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In photometry
Photometry (optics)

Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by a luminosity function that models human b...
, luminous intensity is a measure of the wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
-weighted power
Power (physics)

In physics, power is the rate at which mechanical work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time....
 emitted by a light source in a particular direction per unit solid angle
Solid angle

The solid angle, O, is the angle in three-dimensional space that an object subtends at a point. It is a measure of how big that object appears to an observer looking from that point....
, based on the luminosity function
Luminosity function

The luminosity function or luminous efficiency function describes the average sensitivity of the human eye to light of different wavelengths....
, a standardized model of the sensitivity of the human eye. The SI
Si

Si, si, or SI may refer to :...
 unit of luminous intensity is the candela
Candela

The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function ....
 (cd), an SI base unit
SI base unit

The International System of Units defines seven dimensional analysis SI base units. All other physical units can be derived from these base units: these are known as SI derived units....
.

Photometry deals with the measurement of visible light as perceived by human eyes. The human eye
Human eye

The human eye is a significant human sense organ. It allows humans conscious light perception, vision, which includes color differentiation and the perception of depth....
 can only see light in the visible spectrum
Visible spectrum

The visible spectrum is the portion of the electromagnetic spectrum that is visual perception to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light....
 and has different sensitivities to light
Light

Light, or visible light, is electromagnetic radiation of a wavelength that is Visible spectrum to the human eye , or up to 380?750 nm. In the broader field of physics, light is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not....
 of different wavelengths within the spectrum.






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In photometry
Photometry (optics)

Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by a luminosity function that models human b...
, luminous intensity is a measure of the wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
-weighted power
Power (physics)

In physics, power is the rate at which mechanical work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time....
 emitted by a light source in a particular direction per unit solid angle
Solid angle

The solid angle, O, is the angle in three-dimensional space that an object subtends at a point. It is a measure of how big that object appears to an observer looking from that point....
, based on the luminosity function
Luminosity function

The luminosity function or luminous efficiency function describes the average sensitivity of the human eye to light of different wavelengths....
, a standardized model of the sensitivity of the human eye. The SI
Si

Si, si, or SI may refer to :...
 unit of luminous intensity is the candela
Candela

The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function ....
 (cd), an SI base unit
SI base unit

The International System of Units defines seven dimensional analysis SI base units. All other physical units can be derived from these base units: these are known as SI derived units....
.

Photometry deals with the measurement of visible light as perceived by human eyes. The human eye
Human eye

The human eye is a significant human sense organ. It allows humans conscious light perception, vision, which includes color differentiation and the perception of depth....
 can only see light in the visible spectrum
Visible spectrum

The visible spectrum is the portion of the electromagnetic spectrum that is visual perception to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light....
 and has different sensitivities to light
Light

Light, or visible light, is electromagnetic radiation of a wavelength that is Visible spectrum to the human eye , or up to 380?750 nm. In the broader field of physics, light is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not....
 of different wavelengths within the spectrum. When adapted for bright conditions (photopic vision
Photopic vision

Photopic vision is the visual perception of the eye under well-lit conditions. In humans and many animals, photopic vision allows color vision, mediated by cone cells....
), the eye is most sensitive to greenish-yellow light at 555 nm. Light with the same radiant intensity
Radiant intensity

In radiometry, radiant intensity is a measure of the intensity of electromagnetic radiation. It is defined as power per unit solid angle. The SI unit of radiant intensity is watts per steradian ....
 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
Luminosity function

The luminosity function or luminous efficiency function describes the average sensitivity of the human eye to light of different wavelengths....
. This curve, denoted or , 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.

Luminous intensity should not be confused with another photometric unit, luminous flux
Luminous flux

In photometry , luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
, which is the total perceived power emitted in all directions. Luminous intensity is the perceived power per unit solid angle. Luminous intensity is also not the same as the radiant intensity
Radiant intensity

In radiometry, radiant intensity is a measure of the intensity of electromagnetic radiation. It is defined as power per unit solid angle. The SI unit of radiant intensity is watts per steradian ....
, the corresponding objective physical quantity
Physical quantity

A physical quantity is a physical property that can be Quantitative. This means it can be measured and/or calculated and expressed in numbers. For example, "weight" is a physical quantity that can be expressed by stating a number of some basic measurement unit such as pound or kilograms, while "beauty" is a property that is difficult to desc...
 used in the measurement science of radiometry
Radiometry

In optics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. Note that light is also measured using the techniques of photometry_, which deal with brightness as perceived by the human eye, rather than absolute power....
.

Units

Like other SI base units, the candela has an operational definition
Operational definition

Operational definition is a demonstration of a process — such as a variable, terminology, or object — relative in terms of the specific process or set of Formal verification used to determine its presence and quantity....
—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
WATT

WATT is a radio station broadcasting a News radio-Talk radio-Sports radio format. Licensed to Cadillac, Michigan, it first began broadcasting in 1945....
s per steradian
Steradian

The steradian is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a Plane ....
 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 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
Radiant flux

In radiometry, radiant flux or radiant power is the measure of the total Power of electromagnetic radiation . The power may be the total emitted from a source, or the total landing on a particular surface....
 would be about 18.40 mW, since there are 4p steradians in a sphere. A typical candle produces very roughly one candela of luminous intensity.

Prior to the definition of the candela, 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
England

native_name =|conventional_long_name = England|common_name = England|image_flag = Flag of England.svg|image_coat = England COA.svg|symbol_type = Royal Coat of Arms...
 standard: candlepower
Candlepower

Candlepower is an obsolete scientific unit of luminous intensity based on the light emitted from a candle made to a specified formula.The candlepower as a scientific measure was replaced in 1948 by the international unit known as the candela....
. One candlepower was the light produced by a pure spermaceti
Spermaceti

Spermaceti is a wax present in the head cavities of the sperm whale . Spermaceti is extracted from whale oil by crystallisation at 6 ?C, when treated by pressure and a chemical solution of caustic alkali....
 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
Hefner lamp

The Hefner lamp, or in German Hefnerkerze, is a flame lamp used in photometry that burns amyl acetate.The lamp was invented by Friedrich von Hefner-Alteneck in 1884 and he proposed its use as a standard flame for photometry purposes with a luminous intensity unit of the hefnerkerze ....
. In 1881, Jules Violle
Jules Violle

Jules Louis Gabriel Violle was a France 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 intensity, called the Violle, equal to the light emitted by 1 square centimetre of platinum at its melting point....
 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 is given by where is the luminous intensity in candelas, is the radiant intensity in W/sr, is the standard luminosity function
Luminosity function

The luminosity function or luminous efficiency function describes the average sensitivity of the human eye to light of different wavelengths....
.

If more than one wavelength is present (as is usually the case), one must sum or integrate over the spectrum
Spectrum

A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a Continuum . The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a triangular prism ; it has since been applied by analogy to many fields other than op...
 of wavelengths present to get the luminous intensity:

See also

  • Radiance
    Radiance

    Radiance and spectral radiance are radiometry measures that describe the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction....
  • Intensity