Metal Halide Lumen Maintenance
Over time, the gradual reduction in the quality of light emitted from any High Intensity Discharge lighting system occurs for a number of reasons. Called Light Loss Factors (or LLF), these include lamp lumen depreciation, luminaire dirt depreciation, deterioration of luminaire surfaces, dirt on room surfaces, performance of auxiliary equipment and supply voltage. Ambient temperature generally has no effect on the light output of HPS lamps.
Lumen maintenance curves reflect the average lumen maintenance for all lamp types in a given category when operated at rated wattage. Mean lumens for HID lamps are the average light output throughout the rated life of a lamp. For Metal Halide lamps, mean light output occurs at 40% of Rated Average Life. Information on specific lamp types can be obtained from manufacturers’ literature.
Lumen maintenance of individual lamps depends on many factors, among which are the number of starts, wattage at which the lamp is operated and current wave shape. Fewer lamp starts (for example, when there is a longer operating cycle) will lead to less depreciation in the light output of Metal Halide lamps. The operation of lamps above rated wattage will have an adverse effect on lumen maintenance while slight underwattage operation will have little, if any, effect.
Of all HID lamps, the Metal Halide types tend to experience greater lumen depreciation than either High Pressure Sodium or Mercury Vapor lamps. Frequent starting affects Metal Halide lamps more severely than it affects other HID types. In general, the clear lamp types have somewhat better lumen maintenance than those with a phosphor coating.