How to use caustics

Caustics are formed when light refracts through a transparent surface, producing patterns of focused light on the surface behind it. You can render caustics using photon maps and the physically based rendering algorithm in mantra.

Additionally, using any of the glass materials (glass, bumpy glass, frosted glass, or ice cube) will provided you with faux caustics by default. For better quality caustics, you can turn off the Do Faux Caustics checkbox on the Blur.Density.Shadow tab which will make the shadow completely black, then use photon maps. For more information, see Rendering caustics with PBR.