Photopic (black) and scotopic^[1] (green) luminosity functions. Photopic vision is the vision of the Eye under well-lit conditions Scotopic vision is the Monochromatic vision of the Eye in low light The photopic includes the CIE 1931 standard ^[2] (solid), the Judd-Vos 1978 modified data ^[3] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data ^[4] (dotted). The horizontal axis is wavelength in nm.

The candela (IPA: /kanˈdɛlə/, /-ˈdiːlə/, symbol: cd) 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 (a standardized model of the sensitivity of the human eye to different wavelengths, also known as the luminous efficiency function^[4][5]). The International System of Units (SI defines seven dimensionally independent SI base units. In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a Light source in a particular direction per unit Solid The luminosity function or luminous efficiency function describes the average sensitivity of the human Eye to light of different wavelengths In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency.

Definition

Like other SI base units, the candela has an operational definition—it is defined by a 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 Since the 16th General Conference on Weights and Measures (CGPM) in 1979, the candela has been defined as:^[6]

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10¹² hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures ( CGPM, never GCWM The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. In Radiometry, radiant intensity is a measure of the 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

The definition describes how to produce a light source that (by definition) emits one candela. Such a source could then be used to calibrate instruments designed to measure luminous intensity, for example.

The candela is sometimes still called by the old name candle [1], such as in foot-candle and the modern definition of candlepower. A foot-candle (sometimes foot candle; abbreviated fc lm/ft² or sometimes ft-c is a non- SI unit of Illuminance or Light intensity widely Candlepower (abbreviated as cp) is an obsolete scientific unit of Luminous intensity based on the light emitted from a Candle made to a specified formula

Explanation

The frequency chosen is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometers. Green is a Color, the perception of which is evoked by light having a spectrum dominated by energy with a Wavelength of roughly 520–570- nm. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. The human eye is most sensitive to this frequency, when adapted for bright conditions. Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain In ocular physiology adaptation is the ability of the Eye to adjust to various levels of darkness and light At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye. In Radiometry, radiant intensity is a measure of the In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a Light source in a particular direction per unit Solid The luminous intensity for light of a particular wavelength λ is given by

where I_v(λ) is the luminous intensity in candelas, I(λ) is the radiant intensity in W/sr and 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 total luminous intensity. 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.

A common candle emits roughly 1 cd. A candle is a Light source and sometimes a Heat source consisting of a solid block of Fuel and an embedded wick. A 100 W incandescent lightbulb emits about 120 cd. The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general

Origin

Prior to 1948, there existed a variety of standards for luminous intensity in use 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 standard: candlepower. England is a Country which is part of the United Kingdom. Its inhabitants account for more than 83% of the total UK population whilst its mainland Candlepower (abbreviated as cp) is an obsolete scientific unit of Luminous intensity based on the light emitted from a Candle made to a specified formula 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. Spermaceti (from Greek sperma, seed and cetus, whale is a Wax present in the head cavities of the Sperm whale ( Physeter In many cultures a grain is a unit of measurement of Mass that is based upon the mass of a single seed of a typical Cereal.

It became clear that a better-defined unit was needed. The Commission internationale de l'éclairage (International Commission on Illumination) and the CIPM proposed a “new candle” based on the luminance of a Planck radiator (a black body) at the temperature of freezing platinum. Established in 1913 and based in Vienna, Austria, the International Commission on Illumination (usually known as the CIE for its French name Commission For a general introduction see Black body. In Physics, Planck's law describes the spectral radiance of Electromagnetic radiation In Physics, a black body is an object that absorbs all light that falls on it Platinum (ˈplætɪnəm is a Chemical element with the Atomic symbol Pt and an Atomic number of 78 The value of the new unit was chosen to make it similar to the earlier unit candlepower. The decision was promulgated by the CIPM in 1946:

The value of the new candle is such that the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per square centimetre. M^2 redirects here For other uses see M². CM2 redirects here ^[7]

It was then ratified in 1948 by the 9th CGPM which adopted a new name for this unit, the candela. The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures ( CGPM, never GCWM In 1967 the 13th CGPM removed the term "new candle" and gave an amended version of the candela definition, specifying the atmospheric pressure applied to the freezing platinum:

The candela is the luminous intensity, in the perpendicular direction, of a surface of 1/600 000 square metre of a black body at the temperature of freezing platinum under a pressure of 101 325 newtons per square metre. ^[8]

In 1979, because of the difficulties in realizing a Planck radiator at high temperatures and the new possibilities offered by radiometry, the 16th CGPM adopted the modern definition of the candela. In Optics, radiometry is the field that studies the Measurement of Electromagnetic radiation, including Visible light. ^[9] The arbitrary (1/683) term was chosen so that the new definition would exactly match the old definition. Although the candela is now defined partly in terms of the watt, which is a derived SI unit of power, the candela remains a base unit of the SI system, by definition. The International System of Units (SI defines seven dimensionally independent SI base units. [2]

SI photometric light units

Relationship between luminous intensity and luminous flux

If a source emits a known intensity (in candelas) in a well-defined cone, the total luminous flux in lumens can be calculated by taking the number of candelas, and dividing it by the number in the table below that corresponds to the "radiation angle" of the lamp (the full vertex angle of the emission cone). This article deals with the usage of this term in Optics and Lighting. The lumen (symbol lm) is the SI unit of Luminous flux, a measure of the perceived power of Light. See MR16 for emission angles of some common lamps. MR16 (sometimes referred to as MR-16) is a standard format for halogen Reflector lamps made by a variety of manufacturers Theory Formulas Online converter

• Example: A lamp that emits 590 cd with a radiation angle of 40°: 590/2. 64 = approximately 223 lumens.

If the source emits light uniformly in all directions, the flux can be found by multiplying the intensity by 4π: a uniform 1 candela source emits 12. 6 lumens.

References

1. ^ CIE Scotopic luminosity curve (1951)
2. ^ CIE (1931) 2-deg color matching functions
3. ^ Judd-Vos modified CIE 2-deg photopic luminosity curve (1978)
4. ^ ^{a b} Sharpe, Stockman, Jagla & Jägle (2005) 2-deg V*(l) luminous efficiency function
5. ^ Wyzecki, G. ; Stiles, W. S. (1982). Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. , Wiley-Interscience. ISBN 0471021067.  
6. ^ Base unit definitions: Candela. 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.
7. ^ Barry N. Taylor (1992). The Metric System: The International System of Units (SI). U. S. Department of Commerce, p. 18. ISBN 0941375749.   (NIST Special Publication 330, 1991 ed. )
8. ^ 13th CGPM Resolution 5, CR, 104 (1967), and Metrologia, 4, 43–44 (1968).
9. ^ 16th CGPM Resolution 3, CR, 100 (1979), and Metrologia, 16, 56 (1980).

External links

• Candela: The SI Unit of Luminous Intensity
• The Unit of Luminous Intensity: Candela (cd). 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.