Lighting: Light Output Declines with Time

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Light output from a given fixture declines over time for two reasons. One is that the lamp output itself decreases—an effect known as lumen depreciation. The other is that dirt accumulates on lamps and fixtures. Lumen depreciation data vary 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 frequency of cleaning (Figure 2) and the cleanliness of the environment.

Figure 1: Lamp lumen depreciation

Figure 1: Lamp lumen depreciation

The light output of lamps declines over time.

Source: E Source; data from National Lighting Bureau

Figure 2: Dirt depreciation for a typical fluorescent fixture in a very clean environment

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: E Source

To quantify the effects of dirt accumulation, lighting designers use luminaire dirt depreciation (LDD) factors. The LDD factor curves that were used for many years were based on studies done in the 1950s, when open-window ventilation and smoking were the norm. From 1996 to 1999, the International Association of Lighting Management Companies (NALMCO) conducted a study, with funding from the U.S. Environmental Protection Agency, to update LDD calculations for modern building environments. The results of these studies and new models for LDD were published by the Illuminating Engineering Society of North America (IESNA) as IESNA/NALMCO RP-36-03 in late 2003. Test results show that dirt depreciation is less severe than the old calculations indicated. So, for example, in very clean locations, 8 to 10 percent fewer fixtures are required to provide a specific light level when design calculations are based on new models instead of earlier LDD values.

RP-36-03 describes a process for calculating LDD factors, which depend on the type of fixture in use, how clean the environment is, and the time that has elapsed since the last cleaning. Representative numbers derived from this process are shown in Table 1.

Table 1: Representative luminaire dirt depreciation factors

Table 1: Representative luminaire dirt depreciation factors

To estimate the effect of dirt on the lumen output of a fixture after a certain period of time, multiply the initial lumen output from the fixture by the relative light output, as determined from Figure 1 or the manufacturer's data for a particular lamp, and then multiply by the luminaire dirt depreciation (LDD) factor.

Source: E Source; data from RP-36-03

Copyright 2006 - Platts, a Division of The McGraw-Hill Companies, Inc.