Candela

The candela (/kænˈdɛlə/ or /kænˈdiːlə/; symbol: cd) is the base unit of luminous intensity in the International System of Units (SI); that is, luminous power per unit solid angle emitted by a point light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the standard luminosity function (a model of the sensitivity of the human eye to different wavelengths). A common wax candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured. The word candela is Latin for candle. The old name 'candle' is still sometimes used, as in foot-candle and the modern definition of candlepower. The 26th General Conference on Weights and Measures (CGPM) redefined the candela in 2018. The new definition, which took effect on May 20 2019, is: The frequency chosen is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometres. The human eye, when adapted for bright conditions, is most sensitive near this frequency. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye. The luminous intensity for light of a particular wavelength λ is given by where Iv(λ) is the luminous intensity, Ie(λ) is the radiant intensity and y ¯ ( λ ) {displaystyle extstyle {overline {y}}(lambda )} is the photopic luminosity function. If more than one wavelength is present (as is usually the case), one must integrate over the spectrum of wavelengths to get the total luminous intensity. Prior to 1948, various standards for luminous intensity were in use in a number of countries. These were typically based on the brightness of the flame from a 'standard candle' of defined composition, or the brightness of an incandescent filament of specific design. One of the best-known of these was the English standard of candlepower. One candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. Germany, Austria and Scandinavia used the Hefnerkerze, a unit based on the output of a Hefner lamp. It became clear that a better-defined unit was needed. Jules Violle had proposed a standard based on the light emitted by 1 cm2 of platinum at its melting point (or freezing point), calling this the Violle. The light intensity was due to the Planck radiator (a black body) effect, and was thus independent of the construction of the device. This made it easy for anyone to measure the standard, as high-purity platinum was widely available and easily prepared. The Commission Internationale de l'Éclairage (International Commission on Illumination) and the CIPM proposed a “new candle” based on this basic concept. However, the value of the new unit was chosen to make it similar to the earlier unit candlepower by dividing the Violle by 60. The decision was promulgated by the CIPM in 1946: