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one to employ in determining how much water there is in the pool. A more practical method would be to pump the water into some measuring vessel, as, a barrel graduated in gallons. As a matter of fact, measuring the total lumens emitted by a light source is performed in a manner analogous to the method just referred to. The mean spherical candle-power or the lumen output of an illuminant may be determined at one reading by means of an instrument known as a sphere photometer.
The Foot-Candle.
Light is a cause and illumination the effect or result. Both the lumen and the candle are used to measure the cause, these units applying to the light source itself and not to the point where the light is utilized. To measure the illumination on a newspaper, desk or on a motion picture screen, we employ a unit called the foot-candle. A foot-candle represents an intensity of illumination equal to that produced at a point on a plane which is i foot distant from a source of i candle and which is perpendicular to the light rays at that point.
In Fig. 4, if the source S gives an intensity of i candle along the line Sy^ and if y^ is i foot distant from the source, the intensity of illumina
FiG. 5 The Illumination is less on A'B' than on AB (see footnote)
tion on the plane CD at the point yf is one foot-candle.* The intensity of illumination, measured in foot-candles, is the unit of measurement most intimately associated with our everyday use of light, and a measurement which the eye either consciously or unconsciously is making whenever the faculty of vision is being employed, for the number of foot-candles we have on the screen, other things being equal, determines directly whether or not there is sufficient light.
A working idea of a foot-candle of illumination can be obtained by considering the intensity on a newspaper being read by the light of a candle, the paper being held approximately one foot away from the candle. The foot-candle is a unit applying to a point on a surface; by averaging the foot-candles at a number of points on a plane, we get the average intensity of illumination on that plane.
Care should be taken to avoid confusing the intensity of illumination on a surface as indicated by the foot-candles with the appearance
* If instead of being perpendicular to the beam, the plane y^B is tilted at an angle, as shown in Fig. 5, it will be seen that the light of this beam is spread over a greater area than if the plane is perpendicular, so that the intensity of illumination on the plane is less in proportion to the ratio of the length o^ AB to the length o^ A'B' , or to the cosine of the angle between a perpendicular to the A'B' and the axis of the beam, which is the angle X. If, with the plane in the position AB the illum.ination is 1 foot-candle and the cosine of the angle X is 0.7,, the average illumination on the plane in position A'B' will be only 0.7 of a foot-candle.
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