In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light), in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.
Luminous efficacy is a measurement commonly used in the lighting industry that indicates the ability of a light source to emit visible light using a given amount of power. It is a ratio of the visible energy emitted to the power that goes into the bulb from the electrical line (visible energy emitted is also known as luminous flux, and the units are measured in lumens). Note: in some countries, luminous efficiency is substituted for luminous efficacy. Luminous efficiency, however, is a unitless measure (watts out / watts in) where luminous efficacy is not (lumens out / watts in).