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 LEED for Existing Buildings: Operations & Maintenance (LEED-EBOM) include a credit if the building demonstrates daylight areas over 50% of total floor areas using either simulation or field measurements. However, the field measurements are often based on spot measurements of illuminance levels, which is hard to catch dynamic responses of lighting measurements. Therefore, this study performed a case study using a university building in Washington, D.C. to examine the validity of lighting spot measurements. The case-study building was recently certified under LEED-EBOM, including an IEQ credit 2.4 by demonstrating over 50% of the building’s floor areas achieved daylight illuminance levels over 25 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 9 a.m. to 3 p.m. during the fall and winter months. Thus this study performed spot and continuous measurements of daylight illuminance levels as well as an occupant questionnaire survey regarding daylighting conditions of the building. As a result, 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.