Specifies the threshold the Progressive Lightmapper uses to filter direct light stored in the lightmap when using the A-Trous filter. (Editor only).
The range is 0 through 2. The default value is 0.5. A higher value will increase the threshold, which reduces the amount of noise in the direct lighting component of the lightmap. Too high of a value can cause a loss of detail 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 > Sigma property in the Lighting Settings Asset Inspector.
Additional resources: Lighting Settings Asset.