Hi all,

This is my first post in the optics area, and I'm mostly a novice, so please excuse my ignorance.  I'm working on an optical system with an emitter and detector located on different sides of a plastic tube with a cross-section such that there are flat internal and external surfaces to allow TIR if the media flowing through the tube has the right refractive index.  Imagine a donut, with a flat ceiling inside the ID, and a couple of roof peaks sticking out of the OD.  There's a jpeg attached of the concept.

Theoretically, I can use Snell's Law, Excel, and a generous amount of pencil & paper geometry to optimize angles, dimensions, materials, etc to get the right performance out of the concept, but I was hoping some free software might be out there to help me along.  I've only located one or two free software packages, and they all seem to be built on standard, defined lenses.  I've got an arbitrary 2D shape and I want to trace beam paths through air, into the tube cross-section, and then back out to air again.

How can I import or parametrically draw the shape and trace the beam, accounting for refractive index?  If I can just make sure the beams are going in the right places, I'd be happy.  I'm not terribly concerned about power distribution, dynamics, or any other detailed effects.

Thanks in advance for any insight or advice!

-Adam