[Radiance-general] material trans and glass?

[Thomas Bleicher]

On Wed, Dec 16, 2009 at 10:14 AM, Victor Li <victorpermanent at gmail.com> wrote:

> I know what you mean but i am sitill confused the difference between
> Transmisivity and Transmittance?

It's just how it is. If you want to define the glass material
correctly you have to convert VTL to transittance via formula.
End of existencial uncertainty.

> What is the difference between the first three parameters and trans (the
> second to last parameter) in trans definition?

Obviously the material could have any color. You define the
color of the transmitted and reflected light via the first three
parameters. The actual transmittance is the sixth parameter
(second to last). If you'd set this to 0 you would effectively
get an opaque material of the specified color.

In your example the color is identical to the transmittance
because you set the transmittance parameter to 1 which
eliminates the complexity of finding the right values for all
7 parameters. If your transmittance in less than 1 you have
to calculate the red, green and blue components from their
average value and the transmittance. The formula for this is
given in the Rendering with Radiance book.



[... on difference between glass and trans]

> Actually i run the simulations by two different materials i found the
> illuminance in the room is a little different. Why is it different?

We need to know how much different it is to comment on that.
Can you please quote the numbers you get from the simulations.

One obviouse difference is that the glass material will appear
less translucent at an angle than orthogonal to the surface.
The trans material will keep it's 0.325 transmittance at all
angles.

If you try to compare the two materials you have to use
a method that is not susceptible to these differences.


>  For "dielectric" definition, is anyone can tell me where i can find the
> detailed definition and the instruction fo corresponding parameters?

You can find documentation for Radiance here:

http://radsite.lbl.gov/radiance/framer.html

In particular this guide might be helpful. I think it lists all
material types and parameters.

http://radsite.lbl.gov/radiance/refer/usman1.pdf


Regards,
Thomas