LUX

LUX
Name	LUX
Quantity	Illuminance
Unit	lux
Unitsystem	SI derived unit
Named after	Gustave Eiffel
Derivedfrom	lumen per square metre

LUX LUX denotes the SI derived unit of illuminance, defined as one lumen per square metre. It is used across metrology, photometry, optics, lighting engineering, and architecture to quantify the visible light flux incident on a surface. LUX interfaces with standards bodies such as the International Organization for Standardization, International Electrotechnical Commission, and national metrology institutes including the National Institute of Standards and Technology and the Physikalisch-Technische Bundesanstalt.

Etymology and Terminology

The term traces linguistic roots to Latin via Romance languages and was adopted into scientific usage alongside units such as the lumen and the candela. Terminology around LUX appears in documents from the Comité International des Poids et Mesures, International Commission on Illumination, and early 20th-century texts associated with figures like Hermann von Helmholtz and James Clerk Maxwell. Technical literature distinguishes LUX from related terms such as lumen and candela while linking to standards produced by the British Standards Institution and the American National Standards Institute.

Definition and Measurement

By definition, one LUX equals one lumen per square metre; formally it is the photometric equivalent of the radiant flux density weighted by the photopic luminous efficiency function promulgated by the Commission Internationale de l'Éclairage. The unit connects to fundamental quantities characterized in the SI system; conversion between LUX and radiometric units requires spectral weighting against the V(λ) curve established by the CIE. International committees including the General Conference on Weights and Measures coordinate definitions that link LUX to primary standards maintained by institutions such as the Bureau International des Poids et Mesures and national laboratories like the National Physical Laboratory.

Instrumentation and Measurement Techniques

Measurement of illuminance in LUX commonly employs handheld luxmeter instruments, integrating sphere systems, and calibrated photodiodes traceable to primary standards at laboratories such as the National Research Council (Canada) or the National Metrology Institute of Japan. Photometric sensors incorporate filters matched to the CIE 1931 standard observer and may be calibrated using sources like standard illuminant D65 or Tungsten–halogen lamp standards. Techniques include cosine correction for angular response, use of baffles and stray-light reduction in facilities like integrating spheres used by the European Space Agency, and intercomparison methods practiced at interlaboratory programs organized by the International Laboratory Accreditation Cooperation and the International Bureau of Weights and Measures.

Applications and Standards

LUX is specified in design guidance from organizations such as the International Electrotechnical Commission, the American National Standards Institute, the Royal Institute of British Architects, and the World Health Organization for environments spanning offices, hospitals, streets, sports arenas, and photographic studios. Building codes in jurisdictions influenced by the International Code Council reference illuminance levels in LUX for compliance, and industry standards from bodies like the Illuminating Engineering Society and CIBSE set recommended values. Specialized applications include horticultural lighting standards developed with input from NASA and the Food and Agriculture Organization, museum lighting criteria in collaboration with the International Council of Museums, and occupational safety thresholds promulgated by the Occupational Safety and Health Administration and the European Agency for Safety and Health at Work.

Biological and Environmental Effects

Illuminance measured in LUX is instrumental in studies linking light exposure to circadian physiology researched at institutions such as the National Institutes of Health and Max Planck Society. Thresholds in LUX correlate with melatonin suppression, alertness, and visual performance explored in chronobiology laboratories affiliated with universities like Harvard University and University of Oxford. Environmental light pollution metrics reference horizontal illuminance in LUX in reports by the United Nations Environment Programme and NGOs such as the International Dark-Sky Association; outdoor LUX levels influence ecological studies involving species monitored by organizations like WWF and researchers at the Scripps Institution of Oceanography.

Historical Development and Notable Experiments

The formalization of photometric units leading to LUX evolved through work by pioneers including Gustav Kirchhoff, Hermann von Helmholtz, and committees formed after the Metre Convention. Early comparative experiments on luminous efficiency conducted by researchers at the CIE and in laboratories such as the NIST produced the CIE photopic curves that underpin modern LUX measurement. Notable experiments include intercomparisons of illuminance standards across national laboratories coordinated by the BIPM, field studies of urban nightscapes led by teams from the University of California, Berkeley and landmark daylighting research in the Massachusetts Institute of Technology's Building Technology Program that quantified interior LUX distributions under varied fenestration configurations.

Category:Photometry