[Radiance-general] Re: ies photometry+geometry

[Thomas Bleicher]

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 at 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 at 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
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
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