ABSTRACT
Illumination is the process of lighting an object. Illuminance is the luminous flux density received at a surface; the number of lumens received per square metre of surface. The lumen design method takes into account both the direct and indirect components of illuminance in calculating the average illuminance on the working plane. Light is provided in a building either by the sun or by the use of artificial sources. Historically the day-time lighting of buildings using natural lighting penetrating through windows and/or roof lights has principally concerned the architect. The use of artificial lighting for illuminating interiors at night became the province of electrical engineers and later lighting engineers. The object of Permanent supplementary artificial lighting of interiors (PSALI) schemes is to integrate artificial lighting with daylight in order to compensate for the relative lack of daylight in those parts of a room which are remote from windows.
