Light losses during the use of a lighting installation, caused by the decrease of the luminous flux of lamps, the dirt accumulation on and loss of reflectivity and or transparency of luminaires, and dirt accumulation on or discoloration of room surfaces (in interiors).
Lamp Lumen Depreciation
The light output of virtually all lamp types declines gradually during operation. The causes of light output depreciation are numerous. With incandescent lamps, it is especially the blackening of the bulb – caused by evaporation of the filament. Discharge lamps also suffer from blackening, in this case due to scattering of the electrode material, which settles on the wall of the discharge tube. With fluorescent lamps, high-pressure mercury lamps, and solid state, LED, and OLED light sources with a fluorescent coating, the major cause of light output depreciation is a gradual exhaustion of the fluorescent powder, which slowly loses its effectiveness.
Because of the different causes of lamp lumen depreciation, the actual rate of decline of lumen output is different per lamp type. Lamp manufacturers should be able to supply lamp lumen depreciation curves for their lamps.
The gradual reduction of output of a luminaire is caused by dirt that is gradually deposited on lamps and on or in the luminaires and by a gradual loss of reflectivity of reflectors and mirrors or loss of transparency of refractors due to corrosion and discoloration. Where the effects of dirt accumulation can be offset by regular cleaning, the output loss due to corrosion and discoloration cannot be regained. The rate of output reduction depends on the materials used in the luminaire, the construction of the luminaire, and on the nature of airborne dirt in the atmosphere. The degree of protection provided by the construction of the luminaire is classified according to the international protection code, IP, as described in an international IEC standard.
Room Surface Depreciation
Gradual dirt collection on room surfaces and/or discoloration of these surfaces gradually reduces the interreflected component of light from the installation. This may especially have a noticeable effect in interiors of smaller dimensions where, because of the relative large wall area, interreflection components are relatively large.
The maintenance factor is the ratio of illuminance or luminance produced by the lighting installation at the moment that maintenance operations are being carried out to the illuminance or luminance produced by that installation when new. The maintenance factor takes into account all light losses described above together with the average light loss of the installation due to failed lamps not being replaced. The actual value is of course also dependent on length of the period between maintenance operations. The term light loss factor has sometimes been used instead of maintenance factor. Depreciation factor has been formerly used to designate the reciprocal of the above ratio.