The short answer: The maintenance factor (MF) shows how much usable light an LED installation will lose over time and how much reserve you must plan so the required illuminance is met over the evaluation period. This article names the four exact MF components (LSF, LLMF, LMF, RSMF), shows the formula, explains common mistakes and outlines what you should do as a client. – Dan Flühmann, Sensora Smart AG
In this article:
Tobias Steffen requested two LED lighting proposals. The offers differed significantly in one number: the maintenance factor (MF). He wanted to know what that number means and whether the proposals are comparable.
This article explains the four exact MF components for LED systems, the calculation, and the practical steps you should take as a client to ensure comparable offers and a reliable result.
The MF is a dimensionless number ≤ 1 applied to the initial luminous output to represent expected losses over a defined evaluation period.
Formula: required initial illuminance = target illuminance / MF
A realistic MF prevents underlighting during operation and avoids unnecessary oversizing at installation.
LSF describes the failure of light sources (for LEDs: failed modules or electronics) during the evaluation period. It is a factor between 0 and 1 derived from reliability data or manufacturer failure‑rate information.
LLMF describes the reduction of the LED’s lumen output over time (lumen depreciation). Common references are L80 or L90 and TM‑21 projections; the lighting designer converts these into the LLMF (for example 0.80).
LMF covers the loss due to contamination and ageing of the luminaire’s optical system (covers, reflectors, drivers). Important inputs are ingress protection (IP), the environmental contamination class and planned cleaning intervals. LMF values come from practical tables or manufacturer information.
RSMF represents the reduction of reflectance from ceilings, walls and floors over time. Industrial or tunnel environments typically show larger declines than office spaces. The lighting designer uses tables or measurements to select RSMF values.
Overall formula: MF = LSF × LLMF × LMF × RSMF
All four components should come from manufacturer data sheets, reliability reports or accepted practical tables, and are multiplied to produce the project‑specific MF.
Both mistakes reduce cost‑effectiveness – just in opposite directions.
The numeric determination of MF parameters is the responsibility of a qualified lighting designer. As a client you can and should specify maintenance and cleaning requirements, but the designer must:
Request the four numeric factors from your designer. Only then can you compare bids on an equal basis.
Partly. You can set maintenance and cleaning requirements, but the numeric MF values must be derived by the lighting designer.
Primarily from manufacturer data sheets (LSF, LLMF) and from practical tables or measurements (LMF, RSMF). A competent designer combines these sources and checks plausibility.
No. This guidance is aimed at commercial and industrial projects. For private homes, a simplified approach is usually sufficient.
Sensora Smart AG supports MF calculations, scenario simulations and lifecycle cost (LCC) analyses. Schedule a meeting for a free initial consultation. Project references and resources: sensorasmart.com
About the author: Dan Flühmann is a lighting consultant at Sensora Smart AG, advising Swiss clients on lighting design, MF calculations and lifecycle costs.
Last updated: 27 May 2026