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.
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