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cussion of candle-power distribution of light, for upon this depends in large measure the design of the optical system. It involves a discussion of lumens output per unit of power input, for the source we use must be acceptable from the standpoint of light produced for energy consumed.
A discussion of the efficiency of optical systems involves the use of the unit of light flux, the lumen.
A discussion of screen illumination involves the use of the term lumen for expressing the total light flux reaching a screen, and the term foot-candle for discussing the distribution of the light flux over the area of the screen. The brightness of the screen involves, also, the term candle-power.
A discussion of theater lighting involves the terms candle-power, foot-candle, and lumen. The candle-power per unit of source area must be low in order that glare will be avoided, the foot-candle intensity must be sufficiently high to permit obstacles to be avoided, and the lumen output of the lamps must be sufficient to provide the proper foot-candle intensity.
As I have pointed out, the definitions are simple once their meaning and purpose are thoroughly understood. The following definitions have been accepted by many technical societies throughout the country.
Candle-Power — (C.P.)
The candle-power is the unit in which the luminous intensity of a light source is measured. Measurement of candle-power is not made directly, but is obtained by comparison with that produced by a standard under certain specified conditions.
The present unit of candle-power is referred to as the "International Candle," and is a unit 1.6 per cent smaller than the old "British" or old American candle, which was in use in the United States prior to 1909. The following table gives the relation between the units in use at the present time
I International Candle=i American Candle.
I International Candle=i French Candle, called "Bougie Decimale."
I International Candle=i English Candle, called "Pentane Candle."
If the intensity be measured in all directions in a plane perpendicular to the axis and passing through the center of the filament, the average of the values obtained would be called the mean horizontal candle-power. The axis of an incandescent lamp is assumed to be a line connecting the center of the base and the tip of the bulb.
The average of the candle-power intensities in all possible directions from the source is known as the mean spherical candle-power. Rating in terms of mean spherical candle-power is of supreme importance in comparing lamps having diff"erently shaped filament. The mean spherical candle-power bears a definite relation to the total flux of light, and is, therefore, sometimes used as a measure of that quantity. The mean horizontal candle-power can not satisfactorily be used to indicate the light output of other than vacuum incandescent lamps and lamps without concentrated filaments; because of this limitation it is being superseded by ratings dependent directly upon the total light emitted. (See definition of "Lumen.")
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