The Sky Factor is the ratio of illuminance received directly through unglazed and unframed openings, to illuminance on a horizontal plane due to an unobstructed hemisphere, for a standard uniform sky. The Sky Factor does not include reflected light from outdoor or indoor surfaces.
The Sky Factor indicator is mostly used in UK policies, both for new constructions assesment and to work out whether a proposed new building will affect the 'Rights of Light' of an existing building. The impact of the new building on the 0.2% limit position allows to evaluate the loss of light.