Luminous efficiency is a property of light sources, which indicates what portion of the emitted electromagnetic radiation is usable for human vision. It is the ratio of emitted luminous flux to radiant flux. Luminous efficacy is related to the overall efficiency of a light source for illumination, but the overall lighting efficiency also depends on how much of the input energy is converted into electromagnetic waves (whether visible or not).

The luminous efficiency 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,

Luminous efficacy measures the fraction of electromagnetic power which is useful for lighting. It is obtained by dividing the luminous flux by the radiant flux. Light with wavelengths outside the visible spectrum does not contribute to the efficacy at all, because the luminous flux of such light is zero. Wavelengths near the peak of the eye's response contribute more strongly than those near the edges.
In SI, luminous efficacy has units of lumens per watt (lm/W). Photopic luminous efficacy has a maximum possible value of 683 lm/W, for the case of monochromatic light at a wavelength of 555 nm (green). Scotopic luminous efficacy reaches a maximum of 1700 lm/W for narrowband light of wavelength 507 nm.
In some other systems of units, luminous flux has the same units as radiant flux. The luminous efficacy is then dimensionless. In this case, it is often instead called the luminous efficiency or luminous coefficient and may be expressed as a percentage. For example, it is common to express the luminous efficiency in units where the maximum possible efficacy, 683 lm/W, corresponds to an efficiency of 100%. The distinction between efficacy and efficiency is not always carefully maintained in published sources, so it is not uncommon to see "efficiencies" expressed in lumens per watt, or "efficacies" expressed as a percentage