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
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.
Date: Thu, 31 Jan 2013 20:58:57 +0800
From: "Lars O. Grobe" <firstname.lastname@example.org>
To: Radiance general discussion <email@example.com>
Subject: [Radiance-general] useage of the dielectric material type
Content-Type: text/plain; charset="us-ascii"
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
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,