[Radiance-general] window glass modeling

[Milan Cakanovic]

Hi Thomas,

> ...there is an alternative method to calculate illuminance values on a
> (theoretical) surface thanks to Radiance's "lego bricks" architecture:
> http://sites.google.com/site/tbleicher/radiance/stencil
> (Warning: Content may not be suitable for beginners. Guru guidance advised.)

I looked at this site and immediately got a headache. I still wrestling with 
Basic Radiance, and at this moment, I have no strength for stencil. I am a
beginner and too far away guru.

> The value you choose for -ab depends on the complexity of your scene geometry
> and lighting conditions. From what you have written so far you have an empty
> room with one window and exterior blinds. I assume there is no outside
> obstruction to the window.

There is no outside obstruction to the window. I have an empty room with one
window and exterior blinds, plus four luminaires (but I think that they do not
affect to the complexity).

> Without the blinds you can cover the light distribution within the room
> accurately with an -ab value of 3 or 4. If you add the blinds you will need
> additional "bounces" to account for the light reflected from the blinds. I
> assume you will need at least a value of 6 to achieve reasonable accurate
> results.

I have done some preliminary calculations for an empty room with blinds, but
without a window model, and when I use rtrace with -ab 1 to -ab 8 (for more
accuracy), I get larger and larger values for an illuminance. I assume that the
largest value for an illuminance (-ab 8) is close to correct value. (Similar
situation is explained on p.43, figure 33, Basic Radiance Tutorial, Axel Jacobs).

> Before you do your evaluation calculations you should do some tests to find
> parameters which are accurate enough for your situation:

> 1) Use one typical scene setup and a sky definition without sun (only
> skylight and ground) and calculate the illuminance on one point with
> increasing values for -ab...

> 2) With a reasonably accurate setting for -ab use the same scene and sky but
> define a sequence of 10 or more points from the window to the back of the...

> Your main concern about using multiple points is the processing time.
> However, if you pass all the points to rtrace in one call rtrace will cache..

These are useful advices. I will do what did you say, but after a holiday.

Regards, Milan