Information AboutCandela |
| CATEGORIES ABOUT CANDELA | |
| si base units | |
| photometry | |
| units of luminous intensity | |
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The candela (symbol: cd) is the SI Base Unit of Luminous Intensity (''perceived'' power emitted by a light source in a particular direction). DEFINITION Since the 16th General Conference On Weights And Measures in 1979, the candela has been defined as follows:
EXPLANATION The frequency chosen is in the Visible Spectrum near Green , corresponding to a Wavelength of about 555 nanometers. The Human Eye is most sensitive to this frequency, when adapted for bright conditions. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye. The luminous intensity for light of a particular wavelength is given by : where is the luminous intensity in candelas, is the radiant intensity in W/sr and is the standard Luminosity Function . If more than one wavelength is present (as is usually the case), one must sum or integrate over the Spectrum of wavlengths present to get the total luminous intensity. A common candle emits roughly 1 cd. A 100 W Lightbulb emits about 120 cd. ORIGIN The candela was based on an older unit, the ''candlepower'', which was referenced to the luminous intensity of a "standard Candle " of known composition. This was later replaced by a new unit, the candela, defined in terms of the Black-body radiation emitted by 1/60 of 1 cm2 of Platinum at its melting point. The arbitrary (1/683) term was chosen such that the new definition would exactly match the old definition. The candela was given its modern definition (above) in 1979. Although the candela is now defined in terms of the watt, which is a derived SI unit of power, the candela remains a Base Unit of the SI system {Link without Title} . SI PHOTOMETRIC LIGHT UNITS REFERENCES EXTERNAL LINKS |
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