SV: Re: [Radiance-general] Lux-values for points in a plane

[Thomas Bleicher]

On 14 May 2008, at 08:54, Per Haugaard wrote:
>
> > 2) Don't use a box at all. At the perimeter there is a dark horizon
> > (black walls) which is taken in into the calculation. You could
> > create a sphere of the material around your calculation point  
> instead.
>
> I’m not sure what you mean here.
I would suggest to split your verification of glazing material and  
shading system
into two separate steps to identify possible problems more easily.  
For the material
part I would create a simple scene like this:

#### simple test scene ####

## overcast sky with 10000 lux horizontal diffuse
!gensky 3 21 12:00GMT -c -o 0 -a 51 -m 0 -B 55.866

skyfunc glow skyglow 0 0 4 1 1 1 0
skyglow source sky 0 0 4 0 0 1 180

## material for glass sphere
void glass window_glass
0
0
3 .78 .78 .78

## glass sphere around origin
window_glass sphere glass_sphere
0
0
4 0 0 0 1

#### end of test scene ####

Then use two points (0,0,0) and (0,0,2) to calculate the lux values
inside and outside of the sphere. The direction has to be (0,0,1)
because the scene does not include a ground glow.

If you get around 10000 lux outside of fhe sphere and with the
above material around 7200 lux inside the scene setup is correct
and you can now start modifying the values for the glass material.
The ratio of ouside/inside will give you the transmittance of the
glazing (which is not equal to the values of the glass material!).

When you have verified your glass material do the next step and
use it in a scene. You can now be sure that the material is correct
and any unexpected values are the result of the scene geometry.

>
> > The second problem I can't explane but depending on the size of your
> > box the 'dark horizon' might have some influence here.
>
> My reference point in the room should coincide with the center of  
> the window. I appreciate any help on solving why the ratio between  
> the output and the entered LT-value for the window (0,62 vs. 0,72)  
> differ.

Your reference point might be in the centre of the window but
rtrace samples the whole hemisphere around this point. So if you
have (black) reveals this will influence the calculations even if
you move your calculation point only by 0.05 units.

Without knowing more about your scene it's hard to tell what
exactly causes your values. As a quick help calculate the scene
without glazing and with different glazing values but always
from the same point inside of the room. If these values are correct
it's your geometry that's creating the problems.


Thomas
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