I made a 90-degree prism out of glass with index of refraction of 1.5, which is high enough to produce total internal reflection at 45 degrees. I've done this before with no problem, but suddenly the light seems to just disappear. As far as I can tell the normals are all pointing outward.

So I put a mirror (glossy BDSF) next to it, and it's not reflecting either.

I expect to see two roughy square spots on the green object, but there is no trace of reflection.

I'm sure this is something obvious. I don't use Blender often enough to instinctively set everything correctly, but it seems like the defaults should be set up to allow this simple test to work.

What have I done wrong?

I'm attaching the blend file as well.

enter image description here

enter image description here enter image description here

Left: Prism material, Right: Mirror material.

  • $\begingroup$ Do you have bounce count turned up enough to accomodate light transport? $\endgroup$
    – 3pointedit
    Commented Sep 12, 2016 at 11:56
  • $\begingroup$ @3pointedit Here is a screenshot of the settings in the blend file. I've changed several settings and so far haven't been able to get even the simple mirror to produce a reflected spot on the greenish screen. $\endgroup$
    – uhoh
    Commented Sep 12, 2016 at 12:11
  • 1
    $\begingroup$ Cycles is limited when it comes to caustics correctly. You might want to try a different render engine like luxrender. Read: blender.stackexchange.com/questions/8645/… andhttp://blender.stackexchange.com/questions/24382/how-to-create-and-animate-the-water-reflection-caustics-on-an-object $\endgroup$
    – user1853
    Commented Sep 12, 2016 at 14:30
  • $\begingroup$ @cegaton yep, I recognize that that is good advice. Right now I am most interested in seeing how far I can get within Blender itself. For high quality results, an external rendering platform might indeed be the best way to go. But I do mostly "science" stuff and spend most of my time playing with stuff in Python and using the 3D viewport to see what happens. So for most of what I need to do, imperfect and slightly ragged results are fine. $\endgroup$
    – uhoh
    Commented Sep 12, 2016 at 14:41
  • $\begingroup$ @cegaton It turns out that I seem to have already known this about caustics a year ago and forgot. $\endgroup$
    – uhoh
    Commented Sep 12, 2016 at 15:43

1 Answer 1


You should use way more samples than 201 samples (which you used in your .blend file) to the green wall to illuminate properly in cycles.Thats because tre light source you are using in your scene is way too far and way too strong. Atleast when i put the lamp near the prism the green plate was brighter. U can clearly see in my picture that with about 10K samples the green plate was very brightenter image description here

  • 1
    $\begingroup$ There is a ''how to walk around'' this kind of problem but thats an other question feel free to ask :) $\endgroup$
    – xlxs
    Commented Sep 12, 2016 at 12:23
  • 1
    $\begingroup$ I see - those 42 fireflies are actually the beginnings of the reflected light spots. Since the mirror and prism have nearly 100% reflectivity, the spots will ultimately be roughly as bright as the light hitting the red surface below (except for the Lambertian and the viewing angle), but it takes a long long time to get there, the way cycles is calculating in this particular example. tl;dr: it was working fine. 2000 samples shows improvement Thanks! $\endgroup$
    – uhoh
    Commented Sep 12, 2016 at 12:29
  • 1
    $\begingroup$ OK now that I actually "see the light" I'm going work with it for a bit, and most likely will ask for additional help once I hit a wall. Oh, I didn't notice the little square in the middle of the green plate at first! Got it! $\endgroup$
    – uhoh
    Commented Sep 12, 2016 at 12:46
  • $\begingroup$ It turns out that I seem to have already known this about caustics a year ago and forgot. $\endgroup$
    – uhoh
    Commented Sep 12, 2016 at 15:45
  • $\begingroup$ I've started to wonder about your "how to walk around" comment - but I don't know exactly what question to actually ask to trigger your offer besides. I know there are other renderers that are more suited for caustics (at least speed-wise) and some can be brought into Blender, is that the gist of it or are there goodies within Blender/Cycles proper? $\endgroup$
    – uhoh
    Commented Nov 20, 2016 at 3:13

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .