[Radiance-general] calculation weirdness

[Gregory J. Ward]

Hi Mark,

Thanks for sharing your files with me.  It seems that your light  
fixture has a very tight output distribution, which falls off  
dramatically to the left and right.  The more you subdivide the light  
source, the more accurate your results.  Your second Radiance result  
is the more accurate.  Apparently, AGI32 only sends a single sample  
to the center of the source, which is what you also get from Radiance  
with the default -ds 0.2.

I discovered this by looking up from your sample point 120 inches  
below the light source, which is itself 20 inches in diameter.  A  
perspective view shows that even at this distance, the fall-off is  
quite noticeable.

Bottom line, the rtrace -ds 0.1 result is more accurate.  Whew!

-Greg

> From: "Mark de la Fuente" <MdelaFuente at wmtao.com>
> Date: July 31, 2006 5:56:01 PM EDT
>
> Hi all, I'm running a radiance calculation, and as always, I  
> compare my radiance calcs with AGI to make sure I didn't mess up.   
> I know enough to be dangerous!  :)
>
> My comparison consists of placing a calculation point 10' below an  
> iesfile based light fixture.
>
> AGI calc = 64810 lux
>
> Radiance = 40,822 lux.
>
> ies2rad -di -t white iesfile.rad
> rtrace -w -h -I -ab 0 -aa .1 -ad 2048 -as 512 -ar 512 -ds .1 -aw 0  
> test.oct < input_pt.txt | rcalc -e '$1=47.4*$1+120*$2+11.6*$3'
>
> OK, red flag, I've messed up somehow.  The numbers are off by 30%.
>
> After almost giving up, I ran the same command but deleted the "- 
> ds .1" and got 64844 lux!
>
> What's going on here?  I would think source substructuring "-ds .1"  
> would be better than the default (which I think is .2)  I  
> appreciate your thoughts.
>
> Mark