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@gmx.net>

To: Radiance general discussion <radiance-general@radiance-online.org>

Subject: [Radiance-general] useage of the dielectric material type

Message-ID: <4CCD6425-B096-44B6-8B39-69741B53E1AD@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.