To produce caustic effects a light must be deifined as a source of photon emission. A light can be defined as a source by turning caustics on in the photon tab of the light shader. A light defined to emit photons can be a point, spot or area light.

Photon emission can be controlled by a custom photon emitter shader. Without a photon emitter shader photons are emitted uniformly in all directions which have a chance of hitting objects. Possible directions are controlled by the direction and spread parameters which depend on the light type and usually Mental Ray will predict the optimal directions.

To generate photons using a defined light source the number of photons to be generated and the energy being distributed into the scene must be specified. To adjust effects on caustic appearance with the photon energy controls within the light property editor it is best to adjust the number of emitted photons slowly whilst increasing the energy intensity.