Illumination.
—The development of high-efficiency lamps has caused a radical change in the methods of illumination. With cheaper light came the desire to more nearly approximate the effect of daylight in illumination. This has brought into use indirect illumination, in which the light from the lamp is diffused by reflection from the ceiling and walls of the room. Illuminating engineering is now a business that has to do with placing of lamps to the greatest advantage in lighting any desired space. In large and complicated schemes of lighting professional services are necessary, but in household lighting the required number of lamps for the various apartments are almost self-evident. The lighting of large rooms, however, requires thoughtful consideration and in many cases the only definite solution of the problem is that of calculation.
The Foot-candle.
—The amount of illumination produced over a given area depends not only on the number of lamps and their candlepower, but upon their distribution and the color of the walls and furnishings. In the calculation of problems in illumination, units of measure are necessary to express the amount of light that will be furnished at any point from its source. The units adopted for such purposes are the foot-candle and the lumen.
The Lumen.
—A light giving 1 candlepower, placed in the center of a sphere of 1 foot radius illuminates a sphere, the area of which is 4 × 3.1416 or 12.57 square feet. The intensity of light on each square foot is denoted as a candle-foot. The candle-foot is the standard of illumination on any surface. The quantity of light used in illuminating each square foot of the sphere is called a lumen. A light of 1 candlepower will therefore produce an intensity of 1 candle-foot over 12.57 square feet and give 12.57 lumens. Therefore, if all of the light is effective on a plane to be illuminated, a lamp rated at 400 lumens would light an area of 400 square feet to an average intensity of 1 candle-foot.
To find the number of lamps required for lighting any space, the area in square feet is multiplied by the required intensity in foot-candles, to obtain the total necessary lumens, and the amount thus obtained is divided by the effective lumens per lamp.
The bulletins of the Columbia Incandescent Lamp Works gives the following method of calculating the number of lamps required to light a given space:
Number of lamps = (S × I)/(Effective lumens per lamp)
S (square feet) × I (required illumination in foot-candles) = total lumens.
The total lumens divided by the number of effective lumens per lamp gives the number of lamps required. In using the formula the effective lumens per lamp is taken from the following table: