The lumen (lm) is the International System of Units (SI) measure of luminous flux, which quantifies the total quantity of visible light emitted by a source. It is a fundamental unit in photometry, the field of science that measures and quantifies the characteristics of visible light. While luminous flux describes the total raw output of light energy, it is distinct from illuminance, which measures the amount of light falling upon a surface, and luminous intensity, which measures the light output in a specific direction. This unit is essential for characterizing light sources in fields ranging from illumination engineering to astrophysics, providing a standardized metric for light power output.