Equate lumens to candlepower. Some say lumens divided x 12.57 = candlepower...doesn't make alot of sense to me

Answer 1

Candlepower is a very obsolete unit and to avoid confusions it is better not to use it at all. See http://en.wikipedia.org/wiki/Candlepower .

Candlepower was very long ago replaced by candela http://en.wikipedia.org/wiki/Candela , now the official SI unit of luminous intensity http://en.wikipedia.org/wiki/Luminous_intensity , so we have to use candela instead of obsolete candlepower . (BTW candela approximately, but not exactly, corresponds to the candlepower. The difference is about 2 %.)

Lumen is SI unit of luminous flux http://en.wikipedia.org/wiki/Luminous_flux . The relation between luminous intensity (candela) and luminous flux (lumen) is: „One lumen is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian.“

For steradian, the unit of solid angle, see http://en.wikipedia.org/wiki/Steradian . (The surface of a sphere with radius of one unit is 4π = 12.57 ... that is a number you mentioned above.)

So if we have a light source with luminous intensity 1 candela, then the luminous flux over an angle of 1 steradian is 1 lumen, and the luminous flux over a full sphere is 2π lumens.

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