Objects in the scene transmit and receive photons in order to create a photon map which is used to determine the caustic effect. The appearance of caustic effects in the scene also depends on the relative position of the light and the scene objects.

Caustic effects occur when photons emitted from a light source are reflected or transmitted by a specular surface and are reflected by a diffuse surface to the camera. Highly specular surfaces such as glass which transmit photons are said to cast a caustic and surfaces which diffusely reflect a caustic to the camera are described as receiving a caustic.

To produce caustic effects objects in the scene must be defined as a either a transmitter or receiver of photons and then assigned a material and photon shader appropriate to that purpose. In this scene the glass was the specular surface which casts the caustic and other surfaces surrounding the window are recieve the caustic effects.