by Sylvania, were installed in
the open-office ceiling and two in each
of the private offices. Light levels were
held constant at about 30 foot-candles,
while CCT was maintained at 5,000 kelvin. The study participants— 13 graduate
students—were assigned tasks in these
spaces. While they worked, the CRI was
adjusted from 89 to 68 over a period of
3 seconds. Afterward, a questionnaire
popped up on their computer screens
asking them what they were doing and
whether they noticed a change.
More than 300 of these queries
Practical today, economical tomorrow
prompted a response; 63 percent said
they hadn’t registered a change. Changes
were more likely to be noticed when they
happened in the immediate area and
less likely to be noticed when occurring
in a peripheral area. This suggests an
appropriate application for CRI modu-
lation is unoccupied spaces adjacent to
occupied spaces. These findings were
confirmed in subsequent additional
Therefore, CRI modulation may
represent an untapped energy-saving
control strategy. At the time of the study,
it wasn’t entirely practical, but recent
advances in LED technology have made
it implementable. However, whether it is
economically viable remains question-
able due to the requirements of using
color-mixing LEDs and granular control
zoning. That being said, as LED product
costs continue to decline, its viability
may strengthen in the future.
Alternatively, CRI modulation may
find a niche in general lighting applica-
tions as an indicator and for personalized
aesthetics. For example, in a private
office, the lighting could be zoned so that
peripheral lighting is color-tunable to
user preference and may automatically
change based on activity, such as the light
turning red to indicate the occupant is on
the phone and should not be disturbed.
The first phase of the LED revolution
focused on developing LED products
that matched or exceeded the perfor-
mance of conventional lighting while
saving energy. In the next phase, manu-
facturers will continue to innovate based
on the LED’s unique characteristics,
including color tuning and easy integra-
tion with intelligent lighting control. CRI
modulation is an example of a potential
application that is practical today and
may become economical tomorrow.
Light of Many Colors
COLOR-TUNABLE LIGHT-EMITTING DIODE (LED) lighting has unlocked enormous potential in lighting design, creating new applications. LED lighting offers the
ability to not only adjust light source color appearance (correlated color temperature, or CCT) but also its color rendering properties (color rendering index, or CRI).
DILOUIE, L.C., a lighting industry journalist, analyst and marketing consultant, is
principal of ZING Communications. He can be reached at www.zinginc.com. I S T