Specifies the radius the Progressive Lightmapper uses to filter the direct lighting component of the lightmap when you use the Gaussian filter. (Editor only).
The default value is 1. The value range is 0.0 to 5.0. A higher value increases the strength of the blur, which reduces the amount of noise from direct light in the lightmap.
This setting affects lightmaps generated by the Baked Global Illumination system. It only applies to the the Progressive Lightmapper.
When Unity serializes this LightingSettings object as a Lighting Settings Asset, this property corresponds to the Filtering > Direct Filter > Radius property in the Lighting Settings Asset Inspector.
Additional resources: Lighting Settings Asset.