I'm trying to use luxcore engine to simulate how the light is distributed using various shapes of trasparent plastic (as in moder cars lights).
In order to better understand the light behaviours, i would like to see how a single laser beam behave (visualizing also it's path in both air and plastic).
I found this forum thread viewtopic.php?f=4&t=573 but I cannot reproduce it in an easy way.
My problem is that I cannot see how light move inside the plastic part (IOR 1.5).
As far as I have understood the right setup for this should be:
engine bidir with metropolis sampler, orthographic camera
-single laser beam
-glass material
-interior volume with scattering (IOR1.5) [glass volume]
-exterior volume with scattering (IOR1.0) [dust volume]
-camera volume set to [dust volume]
-world volume set to [dust volume]
In this way I can only see the light (with correct reflection and refraction) outside the plastic part, and not inside (image1).

I could find a workaround using a cube with inner volume set to [dust volume] and touching the plastic part on the front part. The other settings remain unchanged. In this way i could achieve (image2), but I'm not sure it's the right way to do this. There is also a dark area in the first refraction that seems strange to me. There is a screeshot where the larger cube is the [dust volume].


What I'm doing wrong?
You can find attached also the blender file.
Thanks for the help,
Emanuele