[Radiance-general] subtended sun angle in gensky and gendaylit

[Jan Wienold]

Hi Francesco,
dear folk,

I have been in some discussions with a user about gendaylit and 
especially the cygwin- binary of it.
We found out, that the subtended angle of the sun differs from the 
original gendaylit (0.533) version to the one, which we can download on 
http://www.bozzograo.net/radiance(0.5 for cygwin binary).
For that reason, I checked in one of the main references in the solar 
world (Duffie, Beckman: Solar engineering of thermal processes) and 
found 32' for the subtended solar angle, which corresponds to 0.5333.

So I assume you changed the value in the source code, or? But as long as 
you are not adapting the radiance value of the light source (which you 
don't do), you'll loose more than 10% of the flux of the sun by reducing 
the value from 0.533 to 0.5 !!

Is there any reason to use 0.5 instead of 0.533?

And a question to Greg: gensky is also using 0.5. Is there any reference 
to that? But: As long as the overall flux is correct, the difference in 
the angle can be neglected.


For the cygwin-version of gendaylit I would suggest to correct that 
value in your version to 0.533 of the source code. In addition to this, 
I'll try to put a gendaylit version on our Web-page, which can be 
compiled also under cygwin without any modification. If ready, I'll 
announce this here again.

Cheers,

Jan


-- 
Dipl.-Ing. Jan Wienold
Project Manager
Fraunhofer-Institut für Solare Energiesysteme
Thermal Systems and Buildings, Lighting and Daylighting
Heidenhofstr. 2, 79110 Freiburg, Germany
Phone: +49(0)761 4588 5133 Fax:+49(0)761 4588 9133
jan.wienold at ise.fraunhofer.de
http://www.ise.fraunhofer.de

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