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The Candela

The candela is the foundation unit for the measurement of visible light. It is one of the seven foundation SI units. It's formal definition is:

The candela is the luminous intensity, in a given direction. of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
The candela is abbreviated cd and its standard symbol is Iv. The candela is then used to define the lumen and other quantities used in the measurement of visible light. It is approximately equal to the old unit "candlepower" and is generally taken to be equivalent.

For an isotropic source, the relationship between the candela and lumens is 1 cd = 4π lm and the unit relationships is 1 cd = lm/sr.
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