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Luminous Intensity
In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit. Measurement 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. When adapted for bright conditions ( photopic vision), the eye is most sensitive to yellow-green light at 555 nm. Light with the same radiant intensity 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. This curve, denoted ''V''(λ) or \textstyle \overline(\lambda), is based on an average ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Candela
The candela ( or ; symbol: cd) is the unit of luminous intensity in the International System of Units (SI). It measures luminous power per unit solid angle emitted by a light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the standard luminosity function (a model of the sensitivity of the human eye to different wavelengths). A common wax candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured. The word ''candela'' is Latin for ''candle''. The old name "candle" is still sometimes used, as in ''foot-candle'' and the modern definition of ''candlepower''. Definition The 26th General Conference on Weights and Measures ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Steradian
The steradian (symbol: sr) or square radian is the unit of solid angle in the International System of Units (SI). It is used in three-dimensional geometry, and is analogous to the radian, which quantifies planar angles. Whereas an angle in radians, projected onto a circle, gives a ''length'' on the circumference, a solid angle in steradians, projected onto a sphere, gives an ''area'' on the surface. The name is derived from the Greek 'solid' + radian. The steradian, like the radian, is a dimensionless unit, the quotient of the area subtended and the square of its distance from the centre. Both the numerator and denominator of this ratio have dimension length squared (i.e. , dimensionless). It is useful, however, to distinguish between dimensionless quantities of a different nature, so the symbol "sr" is used to indicate a solid angle. For example, radiant intensity can be measured in watts per steradian (W⋅sr−1). The steradian was formerly an SI supplementary unit, but th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Luminosity
Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical object. In SI units, luminosity is measured in joules per second, or watts. In astronomy, values for luminosity are often given in the terms of the luminosity of the Sun, ''L''⊙. Luminosity can also be given in terms of the astronomical magnitude system: the absolute bolometric magnitude (''M''bol) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band. In contrast, the term ''brightness'' in astronomy is generally used to refer to an object's apparent brightness: that is, how bright an object appears to an observer. Apparent brightness depends on both the lum ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Jules Violle
Jules Louis Gabriel Violle (16 November 1841 – 12 September 1923) was a 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 intensity, called the Violle, equal to the light emitted by 1 cm² of platinum at its melting point. It was notable as the first unit of light intensity that did not depend on the properties of a particular lamp. This was much larger than traditional measures such as candlepower, so the standard SI unit candela was originally defined in 1946 as 1/60 Violle. Throughout his life, Violle taught at several colleges including the University of Lyon and the Conservatoire des Arts et Métiers in Paris. He was one of the founders of the Institut d'optique théorique et appliquée and the École supérieure d'optique. He improved and invented a number of devices for measuring radiation, and determined the freezing and melting points of palladium ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 photometric purposes with a luminous intensity unit of the Hefnerkerze (HK). The lamp was specified as having a 40 mm flame height and an 8 mm diameter wick. The Hefner lamp provided the German, Austrian, and Scandinavian standard for luminosity during the late nineteenth and early twentieth centuries. The unit of light intensity was defined as that produced by the lamp burning amyl acetate with a 40 mm flame height. The light unit was adopted by the German gas industry in 1890 and known as the ''Hefnereinheit''. In 1897 it was also adopted by the Association of German Electrical Engineers under the name ''Hefnerkerze'' (HK). Germany moved to using the new candle (NC) from 1 July 1942 and the candela (cd) from 1948. :1 Hefnerkerze is about 0.920 ca ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Grain (mass)
A grain is a unit of measurement of mass, and in the troy weight, avoirdupois, and apothecaries' systems, equal to exactly milligrams. It is nominally based upon the mass of a single ideal seed of a cereal. From the Bronze Age into the Renaissance, the average masses of wheat and barley grains were part of the legal definitions of units of mass. Expressions such as "thirty-two grains of wheat, taken from the middle of the ear" appear to have been ritualistic formulas, essentially the premodern equivalent of legal boilerplate. Another source states that it was defined such that 252.458 units would balance of distilled water at an ambient air-water pressure and temperature of and respectively. Another book states that Captain Henry Kater, of the British Standards Commission, arrived at this value experimentally. The grain was the legal foundation of traditional English weight systems, and is the only unit that is equal throughout the troy, avoirdupois, and apothecaries' system ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Spermaceti
Spermaceti is a waxy substance found in the head cavities of the sperm whale (and, in smaller quantities, in the oils of other whales). Spermaceti is created in the spermaceti organ inside the whale's head. This organ may contain as much as of spermaceti. It has been extracted by whalers since the 17th century for human use in cosmetics, textiles, and candles. Theories for the spermaceti organ's biological function suggest that it may control buoyancy, may act as a focusing apparatus for the whale's sense of echolocation, or possibly both. There has been concrete evidence to support both theories. The buoyancy theory holds that the sperm whale is capable of heating the spermaceti, lowering its density and thus allowing the whale to float; in order for the whale to sink again, it must take water into its blowhole which cools the spermaceti into a denser solid. This claim has been called into question by recent research which indicates a lack of biological structures to support ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
England
England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe by the North Sea to the east and the English Channel to the south. The country covers five-eighths of the island of Great Britain, which lies in the North Atlantic, and includes over 100 smaller islands, such as the Isles of Scilly and the Isle of Wight. The area now called England was first inhabited by modern humans during the Upper Paleolithic period, but takes its name from the Angles, a Germanic tribe deriving its name from the Anglia peninsula, who settled during the 5th and 6th centuries. England became a unified state in the 10th century and has had a significant cultural and legal impact on the wider world since the Age of Discovery, which began during the 15th century. The English language, the Anglican Church, and ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Candle
A candle is an ignitable wick embedded in wax, or another flammable solid substance such as tallow, that provides light, and in some cases, a fragrance. A candle can also provide heat or a method of keeping time. A person who makes candles is traditionally known as a chandler. Various devices have been invented to hold candles, from simple tabletop candlesticks, also known as candle holders, to elaborate candelabra and chandeliers. For a candle to burn, a heat source (commonly a naked flame from a match or lighter) is used to light the candle's wick, which melts and vaporizes a small amount of fuel (the wax). Once vaporized, the fuel combines with oxygen in the atmosphere to ignite and form a constant flame. This flame provides sufficient heat to keep the candle burning via a self-sustaining chain of events: the heat of the flame melts the top of the mass of solid fuel; the liquefied fuel then moves upward through the wick via capillary action; the liquefied fuel fina ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radiant Flux
In radiometry, radiant flux or radiant power is the radiant energy emitted, reflected, transmitted, or received per unit time, and spectral flux or spectral power is the radiant flux per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. The SI unit of radiant flux is the watt (W), one joule per second (), while that of spectral flux in frequency is the watt per hertz () and that of spectral flux in wavelength is the watt per metre ()—commonly the watt per nanometre (). Mathematical definitions Radiant flux Radiant flux, denoted Φe ('e' for "energetic", to avoid confusion with photometric quantities), is defined as \Phi_\mathrm = \frac :Q_\mathrm = \int_ \mathbf\cdot \hat\mathbf\, dA where *''t'' is the time; *''Q''e is the radiant energy flux of the field out of a closed surface \Sigma; *S is the Poynting vector, representing the current density of radiant energy; *n is the normal vector of a point on ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Watt (1736–1819), an 18th-century Scottish inventor, mechanical engineer, and chemist who improved the Newcomen engine with his own steam engine in 1776. Watt's invention was fundamental for the Industrial Revolution. Overview When an object's velocity is held constant at one metre per second against a constant opposing force of one newton, the rate at which work is done is one watt. : \mathrm In terms of electromagnetism, one watt is the rate at which electrical work is performed when a current of one ampere (A) flows across an electrical potential difference of one volt (V), meaning the watt is equivalent to the volt-ampere (the latter unit, however, is used for a different quantity from the real power of an electrical c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
