[Radiance-general] Perez Sky for Daylight Simulation

[Roland Schregle]

escher@climate.bauwesen.uni-dortmund.de wrote:

> I want to generate indoor illuminance values in Lux for a validation
> with measured values of an office room.

Hi Joerg,

I don't wanna discourage you, but you're in for some MAJOR headaches.
Physical validations are a formidable task, to say the least. Errors are
numerous and unavoidable, and you'll have to figure out where they come
from and isolate them. 

We've done a physical validation of the RADIANCE photon map at
Fraunhofer ISE, and I'm still trying to figure out some of the errors in
some instances. We kept everything as simple as possible. Even then,
it's still a nightmare.

We didn't even bother with daylight because of its fluctuations, but
used an artificial source instead. We also gave up using "real" daylight
systems in the measurements after we discovered that most of them have
defects (warping, etc) which vastly affect the light redirection and are
difficult to model. Furthermore, some systems may even give rise to
polarising effects, which cannot be modeled with RADIANCE. In the end,
we resorted to measurements with a simple light shelf.

> To obtain results for illuminance I execute rcalc for the coordinates
> of the real  measured points using a function I found in the mailing
> archive
> 
> rcalc -e '$1=47.435*$1+119.93*$2+11.635*$3'

I assume you're converting irradiance to illuminance here. You can of
course use your measured sky illuminance directly in the simulation
without the need for conversion -- RADIANCE won't care whether it
calculates in watts or lumens. 

Beware that spectral effects can introduce some deviation if you treat
the walls as monochromatic in the simulation. You could roughly estimate
how much the daylight spectrum is modified by wall interreflection
before V(lambda) weighting by the sensors, which I assume is the case in
your setup.

Your validation is also complicated by the wall paint in your office.
Wall paint tends to be off-specular and in some cases even
retroreflective. RADIANCE's gaussian BRDF model will NOT account for
these effects. I suspect the paint's specular component (and no paint is
truly lambertian) contributes significantly to your error.

> Unfortunatly I don't obtain reasonable results

Welcome to the club! :^)

Like I said, you're in for a VERY tough treat here. Life sucks when
comparing SimLife[tm] to RealLife[tm]. Good luck!!!

--Roland


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