The mathematisation of daylighting: a history of British architects' use of the daylight factor

British post-war planning guidance proposed that cities be rebuilt according to scientific principles. Mathematical tools were devised to determine built form; daylight levels within buildings were to be evaluated using a metric called the daylight factor. The daylight factor is still the principal metric used in daylighting guidance, despite recent calls to replace it with other metrics. This article explores whether the Modernist ambition, for buildings to be designed according to mathematically verifiable principles, was realised in relation to daylighting. Specifically, the article explores post-war architects' usage of the daylight factor in designing housing. The article draws on eleven semi-structured interviews with practising and retired architects, lighting consultants and a planning officer. Architects interviewed for the study reported that they did not routinely evaluate daylight factors at the design stage. Interviewees described the process as time-consuming, and often unnecessary as windows can be designed without undertaking a calculation or a photometric model study. Also, planning authorities rarely required architects to prove that proposed dwellings achieved specific daylight factors. However, those architects who were knowledgeable about daylight factors reported that this knowledge enhanced their understanding of the principles of daylighting. The daylight factor also helped to establish objective standards.