[Radiance-general] point calcs for verification: oddness

[Greg Ward]

The d in d^w has to be meters, not feet.  That could account for the 
10x difference.

-G

> From: "Rob Guglielmetti" <rpg at rumblestrip.org>
> Date: December 10, 2004 2:36:34 PM PST
>
> Hi All,
>
> I'm having some trouble understanging something (again).  I wanted to
> verify that an .ies file I have is being used properly, and I'm getting
> some confusing info.  I have an .ies-formatted photometry file for a
> PAR38.  The candela at nadir is 5448.  To do a simple verification that
> all is well I performed the following steps:
>
> ies2rad -o foo -di -t white -m 1 foo.ies #convert file
> oconv foo.rad > foo.oct # create octree
> echo "0 0 -120 0 0 1" | rtrace -h -I -ab 0 foo.oct #calculate 
> illuminance
> at 10' below fixture
>
> ... output from that is:
> 3.276076e+00    3.276076e+00    3.276076e+00
>
> Taking 3.276706 and multiplying by 179 I get 586 lux, or about 10x what
> I'd expect.  I'd assumed that by measuring the light source in a void,
> that the candela at nadir could be used in a simple point calc, 
> unaffected
> by interreflection.  So, shouldn't CBCP*d^2 = illuminance ?
>
> What am I doing wrong here?
>
> - Rob