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CRI Modulation

Traditional lighting systems such as fluorescent and high-intensity discharge (HID) are controllable, though control is predominantly limited to On/Off and, in some applications, dimming. In contrast, LED lighting can deliver not only significant energy savings and improvement in source longevity, but also the ability to implement another dimension of control: color.

In lighting design, color quality is predicted and evaluated using correlated color temperature (CCT), which expresses the shade of white light, and color rendering index (CRI), which expresses how closely a source renders colors compared to an ideal source.

The development of LED lighting enables the ability to adjust CCT using separately dimmable arrays of warm- and cool-CCT LEDs; color mixing RGB+A LEDs; or by adding separately dimmable color LEDs to white light LEDs.

As the color of light can dramatically transform the appearance of spaces, people and objects, this capability offers amazing design possibilities. The primary benefit is aesthetic, though color is now being linked to circadian health.

Another interesting potential is CRI modulation. By mixing red, blue, green and yellow or amber LEDs, we can not only change the shade of white light but also its color rendering.

The primary question, of course, is why we would want to do this. The simple answer is energy savings. By gradually reducing the red component of an RGB+Y (or A) mix, CRI declines as luminous efficacy increases, while both CCT and intensity can be maintained. From 10 to 25 percent energy savings can be realized, depending on the application.

This type of control strategy could be effective under certain conditions. Namely, spaces where 1) the lights must remain On during periods where there is no occupancy, and 2) intensity and CCT must be maintained.

An example of an application where good savings might be realized is an airport concourse. Late at night, the lights must be On and at full output for safety, though much of the concourse is empty. In this application, we could zone the luminaires in the central circulation areas to operate at full output and normal CRI and CCT. In peripheral areas, however, CRI could be reduced to save energy based on a schedule or occupancy.

Energy savings could be improved by increasing CCT as well, but such a change would mix sources with different CCTs in the field of view, which would be objectionable from an aesthetics perspective.

In 2005, a study was conducted at the Massachusetts Institute of Technology (MIT) evaluating CRI modulation as an energy-saving strategy in an open office space and two private offices at MIT’s Media Lab. The research goal was to determine how far CRI could be reduced in both the immediate and peripheral areas before occupants noticed the change and/or found it objectionable.

Eight experimental OSRAM SYLVANIA tunable ceiling-mounted LED panels were installed in the open office and two in each of the private offices. The 13 subjects were graduate students with no prior knowledge of the study. While CCT was maintained at a constant 5,000K and light levels maintained at about 30 footcandles, CRI was gradually changed over three seconds from CRI of 89 to CRI of 68. Fifteen seconds later, an instant message pop-up questionnaire asked the occupant what they were doing and whether they noticed the change.

Out of 320 queries that received responses, 203 indicated occupants did not notice a change. Occupants were most sensitive to changes in their immediate area and when changes occurred simultaneously in immediate and peripheral areas. The least noticed changes were those that occurred solely in peripheral areas. These results suggest CRI modulation is likely to be an acceptable method of saving energy in spaces that must be illuminated at full output even though they’re only partially occupied.

At the time the final study results were released in 2008, CRI modulation was considered a theoretical but not a practical strategy. LED technology has progressed a great deal since then, making it much more viable. However, the requirement of color mixing and careful zoning currently poses an installed cost that challenges this control strategy’s economic viability based on the potential energy savings. That being said, LED and control technology continues to advance in terms of performance and cost, and as it does, CRI modulation may increasingly become viable as an energy-saving lighting control strategy.

In the meantime, energy savings may be realized by applying color in another fashion in occupied spaces—as an indicator. Imagine a private office zoned so lighting outside the occupant’s immediate area becomes a saturated red when he or she is on the phone and therefore should not be disturbed. This would save energy while indicating to coworkers what the occupant is doing. There are many applications where this approach could be viable.

Color tuning has opened a vast potential in lighting design and application, and we are just beginning to pioneer. While these applications focus on aesthetics with some interest in circadian health, this extraordinary emerging dimension of lighting control may also be used to maximize energy savings.