The Render Manager > mental ray (Global Renderer)
The Render Manager > (expand a pass) > mental ray
Set the current pass and choose Render > Render > Renderer Options from the Render toolbar.
Tab: Importons [Properties Reference]
Options: Diagnostics
Importons are like "virtual particles" that are emitted from the camera and bounce towards light. Importons move in the opposite direction to how photons travel. Nor do they distribute energy as photons do. Instead, they store color data that describes the factor with which an illumination at a certain location would contribute to the final image.
When you enable importons for use with global illumination and caustics (optional), the illumination contribution factor is used by mental ray to decide how to distribute its computational efforts in order to get the best quality image given the available rendering resources. In this way, importons provide a supplementary mechanism for improving the rendering quality and performance of photon effects. They can dramatically reduce the size (thus speeding up the lookup) of the global illumination photon map.
When you enable the irradiance particles algorithm, importons are automatically enabled. Importons are used as the primary mechanism for driving the quality of irradiance particles. For more information, see Irradiance Particles.
The importon map is controlled by the Density (number) of importons fired from the camera per pixel, the Merge Distance used to merge importons within a specified radius, and the number of Diffuse Bounces the importons should scatter through. For global illumination, you can also set whether importons are blocked at the first opaque geometry they hit, or get stored for all intersections with geometry as they trace through the scene.
These options are all described on the Photon Global tab of the mental ray Render Options Property Editor [Properties Reference].
