# Radiance-general digest, Vol 1 #40 - 1 msg

Georg,

Thanks for the help, I'm still a little confused though.

First, the efficacy radiance uses for a 32W T8 is 179lumens/W? I'm assuming
that efficacy is for light energy and not power consumed. Is this the same
for all lamps? Most of the time the efficacy of a lamp refers to the power
consumed, a typical T8 having an efficacy of 2900lumens/32W or 90lumens/W.

Also, I agree that the luminous surface is 0.0979m2 (I had missed a length of
4' in my calc). Then you calculated the radiance with

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

This is the total radiant output per steradian per area. However, it seems
like the total steradians you should be dividing by is the entire sphere that
the 16.2W of radiant energy is filling, a sphere being 4*pi steradians. This
gives;

16.2W / (0.0979m2 * 4 * Pi) = 13.17 W/m2/sr

So, I'm still a little confused with the math here, and the efficacy used to
convert lumens to light energy(W).

BTW, I too get those values when I run lampcolor, I was just looking at the
example in the Rendering with Radiance book, which I guess might have a typo.

Thanks for the help,
Zack

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than "Re: Contents of Radiance-general digest..."

Today's Topics:

1. Re: Modeling Lamps (Georg Mischler)

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Message: 1
Date: Thu, 20 Dec 2001 10:58:23 -0500 (EST)
From: Georg Mischler <[email protected]>
To: <[email protected]>

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
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/

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_______________________________________________
[email protected]

Zack Rogers wrote:

First, the efficacy radiance uses for a 32W T8 is 179lumens/W? I'm assuming
that efficacy is for light energy and not power consumed. Is this the same
for all lamps? Most of the time the efficacy of a lamp refers to the power
consumed, a typical T8 having an efficacy of 2900lumens/32W or 90lumens/W.

I'm not sure if efficacy is really the best word here. The value
of 179 is simply the factor that Radiance normally uses for
converting between photometric and radiometric units (lumens per
Watt of radiation in the visible spectrum), taking into account the
response curve of the human eye for a "uniform white" light color.

power consumption of your lamp has no relevance in the
calculation at all. For all we know, you could just as well be
working with a magic oil lamp!

Also, I agree that the luminous surface is 0.0979m2 (I had missed a length of
4' in my calc). Then you calculated the radiance with

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

This is the total radiant output per steradian per area. However, it seems
like the total steradians you should be dividing by is the entire sphere that
the 16.2W of radiant energy is filling, a sphere being 4*pi steradians.
This gives;

16.2W / (0.0979m2 * 4 * Pi) = 13.17 W/m2/sr

Why do we divide by Pi? Good question...
I don't have a degree in math myself, so I can't explain this
very well either. The best hint might be that we're looking at a
discrete direction (a point out of a continuum), and not at a
fraction of the total [hemi]sphere (a subset of that continuum).
Our direction has a solid angle of zero, while your line of
thought assumes a solid angle of 1.

That doesn't really explain the division by Pi, but at least it
shows why a division by 4*Pi for the full sphere (or 2*Pi for the
hemisphere) would be wrong. The factor of Pi can probably be
found somewhere in the derivation of the integral that stands