IES LM-83-12: Approved Method: IES Spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE)
About this book
IES LM-83-12, "Approved Method: IES Spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE)," published in 2012 by the Illuminating Engineering Society, establishes standardized definitions, calculation procedures, and reporting requirements for two climate-based daylighting metrics that have become the primary quantitative tools for evaluating daylighting performance in buildings. The publication emerged from the recognition that traditional static daylighting metrics — particularly the daylight factor, which measures the ratio of indoor to outdoor illuminance under a standardized overcast sky — were inadequate for evaluating the actual performance of daylighting strategies in real buildings under real sky conditions. The daylight factor was developed in the early twentieth century and remains useful for rough comparative assessments, but it cannot capture the dynamic nature of daylighting, the effects of direct sunlight, the benefits of views to the sky, or the variation in performance across different climates and orientations.
Spatial Daylight Autonomy (sDA) measures the percentage of floor area in a space that receives sufficient daylight — defined as at least 300 lux for a minimum of 50 percent of occupied hours annually — from climate-based annual simulation. A space achieving sDA300/50% of 55 percent or higher is considered "nominally daylit" in LEED v4; a space achieving 75 percent or higher is considered "well daylit." These thresholds were derived from calibration studies relating simulated sDA values to occupant satisfaction with daylighting. Annual Sunlight Exposure (ASE) provides a complementary measure of potential glare and visual discomfort risk.
It quantifies the percentage of floor area that receives direct sunlight — defined as more than 1,000 lux for more than 250 occupied hours per year — which is indicative of conditions where glare would likely cause occupant discomfort and prompt the use of blinds or shades that would defeat the daylighting strategy. An ASE value exceeding 10 percent of floor area is considered a threshold of concern in LEED v4. Both metrics require climate-based annual simulation using measured or model weather data for the specific building location, capturing the actual variation in solar radiation and sky conditions across all hours of the year.
This approach, known as Climate-Based Daylight Modeling (CBDM), represents a fundamental advance over the static overcast-sky methods it replaces. LM-83 has been adopted as the daylighting metric standard in LEED v4, the WELL Building Standard, and several other green building certification programs, making it the de facto international reference for performance-based daylighting design. Sources: Illuminating Engineering Society (ies.org); IES LM-83-12 official publication; LEED v4 daylighting credits documentation; Reinhart & Walkenhorst (2001) seminal CBDM paper.