In 1948 the candela was defined based on the radiation of platinum at the temperature of its solidification. As this definition in practice was very complicated to materialize, other ways of making the unit were proposed, until the CIPM adopted the current one, in 1977, which takes into account the color and direction of light.
The candela is, then, the luminous intensity in a given direction of a source that emits monochromatic radiation (of a single color or spectral line) at a frequency of 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
