Definition
Lux (symbol: lx) is the SI unit of illuminance — the amount of luminous flux falling on a surface per unit area. One lux equals one lumen uniformly distributed over one square meter (1 lx = 1 lm/m²). Lux describes how much light actually reaches a surface, accounting for distance (inverse square law: doubling distance quarters illuminance) and angle of incidence (cosine law: light at 60° from vertical delivers 50% of head-on illuminance). Lux is the primary metric for lighting design compliance — standards like EN 12464-1, IES RP-1, and GB 50034 specify maintained illuminance in lux for each space type and visual task. A calibrated lux meter with cosine correction is the standard measurement tool.
Key Data
| Parameter | Value / Explanation |
|---|---|
| Full daylight (direct sun) | 32,000-100,000 lux |
| Overcast day | 1,000-2,000 lux |
| Office desk (EN 12464-1) | 500 lux maintained |
| Warehouse general storage | 150-200 lux |
| Precision assembly | 750-1,000 lux |
| Retail accent (merchandise) | 750-1,500 lux |
| Emergency egress path | ≥1 lux (centerline), ≥0.5 lux minimum per EN 1838 |
Application Guide
Office desk
500 lux maintained, UGR ≤19, uniformity ≥0.6
EN 12464-1 for writing, typing, reading, data processing (Task Area 5.26.2)
Supermarket fresh produce
750-1,000 lux, CRI 90+, 4000K
Color accuracy critical for produce appearance; higher illuminance signals freshness
Hospital operating theatre
1,000-10,000 lux (surgical lamp), CRI 95+
Extreme precision required — surgical lamps deliver 40,000-160,000 lux at the wound
Conclusion & Procurement Recommendation
For B2B lighting procurement, specify maintained illuminance in lux (not initial), including the measurement reference plane (typically 0.8m above floor for offices, floor level for corridors). Key RFQ requirements: (1) Maintained illuminance (lux) per space type per applicable standard, (2) Uniformity ratio (Emin/Eavg ≥ 0.6 for task areas), (3) Measurement grid specification (excluding 0.5m border from walls), (4) Light loss factor assumptions (lumen depreciation, luminaire dirt depreciation, room surface depreciation). Always require a post-installation illuminance verification report before final payment — measured values must meet or exceed the specified maintained levels.