Understanding Lighting Maintenance Factor in Design
If you’ve ever delivered a lighting installation that looked dimmer than the calculations predicted, the Maintenance Factor (MF) is likely where the gap occurred. It’s one of the most important — and most frequently misapplied — parameters in professional lighting design.
What Is Maintenance Factor?
Maintenance Factor is a multiplier (always less than 1.0) applied to initial luminaire output to predict maintained illuminance over the life of the installation. It is the product of four components:
- Lamp Lumen Maintenance Factor (LLMF): LED output reduction over time, expressed as L70 (point at which output reaches 70% of initial)
- Luminaire Dirt Depreciation (LDD): Reduction from dust and contamination on optics, diffusers, and the luminaire body
- Room Surface Dirt Depreciation (RSDD): Reduced reflectance from dirty walls, ceilings, and floors
- Lamp Survival Factor (LSF): For multi-source installations, the proportion of sources still operating at the calculation point
MF = LLMF × LDD × RSDD × LSF
Typical MF Values by Environment
| Environment | Typical MF | Maintenance Assumption |
|---|---|---|
| Clean office | 0.80–0.85 | Annual cleaning |
| Retail | 0.70–0.80 | Bi-annual cleaning |
| Industrial / warehouse | 0.55–0.65 | Dusty, 3-year cycle |
| Clean room / laboratory | 0.85–0.90 | Frequent cleaning |
| Outdoor (IP65) | 0.65–0.75 | 3-year maintenance cycle |
The Most Common Mistake
Using a generic MF of 0.8 for every project regardless of context. A dusty factory with a 5-year maintenance cycle and an assumed MF of 0.8 will be significantly under-lit within 2 years. Conversely, applying a conservative MF of 0.55 in a clean, well-maintained office results in expensive over-specification and unnecessary energy use.
MF in DIALux EVO
DIALux EVO allows MF to be set per room or per luminaire group. Use the LLMF data from the manufacturer’s LM-80 test report for accurate lamp depreciation figures. Calculate LDD based on the luminaire’s IP rating and the cleaning schedule documented in the project maintenance plan.
