Illuminance, measured in lux (lx), is the total luminous flux incident on a surface per unit area. It is the most commonly specified lighting parameter in building codes, lighting design software, and workplace ergonomics standards. Understanding lux — how it differs from lumens (lm) and candelas (cd), how to measure it correctly, and what target levels are required for different environments — is fundamental for any lighting professional. This article provides a comprehensive technical reference covering the photometric distinction between lux, lumens, and candelas, target illuminance levels for over 30 room and task types per GB 50034-2013 and EN 12464-1:2021, measurement instruments and procedures, the inverse-square law, and typical verification protocols for installed lighting systems.
Lux vs Lumens vs Candelas: The Three Fundamental Photometric Quantities
Light measurement distinguishes three quantities that are frequently confused. The table below clarifies each quantity, its symbol, SI unit, and practical meaning.
| Quantity | Symbol | Unit | Definition | Analogy |
|---|---|---|---|---|
| Luminous flux | Φ | Lumen (lm) | Total visible light energy emitted by a source per second | Total water flow from a faucet |
| Luminous intensity | I | Candela (cd) | Luminous flux per unit solid angle in a specific direction | Water flow concentrated into one direction |
| Illuminance | E | Lux (lx) | Luminous flux incident on a unit surface area (1 lx = 1 lm/m²) | Water depth in a pool of a given size |
Mathematical relationships:
- 1 lux = 1 lumen / 1 square meter. A light source emitting 1000 lumens uniformly into a 10 m² area produces 100 lux on that surface.
- Illuminance from a point source (inverse-square law): E = I / d², where I is the luminous intensity (cd) and d is the distance (m) between source and surface, valid for a point source with a beam narrower than approximately ¼ of the distance.
- For extended sources (panels, troffers): E = (Φ × CU × LLF) / A, where CU is the Coefficient of Utilization and LLF is the Light Loss Factor accounting for lamp lumen depreciation (LLD) and luminaire dirt depreciation (LDD).
Luminance (cd/m²) is a fourth photometric quantity — the luminous intensity emitted from a surface per unit projected area — but it is distinct from illuminance. A white wall illuminated to 500 lx may have luminance of 120–150 cd/m² (assuming diffuse reflectance of 75–95 %), while a black wall at the same 500 lx may have luminance of only 10–15 cd/m² (reflectance 5–10 %).
Target Illuminance Levels by Application
Lighting standards worldwide specify recommended or mandatory maintained illuminance levels for different room types and tasks. The two most commonly referenced standards for Chinese and European projects are GB 50034-2013 and EN 12464-1:2021. The table below provides target maintained illuminance (E_m) values with a third column showing the equivalent standard for reference.
| Application / Room Type | Target E_m (lux) | GB 50034 Table | EN 12464-1 Reference | Notes |
|---|---|---|---|---|
| Office — general (desk work) | 500 lx | Table 5.1.1 | §5.26 | At workplane height (0.75 m). UGR ≤ 19. |
| Office — conference / meeting | 500 lx | Table 5.1.1 | §5.27 | On table surface. UGR ≤ 19. |
| Corridor — day lit | 100 lx | Table 5.1.3 | §5.1 | At floor level. |
| Corridor — internally lit | 100 lx | Table 5.1.3 | §5.1 | At floor level. U0 ≥ 0.4. |
| Classroom — general | 300–500 lx | Table 5.2.1 | §5.36.1 | At desk level (0.75 m). UGR ≤ 19. |
| Classroom — blackboard | 500 lx | Table 5.2.1 | §5.36.2 | Vertical illuminance on board surface. |
| Retail — small store | 300 lx | Table 5.3.1 | §5.47 | General ambient. |
| Retail — supermarket | 500 lx | Table 5.3.1 | §5.48 | On shelves (vertical). |
| Retail — boutique / high-end | 300–750 lx | Table 5.3.2 | §5.49 | Accent up to 1500 lx on displays. |
| Museum/gallery — ambient | 200 lx | Table 5.4.1 | §5.37 | Light-sensitive exhibits: 50 lx max. |
| Museum — exhibit accent | 500–1500 lx | Table 5.4.1 | §5.38 | Non-sensitive materials only. |
| Hospital — ward (general) | 100 lx | Table 5.5.1 | §5.57 | At bed level, night. |
| Hospital — examination room | 1000 lx | Table 5.5.1 | §5.60 | On examination table. |
| Operating room | 1000–5000 lx | Table 5.5.2 | §5.62 | Surgical task lighting, dimmable. |
| Industrial — precision assembly | 1000–1500 lx | Table 5.6.2 | §5.20.2 | Very fine tasks. |
| Industrial — general machine work | 300 lx | Table 5.6.1 | §5.20.1 | On workplane. |
| Warehouse — rack face (vertical) | 200 lx | Table 5.6.3 | §5.10.2 | On vertical storage faces. |
| Warehouse — gangway/aisle | 100 lx | Table 5.6.3 | §5.10.1 | Floor level. |
| Residential — living room ambient | 150 lx | §4.2 Table 4.2.1 | — | General activity, at floor level. |
| Residential — kitchen (food prep) | 500 lx | §4.2 Table 4.2.2 | — | On countertop (task). |
| Residential — reading | 500 lx | §4.2 | — | On book/document surface. |
| Parking garage — ramp/entrance | 75 lx | — | §5.8.3 | Transition zone. |
| Parking garage — parking area | 50 lx | — | §5.8.2 | Floor level, maintained. |
| Street — main road (motor traffic) | 15–30 lx | — | EN 13201-2 | Average pavement luminance. |
| Street — footpath / pedestrian | 5–10 lx | — | EN 13201-2 | Average horizontal illuminance. |
| Sports — indoor basketball/volleyball | 300–750 lx | — | EN 12193 | Class III to I. |
| Sports — outdoor football (training) | 200 lx | — | EN 12193 | Horizontal illuminance. |
All values above are maintained illuminance (E_m), defined as the average lux level at the specified reference plane at the end of the maintenance interval (typically before lamp replacement or cleaning). Initial illuminance values are higher, typically by 10–30 %, depending on the Light Loss Factor (LLF) assumed in the design.
Illuminance Measurement: Instruments and Procedures
Field measurement of illuminance is performed using a calibrated lux meter (illuminance meter). The instrument must meet the following requirements per CIE 69-1987 and GB/T 5700-2008 (China) or BS 667 (UK) and DIN 5035 (Germany):
- Photocell type: Silicon photodiode with V(λ) correction filter that matches the CIE standard photopic luminous efficiency function. Class A or Class B per DIN 5032-7 (Class A: error < 3 %; Class B: error < 6 %).
- Cosine correction: The detector must have a cosine diffuser to correctly measure light from all incident angles. Error should be ≤ 3 % for incidence angles up to 60°.
- Measurement range: 0.1 lx to 100,000 lx typical. Resolution 0.01 lx for low levels, 1 lx for high levels.
- Calibration interval: Annual calibration traceable to NIM (China), NIST (US), or PTB (Germany). Typical calibration uncertainty: ±2 %.
Measurement procedure per GB/T 5700-2008:
- All lamps should be operated for at least 100 hours before measurement (to stabilize LED output) and operated for 30 minutes before measurement (to reach thermal equilibrium).
- For general room illuminance, measure at grid points on a 0.5–2.0 m grid (depending on room size and uniformity). A minimum of 10 measurement points per 100 m² is required.
- The sensor must be positioned at the workplane height (0.75 m for offices, 0.85 m for school desks, floor level for corridors, ground surface for outdoor areas).
- The person taking measurements must stand at least 0.5 m from the sensor to avoid shadowing and body reflection errors.
- Calculate average illuminance (E_avg), minimum illuminance (E_min), maximum illuminance (E_max), and uniformity (U0 = E_min / E_avg).
- The measurement uncertainty (k=2) under field conditions is typically ±5–10 %, depending on grid density and meter accuracy.
Inverse-Square Law and Practical Illuminance Calculations
The inverse-square law is the most important calculation tool for estimating illuminance from a point-source luminaire:
E = I / d² × cos(θ)
Where E is the illuminance (lx) on the surface, I is the luminous intensity (cd) of the luminaire in the direction of the surface, d is the distance (m) from the luminaire to the surface, and θ is the angle of incidence measured from the surface normal.
Example calculation: A 30°-beam LED track head with 1500 lm output has a peak luminous intensity of approximately 5500 cd. At 3.0 m distance (straight down, θ = 0°): E = 5500 / 3² = 5500 / 9 ≈ 611 lx at the center of the beam. At 2.0 m distance: E = 5500 / 2² = 1375 lx. This calculation is valid only for the center-beam point and assumes a point-source approximation (distance > 5× the luminaire's largest dimension).
For extended sources (LED panels, linear fixtures), the inverse-square law is not directly applicable within the near field (distance < 5× the fixture's largest dimension). Instead, the zonal cavity method (per IES Lighting Handbook) or lighting calculation software (DIALux, Relux, AGi32) should be used.
Maintained Illuminance and Light Loss Factors
The Light Loss Factor (LLF) accounts for the reduction in illuminance over time. It is the product of several independent factors:
| Factor | Symbol | Typical Value | Description |
|---|---|---|---|
| Lamp Lumen Depreciation | LLD (or LMF) | 0.85–0.95 (LED at 50,000 h) | Reduction in LED package luminous flux over life. L70 = 70 % of initial flux maintained at rated life. |
| Luminaire Dirt Depreciation | LDD | 0.80–0.95 (clean vs dirty) | Accumulation of dirt on lenses, reflectors, and diffusers. |
| Room Surface Dirt Depreciation | RSDD | 0.90–0.98 | Reduced reflectance from painted walls and ceilings over time. |
| Luminaire Ambient Temperature Factor | — | 0.95–1.02 | LED output varies with ambient temperature; factor = 1.0 at 25 °C. |
| Ballast/Driver Factor | BF | 0.90–1.00 | Driver efficiency and power factor correction losses. |
Total LLF = LLD × LDD × RSDD × BF
A typical LLF for an LED office installation with a 3-year maintenance cycle is 0.75–0.85. This means the initial (design) illuminance must be 18–33 % higher than the required maintained illuminance. For example, to achieve 500 lx maintained at end-of-life, the initial illuminance should be 500 / 0.80 ≈ 625 lx.
Common Mistakes in Illuminance Specification
- Confusing lux with lumens. A 5000 lm LED panel does not deliver 500 lx to a room. The actual illuminance depends on room dimensions, ceiling height, surface reflectances, and luminaire distribution. A common rule of thumb: in a typical office, 800–1000 lm per m² is needed to achieve 500 lx maintained, assuming CU ≈ 0.5–0.6 and LLF ≈ 0.8.
- Specifying only average illuminance without uniformity. GB 50034 requires U0 ≥ 0.4 for corridors, U0 ≥ 0.6 for office workplanes, and U0 ≥ 0.7 for precision tasks. High average lux with low uniformity (U0 < 0.4) produces patchy lighting with dark spots.
- Measuring at the wrong reference plane. Corridor illuminance must be measured at floor level, not at table height. Office illuminance must be measured at 0.75 m (desk height), not at the ceiling.
- Ignoring the difference between horizontal and vertical illuminance. For retail shelving, library stacks, and warehouse racking, vertical illuminance on the face of the rack is the critical parameter, not horizontal illuminance on the floor. A typical warehouse might have 200 lx horizontal (floor) but only 50–80 lx vertical on the rack face.
- Assuming steady-state LED illuminance is stable. LED illuminance drops by 5–15 % in the first 100 hours of operation (thermal stabilization), then declines slowly per the L70/L90 depreciation curve. Measurements taken during the first 30 minutes of operation may overstate steady-state illuminance by 3–8 %.
Frequently Asked Questions
What is the difference between lux, lumens, and candelas in simple terms?
Think of a light bulb: Lumens (lm) = how much total light the bulb emits. Candelas (cd) = how bright the bulb appears when you look directly at it from a specific direction. Lux (lx) = how much of that light falls on your desk surface. If you move the bulb further from your desk, the lux drops (less light on the surface) but the candelas in the direction of the desk don't change, and the lumens stay the same.
How many lux do I need for a home office?
GB 50034-2013 recommends 500 lx on the desk surface at 0.75 m height for office tasks. For a home office, 300–500 lx is recommended. If you use a 500 lx task lamp on your desk, the general room ambient can be 150–200 lx. A typical 12–15 m² home office needs approximately 3000–4500 total lumens from ceiling lights to achieve 300 lx ambient.
What tool should I use to measure lux?
Use a calibrated lux meter conforming to DIN 5032-7 Class B or better. The UNI-T UT383 series (approximately ¥150–300 for Class B accuracy) is a common entry-level choice. For professional use, the Konica Minolta CL-200A (approximately ¥15,000–20,000) or Testo 545 (approximately ¥3,000–5,000) provides Class A accuracy with up to 3 % measurement uncertainty.
What is the typical illuminance on a sunny day outdoors?
Direct sunlight at midday: 80,000–120,000 lx. Overcast day (diffuse light): 10,000–25,000 lx. Full moon at night: 0.1–0.3 lx. Indoor ambient (office): 200–500 lx. Home living room at night (lamps on): 100–300 lx.
Is there a relationship between lux and energy consumption?
Not directly. The energy consumption of a lighting system depends on the installed power (watts) and operating hours. A well-designed system achieving 500 lx with 8 W/m² is far more efficient than a poorly designed system achieving 500 lx with 20 W/m². The Lighting Power Density (LPD) metric in W/m², referenced in GB 50034-2013 and ASHRAE 90.1, sets maximum allowable LPD for given illuminance targets. For office buildings, GB 50034 sets LPD limits of 9 W/m² for 500 lx target using LED.
Related Products and Suppliers
For lighting products with certified photometric performance and guaranteed lux levels per GB 50034, explore the following categories:
- Office LED Panel Lights, 500 lx compliance, ≥110 lm/W, LPD ≤ 9 W/m²
- Residential and Commercial Downlights with IES photometric files
- Industrial High Bay Lights, 200–1000 lx target with detailed photometry
- Outdoor Floodlights with lux calculation tools
★ KSIMPEXP Recommendation
KSIMPEXP provides complete lighting design support including DIALux simulation files, photometric data (.ies/.ldt), and maintained illuminance calculations per GB 50034-2013 and EN 12464-1:2021. All luminaires are tested for flux, efficacy, and L70 lifetime in an accredited photometric laboratory. Request a free illuminance calculation and product specification sheet for your project.
Sources: GB 50034-2013, EN 12464-1:2021, CIE 69-1987, GB/T 5700-2008, IES Lighting Handbook (10th ed.), CIE 121-1996, EN 12193, EN 13201-2
Disclaimer: This article is for reference only. Specifications should be verified with current standards and manufacturer data sheets.
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- CIE 013.3-1995 — International Commission on Illumination: Method of Measuring and Specifying Colour Rendering
- CIE S 026:2018 — CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light
- IES TM-30-20 — IES Method for Evaluating Light Source Color Rendition
- IEC 62471:2006 — Photobiological safety of lamps and lamp systems
These standards and reports are cited as authoritative references. Specifications may vary by region and product version.