A building interior filled with daylight is one of the most photogenic and iconic demonstrations of sustainable design. But until recently, it was anybody’s guess as to what a “daylit” space looked like—and anybody’s right to define measures of success as best suited for their project. The lack of consistency not only created confusion among designers, building scientists, owners, and occupants, but it also prevented fair comparisons between built environments and between design options.

In 2000, the U.S. Green Building Council (USGBC) introduced the LEED rating system to standardize metrics for several primary sustainable design characteristics, including daylight. But equitable comparisons of daylight performance still weren’t possible until 2013, when the Illuminating Engineering Society (IES) adopted and published the testing and calculation guide Lighting Measurement 83 (LM-83), Approved Method: IES Spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE).

LM-83 is the first IES-adopted, evidence-based annual daylighting performance metric in the lighting industry. It resulted from a six-year-long research effort by the IES Daylight Metrics Committee (DMC), led by the energy-efficiency consultant Heschong Mahone Group (now a part of the TRC Companies), that included human-factors field research in which subjects answered questions about their visual preferences and comfort levels in more than 60 spaces across three building types (classrooms, offices, and other) and several climate zones.

The DMC, an international team of about 15 daylighting experts, building scientists, designers, manufacturers, and code consultants, also fostered the development of computational methods to run annual daylight simulations of the same spaces studied in the field, vetted the results, and crafted the resulting daylighting guidelines and criteria within LM-83. (Full disclosure: I have chaired the DMC since 2012, served as vice-chair from 2009 to 2012, and was a committee member before that.)

Also known as climate-based daylight metrics or dynamic-daylight metrics, sDA and ASE are now incorporated in common lighting analysis and design software packages, such as Diva-for-Rhino, OpenStudio, Radiance, Daysim, and LightStanza. The creation of LM-83 and these two annual daylight metrics have added rigor and complexity to daylighting design consultation and building performance assessment, and establish performance expectations that can be referenced in project specifications and owner’s requirements.

SDA: Is There Enough Daylight?
Spatial Daylight Autonomy (sDA) examines whether a space receives enough daylight during standard operating hours (8 a.m. to 6 p.m.) on an annual basis using hourly illuminance grids on the horizontal work plane. In lieu of collecting a year’s worth of data in the field, sDA is calculated virtually through computational simulation with precise parameters. It references a local climate file—such as an EnergyPlus data file available from the U.S. Department of Energy—to run hourly illuminance maps in the lighting software packages, and incorporates an algorithm to approximate manual operation of window blinds.

Floor areas, or grid points, in the building model that achieve 300 lux for at least half of the analysis hours count as meeting the daylighting threshold. As a result, sDA values can range from zero to 100 percent of the floor area in question. An sDA value of 75 percent indicates a space in which daylighting is “preferred” by occupants; that is, occupants would be able to work comfortably there without the use of any electric lights, and find the daylight levels to be sufficient. An sDA value between 55 percent and 74 percent indicates a space in which daylighting is “nominally accepted” by occupants. Lighting designers, therefore, should aim to achieve sDA values of 75 percent or higher in regularly occupied spaces, such as an open-plan office or classroom, and at least 55 percent in areas where some daylight is important.

Lighting designers can use sDA values to compare different spaces on equivalent daylighting terms. Based on simulations using LM-83’s methodology, 68.5 percent of the office (top) achieves a minimum of 300 lux for half of the analysis hours, while only 54.3 percent of the classroom (bottom) does. Thus the office would have daylighting that is “nominally accepted” by users, while the classroom would fail the daylight sufficiency criteria by missing the 55-percent sDA mark.
Kevin Van Den Wymelenberg Lighting designers can use sDA values to compare different spaces on equivalent daylighting terms. Based on simulations using LM-83’s methodology, 68.5 percent of the office (top) achieves a minimum of 300 lux for half of the analysis hours, while only 54.3 percent of the classroom (bottom) does. Thus the office would have daylighting that is “nominally accepted” by users, while the classroom would fail the daylight sufficiency criteria by missing the 55-percent sDA mark.

ASE: A Proxy for Glare and Overheating
With higher levels of daylight sufficiency comes the potential for glare and solar heat gain. That’s where Annual Sunlight Exposure (ASE) steps in. Meant to complement sDA, ASE is intended to help designers limit excessive sunlight in a space. While ASE is a crude proxy for glare phenomena, it measures the presence of sunlight using annual hourly horizontal illuminance grids rather than luminance measures, so it is technically not a glare metric.

ASE uses a simulated 1,000 lux as an indicator value for sunlight, but the simulated value can differ significantly from what is measured in the physical world, which considers secondary bounce-off surfaces. Like sDA, ASE values range from zero to 100 percent, with the latter suggesting that the entire floor area of the space in question exceeds the simulated value of 1,000 lux for at least 250 hours per year. Thus, to reduce the potential for glare and thermal stress, designers should aim for low ASE values.

LM-83 provides preliminary guidance for recommended ASE limits, cautioning that spaces with ASE values exceeding 10 percent will likely result in visual discomfort. The DMC is presently refining the criteria for recommended practice of interpreting ASE results, but designers can use ASE immediately to make relative comparisons among proposed design options. In the future, the DMC will provide better criteria to guide absolute performance thresholds using ASE.

The sunlight analysis method implemented in simulations using LM-83’s methodology to calculate ASE is also used to trigger the human operation of manual blinds, which affects the determination of sDA. As a result, areas with high ASE values may have lower sDA scores because the algorithm assumes building occupants will close window blinds or draw the window shades manually if excessive sunlight persists.

A classroom with an exterior overhang and light shelf (Fig. 2a) has a much higher Spatial Daylight Autonomy (sDA) score than a classroom without the features (Fig. 2b) due to the assumption that occupants would deploy manual blinds more frequently in the latter scenario. Adding an overhang and light shelf increases the classroom’s sDA value from 28.1 percent to 54.3 percent, and decreases the Annual Sunlight Exposure from 31.3 percent to 10.1 percent, both of which are improvements according to LM-83. Note that tubular daylighting devices installed at the back of the classroom will likely provide more annual illumination in reality than what is captured through the simulation method.
Kevin Van Den Wymelenberg A classroom with an exterior overhang and light shelf (Fig. 2a) has a much higher Spatial Daylight Autonomy (sDA) score than a classroom without the features (Fig. 2b) due to the assumption that occupants would deploy manual blinds more frequently in the latter scenario. Adding an overhang and light shelf increases the classroom’s sDA value from 28.1 percent to 54.3 percent, and decreases the Annual Sunlight Exposure from 31.3 percent to 10.1 percent, both of which are improvements according to LM-83. Note that tubular daylighting devices installed at the back of the classroom will likely provide more annual illumination in reality than what is captured through the simulation method.

The Feedback
Since publishing LM-83 in 2013, DMC members have been actively educating designers worldwide through presentations and soliciting feedback from early adopters who are implementing sDA and ASE calculations in their software packages, and from designers using the software to inform their work. Feedback has been as varied and
lively as the debate among IES DMC members during the process leading up to the publication of LM-83. But there have been a few repeated questions:

First, can a space with the right amount of daylight for one task, say classroom learning, be adequate for all other tasks, such as office-computer work? This is a limitation of the current version of LM-83. The next iteration of the guidelines may provide more nuanced criteria by space and use type.

Second, why does the algorithm simulating the use of manual blinds in the software programs seem to behave like an automated shading system? After LM-83 was released, several new studies on manual blind use were published. These will inform future versions of LM-83, and likely reduce the frequency of operation from the current manual blind algorithm.

Third, why does LM-83 use the same performance criteria to evaluate a building in Anchorage, Alaska, as it does a building in San Diego, when the former has significantly less potential for annual daylight hours? Since LM-83 only includes surfaces at 300 lux at 50 percent time in its sDA value, the theoretical maximum value for sDA is still 100 percent, even in Anchorage. That said, it is more difficult to achieve the recommended criteria in northern latitudes with fewer hours of daylight.

The Future
For now, sDA and ASE are most applicable to space types similar to those the DMC evaluated in the research—namely classrooms, offices, and other common commercial spaces, such as lobbies and conference rooms. The DMC recommends further refinement of performance criteria by space type, but this requires additional research to provide satisfactory evidence for these new criteria.

A broad way to address questions about LM-83 is to consider the model for building codes and performance certification programs, such as LEED. Building codes for energy and life safety simplify problems and create compliance pathways that can be enforced economically by jurisdictions. Performance certification programs reward projects that exceed best practices and, in doing so, define specific thresholds against which designs can be evaluated.

The DMC wrestled with ways to make sDA and ASE applicable to designers in support of decision making, while also anticipating that code and standard organizations would adopt and implement them. In fact, LEED v4 aims to include sDA for an additional credit beyond the existing daylight-compliance pathways.

Inevitably, in the process of creation, designers will test and challenge the noble efforts of codes and standards committees. Science answers one daylighting performance question and art finds good reason to throw it out and ask different questions. This reality signifies the urgent need for increased funding to support evidence-based research and revisions to refine—or redefine—established metrics and criteria.

In the meantime, lighting designers should reach for high sDA values while implementing strategies to lower ASE values, and strive to lower ASE values without sacrificing sDA. They should do this not because a particular certification program may reward a project with an extra point, but simply because it’s a matter of good design. •

Kevin Van Den Wymelenberg is an associate professor at the University of Oregon, where he directs the Energy Studies in Buildings Laboratory (ESBL). He is also the chair of IES’s Daylight Metrics Committee.

Alen Mahić is a research assistant at ESBL with a focus on digital daylight and energy simulation.

Resources


Below is an introductory list of resources and articles on LM-83 and its development.

“Dynamic Daylight Performance Metrics for Sustainable Building Design,” by Christoph Reinhart, John Mardaljevic, and Zack Rogers, Leukos, 2006. Available at bit.ly/1Rw3uaU.

“Daylight Metrics and Energy Savings,” by John Mardaljevic, Lisa Heschong, and Eleanor Lee, Lighting Research & Technology, 2009. Available at 1.usa.gov/1ReG9ay.

Lighting Measurement 83 (LM-83), Approved Method: IES Spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE), by the IES Daylight Metrics Committee, IES, 2012. Available at bit.ly/1UA26EF.

“Predicting the Daylit Area—A Comparison of Students Assessments and Simulations at Eleven Schools of Architecture,” by Christoph Reinhart, Tarek Rakha, and Dan Weissman in Leukos, 2014. Available at bit.ly/1RgDzyF.

“Annual Daylight Performance Metrics,” by Lisa Heschong and Kevin Van Den Wymelenberg in Building Synapses: Connections in Lighting, 2012.

“A Comparative Study of Spatial Daylit Area Drawings with Annual Climate-Based Simulation Using Multiple Manual Blind Control Patterns, and Point in Time Simulation,” by Amir Nezamdoost and Kevin Van Den Wymelenberg, ASHRAE Energy Modeling Conference, Oct. 2015.