V-Ray supports the rendering of caustics effects by using one of the following techniques: photon mapping or progressive.
The new Progressive caustics solver uses advanced sampling techniques, and is able to trace as many photons as required without suffering the memory constraints of traditional Photon Mapping techniques.
The other method, Photon Mapping, is a two-pass technique. The first pass consists of shooting light particles (photons) from the light sources in the scene, tracing them as they bounce around the scene, and recording the places where the photons hit the object surfaces. The second pass is the final rendering, which is when the caustics are calculated by using density estimation techniques on the photon hits stored during the first pass.
