In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction, based on the luminosity function, a standardized model of the sensitivity of the human eye. This article deals with the usage of this term in Optics and Lighting. In Physics, power (symbol P) is the rate at which work is performed or energy is transmitted or the amount of energy required or expended for Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 The luminosity function or luminous efficiency function describes the average sensitivity of the human Eye to light of different wavelengths The SI unit of luminous intensity is the candela (cd), an SI base unit. The candela (kanˈdɛlə /-ˈdiːlə/ symbol cd) is the SI base unit of Luminous intensity; that is power emitted by a light source in a particular The International System of Units (SI defines seven dimensionally independent SI base units.

Photometry deals with the measurement of visible light as perceived by human eyes. The human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 When adapted for bright conditions (photopic vision), the eye is most sensitive to greenish-yellow light at 555 nm. Photopic vision is the vision of the Eye under well-lit conditions Light with the same radiant intensity at other wavelengths has a lower luminous intensity. In Radiometry, radiant intensity is a measure of the The curve which measures the response of the human eye to light is a defined standard, known as the 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 V(λ) 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, which is the total perceived power emitted in all directions. In photometry, luminous flux or luminous power is the measure of the perceived power of Light. Luminous intensity is the perceived power per unit solid angle. Luminous intensity is also not the same as the radiant intensity, the corresponding objective physical quantity used in the measurement science of radiometry. In Radiometry, radiant intensity is a measure of the A physical Quantity is a physical property that can be quantified In Optics, radiometry is the field that studies the Measurement of Electromagnetic radiation, including Visible light.

Contents 1 Units 2 Usage 3 See also 4 References

Units

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. An operational definition is a demonstration of a process &mdash such as a Variable, term, or object &mdash relative in terms of the specific Process By definition, if one constructs a light source that emits monochromatic 540 THz light, 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. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. The steradian (symbol sr) is the SI unit of Solid angle. It is used to describe two-dimensional angular spans in three- Dimensional space ^[1]

The 540 THz frequency used in the definition corresponds to a wavelength of about 555 nm, which is green light near the peak of the eye's response. Since there are about 12. 6 steradians in a sphere, the total radiant flux would be about 18. In Radiometry, radiant flux or radiant power is the measure of the total power of Electromagnetic radiation (including Infrared, 40 mW, if the source emitted uniformly in all directions. A typical candle produces very roughly one candela of luminous intensity.

In 1881, Jules Violle 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. Year 1881 ( MDCCCLXXXI) was a Common year starting on Saturday (link will display the full calendar of the Gregorian calendar (or a Common Jules Louis Gabriel Violle ( November 16 1841, Langres - September 12 1923, Fixin) was a French Physicist It was superseded by the candela in 1946.

Usage

The luminous intensity for monochromatic light of a particular wavelength λ is given by

where

I_v is the luminous intensity in candelas,

I is the radiant intensity in W/sr,

is the standard 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 of wavelengths present to get the luminous intensity:

See also

• Intensity

References

1. ^ Base unit definitions: Candela. A spectrum (plural spectra or spectrums) is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. In Physics, intensity is a measure of the time-averaged Energy Flux. The NIST Reference on Constants, Units, and Uncertainty. Retrieved on 2008-02-08. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 421 - Constantius III becomes co- Emperor of the Western Roman Empire.

SI photometry units

Quantity	Symbol	SI unit	Abbr.	Notes
Luminous energy	Qv	lumen second	lm·s	units are sometimes called talbots
Luminous flux	F	lumen (= cd·sr)	lm	also called luminous power
Luminous intensity	Iv	candela (= lm/sr)	cd	an SI base unit
Luminance	Lv	candela per square metre	cd/m2	units are sometimes called nits
Illuminance	Ev	lux (= lm/m2)	lx	Used for light incident on a surface
Luminous emittance	Mv	lux (= lm/m2)	lx	Used for light emitted from a surface
Luminous efficacy		lumen per watt	lm/W	ratio of luminous flux to radiant flux; maximum possible is 683. In Photometry, luminous energy is the perceived Energy of Light. The lumen (symbol lm) is the SI unit of Luminous flux, a measure of the perceived power of Light. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units In photometry, the talbot ( T) is a nonstandard unit of Luminous energy, named in honor of the early Photographer William Fox Talbot In photometry, luminous flux or luminous power is the measure of the perceived power of Light. The lumen (symbol lm) is the SI unit of Luminous flux, a measure of the perceived power of Light. The steradian (symbol sr) is the SI unit of Solid angle. It is used to describe two-dimensional angular spans in three- Dimensional space The candela (kanˈdɛlə /-ˈdiːlə/ symbol cd) is the SI base unit of Luminous intensity; that is power emitted by a light source in a particular The International System of Units (SI defines seven dimensionally independent SI base units. Luminance is a photometric measure of the density of Luminous intensity in a given direction M^2 redirects here For other uses see M². CM2 redirects here "Nit (unit" redirects here For the unit of information entropy see Nat (information. In photometry, illuminance is the total Luminous flux incident on a surface per unit Area. LUX is the principal centre for the promotion and distribution of experimental Film and Video works in the UK. In photometry, illuminance is the total Luminous flux incident on a surface per unit Area. Luminous efficacy is a property of Light sources which indicates what portion of the emitted Electromagnetic radiation is usable for human vision. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. In Radiometry, radiant flux or radiant power is the measure of the total power of Electromagnetic radiation (including Infrared, 002 lm/W
SI • Photometry

This article deals with the usage of this term in Optics and Lighting.

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