Define Lux — What Is Lux?

勒克斯是什么

Lux is the number that decides whether a lighting installation passes or fails. A contractor can install fixtures with impressive lumen ratings, but if the measured illuminance at the working plane falls below the specified lux level, the installation does not comply. To define lux precisely — and understand what changes it — is therefore essential for anyone specifying, verifying, or purchasing lighting.

How Lux Works: The Physics of Illuminance

Illuminance is a received quantity. A light source emits luminous flux (lumens); some of that flux lands on a given surface; the flux per unit area of that surface is the illuminance in lux. The base relationship is simple:

E (lux) = Φ (lumens) ÷ A (m²)

Spread 10,000 lm perfectly over a 20 m² room and the average illuminance is 500 lx. Spread the same 10,000 lm over 100 m² and you get only 100 lx. The source did not change — the geometry did. This is the first key to define lux correctly: lux is a property of the source, the distance, and the surface together, never of the source alone.

The Inverse Square Law

For a compact (point-like) source, illuminance falls with the square of distance:

E = I ÷ d²  (E in lux, I in candela, d in meters)

A spotlight producing 2,000 cd on its beam axis delivers 2,000 lx at 1 m, 500 lx at 2 m, and just 80 lx at 5 m. Double the distance, quarter the lux. This single formula connects candela to lux and explains why mounting height dominates lighting design: raising a high bay from 6 m to 8 m cuts task-plane illuminance by roughly 44% unless output or fixture count increases.

The Cosine Law

Light striking a surface at an angle spreads over more area, reducing illuminance by the cosine of the incidence angle: E = (I ÷ d²) × cos θ. This is why illuminance drops off toward the edges of a lit area, and why standards require checking uniformity (U₀ = minimum ÷ average illuminance) as well as average lux. EN 12464-1:2021 requires U₀ ≥ 0.60 on office task areas and ≥ 0.40 in the immediate surrounding area.

Maintained Illuminance

Standards define lux requirements as maintained illuminance (Ēm) — the value below which the average must never fall during the installation's life. Because LEDs depreciate (L80/L90 per IES TM-21-21) and luminaires accumulate dirt, designers apply a maintenance factor (typically 0.67–0.8) so a space specified at 500 lx maintained might be designed to about 625–750 lx initial.

Key Data: Recommended Lux Levels by Environment

Note the enormous range: full sunlight delivers roughly 100,000 lx while a full moon delivers about 0.2 lx — a factor of half a million. Human vision adapts across this range, which is why perceived "brightness" is a poor guide and standards define lux targets objectively for every task.

How Lux Relates to Lumens and Candela

Lux is one of three sibling photometric quantities defined in CIE S 017:2020. To define lux in context, keep the emission-to-reception chain in view:

1. Lumen (lm) — luminous flux: total light emitted by the source in all directions. A property of the lamp. Measured for products in an integrating sphere per IES LM-79-19.
2. Candela (cd) — luminous intensity: flux emitted per unit solid angle in a direction (1 cd = 1 lm/sr). A property of the lamp plus its optics — the shape of the beam.
3. Lux (lx) — illuminance: flux received per unit area (1 lx = 1 lm/m²). A property of the whole situation: source intensity, distance, angle, and surface.

Think of it as a garden hose: lumens are the total water flow from the tap, candela is how tightly the nozzle focuses the jet, and lux is how wet a particular patch of ground gets. Change the nozzle (optics) or step back (distance) and the ground gets less wet — even though the tap flow (lumens) never changed.

Practical conversions worth memorizing:

• From lumens: average lux = delivered lumens ÷ area. (Include utilisation and maintenance factors in real designs.)
• From candela: lux at distance d (on-axis, perpendicular) = candela ÷ d².
• Related unit: the US footcandle (fc) = lumens per square foot; 1 fc ≈ 10.764 lx. North American IES documents often quote footcandles — multiply by ~10.8 to compare with European lux requirements.

Practical Applications: Measuring and Specifying Lux

1. Verifying an Installation

Compliance measurement uses a calibrated lux meter (class B or better per ISO/CIE 19476) placed on the reference plane — typically 0.75 m desk height for offices, floor level for corridors and warehouses. Measurements follow a grid defined in EN 12464-1; the average must meet the maintained illuminance and the minimum must satisfy the uniformity ratio. Measure at night or with blinds closed to exclude daylight when verifying the electric lighting alone.

2. Designing to a Lux Target

Designers use software (DIALux, Relux, AGi32) loaded with each luminaire's photometric file to predict lux point-by-point. The inputs that matter: fixture lumen output (from LM-79 data), candela distribution, mounting height and spacing, room reflectances, and maintenance factor. Never accept a design that quotes only fixture lumens — demand a calculated lux plot for the actual space.

3. Workplace Compliance and Wellbeing

Under-lit workplaces increase error rates and eye strain; over-lit ones waste energy and create glare. EN 12464-1:2021 also introduces "context modifiers" — allowing designers to raise the design level (e.g., 500 → 750 lx) for older workers or low-contrast tasks. When you define lux requirements for a real workforce, consider the task, contrast, and occupant age, not just the table minimum.

4. Horticulture, Security, and Special Cases

Security cameras specify minimum scene illuminance in lux (e.g., 0.1 lx for low-light operation). Horticulture, by contrast, uses PPFD (µmol/m²/s), not lux, because plants respond to different wavelengths than the human eye — a reminder that lux is strictly a human-vision-weighted quantity per the CIE V(λ) function.

Common Mistakes When Working with Lux

1. Confusing lumens with lux. A "10,000-lumen" floodlight sounds impressive but may deliver under 50 lx at 15 m. Always ask: how many lux, at what distance, on what plane?
2. Ignoring maintenance factor. Designing to exactly 500 lx initial guarantees non-compliance within a couple of years as lumen depreciation and dirt accumulate.
3. Measuring at the wrong plane. Standards define lux at the task plane (e.g., 0.75 m), not the floor. Floor measurements read lower and cause over-specification.
4. Forgetting uniformity. An average of 500 lx with dark 150 lx patches fails EN 12464-1's U₀ ≥ 0.60 requirement and fatigues occupants.
5. Comparing lux across mixed units. US datasheets in footcandles understate values 10.8× versus lux. Convert before comparing.

Key Takeaways