[Radiance-general] window glass modeling

[Milan Cakanovic]

Thomas,

> Just a comment here: You should calculate the illuminance at a few points
> around your actual sample point and then average the results, especially when
> you want to use blinds. This will avoid peak values due to direct sunlight
> passing between the slates and hitting your single measurement point.

I know for this problem. I have done some preliminary calculations, and when I
use rtrace with -ab 8 (for more accuracy), calculation takes a long time (4 - 5
min). If I want to calculate illumination for few more points, I will increase
the calculation time for few times. For now, I will just avoid situations when
direct sunlight passing between the slats.

> This is just a general representation of the angular dependent
> transmission/reflection of the glazing. You would have to convert this to a
> format Radiance can understand (but you already have the description above).
> To define a basic glass material you would use the Vtc (visual transmittance)
> for a 0 degree angle (orthogonal to surface). Some of the angular dependence
> is accounted for in the material primitive "glass" so we don't have to fuzz
> about with all the angles. It's not very accurate but for a scene that
> includes blinds and artificial lighting an accurate glazing representation is
> probably less important.
> You have to decide about this of course.

With this statement, you're opened my eyes. I did not know that, how you say,
some of the angular dependence is accounted for in the material primitive
"glass". I can live with "It's not very accurate".

> If you use mkillum you have to replace the window material with the
> definition created by mkillum. You also have to make sure that the polygon
> normal points inwards or you won't get any light.

I know that, but thank you anyway.

> Sounds incredibly complicated and is not at all necessary. There is no
> difference in the definition for glass for sunlight and glass for skylight.
> The various transmission values you have in your Window6 output are
> approximated/accounted for in the base primitive "glass" or the BRTDfunc
> definition.

Again, you're opened my eyes with this statement.

> If you want to split you calculation into sun, sky and ground you can do so
> in the scene definition and later add the individual results. However, your
> scenario is not a good candidate for that because the problem you see does
> not really exist.

Now, when I know all this, I have no need to split my calculation.
"...because the problem you see does not really exist." Well, because I have
bugs (or worms) in my head, the problem existed. :-) If I had known that some of
the angular dependence is accounted for in the material primitive "glass", I
would not have written a bunch of nonsense.

Thank you for all.

Regards and Happy New Year!  Milan