Hi,
Here’s my definitive checklist to get rid of unwanted blotches and splotches when using VRay’s irradiance map GI method. Maybe this is a good topic to turn into a sticky one, as newcomers will keep panicking at their first encounter of the blotch kind and the same questions will be asked over and over.
The blotches can be divided into three categories:
1: General smooth splotchiness in indirectly lit areas of a render.
This phenomenon can roughly be compared to the noise that occurs in areas of a photo where not enough light was present to capture a clear snapshot of that area. In other words: it is simply a shortage of bounced light ray information, resulting in image coarseness in the areas that are indirectly lit.
Solutions:
• In the VRay render settings turn up the amount of hemispheric subdivisions (GI sampling detail), lower the color treshold values and normal treshold values, for a more detailed evaluation of bounced light in the scene. If necessary raise the interpolation samples for more smoothness.
• In the VRay render settings adjust the QMC (Quasi Monte Carlo) sampler parameters: lower the noise threshold and the early termination amount and raise the minimum samples.
2: Smooth bright spots in problem areas of a render (hard edges etc.).
Solutions:
• In the VRay render settings turn up the amount of hemispheric subdivisions (GI sampling detail), lower the color treshold values and normal treshold values, for a more detailed evaluation of bounced light in the scene. If necessary raise the interpolation samples for more smoothness.
• In the VRay render settings adjust the QMC (Quasi Monte Carlo) sampler parameters: lower the noise threshold and the early termination amount and raise the minimum samples.
• Turn on the “Check sample visibility” checkbox in the irradiance map’s advanced parameters rollout. This can especially be a solution if the irradiance map has a low resolution. It works by checking if the sample points in the irradiance map are actually visible from each point before using them in the interpolated lighting solution.
• Check if every element that’s present in the scene has at least some thickness. Objects without any thickness can also be a cause of the smooth bright ‘light leaks’.
3: Harsh white splotches.
Can occur when highlight values are being clamped too far beyond white and/or when certain UVW coordinates at the edges of bitmapped objects cause bouncing light rays to become ‘confused’. 
These kind of blotches do not necessarily occur where the highlights are, but can occur at seemingly random positions in the render.
Solutions: here we go …
• Activate all “Apply” checkboxes in the Cropping/Placement section of your bitmaps (no need to actually specify a cropping region), or run the free MaxScript that automates this, available at http://www.varcht.com/tools.htm.
• Don’t use the default Max Raytrace material or map (use a VRay material or map in stead).
• Don’t use the default Max Reflect/Refract map (use a VRay map or material in stead).
• Avoid overbright highlights by using a shader like Phong and limiting your specular and glossiness values.
• Turn off “Reflective GI caustics” in the VRay renderer settings. It can cause overbright highlights, especially when using a bright HDRI map for your lighting.
• Limit your map output (do not raise your map’s output level to higher than 1.0 and do not raise the map value in front of the map slots to higher than 100).
• Be careful using self-illuminated materials, especially if their output is multiplied.
• In the “Object settings” submenu of the VRay renderer System rollout, turn down the “Receive GI” value of the object(s) showing the harsh white blotch(es).
• If all this still doesn’t work, then you can try using a different GI method than the irradiance map (global photon map or direct computation).
Cheers,
/\/\etin
