Good Daylight: Quantity or Quality?

Average Daylight factor (ADF) can be a valuable tool for assessing the quantity of internal daylight amenity. However, where the quality of the internal daylight needs to be considered, more appropriate metrics can be used such as Useful Daylight Illuminance (UDI).

ADF continues to be the standard metric used to assessing daylight amenity of new buildings. This simple assessment establishes the quantitative relationship between internal illuminance and external illuminance. This assessment is calculated in specialist software with computer generated models.

It is said that today’s current kitchen ADF standards (2%) are derived from an experiment conducted with the Women’s Institute (WI), where WI members concluded that an ADF of 2% was the minimum level of daylight required to read a cookery book. Henceforth, the 2% kitchen ADF standard was set.

The CIBSE Lighting Guide bands ADF results into three categories: under 2% – not adequately lit, between 2% and 5% – adequately lit and over 5% – artificial light generally not required. Whilst these values are useful, it does not tell us the quality of light available throughout the assessed room, only the quantity.  

There are other metrics which can help us assess the quality of daylight amenity in our computer generated models.

For example, UDI enables us to assess and inform design by being able to assess and allow for specific lux requirements, such as ‘100-2000 lux is required the assessed working plane for 75% [of the user defined time/date]’. Using such a metric is a natural progression from the ADF as ‘dark spots’ in rooms can be legitimately excluded from the calculation, or long kitchen/ dining rooms which typically fail ADF standards can be shown to have adequate daylight amenity where the daylight is needed. Additionally, glare can be controlled by setting a lux upper threshold, something which is not possible with ADF.

ADF is clearly a useful and well established tool. However, in better understanding the quality of light in our buildings, the UDI metric has a valuable role to play in the computer based analysis of daylight and is part of a suite of environmental modelling techniques that we employ to improve the quality of buildings.