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In
photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electroma ...
, luminous intensity is a measure of the
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adja ...

wavelength
-weighted
power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. In older works, p ...
emitted by a
light source Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
in a particular direction per unit
solid angle In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position o ...

solid angle
, based on the
luminosity function (black) and scotopic (green) luminous efficiency functions. The photopic includes the CIE 1931 standard (solid), the Judd–Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is wavel ...
, a standardized model of the sensitivity of the
human eye The human eye is a that reacts to and allows . and in the are photoreceptive cells which are able to detect and convey this information to the . Eyes signal information which is used by the brain to elicit the perception of colour, shape, ...

human eye
. The
SI
SI
unit of luminous intensity is the
candela The candela ( or ; symbol: cd) is the of in the (SI); that is, luminous power per unit emitted by a point light source in a particular direction. Luminous intensity is analogous to , but instead of simply adding up the contributions of ever ...

candela
 (cd), an
SI base unit The SI base units are the standard defined by the (SI) for the seven of what is now known as the : they are notably a basic set from which all other can be . The units and their physical quantities are the for , the for , the for , the f ...

SI base unit
. Photometry deals with the measurement of visible light as perceived by human eyes. The human eye can only see light in the
visible spectrum The visible spectrum is the portion of the that is to the . in this range of s is called ' or simply . A typical will respond to wavelengths from about 380 to about 750 . In terms of frequency, this corresponds to a band in the vicinity of ...
and has different sensitivities to
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 ...

light
of different wavelengths within the spectrum. When adapted for bright conditions (
photopic vision Photopic vision is the vision of the human eye, eye under well-lit conditions (luminance level 10 to 108 Candela per square metre, cd/m2). In humans and many other animals, photopic vision allows color vision, color perception, mediated by cone cel ...
), the eye is most sensitive to yellow-green light at 555 nm. Light with the same
radiant intensity In radiometry Radiometry is a set of techniques for measurement, measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power (physics), power in space, as op ...
at other wavelengths has a lower luminous intensity. The curve which measures the response of the human eye to light is a defined standard, known as the
luminosity function (black) and scotopic (green) luminous efficiency functions. The photopic includes the CIE 1931 standard (solid), the Judd–Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is wavel ...
. This curve, denoted ''V''(λ) or \textstyle \overline(\lambda), is based on an average of widely differing experimental data from scientists using different measurement techniques. For instance, the measured responses of the eye to violet light varied by a factor of ten.


Relationship to other measures

Luminous intensity should not be confused with another photometric unit,
luminous flux In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ...

luminous flux
, which is the total perceived power emitted in all directions. Luminous intensity is the perceived power ''per 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 1
steradian The steradian (symbol: sr) or square radian is the SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric sys ...

steradian
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. Luminous intensity is also not the same as the
radiant intensity In radiometry Radiometry is a set of techniques for measurement, measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power (physics), power in space, as op ...
, the corresponding objective
physical quantity A physical quantity is a physical property of a material or system that can be Quantification (science), quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ''numerical value'' ...
used in the measurement science of
radiometry Radiometry is a set of techniques for measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are dependent on the ...
.


Units

Like other SI base units, the candela has an
operational definition An operational definition specifies concrete, replicable procedures designed to represent a construct. In the words of American psychologist S.S. Stevens (1935), "An operation is the performance which we execute in order to make known a concept." ...
—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 
watt The watt (symbol: W) is a unit of power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equa ...

watt
s per
steradian The steradian (symbol: sr) or square radian is the SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric sys ...

steradian
in a given direction, that light source will emit one candela in the specified direction. 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. If the source emitted uniformly in all directions, the total
radiant flux In radiometry Radiometry is a set of techniques for measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are ...
would be about 18.40 mW, since there are 4π steradians in a sphere. A typical candle produces very roughly one candela of luminous intensity. 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 the
English English usually refers to: * English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval England, which has eventually become the World language, leading lan ...

English
standard:
candlepower Candlepower (abbreviated as cp or CP) is an obsolete unit of measurement for luminous intensity In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to hu ...
. One candlepower was the light produced by a pure
spermaceti Spermaceti (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approxima ...

spermaceti
candle weighing one sixth of a pound and burning at a rate of 120 
grains A grain is a small, hard, dry seed A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double albu ...
per hour. Germany, Austria, and Scandinavia used the Hefnerkerze, a unit based on the output of a
Hefner lamp 200px, Hefner lamp or in German Hefnerkerze The Hefner lamp, or in German ''Hefnerkerze'', is a flame lamp used in photometry that burns amyl acetate Amyl acetate (pentyl acetate) is an organic compound and an ester with the chemical formula CH3 ...
. In 1881,
Jules Violle Jules Louis Gabriel Violle (16 November 1841 – 12 September 1923) was a France, French physicist and inventor. He is notable for having determined the solar constant at Mont Blanc in 1875, and, in 1881, for proposing a standard for luminous ...
proposed the '' Violle'' 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.


Usage

The luminous intensity for monochromatic light of a particular wavelength λ is given by I_\mathrm = 683 \cdot \overline(\lambda) \cdot I_\mathrm, where *''I''v is the luminous intensity in candelas (cd), *''I''e is the
radiant intensity In radiometry Radiometry is a set of techniques for measurement, measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power (physics), power in space, as op ...
in watts per steradian  (W/sr), * \overline(\lambda) is the standard luminosity function. If more than one wavelength is present (as is usually the case), one must sum or integrate over the
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum Continuum may refer to: * Continuum (measurement) Continuum theories or models expla ...

spectrum
of wavelengths present to get the luminous intensity: I_\mathrm = 683 \int_0^\infty \overline(\lambda) \cdot \frac \, d\lambda.


See also

*
Brightness Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. It is not necessarily proportional to lumina ...

Brightness
*
International System of Quantities The International System of Quantities (ISQ) consists of the quantities Quantity or amount is a property that can exist as a multitude Multitude is a term for a group of people who cannot be classed under any other distinct category, except for ...
*
Radiance In radiometry Radiometry is a set of techniques for measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are ...


References


Curve data

{{DEFAULTSORT:Luminous Intensity Scalar physical quantities SI base quantities Photometry