1. Unit of Luminous Intensity CANDELA

2. Definition The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10 12 hertz and that has a radiant intensity of 1 / 683 watt per steradian.

3. Explanation The frequency chosen is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometres. 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.

4. Visible Spectrum

5. Luminous Intensity 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.

6. Luminosity Function The luminosity function or luminous efficiency function describes the average spectral sensitivity of human visual perception of brightness. It is based on subjective judgements of which of a pair of different-coloured lights is brighter, to describe relative sensitivity to light of different wavelengths.

7. Solid angle Any area on a sphere which is equal in area to the square of its radius, when observed from its centre, subtends precisely one steradian.

8. Historical Each country had its own, and rather poorly reproducible, unit of luminous intensity. 1909 saw the beginning of unification when the national laboratories of US, France and Great Britain decided to adopt the international candle represented by carbon filament lamps. Germany, at the same time, stayed with the Hefner candle, defined by a flame standard, equal to about nine-tenths of an international candle.

9. Historical Standard based on incandescent lamps, and upon there stability, not fully satisfactory. Units of luminous intensity based on flame or incandescent lamps in use before 1948 initially replace by the ‘new candle’ based on the luminance of a Planckian radiator (a blackbody) at the temperature of freezing platinum. (2045 K)

10. Historical This modification was prepared by the International Commission on Illumination. Ratified in 1948 by the 9 th CGPM (General Conference on Weights and Measures) which adopted a new international name for this unit, the candela (symbol cd).