Skydome lights can use light portals to reduce noise for interior scenes, where the light comes in through relatively small openings. Instead of emitting light, they can be used to guide skydome light sampling. Light portals must be placed to cover all windows, doors and other openings through which skydome light comes into the scene.
Rollover image to view scene without light portal (skydome light only)
The light portal should be oriented so that it is pointing in through the window as indicated below:
Light portal pointing inwards outside window
Skydome lights also have a
portal_mode parameter with options off, interior_only and interior_exterior to respectively turn off portals, block any light outside portals for interior only scenes, and let light outside portals through for mixed interior and exterior scenes. Blocking light outside portals more predictably reduces noise for interior only scenes.