Transactions of the Society of Motion Picture Engineers (1916)

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Foot-Candle. The intensity to which a surface is illuminated is measured in footcandles. The intensity of illumination can be measured by a photometer or it can be calculated when the candle-power, direction and distance of the light source or sources is known. The illumination produced by a source of i c.p. on a perpendicular plane i foot away has an intensity of i foot-candle. When one lumen of flux uniformly covers i square foot of surface, the surface is illuminated to an intensity of I foot-candle. CPX-Sine $ Foot-candles= D2 The c.p. is that of the source in the direction of the surface considered the angle is the angle between the light-ray (when the surface is perpendicular to the ray of light, the angle $ is 90 degrees and sine I is 1. 00). D is the distance in feet from the source to the surface. This relation holds exactly only when the source dimensions are small compared with the distance of measurement. Lumen. The lumen is the unit of light flux. The flux required to light a given surface is proportional to the illumination and to the area of the surface. One lumen will light a surface of i square foot to an average intensity of i foot-candle. The number of lumens required to light a surface to any given illumination is equal to the product obtained by multiplying the area of the surface in square feet by the average footcandles which it is desired to produce. Since a foot-candle of illumination is produced by a source of i c.p. shining on a surface one foot distant, if a source giving i c.p. in every direction is at the center of a hollow sphere of i foot radius every point on the interior surface of the sphere will be illuminated directly from the source to an illumination of I foot-candle. As the interior surface of the sphere of i foot radius has an area of 12.57 square feet and as the i c.p. source directly illuminates this area of 12.57 square feet to i foot-candle, the source must produce 12.57 lumens. Consequently a source of an intensity of i c.p. in every direction, which is i spherical candle-power, has an output of 12.57 lumens. In view of their usefulness in the study of light projection, I suggest that these definitions be incorporated in the Standards of this Society and that this Society make an earnest eff"ort to popularize them in the motion picture industry. 85