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 standards was the English standard: 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 the definition of the candela, a variety of units for luminous intensity were used in various countries.
Note that a typical modern candle produces very roughly one candela while releasing heat at roughly 80 W. If the 1 candela source emitted uniformly in all directions, the total radiant flux would be about 18.40 mW, since there are 4π steradians in a sphere. The frequency of light used in the definition corresponds to a wavelength in a vacuum of 555 nm, which is near the peak of the eye's response to light. By definition, if one constructs a light source that emits monochromatic green light with a frequency of 540 THz, and that has a radiant intensity of 1/683 watts per steradian in a given direction, that light source will emit one candela in the specified direction. Like other SI base units, the candela has an operational definition-it is defined by the description of a physical process that will produce one candela of luminous intensity. Luminous intensity is also not the same as the radiant intensity, the corresponding objective physical quantity used in the measurement science of radiometry. The resulting beam is narrower and brighter, though its luminous flux remains unchanged. If the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of 2 candela. If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1 steradian beam, then the beam would have a luminous intensity of 1 candela. Luminous intensity is the perceived power per unit solid angle. Luminous intensity should not be confused with another photometric unit, luminous flux, which is the total perceived power emitted in all directions. Relationship to other measures Comparison of photometric and radiometric quantities ^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967 ^ Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric or photon quantities.^ The symbols in this column denote dimensions " L", " T" and " J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.Luminous efficacy normalized by the maximum possible efficacy Luminous efficiency, luminous coefficient Ratio of luminous flux to power consumption
The candela per square metre is sometimes called the nit. Luminous flux per unit solid angle per unit projected source area. The lumen second is sometimes called the talbot. Luminous intensity of artificial light sources is typically measured using and a goniophotometer outfitted with a photometer or a spectroradiometer. The curve which represents the response of the human eye to light is a defined standard function y(λ) or V(λ) established by the International Commission on Illumination (CIE, for Commission Internationale de l'Éclairage) and standardized in collaboration with the ISO. Light with the same radiant intensity at other wavelengths has a lower luminous intensity. When adapted for bright conditions ( photopic vision), the eye is most sensitive to yellow-green light at 555 nm. The human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. Photometry deals with the measurement of visible light as perceived by human eyes. The SI unit of luminous intensity is the candela (cd), an SI base unit. In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye.
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