Objects in the scene must be defined as a either a transmitter or receiver of photons in order to cast caustic effects. To define an object as such requires the caustics transmitter or receiver box in the object visiblity options to be ticked on. An object, in this case such as the glass, can be both a transmitter and a receiver. Transmitters and receivers must be geometric objects and cannot be lights.

Caustic transmitting and receiving objects must have a material that contains a photon shader statement. Photon shaders can store absorb, reflect or transmit photons. If the photon shader is missing on the transmitting object photons are not reflected or transmitted by it and if it is missing on the receiving object no photons are stored.

The transmitter in this scene is the glass window. Transmittance is the fraction of light which passes through a material. Caustic casting materials should not be diffuse as diffuse objects scatter the light rather than focusing it which is what causes caustic effects.

Caustic receiving materials in the scene must be diffuse because otherwise no photons are stored. If many objects in this case such as the walls and floor are to be receivers all objects can be multiselcted and assigned as receivers.