[Radiance-general] Modeling Lamps

[Zack Rogers]

Hello,

I am modeling several fluorescent troffers and am a little confused with
the "Glow" primitive commonly used to model lamps.  I have figured out
that it does not pass an illum surface and cause any additional
illumination.  It does, however, change the luminance of the luminaire
when viewed directly (i think only when using the "illum" primitive for
the ies data and not the "light" primitive).  So, I would like to get
this "Glow" correct so that I can compare the luminance ratios of
various fixtures.  In Desktop Radiance's fixture library it seems to
typically use 53.195959 as the RGB radiance value.  I can't figure out
where this number came from.  First of all, what units are these
Radiance values in?  W/ster/m2?  If this is the true than I calculate
for say a T8;

32 W / 4*pi steradians / 2*pi*0.5 sf = 0.8105 W/ster/sf

0.8105 W/ster/sf * (m2/ 10.764sf) = 0.0753 W/ster/m2

This doesn't seem right.  Please correct me if there is any error in my
calculation.

If the radiance vaue is supposed to be photometric and in units of
lumens/ster/m2 than I calculate;

2900  lumens / 4*pi steradians / 2*pi*0.5 sf = 73.46 lumens/ster/sf

73.46 lumens/ster/sf * (m2/ 10.764sf) = 6.824 lumens/ster/m2

Then the Rendering with Radiance book on Page 322 and 323 gives examples
of using the "lampcolor" program and for a daylight fluorescent (? not
sure what this is) with 2900 lumen output it gives RGB values of
0.867251 1.189160 1.066163.

I am confused.  And seeing as how this "Glow" primitive completely
effects the luminance of my parabolic troffers I do not want to continue
my comparison without assurance that I am modeling them correctly.  Any
clarification would be helpful.  Thanks!

Regards,
Zack Rogers