Climate-based daylight modelling is the prediction of any luminous
quantity (illuminance and/or luminance) using realistic sun and
sky conditions derived from standardised climate data. CBDM
evaluations are usually carried out for a full year at a time-step of
an hour or less in order to capture the daily and seasonal dynamics
of natural daylight. Developed in the late 1990s, CBDM has steadily
gained traction, first in the research community then among some
practitioners. The widespread adoption of the Radiance lighting
simulation system and, ultimately, CBDM was due in part to the
outcomes from validation studies which demonstrated the potential
high accuracy of such tools. The EFA criteria are founded on the useful
daylight illuminance (UDI) metric.
The UDI metric is based on occupant responses to absolute daylight
levels as reported in a number of studies. The idea behind UDI is
simple: daylight illuminance below a certain value (around 100 lux)
does not contribute significantly to preferred illumination levels (the
design level or greater), while high daylight illuminances (around
3000 lux or greater) may be an indicator of too much daylight/
sunlight which could result in the lowering of blinds/shades.
The UDI-achieved range of 100 to 3000 lux divides into UDI-supplementary and UDI-autonomous. UDI-supplementary daylight
illuminances in the range 100 to 300 lux are where additional
artificial lighting may be needed for common tasks such as reading.
UDI-autonomous covers the 300 to 3000 lux range where additional
artificial lighting will most likely not be needed. The daylight
autonomy (DA) value for 300 lux is the sum of the UDI-autonomous
and the UDI-exceeded values: in other words, the number of hours
where that illuminance is exceeded. The key distinction between
useful daylight illuminance and daylight autonomy is that UDI
achieved contains an upper threshold value which, when exceeded,
reduces the overall UDI rating.
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