Toshiba luminous intensity standard lamp, facing the PH04B photometer, one of the four primary photometers involved in the realization of the candela. That curve was determined after a series of measurements conducted on a large number of people, so that photometric measurements could be made using standard benchmarks for representing the human eye. In 1924, the International Commission on Illumination (CIE) defined a relative spectral luminous efficiency curve to describe the daytime vision of an average observer. The spectral luminous efficiency function depicts the sensitivity of the human vision system to the various wavelengths. The CIE 1924 standard observer – V (λ) curve In 1979, the 16th CGPM adopted a new definition of the candela based on an energy unit that represented a complete break from the melting temperature of platinum: Any variation in that temperature causes the blackbody radiation, and thus the candela, to vary as well. The candela, for its part, was rendered less reliable by fluctuations in the set temperature at the copper fixed point, which varies depending on the version of the International Temperature Scale (EIT), from which the melting temperature of platinum is extrapolated. In 1967, in order to clarify the context for its implementation, the 13th CGPM slightly modified the definition for the candela, as follows: “The candela is the luminous intensity, in the perpendicular direction, of a surface of 1/600,000 square metre of a black body at the temperature of freezing platinum under a pressure of 101,325 newtons per square metre.”īy that time, radiometric measurements (energy measurements of optical radiation) had become highly effective, thanks in particular to the emergence of electrical substitution radiometers. The candela was introduced in the International System of Units a few years later in 1954 at the 10th CGPM, at the same time as the ampere and the kelvin. In 1946, the International Committee on Weights and Measures (ICWM) drew on this approach by defining the unit of luminous intensity, known as the New Candle, as follows: “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.” That decision was ratified at the 9th General Conference on Weights and Measures (CGPM) in 1948, which also adopted a new name for the unit: the candela, denoted as cd. A few years later, following the work of Max Planck, the radiation from platinum was replaced with the radiation from a black body (full radiator), which was more universal. In 1881, Jules Violle, a professor at France’s Conservatoire National des Arts et Métiers (CNAM), proposed that luminous intensity be defined as the light emitted by a platinum surface heated to its melting point. These references were initially founded on flame standards (oil lamps), but their stability was weak. Photometric quantities and units: the candela (cd) for measuring luminous intensity, the lux (lx) for describing illuminance and the lumen (lm) for measuring luminous flux Hence the decision to adopt the unit of luminous intensity as a base unit. It very quickly became apparent that a standard for “light,” as well as a photometric unit, would be helpful. Those measurements were grouped under the name photometry. The practical value of measuring optical radiation for lighting first became apparent at the start of the 19th century. Has definition The SI unit of luminous intensity, defined as "the luminous intensity, in the perpendicular direction, of a surface of 1/600000 square meter of a blackbody at the temperature of freezing platinum under a pressure of 101325 newtons per square meter.Units derived from the candela: lumen, lux The candela: some historical background Has historical origin in 1933 it was based on the luminous emission of a blackbody at the freezing temperature of platinum (2045 K) Next base SI unit: Kelvin Up: base SI unit, luminous intensity unit Previous base SI unit: ampere Candela concept from the Astronomy knowledge base Astronomy View all facts Glossary HelpĪbstraction > mathematical concept > unit > luminous intensity unit > candela
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