SOWatt? The Standard Observer Watt

A standard observer methodology for evaluating circadian-effective luminaire efficiency

By David Pedler, Mark S. Rea, and Mariana G. Figueiro

Light isn’t just for vision anymore. Lighting researchers and professionals across multiple disciplines agree that an abundance of circadian-effective light in architectural spaces promotes good health, good mood, general well-being, and a good night’s sleep.

The consensus breaks down, however, when competing systems are advanced to quantify light’s circadian effectiveness or otherwise explain how light confers these benefits.

The not-so-good news? If the Dec 2022 issue of designing lighting is any indication, consensus on a metric for quantifying light’s circadian effectiveness doesn’t look like it will be reached anytime soon, neither among the research community nor the lighting industry at large. The great news? For the application at hand, it doesn’t really matter which metric is used to quantify circadian effectiveness. Though we obviously stand behind the circadian stimulus (CS) metric, all the presently available metrics will produce broadly similar results among real people in real spaces. Melanopic EDI (mEDI)? Equivalent melanopic lux (EML)? Which is the better choice? We say, “SOWatt!” (Standard Observer Watt).

Rigorous laboratory and field work, peer review, questioning and revision, publication and debate are part and parcel of the scientific process, to which we maintain a vital, lifelong commitment. But at some point, in practical terms, it all comes down to providing circadian-effective lighting for people in buildings. To date, however, lighting designers and engineers have lacked a salient tool for identifying luminaires that can best meet that relatively simple end, free of competing claims over circadian efficiency functions and metrics.

That is why we have proposed a methodology for evaluating and ranking luminaires in terms of their relative potential for delivering circadian-effective light to the eyes of a standard observer. Our approach emulates the United States Environmental Protection Agency’s (USEPA) automobile fuel efficiency standard methodology that led to the iconic new-car window stickers and transformed the new-car marketplace in response to the 1973 oil crisis and 1975 passage of Energy Policy and Conservation Act.

Facsimile of the 2008 USEPA fuel-economy sticker for a new car in the sport utility vehicle (SUV) “line”.

Today, we face a no less urgent but largely silent crisis — sleeplessness, which the Centers for Disease Control (CDC) has declared a public health epidemic. This epidemic puts tens of millions of Americans at risk for car crashes, medical errors, industrial accidents, and a well-known host of disorders contributing to serious medical problems (e.g., cancer, metabolic diseases, depression) that are associated with disordered sleep. Given that the CDC’s declaration was made almost 10 years ago, the time for a redoubled effort to provide circadian-effective lighting in buildings has long passed.

Every new-car buyer has at least once studied a USEPA fuel-economy sticker, which follows a procedure that measures exhaust emissions from test vehicles on simulated city and highway drives to calculate fuel efficiency. The resulting prosaic window sticker displays the vehicle’s data and position in the range of vehicles in the same vehicle “line” (i.e., compact, sedan, sport utility vehicle, light-duty truck, etc.). We believe our luminaire standard methodology can do for the lighting industry what the USEPA did for the automobile industry.

Simplicity is the key, given the daunting array of factors that can affect the amount and distribution of circadian-effective light reaching a person’s eyes, including the characteristics (e.g., configuration, relative spectral power distribution [SPD], spatial distribution) of the luminaire itself, the reflectance values of the space’s surfaces (i.e., walls, ceilings, floors, desk/ table tops), workstation partition heights, window locations, occupants’ direction(s) of view, and so on. But, choosing among candidate luminaires is governed by a single common denominator—their capacity to deliver circadian-effective light during the day to an occupant’s eyes. There are, of course, collateral issues that must be considered in any lighting design, like glare and energy, but these are easily managed by experienced designers. What is new and sorely needed for current lighting design is an objective way to measure the circadian effectiveness of different luminaire types.

Our proposed standard observer method differentiates spectral from optical factors to facilitate comparisons between the electric power (Watts) needed by luminaires to deliver a criterion circadian stimulus value (SOWatt), whether it be specified in terms of EDI, EML, CS or even photopic illuminance at the eyes.

The proposed Circadian-effective Lighting Estimates tag showing the SOWatt value for a sample luminaire. Like the USEPA sticker, the tag shows the luminaire’s position with respect to the range of values calculated for luminaires in the Ceiling Mounted – Direct Batwing Distribution "line”.

Whatever measurement criterion is used, the lowest resulting SOWatt value within a given luminaire “line” indicates which is more or less efficient for delivering circadian-effective light to a standard observer. Importantly, as is the case with the USEPA sticker, the actual performance of a given luminaire will depend on when (timing), for how long (duration), and under what conditions (partitions, reflectance values of wall, floor, furniture, and ceiling) it is installed and operated. A distinct advantage of this method is that SOWatt values can be calculated virtually using photometric prediction software or through actual photometric testing.

As electric utilities become increasingly interested in the nonenergy benefits of lighting, the SOWatt metric provides a convenient, intuitive, and meaningful scale for supporting and promoting energyefficient circadian-effective lighting.

And nobody can reasonably say, “So what?” to that. ■