Candela

Candela, pronounced kan DEHL uh or pronounced kan DEE luh, is the unit of measurement of luminous intensity, the amount of light produced in a certain direction by a glowing object. The more luminous intensity a light has, the brighter the light appears. Luminous intensity is sometimes called candle power.

The candela is one of seven base units in the metric system. Its abbreviation is cd. One candela is now defined as the luminous intensity from a source producing light at a specific frequency and intensity in a given direction. The frequency of the light is 540,000,000,000,000 hertz and its intensity is 1/683 watt in a solid angle called a steradian. To understand a steradian, imagine a small, uniform light placed at the center of a hollow sphere. Beams of light spread uniformly in all directions and illuminate the inside surface of the sphere. If the area illuminated on the inside of the sphere equals one square foot, then the angle of the light measured near the center of the sphere equals one steradian.

The candela is used to calculate other units of light measurement. These units include lumens and foot-candles. The unit of measurement for luminous intensity was once the candle, the amount of light produced by a certain kind of candle. But scientists found this unit too difficult to standardize. In 1948, the International Commission on Illumination adopted the candela as a standard unit of measure. One candela is slightly less than 1 candle.

Contributor: Ronald N. Helms, Ph.D., Chairperson of Architectural Engineering, North Carolina Agricultural and Technical State Univ.; Lighting Consultant; Expert Witness.

Candela

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