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

[Gregory J. Ward]

Hi Jan,

The 0.5 degree value in gensky is approximate, and could be corrected  
to 0.533.  This is not difficult to do.  The current radiance is  
figured using the 0.5 degree value, and could be adjusted.  I see no  
reason not to make such a correction in the next release.

Other opinions?
-Greg

> From: Jan Wienold <jan.wienold at ise.fraunhofer.de>
> Date: April 4, 2006 4:52:00 AM PDT
>
> 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