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).


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 or objectionable.


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 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 modulation is unoccupied spaces adjacent to occupied spaces. These findings were confirmed in subsequent additional visual experiments.


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 questionable 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 applications 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 performance of conventional lighting while saving energy. In the next phase, manufacturers will continue to innovate based on the LED’s unique characteristics, including color tuning and easy integration with intelligent lighting control. CRI modulation is an example of a potential application that is practical today and may become economical tomorrow.