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.
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.