Candela (symbol: cd) is the SI unit of luminous intensity. It measures the luminous power per unit solid angle emitted in a particular direction. A common wax candle has a luminous intensity of roughly 1 cd. 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.
Definition The 26th General Conference on Weights and Measures (CGPM) redefined the candela in 2018. The new definition, which took effect on 20 May 2019, is:
The candela [...] is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×1012 Hz, Kcd, to be 683 when expressed in the unit lm W−1, which is equal to cd sr W−1, or cd sr kg−1 m−2 s3, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.
Explanation
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. Under these conditions, photopic vision dominates the visual perception of our eyes over scotopic vision. 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
I v ( λ ) = 683.002 l m / W ⋅ y ¯ ( λ ) ⋅ I e ( λ ) , {\displaystyle I_{\mathrm {v} }(\lambda )=683.002\ \mathrm {lm/W} \cdot {\overline {y}}(\lambda )\cdot I_{\mathrm {e} }(\lambda ),}
where Iv(λ) is the luminous intensity, Ie(λ) is the radiant intensity and y ¯ ( λ ) {\textstyle \textstyle {\overline {y}}(\lambda )} is the photopic luminous efficiency 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. 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 luminous efficiency function, the model of the sensitivity of the human eye to different wavelengths, standardised by the CIE and ISO.
Examples A common candle emits light with roughly 1 cd luminous intensity. 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 25 W compact fluorescent light bulb puts out around 1700 lumens; if that light is radiated equally in all directions (i.e. over 4π steradians), it will have an intensity of I V = 1700 lm 4 π sr ≈ 135 lm / sr = 135 cd . {\displaystyle I_{\text{V}}={\frac {1700\ {\text{lm}}}{4\pi \ {\text{sr}}}}\approx 135\ {\text{lm}}/{\text{sr}}=135\ {\text{cd}}.}
Focused into a 20° beam (0.095 steradians), the same light bulb would have an intensity of around 18,000 cd or 18 kcd within the beam.
History 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.
A better standard for luminous intensity was needed. In 1884, Jules Violle had proposed a standard based on the light emitted by 1 cm2 of platinum at its melting point (or freezing point). The resulting unit of intensity, called the "violle", was roughly equal to 60 English candlepower. Platinum was convenient for this purpose because it had a high enough melting point, was not prone to oxidation, and could be obtained in pure form. Violle showed that the intensity emitted by pure platinum was strictly dependent on its temperature, and so platinum at its melting point should have a consistent luminous intensity. In practice, realising a standard based on Violle's proposal turned out to be more difficult than expected. Impurities on the surface of the platinum could directly affect its emissivity, and in addition impurities could affect the luminous intensity by altering the melting point. Over the following half century various scientists tried to make a practical intensity standard based on incandescent platinum. The successful approach was to suspend a hollow shell of thorium dioxide with a small hole in it in a bath of molten platinum. The shell (cavity) serves as a black body, producing black-body radiation that depends on the temperature and is not sensitive to details of how the device is constructed. In 1937, the Commission Internationale de l'Éclairage (International Commission on Illumination) and the CIPM proposed a "new candle" based on this concept, with value chosen to make it similar to the earlier unit candlepower. The decision was promulgated by the CIPM in 1946:
… excerpt ends here. Continue reading the full article.
