Radius - this parameters determines the amount of area (in scene units) where the VRayDirt effect is produced. For further info, please see the Examples section. You can also use a texture to control the radius. The texture intensity is multiplied by the radius to calculate the final radius at a given surface point. If the texture is white at a given surface point, the full radius value is used. If the texture is black, a radius of 0.0 is used.