The above scene shows how light is transported lossless through a lightguide of glass via "total inner reflection". This occurs when light is bouncing at a low angle of incidence onto a surface between an optical dense material (glass) and a material with less density (air). Then the vector of refraction and the vector of reflection point to the same direction, which gives a perfect lossless reflection back into the dense material.

Scene simulation with the Virtual Modeler "VModel"

Because of those many bounces inside the light guide, this scene requires a high trace depth value for specular light-transport (SpecLightDepth) during the lighting pass. Using the same high value (20 in this case) for the second (raytracing, SpecViewDepth) pass (as it is used by default, when using only the version 0.4 SpecDepth syntax) makes the renderer terrible slow.

So for rendering this scene I used a modified release 0.4 where the specular trace depth was limited to 4 only for the second pass. Since release 0.41 there are two different SpecDepth fields in the render-dialog (Light, View).