Generally, the maintenance factor can be determined using the following formula with the respective data of lamp and luminaire manufacturers or data from reference curves.
Maintenance Factor due to reference values of light sources, luminaires and the surrounding room
MF = LLMF · LSF · LMF · RMF
This procedure is described in detail in the publication CIE 97.2:2005 titled "Guide on the maintenance of indoor electric lighting systems". According to EN 12665, the terms used here are defined as follows:
The maintenance factor MF is the ratio of the average illuminance on the working plane produced by the lighting installation after a certain period to the average illuminance produced by a new installation under the same conditions.
The lamp lumen maintenance factor LLMF is the ratio of luminous flux of a lamp at a given time in the life to the initial luminous flux.
The lamp survival factor LSF is the portion out of the total of lamps which remains ready-to-operate at a given time under specified conditions with a defined switching frequency. In LED luminaires, it is mostly determined by total failure of the LED luminaire (see chapter ).
Luminaire maintenance factor LMF is the ratio of the light output ratio of a luminaire at a given time to the initial light output ratio (luminaire light output ratio). For LED luminaires, the initial luminaire light output ratio is LOR = 1.
The room surface maintenance factor RSMF is the ratio of the light reflected by the surfaces of a room after a certain period of use of the lighting installation to light reflected when the installation is considered conventionally as new. The room surface maintenance factor according to EN 12665 describes merely the change in reflectance value of the room surfaces. It causes no change (decrease) of illuminance on the working plane proportional to it, since the direct portion of the light as well as multiple reflections occur.
The room maintenance factor RMF captures the change in utilance U during the period of operation. Utilance therein describes the dependence of the illuminance on the working plane due to the deterioration in surface reflectance with consideration of the direct portion of light and the occurring multiple reflections.
