I'm using an infinite light shining through two sets of slits in a wall. As an infinite light normally 'whites out' using volumetric atmosphere I have reduced the 'atmosphere' setting on the light to 0.5. The light uses raytraced shadows Then in the material room, select the 'atmosphere' material and set the volume density to 0.075, the volume step size to 10.0 and the noise at 0.0. Render using the firefly renderer with ray tracing and shadows on. The interference caused by the light shining through the slits is what causes the effect. Here is the same shot from a different angle to show the slits in the wall, with the atmosphere turned off. I will be making this 'prison cell' available in the free area as soon as it is finished (I want to add some poseable chains)