The same EN 12464-1 lumen-method formula applied to a 500 lux interior office and a 75 lux exterior car park traffic route — two very different target illuminances driving two very different fixture counts.
| Room 1: Open-plan office | 10 m x 8 m, mounting height 2.5 m, target 500 lux |
| Luminaire (office) | 4000 lm, UF=0.55, MF=0.8 |
| Area 2: Car park traffic route | 40 m x 20 m, mounting height 8 m, target 75 lux |
| Luminaire (car park, floodlight) | 20,000 lm, UF=0.45, MF=0.75 |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Office achieved illuminance | ≥ 500 lux | 506.0 lux | ✓ PASS |
| Office luminaire count | n/a (sizing basis) | 23 luminaires | ✓ PASS |
| Car park achieved illuminance | ≥ 75 lux | 75.94 lux | ✓ PASS |
| Car park luminaire count | n/a (sizing basis) | 9 floodlights | ✓ PASS |
Key insight: The lumen method's fixture count scales with the ratio of target illuminance to (luminaire output x utilization x maintenance factor) x area — a large area with a low target (like a car park route) can need far fewer fixtures than a small area with a high target (like a technical drawing office at 750 lux), which is why area alone is a poor proxy for lighting cost or fixture count without also weighing the target illuminance for that specific space type.
Every input in this example is editable in the live calculator — free, no signup.
Open Lighting Design calculator →UF accounts for how much of a luminaire's emitted light actually reaches the working plane, given room proportions, mounting height, and surface reflectances — a low, wide room with light-colored walls has a higher UF than a tall, narrow room with dark surfaces. MF accounts for the light loss expected over the maintenance cycle from lamp lumen depreciation and luminaire dirt accumulation before the next cleaning or relamping — both factors are why the same luminaire in two different rooms is very unlikely to hit the same lux value.
Interior room types are free on this calculator, while exterior/area lighting types (car parks, building exteriors) are grouped as a subscriber feature — reflecting that exterior lighting design typically involves additional considerations (light trespass, glare, pole spacing) beyond the same basic lumen-method illuminance target used here.