Surface brightness is measured in lamberts, one lambert being equal to an average intensity of 1/π candle per square centimeter of a radiating surface. It is measured in lumens, one lumen being equal to the luminous flux per unit solid angle (steradian) emitted by a unit candle. Luminous flux is the radiation given off in the visible range of wavelengths by a radiating source.
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Other quantities of importance in photometry include luminous flux, surface brightness (for a diffuse rather than point source), and surface illumination. This unit is sometimes called the new international candle the official name given to it by the International Commission of Illumination (CIE) is candela. Click the link for more information. radiator at the temperature at which platinum solidifies (2,046°K ). Lampblack, or powdered carbon, which reflects less than 2% of the radiation falling on it, crudely approximates an ideal blackbody a material consisting of a In physics, an ideal black substance that absorbs all and reflects none of the radiant energy falling on it. It is defined as 1-60 of the intensity of one square centimeter of a blackbody blackbody, In 1948 a new candle, about 1.9% smaller than the former unit, was adopted. Later the international candle was taken as a standard not actually a candle, it is defined in terms of the luminous intensity of a specified array of carbon-filament lamps. It was originally defined as the luminous intensity in a horizontal direction of a candle of specified size burning at a specified rate. Since an ordinary candle is not a sufficiently accurate standard, the unit of intensity has been defined in various ways. The intensity of electric lights is commonly given as so many candlepower, i.e., so many times the intensity of a standard candle.
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Photometry (fōtŏm`ətrē), branch of physics dealing with the measurement of the intensity of a source of light, such as an electric lamp, and with the intensity of light such a source may cast on a surface area.