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The key word here is ‘tactile’ – shoe-box studies provide the ob-
server with a genuine feel for the complex relation between the
resulting patterns of illumination and the interplay of source,
space geometry and the reflective/transmissive properties of
the surfaces. In fact, it might be said that the shoe-box model
is an illustration of what climate-based modelling is intended
to achieve. Models should be kept fairly simple for these stud-
ies, with just a few interior surfaces to demonstrate the effect
of obstructions, etc.
The shoe-box model can be used under almost any lighting
condition, though natural light from a window or outside will al-
low for greater variety and subtlety in experienced illumination
conditions than artificial light. If the systematic progression of
sun needs to be evaluated, then the shoe-box model could be
viewed using light from a heliodon in a similar manner to that
used for more carefully prepared scale models, Figure 9.
Quantitative measures of daylight
Daylight in any quantitative sense should begin with absolute
measures such as illumi- nance (lux) or luminance (cd/m
2
).
Students should be given the freedom to relate light meter
readings to their own perceptions of a space under a variety of
conditions. A number of studies have demonstrated that 300
lux of natural illumination is considered adequate by the ma-
jority of building users and also correlates with the notion of a
“well daylit space”. This is something the students could inves-
tigate. An excellent exercise in this regard is the study carried
out by Reinhart and Weissman: “The daylit area – Correlating
architectural student assessments with current and emerging
daylight availability metrics” (Reinhart and Weissman, 2012).
Scene luminance was until recently a more difficult quantity to
measure, and therefore also more challenging to appreciate in
a ‘hands on’ sense. Luminance meters are still fairly expensive,
and, of course, give a luminance reading for only a tiny portion
of the scene – meters typically have an acceptance angle of 1
o
or less. A recent technology called high dynamic range (HDR)
imaging has greatly expanded our capacity to measure and
describe the visual field. A high dynamic range (HDR) image
is one where every pixel contains a luminance reading for that
point in the recorded scene, in other words: a measurement of
luminance, Figure 10. There are a small number of specialist
HDR cameras on the market, however it is possible to cre-
ate HDR images from multiple exposures taken by consumer
digital cameras which can have up to 10 million or more pixels
(Reinhard et al., 2005). Furthermore, the consumer cameras
can be fitted with a full fish-eye lens so the recorded image will
be equivalent to or exceed the human field of view.
HDR imaging can also be used to measure the luminous flux
through building apertures such as windows, facade systems,
light-pipes etc. using a simple technique whereby the lumi-
nance values in the HDR image serve as a proxy measurement
for incident il- luminance (Mardaljevic et al., 2009b). The ap-
proach allows rapid quantification of the luminous flux in light-
fields of arbitrary complexity where the standard measurement
practices would be either time-consuming or impossible to
apply with any certainty due to the practical difficulties of car-
rying out numerous spot measurements covering large areas
under natural (i.e. non-steady) conditions. Put simply, a sheet
of diffusing material is placed over a building aperture and the
distribution of luminance across the diffusing material is mea-
sured using a HDR image. If the relation between incident illu-
minance and the resulting luminance of the diffusing material
is known, then it is a straightforward matter to determine the
luminous output of the window from the HDR image. Ordinary
printing paper can be used for the diffusing material, and the
skills required to carry out the measurements are fairly mod-
est and could be made simple enough for non-experts to use
in under-graduate laboratory practicals. Thus students could
begin to gain an appreciation of windows as “dynamic daylight
luminaires”, allowing into the space a measurable amount
of light depending on the external conditions. Such activities
will complement the developments in daylight simulation and
evaluation where the move is towards absolute measures of
illumination under realistic sky conditions.
The standard evaluation methods
The basis of most guidelines and recommendation is still the
daylight factor (DF). Stu- dents should of course be familiar
with the daylight factor, but the teaching should strive to make
the distinction between the “good daylighting” and what the
daylight factor can tell us. Following on from that, the students
should be made aware of the dangers of ‘compliance chas-
ing’, and also taught to think critically about ‘received wisdom’.
Rule-of-thumb methods should not be overlooked since these
can help with an appreciation of the basics of daylighting de-
sign (Reinhart and LoVerso, 2010).
Students should be encouraged to relate DF values to the likely
occurrence of internal daylight levels using the long-overlooked
method described in a the 1970 CIE document ‘Daylight’ (Com-
mission Internationale de l’Eclairage, 1970). Recently discussed
on an EU standardisation panel was a proposal to move the ba-
sis of daylight evaluation from relative values based on a single
sky (i.e. the DF), to the annual occurrence of an absolute value
for illuminance (e.g. 300 lux) estimated from the cumulative
availability of diffuse illuminance as determined from stan-
dardised climate files. This proposal offers several advantages.