[Radiance-general] Perez Sky for Daylight Simulation

[Greg Ward]

Hello Jorg,

Not knowing anything about your model, it is difficult to say what 
might be the source of your error.  There are so many possibilities, 
it's difficult to even count them.  First off, you should include a 
ground description in your sky description, like so:

skyfunc glow sky_glow
0
0
4 1 1 1  0
sky_glow source sky
0
0
4 0 0 1 180
skyfunc glow ground_glow
0
0
4 1 1 1 0
ground_glow source ground
0
0
4 0 0 -1 180

Or, you could simply increase the coverage of your sky by changing the 
180 to 360.  The result will be the same.  The reason it's important to 
include the ground glow is so that rays outside of your ground plane 
are not counted as black.  I assume your model includes a ground plane 
(a large ring or polygon).  If it does not, and your window is near to 
the ground such that the shadow of the building is important, it can 
make a significant difference to your results.

If you haven't done so already, I strongly recommend you read the 
chapter in "Rendering with Radiance" authored by John Mardaljevic, 
which goes into some detail regarding daylight simulations.  His thesis 
is an even better reference, if you can get a copy.  I didn't see a 
link to it on his website, but there are lots of other useful 
references there:

	http://www.iesd.dmu.ac.uk/~jm/

I do not have personal experience with the Perez model of gendaylit, 
but I can tell you from the experience I do have that real skies almost 
never match simulated ones.  Unless you start from full angular 
measurements of your sky distribution, you cannot really hope for 
agreements between measurments and simulations of much better than 
+/-25%.  This is not the fault of Radiance or of the options you give 
to the simulation, but simply a "reflection" of the difficulty of 
modeling something as naturally variable as skylight.  Sky distribution 
models are designed to provide a basis for comparison between designs 
under "typical" sky conditions, and are ill-suited to matching specific 
skies or physical measurments.  Try taking the same reading two days in 
a row under what you consider to be an overcast sky.  I wouldn't be 
surprised if the ratio you measure between the outdoor and indoor 
illuminances doesn't vary under different real skies by at least 
10-15%.  You can't ask any simulation to get errors below this 
variation no matter how good it is or how well you have modeled the 
sky.  To match the reality of a particular sky, you must measure it.

I hope this is helpful.
-Greg