Light output from a given fixture declines over time for two reasons. One is because the lamp output itself decreases—an effect known as lumen depreciation. The other is because of the accumulation of dirt on lamps and fixtures. Lumen depreciation data varies with the type of lamp, as shown in Figure 1. Data for a particular lamp can be found in technical information available from the manufacturer. The depreciation in light output from dirt accumulation depends on both the frequencey of cleaning (Figure 2) and the cleanliness of the environment (Figure 3).

Figure 1: Lamp lumen depreciation The light output of lamps declines over time. Source: National Lighting Bureau

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Figure 2: Dirt depreciation for a typical fluorescent fixture in a very clean environment Even if fixtures are cleaned every 18 months, dirt buildup can reduce light output by 10 percent between cleanings. Source: Platts

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Figure 3: Fixture dirt depreciation for enclosed fixtures in environments of varying cleanliness Even in a very clean environment, a fixture loses about 15 percent of its light output after three years, due to dirt buildup. Source: IES Lighting Handbook

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