Lighting journal sept 14

Page 17

Daylighting 15

Axel Jacobs believes that CBDM is too complex and will lead to poorly daylit spaces

The climate



aunched in February 2014, the new EFA Daylight Design Guide has already divided the UK’s daylighting community in a manner that until now only a certain yeastbased food spread was thought to be capable of. Two of the seven advisers who contributed to the new EFA guide were swift in publicly distancing themselves from it at the launch event. Daylighting professionals, it seems, are now divided into climate sceptics and climate believers. The panel of advisers to the new guide did not include any of the former. Hitherto, the ‘daylitness’ of a space was expressed as its average daylight factor (ADF). A daylight factor (DF) is simply the ratio of the indoor illuminance divided by the unobstructed outdoor illuminance,

making it a unitless metric. Note that this is in contrast to how levels of artificial light are measured, which is in lux at working plane height. The average illuminance tells us nothing about how well lit a space is, whether by artificial or natural light alone. The ADF of a space does. If it is less than two per cent, the room is considered ‘poorly daylit’ and will need to be artificially lit most of the time. An ADF of five per cent or more indicates that a space is well-daylit, and will not have to rely on electric lighting for the majority of daytime working hours. This is irrespective of facade orientation or site latitude. Representing a worstcase scenario, ADFs are calculated using an overcast sky. The new EFA Guide moves away

Lighting Journal September 2014

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