What is lumen definition?

A lumen is a unit of measurement used to describe the amount of light emitted by a light source. It measures the amount of light that is visible to the human eye. The greater the number of lumens, the brighter the light. The lumen is a SI derived unit of luminous flux, which is the radiant power of light measured in watts per steradian. This unit of measurement is used in the lighting industry to help consumers choose the right light bulbs and fixtures for their needs. The efficacy of a light source is measured in lumens per watt, indicating how efficiently a light source converts electrical energy into visible light.