Luminous intensity is an expression of the amount of light power emanating from a point source within a solid angle of one steradian . For reference, a frequency of 540 terahertz (540 THz or 5.40 x 10 14 Hz ) is specified. The quantities used to express luminous intensity are arcane to most non-scientists. A frequency of 540 THz corresponds to a wavelength of about 555 nanometers (nm), which is in the middle of the visible-light spectrum , and is generally accepted as the frequency and wavelength at which the average human eye is most sensitive. A steradian is the standard unit solid angle; a sphere encloses 4 p (approximately 12.57) steradians.
Decades ago, luminous intensity was measured in terms of a unit called the candle . 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 candela was defined and adopted as the standard unit of luminous intensity. One candela (1 cd) is the magnitude of an electromagnetic field ( EM-field ), in a specified direction, that has a power level equivalent to a visible-light field of 1/683 watt (1.46 x 10 -3 W) per steradian at 540 THz.