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The first originated with the widely-accepted need to reduce
carbon emissions from buildings in order to minimise the antici-
pated degree of anthropogenic climate change. This in turn led
to the formulation of guides and recommendations to encourage
the design and construction of low energy buildings and also for
the retrofit of existing buildings. All these guides contain recom-
mendations on daylighting, invariably founded on the daylight
factor or an equally simplistic schema such as glazing factors.
The non-visual productivity, health and well-being effects related
to daylight exposure are not yet fully understood, and it is not yet
known what the preferred exposure levels should be, nor if exist-
ing guidelines would be effective for these quantities.
Almost concurrent with the emergence of the two key drivers
noted above were a major advance in the way daylight in build-
ings could be modelled; and, the development of numerous new
glazing systems and materials to better exploit daylighting in
buildings. These developments are expected to lead to signifi-
cant changes in the way that daylight in buildings is both evalu-
ated (through modelling) and exploited (by new glazing systems
and materials).
New developments always pose new challenges. At the time of
writing, it is not yet clear how these new developments will be
accommodated within teaching in architecture and engineering.
This article is intended to contribute to that discussion.
Guidelines for daylighting
One of the earliest recorded recommendations for climate-
adapted building design is that attributed to Socrates [469–
399BC]:
Now in the houses with a south aspect, the Sun’s rays penetrate
into the porti- coes in the winter, but in summer, the path of the Sun
is right over our heads and above the roof so that there is shade. If,
then, this is the best arrangement, we should build the south side
loftier to get the winter sun and the north side lower to keep out the
cold winds.
Quoted by Xenophon in Memorabilia Socrates
Over the following two thousand years numerous architectural
styles evolved across the globe in response to the specific
cultural/societal imperatives and driven by advances in build-
ing technology and construction techniques. Daylighting design
remained a rule-of- thumb practice, informed by tradition and
internalised knowledge about what was known to work for that
particular climate and locale. Building apertures were rarely
designed for the sole purpose of providing daylight illumination
since protection from the hot and cold extremes of the prevailing
climate was often the key design concern.
The pressure to accommodate increasing number of people in
cities of the developing world in the late 1800s led to taller and
more tightly-packed building forms, thereby reducing and often
eliminating entirely the direct view of sky from much of the use-
able, internal space. This in part led to the need for some objec-
tive measure of the daylighting performance of a space which
could, if required, function as a tool to evaluate buildings at
the planning stage. Daylight was at that time still the preferred
source of illumination for both manual and clerical work.
Illuminance for Task
The absolute levels of illuminance that are needed for any
particular task depends on the visual acuity required for the
task and, to a lesser degree, the nature of the environment in
which the task is to be carried out. Most developed countries
have produced design guides which give recommended illumi-
nance levels depending on task and/or setting. The following is a
selection of recommendations produced by the British Chartered
Institution of Building Services Engineers (CIBSE) (CIBSE Guide
A, 2006):
• 100 lux for interiors used rarely, with visual tasks confined to
movement and casual seeing without perception of detail, e.g.
corridors, changing rooms, bulk stores, auditoria.
• 200 lux for interiors where the visual tasks do not require
perception of detail, e.g. foyers and entrances.
• 300 lux for interiors where visual tasks are moderately easy,
e.g. libraries, sports and assembly halls, teaching spaces,
lecture theatres.
• 500 lux for interiors where the visual tasks are moderately
difficult and also where colour judgement may be required, e.g.
general offices, kitchens, laboratories, retail shops.
• 1000 lux for interiors where the visual tasks are very difficult,
requiring small details to be perceived, e.g. general inspection,
electronic assembly, retouching paintwork, cabinet making,
supermarkets.
Recommended illumination levels were conceived primarily for
the purpose of designing artificial lighting systems, and not for
the daylighting design of buildings because the variation in the
provision of natural daylight is such that it is virtually impos-
sible to deliver specific natural illumination levels without huge
fluctuations occurring. For buildings therefore, design guidance
was formulated in terms of building properties which are evalu-
ated under a single, static “worst-case” daylight condition: an
overcast sky. This is the basis of the daylight factor described in
following section. It is only with recent advances in daylight pre-
diction techniques that absolute levels of daylight illumination
under varying sky and sun conditions has become a consider-
ation in the evaluation of the daylighting potential of a building.