What does lumen mean?
A lumen is a unit of light output, especially the total amount of visible light emitted by a lamp, bulb, or other source. It is often confused with related measurements such as lux and luminance, but a lumen specifically represents luminous flux. The same word also has a technical biological meaning: the lumen is the hollow or open space inside a tube-shaped structure, such as a blood vessel, the intestine, or another organ. In everyday discussions of household lighting, people commonly use “lumens” when comparing the brightness of bulbs, although perceived brightness can also depend on beam angle, color temperature, and room conditions. The term comes from Latin, where lumen referred to light and also to an opening through which light passes, reflecting the word’s long association with illumination and visibility.
nounA lumen is the SI unit of luminous flux, equal to the amount of visible light emitted by a uniform point source of one candela in each of the directions of one steradian. In biology and anatomy, the term also denotes the open space within a tubular structure, such as a blood vessel or the digestive tract.
- The SI unit of luminous flux, used to express the total amount of visible light emitted by a source.
- The internal space or channel within a tubular biological structure, such as a blood vessel, organ, or passageway.
"The bright bulb produces 800 lumens of light."
"Modern LED bulbs are commonly compared by their approximate lumen output."
"The blood vessel narrows because of a buildup of plaque within its lumen."
“Lumens” is the usual plural for the unit of light; “lumina” is standard in biological and anatomical contexts.
"The fixture is rated at 900 lumens, and the narrowing of the lumen reduced blood flow."
The word lumen originally referred to an opening that admitted light; it now also names one of the modern units used to measure that light.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does lumen come from?
The English word comes from Latin lumen, meaning “light” and, in anatomical contexts, “opening” or “space within a tube.” Latin forms such as lumin-, meaning light, gave rise to English words including illuminate, luminous, and luminosity. The medical sense developed through the comparison of a vessel or organ to a channel with an interior passage.
Related word forms
How lumen is actually used
In lighting and electronics, lumens are standard in both British and American English. The anatomical sense is specialized but well established in biology and medicine.
Easily confused with lumen
A lumen measures the total amount of light emitted, whereas luminance measures how bright an area appears to an observer.