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NORDIC LIGHT & COLOUR
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behaviour in daylit offices with user operated shading devices.
Daylight illuminances in the range 100 to 500 lux are consid-
ered effective either as the sole source of illumination or in
conjunction with artificial lighting. Daylight illuminances in the
range 300 to around 2,000 or maybe 3000 lux are often per-
ceived either as desirable or at least tolerable.
The range limits for UDI depend to a degree on the particular
application, and, at the time of writing, there is no consensus
on what precise values the upper and lower range limits should
have. Nonetheless, the UDI scheme combines intuitive simplic-
ity with rich information content. For the example shown later
in this article, UDI was defined as the annual occurrence of
daylight illuminances that are between 100 and 3000 lux (PAGE
REF). The UDI range is further subdivided into two ranges
called UDI-supplementary and UDI-autonomous. UDI-supple-
mentary gives the occurrence of daylight illuminances in the
range 100 to 300 lux. For these levels of illuminance, additional
artificial lighting
may
be needed to supplement the daylight
for common tasks such as reading. UDI-autonomous gives the
occurrence of daylight illuminances in the range 300 to 3000
lux where additional artificial lighting will most likely not be
needed. The UDI scheme is applied by determining at each
calculation point the occurrence of daylight levels where:
• The illuminance is less than 100 lux, i.e. UDI ‘fell-short’
(or UDI-f).
• The illuminance is greater than 100 lux and less than 300 lux,
i.e. UDI supplementary (or UDI-s).
• The illuminance is greater than 300 lux and less than 3000
lux, i.e. UDI autonomous (or UDI-a).
• The illuminance is greater than 3000 lux, i.e. UDI exceeded
(or UDI-e).
The UDI schema was one of the metrics used in a recent, wide-
ranging evaluation of daylight provision for residential buildings
(Mardaljevic et al., 2011).
Can daylighting be adequately described by a single daylight
metric?
The question posed above cannot be answered definitively
since “daylighting” is not a well defined property. Notions as to
what constitutes “adequate” in this regard are similarly vague
also. Although an ill-defined term, there is probably general
acceptance that a space with good daylighting is one that
minimises visual discomfort and provides high levels of visual
quality under solely or predominantly daylight conditions fre-
quently throughout the year. Thus “good daylighting” is some
aggregate measure over the year of the degree and frequency
of occurrence of instantaneous conditions that are deemed to
offer good visual comfort and quality. Eventually, many inputs
may be combined into one composite performance metric. In
the meantime, studying separate dimensions of the daylit envi-
ronment independently is likely to be more informative.
Daylight and architectural pedagogy
Most architecture students will have been taught the daylight
factor method at some point in their studies. The daylight fac-
tor is likely to be the only quantitative measure of daylight per-
formance that the students encounter. A question rarely asked
is: “what do the students actually
learn
about daylight perfor-
mance from the daylight factor?” Hands-on use of the daylight
factor method is certainly useful to engage students with the
issues and problems encountered when trying to evaluate
the daylighting performance of a design. However, if the DF
method is not taught critically, then the student may complete
her studies not appreciating that the DF is an indicator or proxy
for daylight performance rather than an actual measure of it.
The following sections describe suggestions for a hypothetical
curriculum for teaching daylight to architecture students. It is
speculative and incomplete, and therefore not intended to be
a model for any actual curriculum. Instead, its purpose is to
engender wider debate amongst interested parties: teachers,
practitioners, researchers and the students themselves.
Daylighting basics
The first steps in teaching daylighting should be to impart
a qualitative understanding of the spatio-temporal dynam-
ics of daylight in a space, and how these are determined by
the building form and the available illumination. Whilst this
might appear a somewhat challenging task, it is in fact rather
straightforward. The simple ‘shoe-box’ model is ideally suited
for this purpose. Fitted with a door security (or ‘peep-hole’)
viewer, the student can rapidly experience changes in daylight
illumination that, in an actual room, happen too slowly to give
a ‘tactile’ feel for the dynamics of illumination. In addition to
obvious relations between say geometry and patterns of direct
solar ingress, much more subtle interplays of reflected light
within the ‘space’ can be perceived. In the shoe-box model a
window is merely a hole in the sides or the top. However, if the
holes are left with a connecting ‘hinge’, then the windows can
be opened and closed at will by the user who will immediately
‘see’ the effect on daylight patterns of various multiple aper-
tures. Similarly, a piece of thin tissue paper can be placed over
a hole to reveal the effect of replacing clear glass (i.e. a ‘hole’)
with a diffusing material. The same can be done with innova-
tive glazings (e.g. prismatic films) provided that any repeating
structure is not too large compared to the size of the aperture,
Figure 8.