Daylight Dialect

Overview

This Technology column ran in the March 2008 issue of Architectural Lighting and was posted online on 11 March 2008. Its author, Kevin Van Den Wymelenberg, then directed the University of Idaho's Integrated Design Lab in Boise. The piece asks a deceptively basic question: when an architect, lighting designer, client or contractor calls a space "daylit," do they mean the same thing? His answer is that they usually do not, and that the absence of a shared vocabulary and agreed metrics is a real obstacle to buildings that perform as intended.

Key Findings

• The column opens with a December 2005 meeting in Portland, Oregon, where a room of daylighting specialists reviewing a Heschong Mahone Group photocontrols study could not agree on what qualifies a space as daylit.
• A 2005 National Research Council of Canada survey of more than 150 architects and engineers offered five competing definitions of daylighting. Architects favored the ones built on occupant comfort; engineers favored the ones built on lighting energy, whole-building energy, peak demand or economics.
• The author offers his own working definition: a daylit space uses daylight as the primary daytime light source for the occupants' visual tasks, feels visually and thermally comfortable and connected to the outdoors, and persistently delivers lighting energy and peak-demand savings. He deliberately leaves the quantities unspecified because any single threshold would fail across space types.
• Daylight factor (DF), the dominant metric for a century, has recommended thresholds ranging from 0.1 to 10 depending on occupancy, region and era. Because it assumes an overcast sky, it says little about sunny climates, visual comfort, time of day, views, heat gain or realized energy savings.
• Three climate-based "dynamic" metrics are emerging: daylight autonomy (proposed in Switzerland in 1989, refined by Christoph Reinhart from 2001 to 2004), useful daylight illuminance (John Mardaljevic, 2005, binning hours into 0 to 100, 100 to 2,000 and over 2,000 lux), and daylight saturation percentage (Collaborative for High Performance Schools, 2006, using 40 to 400 footcandle limits with a penalty for overlit hours).
• The IES Daylight Metrics subcommittee had set itself a 2010 target for an agreed suite of metrics. The column notes that if it succeeded, it would have taken more than 100 years to settle what "daylit" means, and closes with a nine-entry glossary of the terms discussed.

Why It Matters for Daylighting

For toplighting in warehouses, distribution centers and manufacturing floors, this column explains why savings claims and code language so often talk past each other. A prismatic-skylight layout that reads as "daylit" to a lighting designer looking at illuminance uniformity may look inadequate to an energy engineer measuring hours of daylight autonomy, and vice versa. The dynamic metrics the author previewed did arrive: the IES published spatial daylight autonomy and annual sunlight exposure as LM-83 in 2012, and LEED v4 adopted them. Anyone reading a daylighting simulation report for a skylight retrofit today is reading the vocabulary this piece argued for. The author's warning that contractors and occupants must understand the design intent, or the space will not function as daylit, applies with particular force to photosensor controls in industrial buildings.

About the Source

Architectural Lighting (later folded into Architect magazine's technology section) was the leading U.S. trade publication for lighting designers. Kevin Van Den Wymelenberg holds architecture degrees from the University of Wisconsin-Milwaukee and the University of Washington and is now a professor at the University of Oregon, where he directs the Institute for Health in the Built Environment; he co-authored the Routledge book Daylighting and Integrated Lighting Design. The library copy is a four-page browser printout from August 2014. The full article, with its glossary, remains online at Architect magazine.