ies photometry+geometry

Great post, Rob!

Just one small addition:

steve michel wrote:

A linear fixture is exactly what I had in mind. For a while I even searched for 'bare bulb' photometry files from big lamp manufacturers (GE or Philips) but none exist for fluor tubes (only par and mr16 spots). But is the illum distribution method another one or a subset of using flatcorr and lboxcorr techniques you describe??

No photometric files exist for bare fluorescent tubes probably because you can't really test that condition, nor would anyone want to spend the money to do so. A fluorescent tube will always be in a fixture, even if it's a simple striplight -- a simple ballast housing and two lampholders.

If all you need is a T5 tube without complex reflector geometry behind it
you can create a cylinder with the right brightness to match the output
lumens of your lamp. The 'lampcolor' script calculates the right RGB values
for you. Here is a sample session for a T5 35W lamp:

ble@zwielicht:~ $lampcolor
Program to compute lamp radiance. Enter '?' for help.
Enter lamp type [WHITE]:
Enter length unit [meter]:
Enter lamp geometry [polygon]: cylinder
Cylinder length [1]: 1.449
Cylinder radius [0.1]: 0.008
Enter total lamp lumens [0]: 3300
Lamp color (RGB) = 80.569890 80.569895 80.569895
Enter lamp type [WHITE]: ble@zwielicht:~ $

The RGB values of 80.57 now go in a definition for the light material
which in turn is used by the cylinder definition:

## light material for 35W T5 lamp
void light light_35W_T5
0
3 80.57 80.57 80.57

## light applied to cylinder centred at (0,0,0) and aligned with x-axis
light_35W_T5 cylinder lamp_T5
0
7 -0.7245 0 0 0.7245 0 0 0.008

These four lines will create a bare T5 lamp that can be used as light
source in Radiance scenes. The output distribution of a real lamp might
be a tiny bit different (at the end caps for example) but I'd say it's
close enough for lighting calculations.

You should not stick this lamp in complex reflector geometry and let
Radiance do the calculations of the resulting distribution for you.
The algorithms in Radiance are not suited for this kind of calculation.

Regards,
Thomas

···

On 27 Apr 2007, at 01:21, Rob Guglielmetti wrote:

Thomas,

Thanks for the tip. Using bare lamp photometry can be useful in architectural lighting geometry (eg. cove lighing, cornices etc..) that don't require complex fixtures other than a lamp holder and wiring.

Steve

···

From: Thomas Bleicher <[email protected]>
Reply-To: Radiance general discussion <[email protected]>
To: [email protected],Radiance general discussion <[email protected]>
Subject: [Radiance-general] Re: ies photometry+geometry
Date: Sat, 28 Apr 2007 11:39:43 +0100

Great post, Rob!

Just one small addition:

On 27 Apr 2007, at 01:21, Rob Guglielmetti wrote:

steve michel wrote:

A linear fixture is exactly what I had in mind. For a while I even searched for 'bare bulb' photometry files from big lamp manufacturers (GE or Philips) but none exist for fluor tubes (only par and mr16 spots). But is the illum distribution method another one or a subset of using flatcorr and lboxcorr techniques you describe??

No photometric files exist for bare fluorescent tubes probably because you can't really test that condition, nor would anyone want to spend the money to do so. A fluorescent tube will always be in a fixture, even if it's a simple striplight -- a simple ballast housing and two lampholders.

If all you need is a T5 tube without complex reflector geometry behind it
you can create a cylinder with the right brightness to match the output
lumens of your lamp. The 'lampcolor' script calculates the right RGB values
for you. Here is a sample session for a T5 35W lamp:

ble@zwielicht:~ $lampcolor
Program to compute lamp radiance. Enter '?' for help.
Enter lamp type [WHITE]:
Enter length unit [meter]:
Enter lamp geometry [polygon]: cylinder
Cylinder length [1]: 1.449
Cylinder radius [0.1]: 0.008
Enter total lamp lumens [0]: 3300
Lamp color (RGB) = 80.569890 80.569895 80.569895
Enter lamp type [WHITE]: ble@zwielicht:~ $

The RGB values of 80.57 now go in a definition for the light material
which in turn is used by the cylinder definition:

## light material for 35W T5 lamp
void light light_35W_T5
0
3 80.57 80.57 80.57

## light applied to cylinder centred at (0,0,0) and aligned with x-axis
light_35W_T5 cylinder lamp_T5
0
7 -0.7245 0 0 0.7245 0 0 0.008

These four lines will create a bare T5 lamp that can be used as light
source in Radiance scenes. The output distribution of a real lamp might
be a tiny bit different (at the end caps for example) but I'd say it's
close enough for lighting calculations.

You should not stick this lamp in complex reflector geometry and let
Radiance do the calculations of the resulting distribution for you.
The algorithms in Radiance are not suited for this kind of calculation.

Regards,
Thomas

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