[Radiance-general] Accurate definition of trans material

[Phillip Greenup]

Hi Tim,

I think your answer is transmittance all the way.

If you take a look at the reflection equation, there's a term there for the transmitted indirect component, Tau(a)/pi*A(t).  A(t) is the indirect irradiance on the opposite side of the material (W/m2).  It's a similar story for diffusely transmitted direct components, except with Tau(d).  So, the luminance of the transmitted component is determined by Tau, with no additional factors relating transmissivity to transmittance.  This is the same Tau (Td) that was referred to in Rob's message from yesterday.

I don't know whether this makes any sense for anyone, but the result is that you're looking at transmittance, not transmissivity.

Phil.

>>> Tim.Hanson at arup.com 10/03/2004 1:26:26 am >>>
Hi Rob - and others,

Please can you clarify the following re- trans material, as myself and my
colleagues have been arguing over whether to use transmission or
transmissivity for diffuse transmission and/or specular transmission when
setting the parameters for the trans material based on radiance tests.

Do you consider transmissivity of the diffuse transmittance only, and use
the pure transmission of specular component?  The reason I ask is that we
set up a small test which measures the luminance of a trans material and the
overall light transmission through a trans material with different settings.
Using a specular transmission (Rs) of 0.5 (50%) - i.e. not converting to
tranmissivity, and NO diffuse transmission/transmissivity, we get exactly
half the source luminance on the side opposite the light source on the trans
material using rtrace.  i.e. tranmissivity not used for specular component.

Working out the diffuse transmission is a little more tricky, but looking at
the maths behind the settings for the trans material, I don't quite
understand whether transmissivity or transmittance should be used - it
depends on how the internal workings of radiance interprets the parameters
and therefore I can't be sure.

(As I understand, transmissivity is the proportion of light that travels
through the medium before inter-reflection within the medium).

So - the question could be written :

Do I use specular transmission with diffuse transmissivity when setting the
trans parameters?

Also - if transmissivity is used for the transmitted light, why is a factor
(such as transmissivity) not applied to the reflected part also (due to
inter-reflections)?

Thanks for any insight - I know there's been many questions and answers on
this subject, but I'm still confused!

Tim