Usability of Digital Cameras for Verifying Physically Based ...

2.2.6 Evaluation of Four Daylighting Software Programs
In order to help architects and lighting designers in choosing the most accurate
daylighting software, Ubbelohde and Humann [UH98] did a comparative evalu-
ation of four daylighting programs: Lumen Micro [Lum06], SuperLite [Sup06],
Radiance [War94] and Lightscape [Lig06]. The comparisons were based on a
model of an existing building in San Francisco. A CAD program was used to
generate a standard DXF file of the architectural drawings. A LiCor 210S photo-
sensor was used to take measurements in one of the offices of the building. The
authors tried to find real skies as close to CIE standard skies as possible. In each
program, overcast sky and clear sky conditions were specified for the times and
dates of the real building metering.
The results of all four programs differed significantly from the on-site meas-
urements, especially for Lumen Micro and SuperLite. The predictions were 10 to
20 times too high. The authors concluded that this is because the original DXF
file could not be used with these programs and for that reason the geometry had
to be simplified. Radiance and Lightscape yielded much better results. An exact
match would be unlikely because of the difficulty (or impossibility) to find real
sky conditions that exactly correspond to CIE standard skies.
2.2.7 Validation of Daylight Simulations using Radiance
In 1999, Mardaljevic [Mar99] finished his PhD on the validation of daylight sim-
ulation using the Radiance rendering system. In the course of his work he invest-
igated whether Radiance yielded reliable results for daylight modeling of archi-
tectural scenes. The measurements required were accomplished within the scope
of the International Daylight Measurement Programme (IDMP) organized by the
CIE. These particular measurements were taken from July 1992 to July 1993. On
the one hand, the sky was scanned by mounting a sky monitoring apparatus on top
of a building. During daylight hours, 150 readings were made every 15 minutes.
On the other hand, the illuminance inside two rooms of the same building was
measured using photocells at the same time. Six photocells were positioned 0.7m
above floor level along the center line of each room (see figure 14). The second
room was used to test the effect of several different innovative glazings. The
simulation was based on the recordings of the scan of the sky. The result of the
simulation could therefore directly be compared to the values that were measured
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