The Intensity of Light

Author: J.B. Hoag

The practical unit of luminous flux of light is called the lumen. The International Candle emits a total of 4π lumens in all directions. The luminous flux of light, L, in lumens, which passes through a given area A, in square feet, normal to the path of the light rays at a distance D (in feet) from a source of candlepower C, is given by the following equation:

L = CA/D².

The "illumination" I received on a given surface must not be confused with the intensity of the light which was given out at the source. Inasmuch as the light which reaches a surface decreases inversely as the square of the distance D from the source, the illumination I in foot-candles is given by the equation: I = C/D².

The illumination of a surface placed at right angles to the light rays and 1 foot from a source whose intensity is 1 candlepower is called 1 foot-candle. Ordinary lighting amounts to somewhere between 2 and 15 foot-candles. For office work it should be between 5 and 30, and for detailed work, between 15 and one hundred foot-candles. A 60-watt, 110 volt frosted lamp will give about 30 foot-candles at a distance of 14.5 inches from the center of the bulb. A light intensity of 0.2 lumens will pass through an opening 1 square inch in area in a surface which is illuminated by 30 foot-candles.