A photon map™ is a technique to generate the indirect illumination effects of caustics and global illumination when you render with the mental ray renderer. When it calculates indirect illumination, the mental ray renderer traces photons emitted from a light. The photon is traced through the scene, being reflected or transmitted by objects, until it strikes a diffuse surface. When it strikes a surface, the photon is stored in the photon map.
Generating photon maps is time-consuming. To improve performance, you must explicitly specify:
- Which lights emit photons for indirect illumination.
- Which objects can generate caustics or global illumination.
- Which objects can receive caustics or global illumination.
The settings for generating and receiving caustics are on the Object Properties dialog 
mental ray Panel.
The photon map stores photons only for objects that can receive caustics, global illumination, or both.
To further reduce the time required to generate a photon map, photons are limited by the Trace Depth controls. These limit the number of times a photon can be reflected, refracted, or both.
In animations, another way to save time is to reuse the photon-map file. If lighting doesn't change over the course of an animation, use the Photon Map controls.
The mental ray renderer saves photon maps as PMAP files. Photon map controls are on the Render Setup Dialog Indirect Illumination panel Caustics And Global Illumination rollout.