=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: Watts per lamp 25 40 60 160 150 250 Effective lumens per lamp 95 160 250 420 630 1090 Lumens per watt 3.8 4.0 4.2 4.2 4.2 4.3 The size of the units is a matter of choice since six 400-lumen units are equal to four 600-lumen units in illuminating power, etc. In deciding upon the proper size of lamps to use, consideration must be taken of the outlets if the building is already wired. In general the fewest units consistent with good distribution will be the most economical. The table shows the lumens effective for ordinary lighting with Mazda lamps and clear high-efficiency reflectors with dark walls and ceiling. Where both ceiling and walls are very light these figures may be increased by 25 per cent.