[Radiance-general] Modeling Lamps

[Georg Mischler]

Zack Rogers wrote:

> 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!


Reality is often much less complicated than expected. ;)


You give a luminous flux of 2900 lm for your type of 32W T8 (this
can vary with color/quality). Given the standard efficacy used by
Radiance, this amounts to 16.2 W of light energy (which can't be
directly determined from the electrical consumption!)

The tube has a circumference of approximately 81.7 mm and a
luminous length of around 1.198 m. This results in a luminous
surface of 0.0979 m2.

The radiance of this tube is therefore:

  16.2W / (0.0979m2 * Pi) = 52.7 W/m2/sr

This is what the average rgb value of your glow material should
be based on. The luminance of the tube then amounts to 7684 cd/m2.

Btw: If I feed the lampcolor program with those values, then I
get very different results than you do:

$> lampcolor
Program to compute lamp radiance.  Enter '?' for help.
Enter lamp type [WHITE]: daylight fluorescent
Enter length unit [meter]:
Enter lamp geometry [polygon]: cylinder
Cylinder length [1]: 1.198
Cylinder radius [0.1]: 0.013
Enter total lamp lumens [0]: 2900
Lamp color (RGB) = 35.450659 48.609340 43.581585

0.265*35.450659 + 0.670*48.609340 + 0.065*43.581585 = 44.8

This is 0.85 of the 52.7 above. Lampcolor has a depreciacion
factor for each lamp type, possibly accounting for aging (the
actual purpose of the factor is not documented).


-schorsch

-- 
Georg Mischler  --  simulations developer  --  schorsch at schorsch.com
+schorsch.com+  --  lighting design tools  --  http://www.schorsch.com/