The candela (abbreviation, cd) is the standard unit of luminous intensity in the International System of Units ( SI ). It is formally defined as the magnitude of an electromagnetic field , in a specified direction, that has a power level of 1/683 watt (1.46 x 10 -3 W) per steradian at a frequency of 540 terahertz (540 THz or 5.40 x 10 14 Hz ).
Originally, luminous intensity was measured in terms of units called candles. This expression arose from the fact that one candle represented approximately the amount of visible radiation emitted by a candle flame. This was an inexact specification because burning candles vary in brilliance. So, for a time, a specified amount of radiation from elemental platinum at its freezing temperature was used as the standard. Late in the 20th century, the current definition and terminology were adopted.
The quantities comprising the specification of the candela are obscure to some non-scientists. An EM-field power level of 1.46 x 10 -3 W is small; the radio-frequency (RF) output of a children's toy two-way radio is several times that much. A frequency of 540 THz corresponds to a wavelength of about 556 nanometers (nm), which is in the middle of the visible-light spectrum. A steradian is the standard unit solid angle in three dimensions; a sphere encloses 4 pi (approximately 12.57) steradians.