An analysis of the (BRE) average daylight factor and limiting depth guidelines as design criteria

Following the study by V H C Crisp and P J Littlefair (CIBS, 1984)1 on Average Daylight Factor Prediction, the arguments for the use of average daylight factor as a design criterion are reviewed in light of new experimentalassessments. Part of the experiment carried out in 1984 with a physical scaled model under artificial sky, is repeated using Radiance.It is shown that ADF is very sensitive to light distribution,hence the necessity to measure it with a tighter sensorgrid spacing, than the one used previously, in order to avoid area weighted ADF and to increase accuracy.It is shown that the new readings taken using Radiance and a tighter measurement grid, which starts right from the window-wall, can differ from the area weighted ADF by +30% to +5% as the window head increases in height. Conversely the modified Lynes (BRE) ADF formulaassesses with greater precision the ADF for lower window heads 10% difference, up to 20% difference for higher ones, if compared with the new sets of Radiancereadings for the proposed test rooms.It should be also pinpointed to the fact that the test rooms modelled are based on 1954 typical post war classroom geometry, with ideal window shape, runningfrom wall to wall and cill to ceiling, and without any obstructions, while the ADF is currently used to assessdwellings in urban scenarios.It is also suggested that the position and shape of the window influences the distribution of light over the working plane and therefore the reading of the ADF, by +20% for the cases tested.This difference cannot be taken into account either within the BRE ADF formula or within the limiting depth criterion.Hence an eventual higher ADF does not necessarily equate to a better uniformity of light over the working plane and therefore does not ensure any energy savingsfor example, amongst other benefits that could be achieved by a correct understanding of its use.