SOWatt, So What? A new methodology for gauging circadian-effective lighting

By Craig DiLouie | Dec 11, 2023
The Light and Health Research Center (LHRC) proposed a new tool to help facilitate circadian-effective lighting adoption in commercial buildings.

The Light and Health Research Center (LHRC) proposed a new tool to help facilitate circadian-effective lighting adoption in commercial buildings. Called SOWatt, it offers a method for ranking luminaires according to how efficiently they deliver circadian-effective light to a standard observer (SO), or simulated position of a building occupant. The goal is to make circadian-effective lighting more actionable by narrowing down the information lighting practitioners need to select the right luminaires for the job.

Light’s nonvisual effects

To incorporate light’s nonvisual effects into design, metrics and tools were needed. Today, two major approaches are used to quantify circadian-effective light: equivalent melanopic lux, which is promulgated by the WELL building rating system, and circadian stimulus (CS), which is used by the General Services Administration. Both focus on delivering sufficient vertical illumination to occupants’ eyes to stimulate the biological clock, with the light’s spectral composition playing an enhanced role. Generally speaking, the goal of circadian-effective lighting is about 400 lux (roughly 40 foot-candles) falling on the occupant’s eyes during the day, ideally in the morning for day-active people.

The discovery and characterization of light’s nonvisual effects was revolutionary. There also appears to be a significant need for lighting design to contribute to circadian health, based on research suggesting a significant number of Americans have trouble getting to sleep and enjoying a full night’s rest. Nonetheless, aside from certain markets like healthcare buildings, demand for circadian-effective lighting has been low. Reasons range from lack of awareness to a perception of heavy lifting and complexity.

The LHRC hopes SOWatt will help. This method has a two-step process. First, the luminaire intensity distribution is positioned in the standard room and the illuminance at the SO’s eyes is determined. Then the illuminance is converted into a CS value. Typically, the CS value produced is lower than the 0.3 threshold, so the watts required to increase the illuminance needed to reach 0.3 is computed. The luminaire requiring the fewest watts to reach the CS criterion is the most efficient.

The luminaire information is reported on a label, enabling lighting practitioners to compare similar types in terms of their efficiency for delivering circadian-effective light to the SO. While based on CS, the SOWatt method can be used with any popular circadian light metrics.

Energy-efficient options

Using SOWatt, it may become apparent to lighting designers that the most energy-­efficient/circadian-effective options involve dimmability based on time of day and ability to produce vertical illumination (without direct glare) at typical viewer eye locations. The second part may involve placing the luminaire closer to the occupant within a layered design approach. The light source’s spectral power distribution can play a supportive role, but again, the volume of light and its timing carries far more weight.

“Arguably, any dimmable product that can be adjusted up and down can be used to deliver circadian lighting,” said Mariana G. Figueiro, LHRC director and professor of population health science and policy at the Icahn School of Medicine at Mount Sinai. 

“The most energy-efficient way to do that is by bringing the source close to the eyes. This strategy provides an efficient layer of light to augment ambient illumination from ceiling luminaires. Circadian lighting is certainly actionable, but the transformation to it may be slow if we limit our thinking to ceiling luminaires. We need to think more about local luminaires that can effectively deliver circadian lighting to users,” she said.

Adoption of SOWatt has been slow, reflecting the overall lack of demand for circadian-effective lighting. 

“We have yet to see wide adoption of the SOWatt methodology, likely because circadian lighting is new to specifiers and because it is a very different approach than the kind typically taught in lighting education programs,” said Mark S. Rea, professor of population health science and policy at the Icahn School of Medicine at Mount Sinai. He hopes that will change.

 “Most specifiers get their technical information from manufacturers, and manufacturers trying to compete in the ‘human-centric’ area differentiate themselves on the spectral power distribution of the light source, not on the intensity distribution of the luminaire,” he said. “Sadly, then, most specifiers are focused on ‘white’ light ­spectra—which makes very little difference for designs aimed at delivering circadian-­effective lighting—and not on optics.”

“The lighting industry needs to educate the public about the benefits of circadian-­effective lighting, and it needs to educate itself about how to deliver circadian-­effective lighting efficiently, inexpensively and without glare. The SOWatt methodology is, we hope, a stepping stone to wider application of ­circadian-effective light in buildings,” Figueiro said. / Travis

About The Author

DiLouie, L.C. is a journalist and educator specializing in the lighting industry. Learn more at and





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