Photometry deals with the measurement of visible light as perceived by human eyes. The human eye can only see light in thevisible spectrum and has different sensitivities tolight of different wavelengths within the spectrum. When adapted for bright conditions (photopic vision), the eye is most sensitive to yellow-green light at 555 nm. Light with the sameradiant intensity at other wavelengths has a lower luminous intensity. The curve which represents the response of the human eye to light is a defined standard functiony(λ) orV(λ) established by theInternational Commission on Illumination (CIE, forCommission Internationale de l'Éclairage) and standardized in collaboration with theISO.[1]
Luminous intensity of artificial light sources is typically measured using agoniophotometer outfitted with aphotometer or a spectroradiometer.[2]
^The symbols in this column denotedimensions; "L", "T" and "J" are for length, time, and luminous intensity respectively, not the symbols for theunits litre, tesla, and joule.
^Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric orphoton quantities. For example:USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
^abcAlternative symbols sometimes seen:W for luminous energy,P orF for luminous flux, andρ for luminous efficacy of a source.
Comparison of photometric and radiometric quantities
Luminous intensity should not be confused with another photometric unit,luminous flux, which is the total perceived power emitted in all directions. Luminous intensity is the perceived powerper unit solid angle. If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1steradian beam, then the beam would have a luminous intensity of 1 candela. If the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of 2 candela. The resulting beam is narrower and brighter, though its luminous flux remains unchanged.
Like other SI base units, the candela has anoperational definition—it is defined by the description of a physical process that will produce one candela of luminous intensity. 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 persteradian in a given direction, that light source will emit one candela in the specified direction.[3]
The frequency of light used in the definition corresponds to a wavelength in a vacuum of555 nm, which is near the peak of the eye's response to light. If the1 candela source emitted uniformly in all directions, the totalradiant flux would be about18.40 mW, since there are 4π steradians in a sphere. A typical moderncandle produces very roughly one candela while releasing heat at roughly80 W.
Prior to the definition of the candela, a variety of units for luminous intensity were used in various 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 standards was theEnglish standard:candlepower. One candlepower was the light produced by a purespermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. Germany, Austria, and Scandinavia used theHefnerkerze, a unit based on the output of aHefner lamp.[4] In 1881,Jules Violle proposed theViolle as a unit of luminous intensity, and it was notable as the first unit of light intensity that did not depend on the properties of a particular lamp. All of these units were superseded by the definition of the candela.
If more than one wavelength is present (as is usually the case), one must sum or integrate over thespectrum of wavelengths present to get the luminous intensity:
