[Radiance-general] material trans and glass?

[Pueltz, Gunter]

Hi Andy,
 
I followed your link and studied the description of the material GLASS in
the rayfront manual (see below).
I found, that GLASS is a special form of dielectric for modelling a thin
glas pane without internal reflections within the glas pane itself.
GLASS produces one ray, which is transmitted, and a second ray, which is
reflected - so far, so good.
 
Now my question: I am looking for a diagram or an equation of GLASS, which
shows the angle-dependancies. 
It is well known, that with more normal incidence transmission of rays is
increasing and reflection is decreasing;
with more sloped incidence angle transmission of rays decreases and
refelctions increases ....
 
Where can I find a documentation about this angle dependancies of the
material GLASS ?
 
-Gunter  

-----Original Message-----
From: radiance-general-bounces at radiance-online.org
[mailto:radiance-general-bounces at radiance-online.org]On Behalf Of Andrew
McNeil
Sent: Friday, December 11, 2009 5:35 PM
To: Radiance general discussion
Subject: Re: [Radiance-general] material trans and glass?


Hi Victor,

Transmisivity is the amount of light NOT absorbed in one pass through the
glazing.  Glass is a special case of dielectric optimized for glass that
accounts for angular dependence of transmission and inter-reflection between
front and back surfaces  (but is only modeled as a single surface in
radiance).  The reason for using transmissivity I believe is related to the
desire to maintain a valid input range of 0-1 for the parameters instead of
something like 0 - 0.92.

You can find the equation to get transmisivity given transmittance at the
bottom of this page:
http://www.schorsch.com/rayfront/manual/dielectricdef.html
<http://www.schorsch.com/rayfront/manual/dielectricdef.html>  

A glazing with reported VLT of 0.325 has a transmisivity of 0.354 so the
material definition would be as follows:
void glass glazing.325
0
0
3  0.354  0.354  0.354

And your other question - The first three parameters of trans is color which
affect transmission and reflection.  Trans does not include inter-reflection
or angular transmission properties exhibited by glass so the input values
are transmission.
Also on this page there is a nice diagram describing how trans works:
http://www.schorsch.com/rayfront/manual/transdef.html
<http://www.schorsch.com/rayfront/manual/transdef.html> 

Andy




On 12/10/09 7:22 PM, "Victor Li" <victorpermanent at gmail.com> wrote:



Sorry i made a mistake. I just want to compare the glass 3     0.325 0.325
0.325  and trans 7     0.325 0.325 0.325 0 0 1 1 (not 0.3). 

In trans, the  transmissive specularity is 1 which means the total daylight
transmitted through the material will be specularly transmitted, like glass.
So i defined the   transmissive specularity is 1. 

Actually i had a look the webpage and i define the trans followed the
instruction in the page. 

Note also that the Radiance glass material uses "transmissivity" not
transmittance, 
so does trans use "transmittance" in the first three paremeters? What is the
difference between transmittance and transmissivity?





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