Daylight Metrics

Useful Daylight Illuminance

The core of this section is based on material contributed by John Mardaljevic (Loughborough University, UK)

The useful daylight illuminance (U.D.I.) scheme is founded on occupant responses to daylight levels, as reported in several studies. First published in 2005, the UDI scheme had 100 and 2,000 lux as the lower and upper bounds for useful daylight illuminance achieved. The 2,000 lux value was revised upwards to 3,000 lux a few years later when data from more contemporary studies became available. Setting the UDI range boundaries was, of course, a matter requiring some judgement since the various studies reported a scatter of values for a preferred upper limit. In comparison with more recent studies, the pre-2000 reports tended to suggest a lower tolerance to high ambient daylight illuminance levels. Also, the studies – then and now – were invariably carried out in office spaces. The visual display technology commonly used prior to the mid-90s (e.g. CRT screens) tended to be more prone to glare issues than that used today for three reasons: lower intrinsic brightness; less effective anti-reflective coatings; and, curved screens that could reflect light received from a wide angle. Modern screens are generally much more forgiving of higher ambient daylight levels. This could well explain why more recent studies generally report higher values than 2,000 lux as an upper limit which may prompt the lowering of blinds (in largely side-lit spaces).

The UDI achieved range of 100 to 3,000 lux can be further subdivided into two ranges called UDI-supplementary and UDI-autonomous. UDI-supplementary gives the occurrence of daylight illuminances in the range 100 to 300 lux. For these levels of illuminance, additional artificial lighting may be needed to supplement the daylight for common tasks such as reading. UDI-autonomous gives the occurrence of daylight illuminances in the range 300 to 3000 lux where additional artificial lighting will most likely not be needed. The UDI scheme is applied by determining at each calculation point the occurrence of daylight levels where:

• The illuminance is less than 100 lux, i.e. UDI not achieved.

• The illuminance is greater than 100 lux and less than 300 lux, i.e. UDI supplementary.

• The illuminance is greater than 300 lux and less than 3,000 lux, i.e. UDI autonomous.

• The illuminance is greater than 3,000 lux, i.e. UDI exceeded.

Note that, for any sensor point, the daylight autonomy value for 300 lux is equal to the sum of the UDI autonomous and the UDI exceeded values. The 100 – 3,000 lux UDI achieved range is sometimes referred to as UDI combined.

Case study: The Priority Schools Building Programme daylight criteria

The The Priority Schools Building Programme daylight criteria (PSBP) requirement specifies that the space-averaged value for the occurrence of illuminances in the range 100 to 3,000 lux during the period 08h30 to 16h00 is 80%. It appears that the 80% criterion was based on a series of parametric tests carried out by the daylight specialists, evaluating a number of designs for different orientations. The space-averaged UDI value is determined by first predicting the annual time-series of daylight illuminance values at each ‘sensor’ point on a grid that covers the workplane, with a 0.5 m perimeter gap between the workplane and the walls. Then, for each grid point the occurrence of illuminance values within each of the UDI ranges is determined either as number of hours or as a percentage of the evaluation period, i.e. 08h30 to 16h00 for every day of the year.

Relevant publications

• A Nabil and J. Mardaljevic. Useful daylight illuminance: a new paradigm for as- sessing daylight in buildings. Lighting Research and Technology, 37(1):41–57, 2005.

• A. Nabil and J. Mardaljevic. Useful daylight illuminances: A replacement for day- light factors. Energy and Buildings, 38(7):905–913, 2006.

• David Lindelöf and Nicolas Morel. Bayesian estimation of visual discomfort. Build- ing Research & Information, 36(1):83–96, 2008.

• J. Wienold. Daylight Glare in Offices. PhD thesis, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany, 2009