Evolution of SI Base Unit Candela: Quantifying the Light Perception of Human Eye

The redefinitions of the seven SI base units, as approved and adopted by the 26th CGPM, now depend on the seven defining constants, respectively. Candela is one of the seven base units, which is related to the measurement of light as perceived by the human eye. It is the only base unit to be related to the human physiology and gives a physical base to the phenomenon of vision. Over the period of time, Candela has evolved from its candle-based definition to being defined in terms of the defining constant, K-cd (maximum luminous efficacy of photopic vision). The present article describes the evolution of Candela and briefly discusses the current requirement for Quantum Candela.