Light Flux

Author: E.E. Kimberly

The light flux falling upon 1 square foot of the interior sphere surface illuminated as just described is called a lumen. The area of a sphere of 1-foot radius is 4π square feet. Therefore, if a hollow sphere 1 foot in internal radius be illuminated from its center by a point light source of 1 standard candle, the total light flux emitted will be 4π lumens. The ability of a lamp to produce light is measured, not by its ability to illuminate an object in some particular direction from it, but by its total output in lumens. Hence, lamps are usually rated in lumens rather than in candle-power.

Out-of-doors on a very bright day the illumination may be as great as 10,000 foot-candles, while on a dark day it is sometimes as low as 200 foot-candles. The eye readily adapts itself to this wide range of intensity by contraction or dilation of the pupil to admit less or more light. In buildings without skylights the illumination seldom exceeds 100 foot-candles. Near windows the illumination may be as great as 50 to 100 foot-candles. This intensity is satisfactory for fine assembly work, inspection of small parts, etc. Drafting boards should have about 50 foot-candles. Office work and general reading requires 20 to 50 foot-candles. Illumination intensities of less than 10 foot-candles are unsatisfactory for any except general lighting such as used in hallways, auditoriums and large manufacturing assemblies. Local high-intensity lighting should be accompanied by some degree of general illumination.

Scientific research has shown that, while intensities somewhat less than those just suggested may be used with good results, the fatigue of workmen and their tendencies toward accidents increase rapidly with decrease of intensities below those recommended.