To ensure that the required illuminance is provided over a period of time, the lighting design includes a maintenance factor MF that takes into account the reduction of luminous flux. The new value for the illuminance of an installation is calculated from the maintained value of illuminance, and the maintenance factor. The maintenance plan specifies the cleaning frequency of the luminaires and the room and the lamp replacement. The maintained value of illuminance thus depends on the luminaires, the lamps and the room conditions.