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Candela (cd) is a standard unit of measurement defined in the International System of Units (SI) for luminous intensity. Technically since 1979, the candela has been defined by the General Conference on Weights and Measures (CGPM) as a luminous intensity in a given direction from a source, which emanates monochromatic radiation frequencies of 540 x 1012 hertz and radiant intensities in the same direction of 1/683 watts per steradian. A steradian is a SI unit of solid angular measure. According to the CGPM definition, a steradian is equal to one candela for measuring luminous intensity.
Candela is defined by the CGPM as the level of an electromagnetic field, in a specified direction, with 1/683 watt (1.46 x 10 -3 W) per steradian and frequency of 540 terahertz.
The luminosity function is the average visual sensitivity of the human eye to diverse wavelengths. It is standardized by the International Commission on Illumination (CIE) and can be used to convert radiant energy into luminous.
The older term for candela is candle, such as foot-candle and candlepower. Prior to 1948, luminous intensity was generally measured by the brightness of the flame from a "standard candle." These terms are defined as follows:
The frequency chosen in the optical spectrum that is visible to the human eye is near the color green. According to the CIE, the human eye is most sensitive to the green frequency.
To some professionals the CGPM specifications are unclear. The EM-field power level of 1.46 x 10 -3 W is too small. The frequency of 540 THz resembles a wavelength of about 556 nanometers (nm), which is in the middle of the visible-light spectrum.