This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Illumination may be defined as the quality and quantity of which aids in the discrimination of outline and the perception of color. not only the quantity, but the quality of the light, as well as the arrangement of the units, must be considered in a complete study of the subject of illumination.
The law of inverse squares - namely that the illumination from a given source varies inversely as the square of the distance from the source - shows that the illumination at a distance of two feet from a single candle-power unit is .25 foot-candles. For further consideration of the law of inverse squares, see "Photometry."
Fig. 51. Anti-parallel Feeding System.
Fig. 52. Three-wire System.
Illumination may be classified as useful - when used for the ordinary purposes of furnishing light for carrying on work, taking the place of daylight; and scenic - when used for decorative lighting such as stage lighting, etc. The two divisions are not, as a rule, distinct, but the one is combined with the other.