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NORDIC LIGHT & COLOUR
77
The Daylight Factor
Design guidelines worldwide currently recommend daylight
provision in terms of the longestablished daylight factor (DF)
(Hopkinson, 1963). It appears that the daylight factor, or at
least its precursor, was first proposed in 1895 by Alexander
Pelham Trotter (1857-1947) (Love, 1992). The origins of the
daylight factor are actually somewhat hazy since there does not
appear to have been a seminal paper introducing the approach.
The reference to its introduction in 1895 appears to be anec-
dotal and recalled a number of years later. The daylight factor
was conceived as a means of rating daylighting performance
independently
of the actually occurring, instantaneous sky con-
ditions. Hence it was defined as the ratio of the internal hori-
zontal illuminance E
in
to the unobstructed (external) horizontal
illuminance E
out
, usually expressed as a percentage, Figure 2:
However, the external conditions still need to be defined since
the luminance distribution of the sky will influence the value of
the ratio. At the time that the daylight factor was first pro-
posed it was assumed that heavily overcast skies exhibited only
moderate variation in brightness across the sky dome, and so
they could be considered to be of constant uniform) luminance.
Measurements revealed however that a densely overcast sky
exhibits a relative gradation from darker horizon to brighter ze-
nith; this was recorded in 1901. With improved, more sensitive
measuring apparatus, it was shown that the zenith luminance
is often three times greater than the horizon luminance for
some of the most heavily overcast skies (Moon and Spencer,
1942). A new formulation for the luminance pattern of overcast
skies was presented by Moon and Spencer in 1942, and it was
adopted as a standard by the International Commission for
Illumination (CIE) in 1955. Normalised to the zenith luminance
L
Z
, the luminance distribution of the CIE standard overcast sky
has the form:
where is the luminance at an angle from the horizon and
L
Z
is the zenith luminance (Figure 2).
The luminance of the CIE standard overcast sky is rotationally
symmetrical about the vertical axis, i.e. about the zenith. In
other words, the illumination that the standard overcast sky
delivers to an internal space will be same regardless of the
compass orientation of the building. And, since the sky is fully
overcast, there is no sun. Thus for a given building design, the
predicted DF is insensitive to either the building orientation
(due to the symmetry of the sky) or the intended locale (since
it is simply a ratio). Because the sun is not considered, any
design strategies dependant on solar angle, solar intensity, or
redirection of sunlight can have no influence on the daylight
factor value.
Dissatisfaction with the standard guidelines
Many guidelines give an average daylight factor as the recom-
mended target. For exam- ple, in BS8206-2 the guidance states
that to “have a predominantly daylit appearance
. . . [the] average daylight factor should be at least 2%” (BSI,
2008). In a similar vein, the Building Research Establishment
Environmental Assessment Method (BREEAM) states that “.
. . at least 80% of floor area in occupied spaces has an aver-
age daylight factor of 2% or more”.
1
These guidelines can have
a major influence on key aspects of the building design. For
example, school buildings with a significant element of prefab-
rication have been designed to conform to BREEAM daylight
specifications, Figure 3 (Jansen, 2011).
Note however that, even with something as seemingly straight-
forward as an average daylight factor specification, the results
are open both to interpretation and ‘gameplaying’. The average
can be a quite misleading quantity when applied to daylight
dis- tributions, especially for spaces illuminated from verti-
cal glazing on one wall where the very high DFs close to the
windows can significantly influence the average DF value. The
2011 revision of Lighting Guide 5: Lighting for Education rec-
ommends that there is a 0.5m border width (i.e. perimeter) be-
tween the sensor points and the walls/glazing (LG5 CIBSE/SLL,
2011). However, note that LG5 is stated in terms of a
border
,
whereas the BREEAM guidance recommends a percentage of
the floor area for the sensor points, but not where it should be
placed. Thus, with BREEAM, the user could in principle choose
where to place the 80% coverage sensor plane – leading to
significant variation in the outcome. An 80% coverage sensor
plane ‘pushed-up’ against the glazing would result in a mark-
edly higher average DF compared to one that was centrally
placed, and greater still compared to one placed at the rear of
the space. The median DF would be far less sensitive to such
placement issues (Mardaljevic, 2013).
The impossibility of holistic daylighting design with a
‘fractured’ methodology
In the half-century or more since the daylight factor was first
formulated it has become the the dominant metric, in fact,
often the sole quantitative measure of natural illumination
used to evaluate building designs. An indication perhaps of the
ubiquity of the daylight factor is its appearance as a measure