[Radiance-general] usage of rtrace within a material

[Greg Ward]

Hi Lars,

The physics of this situation is unusual, and the Radiance source code does apply a correction for the "solid angle ratio" caused by lensing at the surface.  However, Radiance doesn't track source directions properly inside a dielectric or through a refracting object -- it naively sends rays in a straight line towards the source, missing it in cases where significant refraction occurs.  This is why a straight ambient calculation works but the direct calculation does not.

Cheers,
-Greg

> From: Wendelin.Sprenger at ise.fraunhofer.de
> Date: January 31, 2013 8:56:14 AM PST
> 
> 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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