Lighting design: maintained illuminance and maintenance factors
For interiors and certain outdoor lighting applications, standards stipulate illuminance values that need to be maintained. These maintained values and the maintenance factors on which they are based must be taken into account in any lighting design.
Getting the lighting level right is crucially important for visual performance. It depends on the illuminance – in road lighting the luminance L – and reflectance of illuminated surfaces.
The general rule is: the lower the reflectance and the more difficult the visual task, the higher the illuminance needs to be.
Maintained illuminance
Maintained illuminance is the value below which the average illuminance is not allowed to fall. When a lighting installation is planned, account needs to be taken of the fact that luminaires, lamps and rooms age and become soiled in the course of time. As a result, illuminance decreases. To compensate for the loss, every new installation needs to be designed for higher illuminances (= illuminance on installation).
The lighting designer takes account of the decrease by applying a maintenance factor:
maintained illuminance = maintenance factor x illuminance on installation.
Examples | |
Circulation areas | 100 lx |
Office | 500 lx |
Operation field | bis 100.000 lx |
Maintenance factor
The maintenance factor depends on the type of lamps and luminaires installed, on the dustiness/dirtiness of the room or surroundings as well as on the method of maintenance employed and the maintenance interval. In many cases, not enough is known at the design stage about the operating conditions likely to impact on a lighting installation, so assuming a three-year maintenance interval, the following maintenance factors are generally applied:
- in clean rooms: maintenance factor 0.67
- in dirty rooms: maintenance factor 0.5
The calculation plane is the surface on which the illuminance should be realised. The recommended plane for office workplaces is 0.75 metres above floor level and for circulation areas no more than 0.1 metres.
Maintained illuminance required by standards
The maintained illuminance required for various rooms, tasks or activities in indoor work premises is set out in DIN EN 12464-1 and the requirements for outdoor work premises in DIN EN 12464-2.
