From AMS Glossary
The ratio of the radiant energy sensed by the average human eye at a particular wavelength to that received.
This ratio reaches a maximum inside the visible portion of the spectrum and falls to zero outside it. Luminous efficiency is dimensionless but is often given the units of lumens per watt. Photometric quantities are obtained by multiplying the corresponding radiometric quantities by the luminous efficiency and so often bear the adjective luminous, for example, luminous flux. However, when the radiometric quantities, radiance and irradiance, are transformed into photometric ones, they are given the special names luminance and illuminance. The luminous efficiency of cones differs from that of rods.