The chief drawback of daylighting is its inconsistency, especially its total unavailability after dusk and before sunrise. Artificial lighting can be instantly and constantly available, is easy to manipulate and can be controlled by the occupants of a building. However, daylighting and artificial lighting should be regarded as complementary. Artificial lighting is used mainly for night-time illumination and as a daytime supplement when daylighting alone is insufficient.
An acceptable balance of brightness within a building can be accomplished by an integration between the design of natural daylight sources and artificial supplementary lighting to provide the combined level of light appropriate to a specific visual task. During daylight hours natural light should appear dominant wherever possible.
However, quite apart from artificial light sources supplementing lighting levels, the use of artificial lighting in a building could lead to more flexible internal planning arrangements and to the incorporation of fewer or smaller windows. Thus daytime supplementary artificial lighting schemes directly affect the appearance of a building and its economy of construction. Against this must be levied the probability of greater energy usage, although reference has already been made to the effects which artificial lighting installations have upon the heating load for a building, and the possible economic advantages obtained by the recycling of heat generated by lamps, etc.