From iRay dev blog the Iray Development team has shown a new feature of Coming iRay version: Light...
>>Download Free iray studio scene
>>iray Material for 3ds Max
Nvidia is previewing Light Path Expressions, a new feature for “instant relighting and non-physical effects” in iray. LPEs will offer users a way to tweak the look of an image without re-rendering, by splitting out specific lighting components (for example, caustics thrown by an area light) into separate passes.
LPEs are supported in iray 3.0 and iray 2013, and will presumably be integrated in to 3ds Max and Maya when Autodesk releases its annual updates early next year.
Starting with iray 3.0 and the Iray Photoreal mode in Iray 2013, we support Light Path Expressions (LPEs), which allow you to do this separation. An LPE is a regular expression that matches some light transport paths that iray generates, but not others. Each result buffer can be associated with a LPE so that only paths which match the expression end up contributing to that buffer. Iray also allows you to render several buffers with different LPEs at the same time at almost no additional runtime cost. LPEs can distinguish between different surface properties such as diffuse or glossy, reflection or refraction, types of light sources, and names.
Get more detail here!
Iray - more about Iray