LuxEdit

Lux

Lux is the SI-derived unit of illuminance, describing how much luminous flux reaches a surface. In practical terms, it measures the brightness of light as perceived by the human eye on a given area. The standard definition is that one lux equals one lumen per square meter (1 lm/m^2), and the symbol for the unit is lx. The term itself comes from the Latin word lux, meaning light. In everyday use, lux helps architects, engineers, and photographers plan spaces and systems to achieve the right level of perceived brightness. Lumen and Illuminance are closely connected concepts in photometry, with lux representing the flux per unit area on a surface.

Definition and physical basis Lux is an illuminance quantity: it quantifies the luminous flux incident on a surface per unit area. Mathematically, lux = lumen per square meter, so 1 lx = 1 lm/m^2. The lumen is itself a measure of luminous flux, a photometric quantity that describes the total amount of light energy perceived by the human eye. The relationship among luminous flux, luminous intensity, and illuminance is mediated by the geometry of the emitting or receiving surface and by human visual response. The candela, the base SI unit of luminous intensity, anchors this system, while the lumen and the lux provide practical ways to express how bright a scene will appear to an observer. For a reference radiometric concept, see the luminous efficacy function V(λ), which weights radiant power by the eye’s sensitivity; this weighting underlies the use of lux as a perceptual measure. Candela, Luminous flux, Luminous efficiency function

Measurement, instrumentation, and standards Lux is measured with photometers or lux meters, devices calibrated to respond to light in a way that mirrors human vision. Modern meters often incorporate spectral weighting to follow V(λ) and may report both photometric illuminance (lux) and, for specialized tasks, spectral data. Because lux is a perceptual unit, two light sources with the same radiant power can yield different lux readings if their spectral content differs. Standards for measurement come from international photometry authorities and national bodies, with the International Commission on Illumination (CIE) providing widely adopted guidelines. The relationship between lux and other photometric units—lumen (luminous flux), candela (luminous intensity)—is central to accurate lighting design and measurement. Photometry, Illuminance meter, CIE

Applications and everyday use Lux appears in a wide range of design and evaluation tasks, reflecting how people experience light in real spaces and contexts.

  • Architecture and interior design: Lighting designers specify illuminance targets to ensure readability, comfort, and visual performance in offices, classrooms, hospitals, and retail environments. Recommended levels vary by task and space, with typical office environments often aiming for hundreds of lux on work surfaces. Illuminance

  • Workplace safety and productivity: Proper illuminance supports accuracy and reduces fatigue, while over-illumination can cause glare. Standards bodies publish task-based guidelines to balance efficiency, safety, and comfort. See discussions linked to CIE and related standards. CIE

  • Photography, film, and display technology: In photography and filmmaking, lux (and its complementary concept, exposure) helps determine lighting setups and camera settings to achieve the desired mood and detail. Display brightness in consumer devices is also described in lux to communicate real-world viewing conditions. Exposure (photography)

  • Horticulture and human factors: In plant growth, the relevant metric is often photosynthetically active radiation (PAR), measured in micromoles per square meter per second rather than lux, because lux weights light for human vision rather than plant biology. This has spurred discussion about when lux is an appropriate proxy for plant lighting versus when PAR is preferred. Photosynthetically active radiation

  • Industrial and outdoor lighting: Roadway, street, and industrial lighting use lux targets to ensure visibility and safety for pedestrians and operators, with different categories for zone lighting, control of glare, and energy efficiency. Lighting design

Limitations and debates Lux as a unit reflects human visual perception through the standard photopic response, which means it prioritizes how bright things appear to the average daytime human eye. This makes lux less suitable for other biology (e.g., non-human vision) or for non-visual outcomes (e.g., plant growth). In horticulture and some scientific contexts, practitioners often prefer measurements like PAR or spectral irradiance that do not heavily weight the eye’s response. The ongoing debate centers on whether lux remains the best universal measure for mixed-use environments or whether task-specific metrics should take precedence in design and regulation. Luminous efficiency function, PAR

Historical notes The concept of illuminance as a measurable, perceptual quantity emerged as photometry matured in the 19th and 20th centuries. The lux, as a formal SI unit for illuminance, was adopted with the broader standardization of SI derived units in the mid-20th century, underscoring the push to quantify lighting in practical, real-world terms. The evolution of units like lux and their interrelations with lumen, candela, and related quantities reflects the broader aim of aligning scientific measurement with human experience and engineering needs. SI units, Luminous flux

See also - Illuminance - Lumen - Candela - Photometry - Luminous efficiency function - PAR - Exposure (photography) - Lighting design - Color temperature - Display brightness