[Radiance-general] useage of the dielectric material type

[Greg Ward]

As to your third question, the transmission is a function of reflection (which is a function of angle) and the amount of dielectric material the ray has passed through, which undergoes exponential absorption with distance.  This is why transmission coefficients are given as a ratio to distance traveled, and there is a RAY struct member to hold this quantity called cext (for coefficient of extinction).  Beware that there are a few conditions under which this coefficient will not be properly accounted for, such as a surface and a light or glow source both within a refracting medium.  Since the ray never crosses any material boundary on its way to the source, it doesn't even know it's in an absorbing medium.  The same holds true for the "mist" type, which is why it's important for light sources and other significant contributors to lie outside the mist volumes.

Best,
-Greg

> From: "Lars O. Grobe" <grobe at gmx.net>
> Date: January 31, 2013 4:58:57 AM PST
> 
> Hi all,
> 
> while it is one of the most fundamental material types in Radiance, dielectric is hardly used in simulations with Radiance as far as I know. I once or twice had to use it, but never for critical parts of a model.
> 
> Now, I need to find the irradiance in (!) a glass pane on the second surface S2 in this simple sketch:
> 
> |        |
> |        |
> | o      |    * source
> | sensor |
> |        |
> 
> S2      S1
> 
> The glass material applied to a flat surface through S1 obviously does not work here, as it includes reflection at S2 into the plane through S1. That would lead to an underestimate of about 4% at normal incidence. What I did is to model S1 as a dielectric, S2 as plastic, and ran the rtrace command with the -i switch, location as marked by the o-letter in the sketch and the view vector towards S2.
> 
> - My first question - is this a valid model in Radiance, with a volume having a dielectric interface on one, a plastic surface on the other side?
> 
> - Second question, can I expect rtrace to calculate a valid irradiance reading under these circumstances?
> 
> - Third question, and that is funny - how do I find the transmissivity parameters for a glass, as dielectric would expect them, if I have transmission T measured?
> 
> If someone here could share some insight how to find a valid dielectric description from a typical transmission measurement for the visible spectrum, that would be of great help for me....
> 
> Cheers and TIA,
> Lars.
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