[Radiance-general] material trans and glass?

[Thomas Bleicher]

On Sun, Dec 13, 2009 at 3:36 AM, Victor Li <victorpermanent at gmail.com> wrote:

> From the web "http://www.schorsch.com/rayfront/manual/dielectricdef.html" ,
> " Transmittance is the total light transmitted through the pane including
> multiple reflections"
>
> So the difference between Transmisivity and Transmittance is the light
> reflected?

I would go with this definition. But the reflections here are those that
happen between two parallel interfaces of a refracting material as the
front and back of a sheet of float glass. Some light is reflected at
the back of the sheet when the light exits the glass. This light will
travel back through the glass to the front and again a small amount
will be reflected towards the back and so on.

The formula to calculate transmissivity from the transmittance of glass
(VLT) takes these inter-reflections into account and so you can model
the two interfaces of a single pane of float glass with one single polygon
in Radiance and you don't have to calculate the reflections during the
ray tracing process.

> According to Andrew's email, for glass with VLT of 0.325
> has a transmisivity of 0.354. The glass is defined as
>
> void glass glazing.325
> 0
> 0
> 3  0.354  0.354  0.354
>
> For trans with VLT 0.325 and no diffse transmission.
> Can we difine it as the following?
>
> void trans window.0.325
>
> 7     0.325 0.325 0.325 0 0 1 1

The correct definition would be
void trans window.0.325
0
0
7 0.325 0.325 0.325 0 0 1 1

This produces a dark clear material without any specularity that has
a VTL of 0.325 (or nearly as far as I can tell from my tests). But it
is not the same as glass.

> For glass, the light transmitted would be specular transmitted,
> is it right? If right, what is the differrence between above meteirals
> definition - glass glazing.325 and trans window.0.325?

The most obvious one is that "glass" is a specialised "dielectric"
material type which modulates the reflection according to the
angle and refraction index of the material. So if you want to
reflections/refractions of glass with another material you should
start with "dielectric" not "trans"

The "window.0.325" material defined above has not specularity
and no roughness. The glass material has a visible specularity.
Just create a small test scene with a bubble of glass/trans and
a light source (like the sun) and you will see the difference.

If you don't care about the particular refraction of glass and
specular refraction and only focus on VTL you could say the
materials are similar or equal depending on your use.

Regards,
Thomas