Overview — The lumen (symbol: lm) is the SI derived unit used to express luminous flux, a measure of the perceived power of light as seen by the human eye. It quantifies the total amount of visible light emitted by a source in all directions or within a specified solid angle. The lumen is tied to human visual sensitivity, not simply radiated energy.

Definition and characteristics

By definition, one lumen equals one candela emitted into a solid angle of one steradian. This relationship is often written lm = cd·sr. For formal context see the SI description: SI derived unit. The candela component is an intensity per unit solid angle; the steradian is the SI unit of solid angle (see steradian). The candela itself is defined in terms of monochromatic radiation near 540×1012 Hz, linked to a conventional luminous efficacy of 683 lm per watt, which ties photometric and radiometric quantities (see candela).

Lumens measure luminous flux (total visible light), whereas lux measures illuminance (lumens per square meter) and candela measures luminous intensity (lumens per steradian). Radiometric units such as watts measure physical power across all wavelengths, while lumens weight that power according to the human eye's spectral sensitivity (the photopic response).

Practical uses and examples

  • Lighting specifications: manufacturers list lumens to indicate how bright a bulb or LED appears, allowing consumers to compare products regardless of technology.
  • Everyday examples: typical household bulbs are often rated in the hundreds to thousands of lumens; a single candle is commonly approximated to produce on the order of ten lumens.
  • Standards and design: architects and lighting designers use lumens and lux levels to plan illumination for tasks, public spaces, and safety requirements.

Measurement and notable facts

Measuring luminous flux requires instruments that account for the eye's sensitivity curve, such as photometers and integrating spheres. Because lumens reflect human perception, two light sources with equal radiant power can have different lumen ratings if their spectral outputs differ. The lumen is an internationally recognized SI derived unit and is central to modern lighting technology and energy-efficiency labeling.