When the light from a lamp falls on a surface it illuminates it, and there is said to be a certain intensity of illumination. This is measured in foot-candles, 1 foot-candle being the illumination produced on a white screen held vertically at a horizontal distance of 1 ft. from a standard candle.
The illumination decreases very much with distance, because if a screen is moved twice as far away from a lamp, the same amount of light has to illuminate four times as great an area. Thus a lamp placed 1 ft. from a white screen produces 10 foot-candles of illumination, but if moved to a distance of 2ft., it only gives a quarter of that illumination, that is 2.5 foot-candles.
As well as depending on the distance from the source of light, illumination varies very much according to the nature of the surface. It is estimated that if a given lamp is put in a room with light walls, there is 50 per cent more illumination than if the same lamp were put in a room with very dark walls. The light walls reflect the light while dark ones absorb it, just as bright and dull surfaces reflect and absorb radiant heat. It follows, then, that when the lighting of a room is considered, the nature of its walls as well as its size will determine what strength of lamp is required.
A third consideration is the purpose for which a room is to be used. In passages, only 1/2 – 1 foot-candle of illumination is needed, while in a sitting-room where people are reading, there should be 3 foot-candles. For dark needlework, at least 8 foot-candles are required, and preferably an extra lamp should be placed near the needlewoman.
An excerpt from:
Everyday Domestic Science
by I. C. Joslin, & P. M. Taylor