Unit: Lumen/Watt (lm/W)
The luminous efficacy is defined as the ratio of the emitted luminous flux to the expended electric power of a lamp.
The luminous efficacy describes the efficacy of a lamp. It is expressed as the ratio of the emitted luminous flux in lumen and the power used in watts. The theoretically attainable maximum value assuming complete conversion of energy at 555 nm would be 683 lm/W. The luminous efficacies that can actually be attained vary depending on the lamp, but always remain far below this ideal value.