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NORDIC LIGHT & COLOUR
81
1966). The argument has some credence, however on its own
it fails to address the patently obvious fact that humans also
require that illumination be within absolute levels in order to
carry out various tasks, to safely negotiate hazards, etc. An-
other major issue with the daylight factor is that actual daylight
illumination conditions deviate markedly from that described
by the overcast sky paradigm. This is so even for Northern
Europe where there is a commonly held belief that skies are
‘mostly’ overcast and so use of the daylight factor as a basis
for evaluation is justified. A paper by Littlefair in 1998 gives
annual cumulative internal illuminance measurements for a
point in similar rooms with North and South facing glazing
(Littlefair, 1998). The rooms were un-shaded and un-occupied.
An illuminance of 200 lux was achieved for approximately 58%
and 68% of the year for the North and South facing spaces
respectively. However, an illuminance of 400 lux was achieved
for only 12% of the year for the North facing space with more
than four times that occurrence (51%) for the South facing
space. Of course, for sunnier climates the effect of orientation
on daylight illumination will be greater still.
An unfortunate consequence of the long-standing and often
uncritical use of the daylight factor is that the terms daylight
(as defined in the Introduction) and skylight are often used
interchangeably. This leads to confusion where precise defini-
tions are required. Some of this muddle has resulted from
the conflation of ‘daylight’
per se
with what is predicted by the
daylight factor. For example, expressions such as: “the daylight
factor was used to evaluate daylight levels”, are common in
both research and practice literature. The daylight factor is
precisely what it was defined to be: a ratio of illuminances
under a specific sky condition. The daylight factor therefore is,
in reality, a proxy for actual daylight illumination. Thus what
the daylight factor communicates is in fact very different from
the actual illumination levels that result from the full range of
naturally occurring sun and sky conditions.
Extending the basis of the daylight factor approach by incre-
mental means has proved problematic. It is a straightforward
matter to use in a daylight simulation non-overcast sky condi-
tions, for example, the CIE clear sky luminance pattern with
sun, Figure 5. The inset images show the illuminance for the
four components of daylight illumination as described in Figure
1. These images reveal the complexity of illumination that
ex- ists in even the simplest of spaces under realistic daylight
conditions. To be useful for evaluation purposes however, the
luminous output of the sun and sky must be known since ab-
solute values and not ratios must be considered. Extending the
daylight factor notion of ratios to non-overcast skies with sun
results in essentially meaningless values and should be avoid-
ed. When absolute values for luminous quantities are predicted
(e.g. lux at the work plane) then the values used to normalise
the output from and sky must be justified, e.g. diffuse horizon-
tal illuminance for the sky and direct normal illuminance for
the sun. Ideally, these should be based typical values for, say, a
clear, sunny day in summer. The predicted quantities however
will be of very limited value for any estimation of prevailing
daylight levels in the building since they are indicative only of
conditions for particular sun and sky conditions occurring at a
particular time of the day/year. In other words, such an evalua-
tion would offer merely a single “snapshot” of the multitude of
naturally occurring daylight conditions due to all the possible
combinations of sun and sky conditions occurring at various
times throughout the year. Estimating overall daylighting per-
formance from snapshot evaluations could be highly mislead-
ing. The parameters governing the availability of daylight do
not lend themselves to any form of averaging. Whilst it can be
informative to determine, say, a monthly average for a scalar
quantity such as temperature, illumination is strongly depen-
dent on the directional character of the incident light. Associ-
ated with every non-overcast sky and sun condition are the
solar altitude and azimuth which, of course, vary continuously
throughout the day. In terms of providing a basis for predict-
ing measures of illumination, the notion of ‘average’ days is
less than useful because an ‘average’ sun position would give
entirely misleading patterns of illumination.
The true nature of illumination from the sun and sky for any
particular locale can only be appreciated by examining the
luminous output from both the sun and the sky over a period of
a full year. The principal sources of annual climate data are the
standard weather files which were originally created for use
by dynamic thermal modelling programs (Clarke, 2001). These
datasets contain averaged hourly values for a full year, that is,
8760 values for each parameter. The key daylight parameters
stored in the weather files are the diffuse horizontal illumi-
nance and the direct normal illuminance. The diffuse horizon-
tal illuminance is the visible part of the radiant energy from the
unobstructed sky that is incident on a horizontal surface. The
direct normal illuminance is the visible light energy from the
sun that is incident on a surface which is kept normal to the
beam of radiation, i.e. the photocell always ‘points’ directly to
the sun.
A visualisation of the illuminance data from a standard weather
file is given in Figure 6. The time-series data of 8760 values has
been rearranged into an array of 365 days (x-axis) by 24 hours
(y-axis). The shading at each hour indicates the magnitude of
the illuminance – see legend – with zero values shaded grey.
Presented in this way it is easy to appreciate both the prevail-