358. Photometry.—It is desirable at times to compare the intensities of illumination produced by light from different sources. This is done to determine the relative cost or effectiveness of various illuminants such as candles, kerosene and gas lamps, and electric lights The process of determining the relative intensity of lights or lamps is called photometry. (Photos = light.)
The unit for measuring the power of light is called a candle power. It is the light produced by a sperm candle burning 120 grains per hour. An ordinary gas light burns 5 or more cubic feet of gas per hour and yields from 15 to 25 candle power. A Welsbach gas lamp, consuming 3 cu. ft. per hour, produces 50 to 100 candle power.
Instead of using candles, for practical photometry, incandescent lamps standardized by the Bureau of Standards are used for testing or calibration purposes.
It is necessary to distinguish between the intensity of a luminous body, i.e., as a source of light, and the intensity of illumination upon some surface produced by a light. It is considered that two sources of light are of equal intensity if they produce equal illumination at equal distances.
359. Law of Intensity of Light.—A device for measuring the candle power of a light is called a photometer. Its use is based upon the law of intensity of light. The intensity of illumination of a surface is inversely proportional to the square of its distance from the source of light. This relation is similar to that existing between the intensity of a sound and the distance from its source. The following device illustrates the truth of this law in a simple manner.
Fig. 350.—The light spreads over four times the area at twice the distance.
Cut a hole 1 in. square in a large sheet of cardboard (K) and place the card in an upright position 1 meter from an arc light or other point source of light (L). Now rule inch squares upon another card (M) and place it parallel to the first card and 2 meters from it. (See Fig. 350.) The light that passed through the hole of 1 sq. in. at a distance of 1 meter is spread over 4 sq. in. at a distance of 2 meters. Therefore, the intensity of illumination on each square inch of M is one-fourth that upon the surface of K. If M is placed 3 meters from the light, 9 sq. in. are illuminated, or the intensity is one-ninth that at 1 meter distance.
