Useful Daylight Illuminance

Useful Daylight Illuminance (UDI) is a modification of Daylight Autonomy conceived by Mardaljevic and Nabil in 2005. This metric bins hourly time values based upon three illumination ranges, 0-100 lux, 100-2000 lux, and over 2000 lux. It provides full credit only to values between 100 lux and 2,000 lux suggesting that horizontal illumination values outside of this range are not useful. There is significant debate regarding the selection of 2,000 lux as an ‘upper threshold’ above which daylight is not wanted due to potential glare or overheating. There is little research to support the selection of 2,000 lux as an absolute upper threshold. The graphical percent values represent the percentage of the floor area that that meets the UDI criteria at least 50% of the time.

• Nabil A, & Mardaljevic J. (2005a). Useful Daylight Illuminance: A New Paradigm to Access Daylight in Buildings. Lighting Research & Technology, 37(1), 41-59.
• Nabil A, & Mardaljevic J. (2005b). Useful Daylight Factors. Energy and Buildings, 38(7).
• Mardaljevic, J. (2006). Examples of Climate-Based Daylighting Modelling. In Paper No. 67. Presented at the CIBSE National Conference 2006: Engineering the Future, Oval Cricket Ground, London, UK: Institute of Energy and Sustainable Development (IESD).
• Reinhart, C. F., Mardaljevic, J., & Rogers, Z. (2006). Dynamic Daylight Performance Metrics for Sustainable Building Design. Leukos, 3(1), 7-31.