Luminous Intensity


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Luminous Intensity

Luminous Intensity (I)(cd)

Photometry, the first measure the amount of light intensity, is currently the international system of units (SI units) of the basic content, the unit for the candle (candela), as one of the seven basic units of the international system of units, defined as "a candle for 540 x 1012 Hz frequency (Hz) monochromatic light a radiation in a certain direction per steradian radiation flux is 1/683 watts (W) of the luminous intensity. The frequency of 540 x 1012 Hz wavelength is about 555 nm. on the other hand, the" candle "can be said to be the only and the role of biological (Human Visual Basic) units.

Early to determine the source surface brightness (brightness) only the human eye. When people use a standard lamp (or a candle) and a lamp for measuring phase comparison, one lamp distance can be adjusted until the two lamps were found with the same eyes bright, and then calculate the light the intensity measured light. Then the light intensity has no unit for the unit of "candlepower", referred to as CP, Chinese for "candle", but the existing legal measurement units and "candle" (candela) is not the same.

In 1979 Candela was defined as "a candle in the frequency of 540 x 1012 Hz (555 nm wavelength) of light emitted from the monochromatic radiation in a certain direction, each radian of light radiation flux is 1/683 watts." the intensity of light emitted by the light source unit in a given direction on the solid angle of the flux, also known as luminescence intensity. The symbol I v. The luminous flux per unit solid angle, light represents the specific angle can be received, its value is not measured with distance change, the unit by candlelight (CD) said.

Or called Luminous intensity. The luminous flux of each solid angle which means the luminous flux can be received in the specific angle which value will not change by measuring distance. The unit is CD.

Schematic diagram of Candela. (from

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