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2016-04-07T16:22:36+05:30
luminous intensity is an expression of the amount of light power emanating from a point source within a solid angle of one steradian . the quantities used to express luminous intensity are arcane to most non-scientists. decades ago, luminous intensity was measured in terms of a unit called the candle . this expression arose from the fact that one candle represented approximately the amount of visible radiation emitted by a candle flame. this was an inexact specification, because burning candles vary in brilliance. so, for a time, a specified amount of radiation from elemental platinum at its freezing temperature was used as the standard. late in the 20th century, the candela was defined and adopted as the standard unit of luminous intensity. 
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2016-04-07T16:23:39+05:30
In photometry,luminous intensity means the measure of the wavelenghth
weighted power emitted  by a light sourse in a particular direction,,,

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.

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 tolight of different wavelengths within the spectrum. When adapted for bright conditions (photopic vision), the eye is most sensitive to greenish-yellow 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 theluminosity function. This curve, denoted V(λ) or , is based on an average of widely differing experimental data from scientists using different measurement techniques. For instance, the measured responses of the eye to violet light varied by a factor of ten.

The luminous intensity for monochromatic light of a particular wavelength λ is given by

where

Iv is the luminous intensity in candelas (cd),Ie is the radiant intensity in watts per steradian (W/sr), 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 wavelengths present to get the luminous intensity:

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