Lumen

Lumen is the SI unit of luminous flux, symbol lm. It measures the total amount of visible light emitted by a light source per unit time. The lumen is weighted by the human eye’s sensitivity across different wavelengths, using the photopic luminous efficiency function, so light that looks brighter to people contributes more lumens.

In anatomy, lumen refers to the interior open space inside a tubular structure, such as a blood vessel, intestine, or airway. The lumen is the channel through which contents pass, while the surrounding walls comprise the structure enclosing it.

In practical terms, lumens describe the brightness of lighting products. Lighting specifications emphasize lumens rather than watts to convey perceived brightness. For example, modern LED bulbs often emit several hundred to over a thousand lumens, while traditional incandescent bulbs of similar perceived brightness correspond to different wattages. A lumen value alone does not indicate color temperature, distribution, or beam angle, which also affect perceived brightness.

Lumen is also the name of a PHP micro-framework developed by the Laravel project for building lightweight web APIs and microservices. It is designed to be fast and minimal, sharing components with Laravel while omitting some features not needed for small-scale applications.

Etymology and scope: the term lumen comes from Latin lumen, meaning "light" or "opening." In science, lumen can refer to light output (as a photometric unit) or to the interior cavity of a tubular organ in biology. The word has related forms such as lumina in plural uses.