What does lux mean?
Lux is the international unit used to express illuminance: how much visible light falls on a surface. One lux is equivalent to one lumen of light distributed evenly across an area of one square metre. In everyday lighting discussions, lux is often treated as a measure of how bright a place appears, although the technical measurement concerns illuminance rather than a light source’s total output or power consumption. The unit is important in photography, museum conservation, workplaces, and architectural lighting because minimum lux levels can be specified for visibility, safety, or protection of sensitive objects. For example, offices may be designed to provide several hundred lux of light on desks, while galleries often restrict illumination to prevent damage to vulnerable materials. Lux was adopted as part of the International System of Units and derives from the Latin word for light, the same root found in terms such as illuminate and lumen.
nounThe SI unit of illuminance, equal to one lumen of light flux distributed uniformly over an area of one square metre. The term is also used informally for the amount of light falling on a surface.
- The SI unit of illuminance, equal to one lumen per square metre.
- The amount of visible light falling on a surface, commonly used in lighting design, photography, and workplace standards.
"The desk is equipped with a dimmable lamp that provides 500 lux of light."
"The museum requires at least 150 lux for the display of delicate documents."
"The camera automatically adjusts its exposure when the scene is illuminated by fewer than 10 lux."
The plural is normally unchanged, as in “a light level of 100 lux.”
"The gallery limits illumination to 50 lux."
The word lux enters the dark: it comes from the Latin for “light” and names the modern standard for measuring brightness on a surface.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does lux come from?
Lux is the Latin noun meaning “light,” “brightness,” or “light” in the sense of radiance. It entered English in the early 20th century as the name for the unit of illuminance. Its root also underlies Latin words such as lumen and lucidus and English derivatives including illuminate and luminous.
How lux is actually used
Technically, lux is a unit of measurement rather than a synonym for brightness in the everyday sense. It is commonly used with lighting levels and illuminance; the plural is normally unchanged.
Easily confused with lux
A lumen measures the total amount of emitted light, whereas a lux measures how much of that light illuminates a particular surface.
A watt measures power consumption, while a lux measures the resulting illuminance on a surface.