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, branch of physics dealing with the measurement of the intensity of a source of light, such as an electric lamp, and with the intensity of light such a source may cast on a surface area.
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Luminous intensity expressed in candelas. The term refers only to the intensity in a particular direction and by itself does not give an indication of the total light emitted. The candlepower in a given direction from a light source is equal to the illumination in footcandles falling on a surface normal to that direction, multiplied by the square of the distance from the light source in feet. The candlepower is also equal to the illumination of metercandles (lux) multiplied by the square of the distance in meters.
The apparent candlepower is the candlepower of a point source which will produce the same illumination at a given distance as produced by a given light source.
The mean horizontal candlepower is the average candlepower of a light source in the horizontal plane passing through the luminous center of the light source.
The mean spherical candlepower is the average candlepower in all directions from a light source as a center. Since there is a total solid angle of 4π (steradians) emanating from a point, the mean spherical candlepower is equal to the total luminous flux (in lumens) of a light source divided by 4π (steradians). See Luminous intensity, Photometry