Light traps

This one is just a simple light dispersion test I did a couple of months ago. I showed it on fryrender forum but forgot to post it here. It was inspired by Heath’s idea of making simple mirror traps and filling them with dispersed light. More info here.

The scene is simple so I’ll try to explain how to make one, but firstly let me show you the final image.


Full rez detail view:


The idea is simple: Cast the directional light on the prism and let the dispersed light enter an enclosed space with reflective walls. The refracted light rays will bounce around and produce beautiful caustic patterns on a white floor. Here’s a quick guide how to setup the scene.

OK, so first of all you need a black environment. Then make a simple plane somewhere in space and give it a white diffuse material. We will treat it as our screen.

Now model a prism and place it just above a plane. Make sure nothing is intersecting but keep the distance as small as it is possible to prevent light leaks. Give it a simple dielectric material. Make sure IOR is set to 1.5~1.6. Fryrender uses Abbe parameter to control dispersion, turn it on and set around 2.0. Eventually I changed one of the prisms walls into mirror to reflect some light back and produce more interesting effect.

Now make the trap. I used a cylinder with top and bottom faces deleted and another one placed inside as the additional reflector. Basically you want a space without the ceiling with a prism fitted into wall (not intersecting but close enough), just as showed on images below.



  1. parabolic mirror
  2. emitter
  3. prism
  4. light trap
  5. camera facing down
  6. additional reflector



This one is really simple but you can experiment with different traps. Concave shapes results in more scattered caustics. You can also observe the effect of caustics nicely flowing along with smooth convex curves.

It’s time to cast some light. We need a direct beam of light and it’s not so easy in fryrender. As you can see I used a combination of  a parabolic mirror and a small emitter (facing the mirror) to produce a straight beam just like Hethe did in his example. There are more ways to create direct light in Fry. I could use a long tube with a small hole and emitter inside or a converging lens.

I tried all of them and it turned out that parabolic mirror is the most efficient. If you have patience for long renders you can try a lens, it will look better but will be more noisy too.

To make a correct parabolic mirror or a lens you’ll need to calculate it. You can do the math, or download my models (may need some slight emitter distance adjustments) – mirror, lens

Use IOR 1,5 for the lens and don’t miss tiny emitter polygons.

I hope it all makes sense 🙂 If you have any questions don’t hesitate to ask here or through ICQ.



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