Re-thinking daylighting spot measurements

Lighting is known as one of the major energy consumers in U.S. commercial buildings. To reduce the use of artificial lights in buildings, the existing green building rating systems such as Leadership in Energy & Environmental Design for Existing Buildings: Operations & Maintenance (LEED EBOM) include a credit that requires a building to demonstrate that at least 50% of total floor areas utilize daylighting based on field measurements. However, the field measurements are often based on spot measurements of illuminance levels, which do not easily track dynamic responses of lighting measurements. Therefore, to examine the validity of lighting spot measurements, this study performed spot and continuous measurements of daylight illuminance levels as well as an occupant questionnaire survey regarding daylighting conditions of a university building in Washington, D.C. The case-study building was recently certified under LEED 2009 EBOM, including an Indoor Environmental Quality (IEQ) credit 2.4 by demonstrating over 50% of the building?s floor areas achieved daylight illuminance levels over 25 foot-candles (fc). Based on this, the building installed an automatic shut-off switch of the lamps in daylight areas from 7 a.m. to 6 p.m. during the spring and summer months and from 9 a.m. to 3 p.m. during the fall and winter months. According to illuminance measurements and occupant questionnaire survey, it was found that the current lighting conditions of the building did not provide proper illuminance levels or visual comfort throughout the daylight spaces based on the values (i.e., 30 fc) provided by the Illuminating Engineering Society of North America (IESNA). In addition, occupants expressed dissatisfaction with their visual comfort. This indicates that the current rating systems should reconsider the existing lighting spot measurement protocols by providing methods to collect and analyze dynamic responses of lighting measurements.