Candela

The candela is the base unit of luminous intensity in the International System of Units, roughly the brightness of a single ordinary candle in a given direction. It is defined by fixing the luminous efficacy of light at a frequency of 540 terahertz, a shade of green near the peak of human eye sensitivity, at exactly 683 lumens per watt, which ties the unit to both a fixed physical constant and the measured response of the human eye, the only SI base unit defined with reference to human perception.