Caustics occur when focused specular light reflects or refracts onto a diffuse surface. Most surfaces are usually diffuse or partially diffuse and all objects in the scene were assigned diffuse materials in order to produce caustics effects. Diffuse surfaces reflect light uniformly in all directions and indirect illumination of diffuse surfaces is generally calculated with radiosity.

Photon shaders store photons only if the surface has a diffuse component since indirect illumination of completely specular surfaces wihtout a diffuse component is computed using ray tracing. The room surfaces such as the window ledges were given a phong shader with the specular turned off in order to best show up the caustic effects.

Adjusting the diffuse color value of the receiving surfaces affects the caustics. By decreasing this value the caustics appear brighter. Giving a diffuse value of 0 allows no surfaces for the photons to be stored and therefore no caustics appear. If the specular is turned on the caustics do not appear in the area of specular highlight.