#LA CANDELA 2 FULL#

The value of the new candle is such that the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per square centimetre. The decision was promulgated by the CIPM in 1946: 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 Commission Internationale de l'Éclairage (International Commission on Illumination) and the CIPM proposed a 'new candle' based on this basic concept. This made it easy for anyone to measure the standard, as high-purity platinum was widely available and easily prepared. The light intensity was due to the Planck radiator (a black body) effect, and was thus independent of the construction of the device. Jules Violle had proposed a standard based on the light emitted by 1 cm 2 of platinum at its melting point (or freezing point), calling this the Violle. It became clear that a better-defined unit was needed. Germany, Austria and Scandinavia used the Hefnerkerze, a unit based on the output of a Hefner lamp. 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.

One of the best-known of these was the English standard of candlepower. 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. Prior to 1948, various standards for luminous intensity were in use in a number of countries.

Focused into a 20° beam (0.095 steradians), the same light bulb would have an intensity of around 18,000 cd within the beam.

The luminous intensity for light of a particular wavelength λ is given by At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye. Under these conditions, photopic vision dominates the visual perception of our eyes over the scotopic vision. The human eye, when adapted for bright conditions, is most sensitive near this frequency. The frequency chosen is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometres. The old name 'candle' is still sometimes used, as in foot-candle and the modern definition of candlepower. 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. A common wax candle emits light with a luminous intensity of roughly one candela. 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). It measures luminous power per unit solid angle emitted by a light source in a particular direction. The candela ( / k æ n ˈ d ɛ l ə/ or / k æ n ˈ d iː l ə/ symbol: cd) is the unit of luminous intensity in the International System of Units (SI).

The photopic includes the CIE 1931 standard (solid), the Judd–Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). Photopic (black) and scotopic (green) luminosity functions.

La candela 2