[Radiance-general] material trans and glass?

[Andrew McNeil]

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

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

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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