[Radiance-general] usage of rtrace within a material

[Wendelin.Sprenger at ise.fraunhofer.de]

Dear Lars,

I had the same problem approximately two years ago: rtrace -I does not 
give you the right answer when being applied after a dielectric surface. 
With the -i option of rtrace, I do not have any experience.
I suggest a workaround for your problem. As you did, I would chose a 
dielectric surface followed by a plastic surface. For rtrace -I inside a 
material, in theory the integral

E = int [ L cos(theta_m) d Omega_m ]

has to be calculated, m naming the angles inside the material. 
Unfortunately, at least the specular path does not remember the light 
refraction for the rtrace -I calculation. However, if you substitute the 
angles in the integral by the old ones before the material, rtrace -I 
works. The substitution theta_m=arcsin(n1/n2*theta), including 
cos(theta_m)=sqrt[1-sin^2(theta_m)], leads to a complicated integral that 
finally can be simplified to

E = n1^2/n2^2 int [ L cos(theta) d Omega ]

rtrace -I only calculates this integral. With the correction factor 
n1^2/n2^2, the rtrace -I results can be applied. 
The ambient path, however, seems to work perfectly without any correction 
factor.
I haven't done any research on what is exactly going on in the source 
codes. However, the results are fine.
I do not have an answer for your third question.

Cheers,
Wendelin





Original message:

Date: Thu, 31 Jan 2013 20:58:57 +0800
From: "Lars O. Grobe" <grobe at gmx.net>
To: Radiance general discussion <radiance-general at radiance-online.org>
Subject: [Radiance-general] useage of the dielectric material type
Message-ID: <4CCD6425-B096-44B6-8B39-69741B53E1AD at gmx.net>
Content-Type: text/plain; charset="us-ascii"

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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