Lumens

Lumens, symbol lm, is the SI unit of luminous flux, a measure of the perceived power of light. Luminous flux represents the total amount of visible light emitted, transmitted, or received by a source. It is defined in relation to luminous intensity I measured in candelas (cd) and solid angle Ω in steradians, with Φv = I × Ω. Equivalently, 1 lumen equals 1 candela steradian.

Luminous flux is weighted by human vision through the standard luminous efficiency function V(λ), which describes the eye’s relative sensitivity to different wavelengths. This weighting distinguishes lumens from radiant flux, which measures energy without regard to color sensitivity. By convention, 1 watt of radiant flux at 555 nanometers (green light where the eye is most sensitive) equals 683 lumens; other wavelengths yield lower luminous flux according to V(λ).

In practice, lumens are used to rate light sources such as bulbs and luminaires. The related unit lux measures illuminance, defined as lumens per square meter. Typical values vary by technology and form factor: a 60-watt incandescent or its LED equivalent is commonly around 800 to 1,100 lumens, while smaller or higher-efficiency lamps may deliver fewer lumens or more.

The lumen provides a photometric measure of brightness that facilitates comparisons across different lamps and spectra. Luminous efficacy, expressed in lumens per watt, relates lumen output to electrical power and is a key metric in lighting efficiency.