The efficiency of a light source is determined by the luminous flux [lm] emitted per unit of power consumed by the light source [W]: lumen/Watt [lm/W].
The luminous efficiency also takes into account the perception of light by the human eye. This perception varies with the wavelength.

1.3.1 Transition from radiometry to photometry
>>Radiometry studies the electromagnetic waves in general
>>Photometry pertains to electromagnetic waves visible to the human eye, that is, light.

1.3.2 Luminous flux perceived by the human eye for 1W of radiated energy
In day vision, a light source that radiates 1W of power will be perceived differently based on the emitted wavelength (color). In day vision, a wavelength of 555nm (yellow-green) is best perceived by the eye.
The International Commission for Lighting has defined the luminous flux perceived by 1W radiated at a wavelength of 555nm as being equivalent to a flux of 683 lumens.

Figure 5: Luminous flux perceived by the human eye for a radiation of 1W at different wavelengths under day vision and night vision (International Commission for Lighting)

In night vision(2) the 507nm blue-green wavelength is best perceived by the human eye (1700lm/W).