Unit of Luminous Intensity the Candela

The base unit candela allows one to establish a quantitative relation between radiometric and photometric measurements of light intensities. In physics and chemistry, the intensities of radiation fields of various natures are normally determined by radiometry in visual optics, in lighting engineering, and in the physiology of the visual system, however, it is necessary to assess the intensity of the radiation field by photometric means. 

There are, in fact, three different ways to quantify the intensity of a radiation beam. One way is to measure the radiant intensity 4, defined as the radiant flux A J e per unit solid angle A12 of the beam. The subscript e  

Another way to quantify the intensity of a beam of radiation is to measure the particle intensity Ip, which is defined as the particle flux Pp divided by the solid angle A72 of the beam. The subscript p stands for particle . The particle flux itself is measured by counting 

In addition to these two radiometric assessments of the beam intensity, for beams of visible light there is a third possibility, which is to quantify the intensity of the beam by the intensity of visual perception by the human eye. Physical quantities connected with this physiological type of assessment are called photometric quantities, in contrast to the two radiometric quantities described above. In photometry, the intensity of the beam is called the luminous intensity 7y. The subscript v stands for visual . The luminous intensity 7v is an ISO recommended base quantity the corresponding SI base unit is the candela (cd). The luminous flux is determined as the product of the luminous intensity and the solid angle. Its dimensions therefore are luminous intensity times solid angle, so that the SI unit of the luminous flux v turns out to be candela times steradian (cd sr). A derived unit, the lumen (Im), such that 1 Im = 1 cd sr, has been introduced for this product. 

The history of the base unit candela is as follows. Before 1948, the units for photometric measurements were be based on flame or incandescent-filament standards. They were replaced initially by the new eandle based on the luminance of a Planckian radiator (a black-body radiator) at the temperature of freezing platinum. This modification was ratified in 1948 by the 9th CGPM, which also adopted the new international name for the base unit of luminous intensity, the candela, and its symbol cd. The 13th CGPM gave an amended version of the 1948 definition in 1967. 

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