Craig’s Lighting Articles, Interviews + Opinion, Light + Health

Interview: A New Method for Evaluating Circadian-Effective Lighting

For an upcoming article for Electrical Contractor Magazine, I recently had the opportunity to interview Mariana G. Figueiro, PhD, Director of the Light and Health Research Center (LHRC) and Professor in the Department of Population Health Science and Policy at the Icahn School of Medicine at Mount Sinai, and Professor Mark S. Rea. The topic: a new methodology (called SOWatt), which specifiers could use to evaluate the relative energy efficiency of luminaires in delivering circadian-effective light. Transcript follows.

DiLouie: According to a Data Bridge Market Research report, the value of global demand for circadian-effective lighting is expected to increase from $261 million in 2022 to $1.1 billion by 2030. How would you characterize demand for circadian-effective lighting in the United States in 2023?

Figueiro and Rea: Demand is different than need. We would say the current demand for circadian-effective lighting in the commercial sector is low but the evidence for the importance of circadian-effective lighting and the need to implement it in the built environment is high. The challenge is to bridge need and demand, but until the lighting industry provides that bridge, lighting in 2030 will not increase as expected.

DiLouie: What are the most popular current applications? What is the full scope of applications in existing and new buildings? Are you seeing any early adopters?

Figueiro and Rea: We are seeing an increase demand in healthcare applications. The clients in health care are better educated about the positive outcomes associated with circadian-effective lighting and therefore they are providing the demand. The product availability in healthcare applications is also greater than in, for example, commercial applications.

DiLouie: How actionable is circadian-effective lighting design regarding available products, controls, and spectral choices, and with limitations such as prevailing energy codes? What does a typical design look like today?

Figueiro and Rea: In simple terms, circadian lighting is about providing bright days and dark nights. So, arguably, any dimmable product that can be adjusted up and down can be used to deliver circadian lighting. 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.

DiLouie: Looking at the scientific underpinnings of circadian-effective lighting design, what do we know for certain at this point? What are we still arguing about or are otherwise still learning?

Figueiro and Rea: We know for certain that bright days and dim evenings entrain our biological clock to our local position on Earth. Entrainment leads to better sleep at night, less sleepiness during the day, reduced depression, and better health outcomes. Some are arguing about how to measure “bright” and “dim” but the differences between metrics are small and a distraction from making a difference in peoples’ lives.

DiLouie: What metrics are available for evaluating circadian-effective light stimulus, and how are they being used in various building rating systems and other programs?

Figueiro and Rea: There are two basic approaches to quantifying light as a stimulus for the human circadian system and each of these approaches deals with light spectrum and light level differently. The first approach, promulgated by WELL, weighs the electromagnetic spectrum in terms of the action spectrum of a single photopigment, melanopsin. The recommended amount, or light level, during the day is based upon “expert” opinion. The second approach, used by GSA, weighs the electromagnetic spectrum in terms of the spectral sensitivity of the neural channel exiting the retina that stimulates the biological clock. The recommended amount of light during the day is based upon the strength of the neural channel response for stimulating the biological clock. Functionally, however, both approaches can be approximated by providing 400 lx of “white” light at the eyes during the day.

DiLouie: LHRC has proposed a methodology for evaluating and ranking luminaires based on relative potential to deliver circadian-effective light. Please describe this methodology.

Figueiro and Rea: SOWatt is a methodology for ranking luminaires in terms of their efficiency in delivering a criterion level of circadian-effective light to a standard observer (SO-) simulating the location of the eyes of a building occupant. The most-efficient luminaire is the one requiring the fewest electric watts (-Watt) to deliver the criterion level of circadian-effective light. Our approach emulates the United States Environmental Protection Agency’s iconic new-car window stickers started in the mid-1970s, which compared fuel efficiency between models within a particular “car line” (e.g., SUV, compact, or light truck). The SOWatt method compares electrical efficiency within a particular “luminaire line” (e.g., direct, downlight, direct/indirect) to help specifiers balance various options to achieve different lighting design objectives.

DiLouie: The methodology is based on a standard observer watt, or SOWatt. What does this mean, and why was this chosen as a metric? How does it in turn relate to metrics used to measure design circadian stimulus?

Figueiro and Rea: The amount of circadian-effective light needed to reach the SOWatt criterion is measured in terms of the circadian stimulus (CS) metric. Luminaires are compared in terms of the electric watts needed by the luminaire to provide a CS = 0.3, a criterion amount of circadian-effective light that has been shown to support circadian entrainment and better nighttime sleep in numerous field studies. SOWatt separately quantifies the impact of both spectral and optical factors, showing how each factor affects overall luminaire efficiency. But SoWatt is not necessarily dependent on CS, so other spectral metrics like EML and even photopic illuminance could be used to compare luminaire efficiency independent of luminaire optics.

The SOWatt methodology is, we hope, a stepping stone to wider application of circadian-effective light in buildings. 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.

DiLouie: Circadian-effective lighting design encompasses many factors that include the luminaire’s optical characteristics but also how it’s applied to a space and the space itself. By choosing a “good” SOWatt rating for luminaires, what bases do we have covered?

Figueiro and Rea: Delivering circadian-effective light is only part of any design. Many other factors, ranging from cost to appearance, determine the final design solution.
A fuel economy label might affect a buyer’s choice among SUVs, but it probably plays very little difference in a buyer’s decision to by a compact car or an SUV. Similarly, SOWatt can influence a specifiers choice among down lights, but it probably plays little difference in the decision to use indirect or direct lighting.

DiLouie: The idea is to produce a luminaire label. What would this proposed label include in terms of information, how should it be read and used, and overall what does it tell us?

Figueiro and Rea: This is the label we propose. There may be too much information in the label, but it does provide specifiers with the information they need to select the most efficient luminaire within a luminaire type, or “line.”


DiLouie: What has the response been so far to the proposed labeling? Have any manufacturers or other industry organizations gotten behind it? What is the overall level of interest?

Figueiro and Rea: There has been little interest, unfortunately. Specifiers get their information from manufacturers, and manufacturers trying to compete in the “human centric” area differentiate themselves on the spectral power distribution of the luminaire light source, not on the intensity distribution of the luminaire. Sadly then, most specifiers are focused on spectrum, which makes very little difference for designs aimed at delivering circadian-effective lighting, not on optics.

DiLouie: If the label isn’t available, how should practitioners evaluate luminaires to approximate the same result?

Figueiro and Rea: Again, the SOWatt label is designed to differentiate among luminaires of the same type, not across luminaire types. So within a ceiling luminaire type, intensity distributions that provide a better balance between vertical and horizontal illuminance will, in general, be more efficient at providing a criterion amount of circadian-effective light to the occupant’s eyes.

DiLouie: What other tools can electrical contractors use to simplify application of circadian-effective lighting to spaces?

Figueiro and Rea: The best way to provide circadian-effective lighting is to embrace the concept of layers of light, with each layer meeting a different design objective. A common mistake is trying to deliver both ambient lighting and circadian-effective light from a single fixture in the ceiling. Local lighting at workstations can efficiently provide circadian-effective light to an occupant while the ceiling luminaires can provide relatively low ambient lighting. Trying to do it all with one ceiling luminaire will either over-light the space or under-light the occupant’s eyes. The concept is not much different than the task-ambient technique proposed by regulators in California.

DiLouie: If you could tell the entire electrical contractor community just one thing about how to evaluate circadian-effective lighting, what would it be?

Figueiro and Rea: Take an illuminance measurement where the occupant’s eyes will be located – if you measure 400 lx, you’re good to go.

DiLouie: Is there anything else you’d like to add about this topic?

Figueiro and Rea: Education is so important in any new area of application. The lighting industry needs to educate the public about the benefits of circadian-effective lighting and the lighting industry needs to educate itself about how to deliver circadian-effective lighting efficiently, inexpensively and without glare.


ArchLIGHT Summit – 2023
Circular Lighting Live
Light! Design Expo – IES San Francisco
DLC Controls Summit 2023
SALC IES – Street & Area Lighting Conference
Click For More


Colonial Electric – Inside Sales -Remote.








%d bloggers like this: