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NORDIC LIGHT & COLOUR
75
Introduction
Daylight in buildings is the natural illumination experienced by
the occupants of any man-made construction with openings
to the outside, e.g. dwelling, workplace, etc. The quantity and
quality of daylight in buildings is continually varying due to the
natural changes in sun and sky conditions from one moment
to the next. These changes have components that are: random
(e.g. individual cloud formations); daily (i.e. progression from
day to night); and, seasonal (e.g. changing day length and pre-
vailing weather pat- terns). For any given sky and sun condition
the quantity and character of daylight in a space will depend
on: the size, orientation and nature of the building apertures;
the shape and aspect of the building and its surroundings; and,
the optical (i.e. reflective and transmissive) properties of all the
surfaces comprising the building and its surroundings.
Daylight may arrive at a point inside a building either directly or
indirectly from the luminous source, i.e. from the sun or from
the sky. Direct illumination generally results from having an
unobstructed view of the source. Indirect illumination is when
the light arrives at the point following one or more reflections.
Thus, strictly speaking, there are direct and indirect compo-
nents of illumination from both the sun and the sky, Figure 1.
Although the sun and the sky are both luminous sources, direct
sunlight when present is given special consideration because
of the small angular size of the sun and its potentially large
contribution to illumination (and also its heating effect). Thus
illumination from direct sunlight is commonly referred to as
“direct light”. In contrast, light from the sky - arriving either di-
rectly or indirectly - is commonly referred to as “diffuse light”.
Sunlight that has undergone one or more diffuse reflections is
also commonly referred to as “diffuse light”. Note, the mode
of reflection of the direct sunlight is important: a specular (or
“mirror”) reflection of sunlight will produce a redirected beam
of direct light rather than diffuse light. For reflections (and
transmissions) that are part-specular and part-diffuse, the
distinction between direct and diffuse light can become lost.
Reflections can occur either internal or external to the building
space under consideration.
The purpose of the very earliest shelters – the forerunners of
buildings – was to protect from the elements. The first build-
ings to include deliberate elements of daylighting design were
often places of worship, many of which survive to this day. Only
when glass became relatively commonplace in the 17th century
did the provision of daylight for everyday buildings become a
consideration. Window design was invariably tailored to the
prevailing climate, e.g. small with deep reveals for locales
where the solar component of daylight needed to be controlled
to prevent overheating. As the cities of the industrialised world
became more populous, building densities increased and the
provision of daylight for buildings became a planning issue.
This eventually resulted in the formulation of the daylight factor
which was intended to be a measure of the daylighting poten-
tial of a building, and which could be predicted at the design
stage using a variety of methods (Hopkinson et al., 1966). De-
vised in the first half of the 20th century, the
daylight factor
still
forms the basis of many guidelines and recommendations for
building design. It is discussed in details in a later section.
Advances in glass making and window technology in the
second half of the 20th Century allowed architects to design
buildings where the perimeter wall could be almost entirely
glazed. Commercial buildings in particular became larger with
deeper plan de- signs so that, despite the highly transparent
facade, many occupants were situated far from the windows
and so received little daylight. A conspicuous icon symbolising
modernity and prosperity, these designs became the exem-
plars for architects all over the world, and now many cities
feature highly-glazed buildings regardless of the local climatic
conditions. Thus the daylighting characteristics of office build-
ings in particular tended to be dictated by considerations of
architectural style rather than climate-adapted design. These
trends were not hindered by the continued reliance on the day-
light factor as an evaluative scheme since the measure is itself
climate and orientation insensitive.
For the majority of buildings it is incumbent on the occupants
to moderate the internal daylight conditions using some form
of blinds or shades. Occupants will deploy blinds/shades in an
effort to moderate the internal environment according to their
perceptions of both visual discomfort (e.g. daylight glare) and
thermal discomfort (e.g. to avoid direct sun) which vary greatly
from person to person. Also, once deployed, blinds/shades will
tend to remain closed long after the external condition has
passed. Thus it is common to see blinds closed for much of the
occupied time. Consequently, the potential to exploit daylight-
ing is often not realised because the blinds are left closed most
of the time and the electric lights are left switched on.
Towards the end of 1990s, the daylighting of buildings began to
achieve greater atten- tion for a number of reasons. The two
most important drivers were:
1. the widespread belief that the potential to save energy
through effective daylighting was greatly under-exploited;
and,
2. the emergence of data suggesting that daylight exposure has
many positive produc- tivity, health and well-being outcomes
for building occupants.