The frequency chosen is in the visible spectrum near green. The human eye is most sensitive to this frequency. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye (called the V-lambda curve in the reference below).
Historically, the candela was a fundamental unit of the SI. It was defined in terms of the black-body radiation emitted by 1/60 of 1 cm2 of platinum at its melting point. The modern definition is no longer fundamental because it is based on another SI unit of power, the watt. Traces of its history remain, however. The arbitrary (1/683) term was chosen such that the new definition would exactly match the old definition.
Candlepower is a way of measuring how much light is produced by a light bulb, LED or by striking an arc in a Carbon-Arc spotlight.
Candlepower, or CANDELA is a measure of how much light the bulb produces, measured at the bulb, rather than how much falls upon the thing you want to light up.
Candlepower is a measure of light taken at the source-not at the target.
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