Complex sentence: Openings where light can exit or enter, normally called ports, that are used for detectors and sources must be small, less than about 5% of the surface area of the sphere, for the theoretical assumptions to be valid.
Split into simpler sentences: Openings where light can exit or enter, used for detectors and sources, are normally called ports. The total area of all ports must be small, less than about 5% of the surface area of the sphere, for the theoretical assumptions to be valid.