[Radiance-general] point calcs for verification: oddness

[Rob Guglielmetti]

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