UNIT VI SECTION 6 ILLUMINATION IS THE CHIEF APPLICATION OF ARTIFICIAL LIGHT Introduction. Illumination deals with the problems of what kind of light sources to use for a given purpose, how much light is required, how the amount of light present can be measured, and where light sources should be placed. Tremendous strides have been made in the science of illumination, and yet few homes are properly illuminated. Proper illumination is desirable because it saves eyestrain. In this age, when the eyes are used so much in reading and close work, it is very important that everything possible be done to relieve the eyes from unnecessary strain. Illuminating Power Is Now Measured in Terms of Lumens. The value of any light source for illumination depends upon how much light it gives out. The unit of intensity is called the candle, which is equal to the intensity of a standard candle burning under -~:3Q-:::::::_t Fig. 215. Light intensity is inversely proportional to the square of the distance from the source. specified conditions. The international candle is now defined to be the light with an illuminating power equal to one tenth of that of a standard gas light produced by a Harcourt pentane lamp. Obviously the intensity of the light from a burning candle depends upon the distance one is from it, so that one must incorporate the idea of distance into the measurement of illuminating power. Afoot-candle is the most con- 442
ILLUMINATION 443 venient unit of intensity; it represents the intensity of a candle light at a distance of one foot from the candle. The lumen is now replacing candle power in measuring illumination. The lumen is really a light energy unit; it measures the quantity as well as the intensity of light because it combines the idea of area with intensity. The lumen is the amount of light falling upon a surface which has an area of one square foot when every point of the area is one foot from the light of one standard candle. The lumen corresponds to the ampere used in measuring the rate of flow of electricity, in that it measures the rate of flow of light energy. Light intensities are expressed in lumens per square foot or in foot-candles. The Measurement of Illumination Is a Common Occurrence, Especially in Photography. The strength of a light source is measured by a photometer. In the simple photometer used by Rumford, the distance of two light sources from a rod is varied until the shadows are of equal intensity. The relative strengths of the two light sources can then be computed on the basis of their relative distances from the rod. The intensity of light is now measured for photographic purposes by means of light-meters, which the photographer calls exposure meters. These are simply photoelectric cells in circuit with sensitive galvanometers which measure the electric current from the photoelectric cell when light shines upon it. Photoelectric cells will be discussed in a later section. The intensity of light may also be measured by actinometers, which measure the amount of a substance produced in a chemical reaction in a given time as a result of exposure of the reactants to light. The Amount of Light Required Depends upon the Person and What He Is Doing. Different persons require different amounts of light, depending upon their sensitiveness to light. Certain conditions of eyestrain will cause such a sensitivity to light that people so afflicted can scarcely stay in the room that is adequately illuminated for other people. As a rule, older people require more light than young people. The amount of light required for good illumination also depends upon the nature of the operation involved. The following values represent good lighting practice: 3 foot-candles — stairways, passageways, etc. 3-10 foot-candles — power plants, elevators, etc. 5-10 foot-candles — general household-work.