Luminous efficacy is the ratio of luminous flux to applied power and is expressed in output lumens per watt (lm/W). Unless otherwise noted, luminous efficacy indicates the ability of a light source or lighting system to convert electrical power to light perceptible by the human eye. Luminous efficacy of a light source or lighting system should not be confused with luminous efficacy of radiation (LER). The conversion by the human eye from optical power (radiant flux in watts) to luminous flux also accompanies a ratio in lumens per watt (lm/W), which is known as the luminous efficacy of radiation (LER). LER describes the upper limit of the efficacy of a light source based on its spectral power distribution (SPD).