Lux (lx): The SI Unit of Illuminance

Introduction

The lux (lx) is the SI derived unit of illuminance, representing the luminous flux incident per unit area. Defined as one lumen per square meter (1 lx = 1 lm/m²), lux quantifies the illuminance of surfaces and environments. While the candela measures directional intensity and the lumen the total emitted light, the lux describes how much light arrives at a surface—a critical consideration in visual tasks, architecture, horticulture, and safety. This article provides an in-depth examination of lux, including its definition, relation to other photometric quantities, measurement principles, applications, and significance.

Definition and Photometric Relationships

Illuminance E_v is defined by:

where dΦ_v is the luminous flux incident on an element of area dA. The unit lux equals one lumen distributed uniformly over one square meter. Because luminous flux already incorporates the eye’s spectral sensitivity, lux also reflects perceived brightness. Illuminance relates to luminous intensity and distance via the inverse square law and cosine law:

• Inverse square law: For a point source with luminous intensity I, illuminance at distance r on a surface normal to the light is E = I / r².
• Cosine law (Lambert’s law): When the surface is tilted, E = (I / r²) cos θ, where θ is the angle between the incident light and surface normal.

These relationships show that illuminance depends not only on light output but also on geometry.

Historical Context

Historically, lighting evaluation was qualitative. With the industrial revolution, factory lighting needed quantification. Early units like the foot-candle—lumens per square foot—emerged. In the metric system, the phot (10 000 lux) and metre-candle preceded the lux. By the mid-20th century, the lux and lumen were formalized in photometric systems, aligning with the candela. The lux replaced the phot in the SI, being more suitable for real-world illuminance levels and convenient for decimal scaling. The Lux Committee of the Illuminating Engineering Society codified recommended illuminance levels for various tasks, promoting the lux as the principal measure.

Measurement Techniques

Illuminance meters, or lux meters, consist of a photodiode or photometer head with a spectral responsivity matched to the V(λ) curve and a cosine-corrected diffuser to account for incident angle. Key considerations:

• Spectral mismatch. The sensor’s spectral response should closely follow V(λ) to avoid errors when measuring light sources with different spectra (e.g., LEDs vs halogen).
• Cosine correction. A high-quality diffuser ensures the meter responds according to the cosine law. Poor cosine response leads to errors when light arrives at oblique angles.
• Calibration. Lux meters are calibrated against standard photometric lamps in accredited laboratories. Calibration should specify uncertainties and the reference spectrum.
• Stray light and environment. Measurement environments should avoid stray reflections. For outdoor measurements, shading and orientation matter.

For complex lighting systems (e.g., LED arrays or mixed spectra), goniophotometric data and software calculate illuminance at points in a space, considering multiple light paths and reflections.

Applications

Architectural and indoor lighting design. Illuminance is crucial for visual comfort, productivity, and safety. Standards specify minimum and recommended illuminance levels for tasks:

• Offices and classrooms: 300–500 lx to support reading and writing.
• Hospitals and laboratories: 500–1000 lx for detailed tasks and hygiene.
• Industrial facilities: 300–1000 lx, depending on task complexity.

Lighting designers simulate illuminance distributions using CAD tools, verifying compliance with standards.

Outdoor and street lighting. Roadway illumination standards specify minimum illuminance and uniformity to ensure visibility and reduce accidents. Typical street lighting provides 5–20 lx on the pavement.

Museums and galleries. Lux levels are controlled to preserve sensitive artwork. Illuminance may be limited to 50–200 lx, with UV filtering and timed lighting to prevent fading.

Horticulture. Plant growth depends on photosynthetically active radiation (PAR). Although lux measures human-perceived light, horticulture sometimes uses lux as an approximate guideline. However, PAR meters are preferable because plant responses differ from human vision.

Safety and regulations. Emergency lighting systems must provide minimum illuminance levels on escape routes (e.g., 1 lx), and building codes require specific lux values in stairwells and corridors.

Photography and cinematography. Lux affects exposure settings. “Scene brightness” measured in lux guides camera exposure, especially in low-light filming.

Significance and Trends

Illuminance measurement underpins lighting design, energy efficiency, and occupant well-being. The lux is a key metric in green building certifications, ensuring adequate lighting without waste. As LED technology allows dynamic control of intensity and spectrum, precise illuminance measurement ensures that adaptive lighting systems maintain appropriate lux levels while modulating color temperature. The increasing awareness of circadian lighting has sparked interest in alternative metrics (e.g., melanopic lux) that weight light according to non-visual photoreceptors. Even so, lux remains the fundamental unit for quantifying light on surfaces, bridging photometry with practical human needs. Future lux meters may incorporate multispectral sensors and calculations of additional biological weighting functions, but the unit’s conceptual simplicity—lumens per square meter—will continue to serve as the backbone of lighting measurement and specification.

Related resources on CalcSimpler

Keep illuminance analysis aligned with surrounding photometric standards.

• Candela (cd): The SI Unit of Luminous Intensity

  Link directional luminous intensity to the illuminance that surfaces receive in real environments.

  Read more ↗

• Lumen (lm): The SI Unit of Luminous Flux

  Connect total luminous output with the per-area illuminance targets you manage.

  Read more ↗

• ISO 80000-7: Quantities and Units of Light and Radiation

  Revisit the standard that defines illuminance, luminous exposure, and related photometric quantities.

  Read more ↗

• The Steradian (sr): The Geometry of Direction

  See how solid angle calculations underpin conversions between luminous intensity, flux, and illuminance.

  Read more ↗

Calculators to apply lux measurements

Use these tools to convert, communicate, and plan illuminance levels.

• Lux to Lumens Calculator

  Convert illuminance readings into total luminous flux for compliance checks.

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• Lux to Foot-Candles Converter

  Express lux measurements in foot-candles when coordinating with US-based teams.

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• Room Lighting Lumens

  Reverse-engineer target lux levels into lamp output recommendations.

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