Method
Lumen-method core using explicit illuminance, luminaire output, utilization and maintenance factors.
Equations
A = L · W
Φ = E · AUF · MF
N′ = ΦΦL
N = ⌈N′⌉rounded up
Eav = N · ΦL · UF · MFA
Symbols and units
| Symbol | Quantity | Unit |
|---|---|---|
| Inputs | ||
| L | Room length | m |
| W | Room width | m |
| E | Target average illuminance | lux |
| ΦL | Luminous flux per luminaire | lm |
| UF | Utilization factor | 0–1 |
| MF | Maintenance factor | 0–1 |
| Results | ||
| A | Room area | m² |
| Φ | Required lumens | lm |
| N′ | Raw luminaire quantity | count |
| N | Recommended luminaires | count |
| Eav | Resulting average illuminance | lux |
Assumptions and limits
- No default lux, utilization or maintenance factors.
In practice
- The target illuminance comes from the lighting standard for the task. The utilization factor depends on the luminaire, room index and surface reflectances, so take it from the luminaire photometric data.
Worked example
The input values are the calculator's own verification test, run through the calculator. They show the working; they are not design values.
Inputs
| L | Room length | 10 m |
| W | Room width | 5 m |
| E | Target average illuminance | 500 lux |
| ΦL | Luminous flux per luminaire | 1,000 lm |
| UF | Utilization factor | 0.8 0–1 |
| MF | Maintenance factor | 0.8 0–1 |
Results
| A | Room area | 50 m² |
| Φ | Required lumens | 39,062.5 lm |
| N′ | Raw luminaire quantity | 39.0625 count |
| N | Recommended luminaires | 40 count |
| Eav | Resulting average illuminance | 512 lux |
A preliminary check for engineer review, not an engineering approval. The project calculation to the applicable codes, authority requirements and specification governs.