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. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/