[BLDG-SIM] Radiance: verifying simulations with reality?

[Caroline Prochazka]

I am a graduate student of architecture at the
University of Waterloo. I am using Energy-10, Ecotect
and Desktop Radiance to study the role of daylighting
with respect to heating and cooling energy for two
existing Canadian 'green' buildings. This posting is
only one of my many questions, which recently arose
from closer examination of my models.

RADIANCE: I have recently run a few simplified test
models and compared my results to true exterior light
conditions using a hand-held light meter. The results
are somewhat frustrating, but interesting nonetheless.
My questions here are particularly: 
1. How does Radiance calculate the external light
levels? Is it simply the calculation for CIE
standards?
2. On a perfectly clear morning in a low pollution
area (assumed turbidity=2) my light meter reading read
approx. 55600 lux. A Radiance test (modelling an
unobstructed horizontal plane outdoors) for the same
date and time, and lat. long settings, with
turbidity=2 produced an exterior light level of 47000
lux (15% off the true reading!). While there may have
been some diffuse reflection off nearby buildings,
there was certainly no direct reflection or shadow on
the light meter. This seems to be much too large a
difference.
3. On what seemed to be a uniformly overcast day, the
exterior readings gave external illuminance of 19250
lux, while Radiance simulated an unobstructed
horizontal plane at 9944 lux (overcast sky) and 13108
lux (uniform sky)...differences of 48% and 32%
respectively.

I know Radiance must be addressing some
average/typical conditions, so how much variation from
the simulation is it reasonable for me to expect? Will
this change for different sky types?

Regards,
Caroline

=====
t.    519.822.3832
e.    cjprocha at yahoo.com
web:  http://www.geocities.com/cjprocha

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