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NORDIC LIGHT & COLOUR
79
for compliance in the most unexpected places. The daylight
evaluation in the first edition of the Estidama Pearls Design
System for Abu Dhabi was founded on daylight factors, i.e. the
CIE standard overcast sky (Estidama, 2009). A quick examina-
tion of the standard climate for Abu Dhabi reveals that it is
almost
never
overcast in that region of the United Arab Emir-
ates. This, not unexpected observation, suggests that at least
in some instances the daylight factor has indeed been applied
as a matter of routine without consideration of the prevailing
climatic conditions.
As noted in the Introduction, it is in fact the non-energy related
considerations of daylight that are likely to have the greatest
influence on building design in the future. The studies that have
claimed improved academic achievement for, say, classrooms
with “good daylighting” have gained prominence in design
circles and are now influential when fundamental decisions
regarding performance criteria are made (Heschong, 2002).
But, whilst the message regarding ‘good daylighting’ is being
taken notice of, the implementation is often poor if not actu-
ally counterproductive – that is, it could result in
worse
rather
better daylight performance. In part this occurs because “good
daylighting” is often taken to mean
more
daylight, which in turn
is taken to mean
higher
daylight factors. In practice, this often
results in spaces where the occupants habitually lower blinds/
shades to control for visual and/or thermal discomfort – any
daylight benefit is lost and the lights are usually left switched
on. Such outcomes should not be too surprising because reli-
ance on the daylight factor has not encouraged practitioners
to think of the luminous environment as one that should be
“well-tempered”, i.e. avoiding too much as well as too little.
Consider the following two criteria taken from the “Construc-
tion Requirements” for a UK hospital:
• “minimise direct solar gain to avoid the requirement for air
conditioning / comfort cooling;”
• “maximise daylight factor in patient areas;”
Using the standard ‘toolset’ i.e. daylight factors and solar pen-
etration / heat gain study, these two criteria are impossible to
reconcile. Hardly surprising since one method uses a (single)
sunless sky and the other a skyless sun, Figure 4.
The marginalisation of the expert daylight designer
A half-century or more of often uncritical use of the DF has
unfortunately led to a conflation in many minds of actual
daylighting performance with what the daylight factor tells us.
The DF is of course a proxy for daylight, but how good or bad
a proxy depends on those important parameters that the DF
approach cannot account for: prevailing climate (meaning the
totality of sky and sun conditions) and building/site orientation.
The expert daylight designer does of course appreciate these
intrinsic deficiencies. If sufficiently experienced, the designer
can roughly guesstimate the likely daylighting performance of
the space and so recommend suitable facade treatments to
temper the luminous environment. Thus the expert intuits what
is called the spatio-temporal dynamics of natural illumination.
We of course shouldn’t be surprised to learn that the designer
recommends different treatments for the north, south and
east/west elevations. Nor that the advice would change if the
building were relocated from, say, Stockholm to Madrid. After
all, ‘climate-adapted design’ is a notion that relates closely to
vernacular architecture. The designer will probably also carry
out a daylight factor analysis because it is easy to do and they
can charge the client for it – even if they take minimal notice of
it themselves. If however the client demands that the daylight
credit from a particular guideline document
(e.g. BREEAM, LEED
2
, etc.) must be achieved, then the success
of the design will hinge to a large degree on the nature of the
‘target’ sought - invariably some measure based on the day-
light factor. In which case, the best the expert designer can do
is try to make good the failings that might, and often do, result
from compliance chasing. The client may even decide that the
expert is not required since the facade treatment will be ‘opti-
mised’ by someone using a lighting simulation tool: tweaking
here and there until the compliance target is reached. This has
led one notable lighting expert to conclude that:
“. . . the only people who have a chance of getting it right are those
who ignore everything the lighting profession proclaims through
daylighting codes, stan- dards and recommended practice docu-
ments.”
(Cuttle, 2012)
Such sentiments are understandable. However, if the stan-
dards are proving to be insufficient to ensure a high likelihood
that a good daylighting design is achieved, then we should
look to improving them rather than ignoring or ditching them
altogether.
The Daylight Factor and Actual Daylighting Conditions
The daylight factor was formulated long before the computa-
tion of actual illumination levels became a practical possibility.
Thus the simplifications inherent in the formulation were back
then a necessary expediency. Also, it was believed that the day-
light factor expressed the efficiency of a room and its window(s)
as a natural lighting system because humans perceive relative
rather than absolute luminances. In other words, the daylight
factor provides a better indicator of the luminous environment
experienced by humans than absolute illuminances because
it expresses the amount of light in a space relative to the light
that would be seen outdoors in sidelit spaces (Hopkinson et al.,