Accuracy specifies the number of photons which are computed when producing indirect illumination such as caustics. The number specified is the number of photons which are considered by the renderer within the area represented by the Radius. They do not affect the photon map itself but change the number of photons considered from the photon map when rendering therefore changing this value does not require the photon map to be rebuilt.

Accuracy values can range from 1 to 4000 and the default value set in XSI is 100. Specifying an accuracy value higher than the number of photons actually stored in the photon map causes Mental Ray to continuously search for photons so that the rendering process will never finish. The number of photons considered by the renderer within the radius are used to define the appearance of the caustic effect. Generally lower accuracy values are more grainy and larger values are more blurry.

Refinement of the effect depends on the scene. Low values can be used for some circumstances such as small highly localised settings if large bright spots are required. Higher values will produce a smoother effect and will reduce a grainy effect more than increasing the number of photons emitted. Numbers at the highest end of the range do not produce a significant differance to a localised caustic effect. Render times are increased with higher accuracy settings as the renderer has to search for higher numbers of photons in the photon map.