Lighting systems are designed to provide specific light levels on various surfaces within a room or facility. Over time the efficiency of these systems can be seriously reduced by lamp depreciation and by the accumulation of dirt and dust on the reflecting and transmitting surfaces of the lighting equipment. To compensate for the gradual depreciation of any lighting system, a lighting designer should understand certain technical factors associated with depreciation and recommend specific maintenance procedures to lessen the severity of this problem.
The first step in addressing this issue in your design is to understand the amount and kind of dirt that is usually found in the type of space you're working in. The Handbook of Illuminating Engineering Society of North America (IES Handbook) includes a table that lists five degrees, or classifications, of dirt conditions that are applicable for spaces that range from offices to a rubber processing plant (Table here).
Once you've properly identified the space, the next step is to use the lumen method to calculate the average uniform horizontal illumination for the luminaire system to be installed. This method uses the definition of a footcandle, which is the illuminance on a surface of one square foot in area, with a uniform distributed flux of one lumen, to derive the following equation:
where, E is illuminance in footcandles, Lu is lumens, and A is the area in square feet. For the purposes of this calculation, the surface is a horizontal plane 2.5 feet above the floor, which is the standard height of a desktop.
Since the illuminance level applies to the workplane and covers the whole room, E is the total luminous flux that falls on the workplane, divided by the area of the room (E=Luf÷A).
While this simple formula calculates the initial lumens produced by the lamps within the fixtures, as soon as a new lighting system is energized, the light level starts to gradually decrease. For that reason, it's necessary to initially provide an illuminance level above the minimum specified level to compensate for the light losses and to ensure that a minimum level will be kept over a specific time period.
This depreciation, or slow reduction in light output, is defined as the light loss factor (LLF), which is the ratio of the illuminance when it reaches its lowest level, just before some corrective action is taken, when compared to the initial light level. Thus, LLF and some other factors are added to the basic formula to provide greater accuracy in your initial design, and in planning an appropriate lighting maintenance schedule.
The elements that contribute to LLF are divided into two categories, unrecoverable and recoverable. The unrecoverable factors refer to equipment and site conditions that can't be changed, such as the ballast factor and system voltage. The three recoverable factors are as follows:
Room surface dirt depreciation. This value accounts for dirt or dust that accumulates on all of the room surfaces — especially on the upper walls and ceiling. The IES Handbook includes a reference table that lists various room surface dirt depreciation (RSDD) factors for direct, semi-direct, direct-indirect, semi-indirect, and indirect luminaire types. Understandably, periodically cleaning or repainting these room surfaces will lessen the overall impact of RSDD.
Lamp lumen depreciation. As a lamp ages, the amount of light it produces declines on a continuing basis. The lamp lumen depreciation (LLD) factor is the fraction of initial lumens produced at a specific time during the life of the lamp. For the LLD factor to be accurate in setting up a maintenance schedule, you should know the expected time of relamping and use it in your calculations. Often, especially in indoor office areas, cleaning of the fluorescent luminaires is done in combination with relamping.
Luminaire dirt depreciation. This value accounts for contaminant buildup (i.e. dust, smoke film, oil, and dirt) on the surfaces of fixtures, lenses, and lamps, which reduces the overall quantity of light produced by the fixture. Even though fixture manufacturers provide luminaire dirt depreciation (LDD) figures for their products, LDD is a difficult factor to determine. This is where the dirt conditions table in the IES Handbook is most useful. However, it's important to note that these figures always assume the use of a regular maintenance program, which includes regularly scheduled cleaning and relamping practices. See the Figure in the Sidebar below.
Nevertheless, you should always estimate a figure for LDD in order to calculate a maintained light level over a certain time period. You can use either the mean LDD value, in which case the design level will be the average over the relamping period, or the end-of-life relamping value, in which case the initial design level is reached only when the system is cleaned and relamped. Generally, the mean value is used in an indoor lighting design.
Practical ideas. It's important that you realize that an estimate of the effect of dirt depreciation is important even for a relatively clean area, such as a modern office. One reason is that the new high-performance T8 fluorescent lamps can provide an average rated life of up to 30,000 hours. With this extended life, it's easy for building owners and maintenance staffs to delay or even forget about fixture cleaning schedules in the office environment. But dirt or oily film accumulation on luminaire surfaces can still cause a significant reduction in useful lumen output in these relatively clean settings.
One way to combat this problem in the design stage of the project is to study and compare various fixture models, since the luminaire design, the lamp type and size, and the luminaire reflector finish all determine how much dirt will adhere to the luminaire over time. This is especially important in indoor industrial settings.
For example, industrial high intensity discharge (HID) luminaires can be divided into four types:
- Open-bottom reflector unit. With no lens, these units can accumulate a great deal of dirt over short periods of time.
- Open-bottom reflector with vents on top of reflector. These units permit an upward flow of air, called a convection current, through the luminaire, which reduces the accumulation of dirt on surfaces to a certain degree.
- Open-bottom reflector with a lens or diffuser. These fixtures allow some airborne dirt to enter the reflector compartment, since a tight seal isn't provided.
- Enclosed and gasketed luminaire These units feature a silicone rubber gasket at the lens perimeter and strong latches to hold the lens in place. This tight seal is designed to block entry of airborne dirt into the optical assembly. However, dirt will still accumulate on the bottom of the lens, diffuser, or refractor of a sealed luminaire.
Outdoor HID lighting systems have their own considerations. For example, enclosed and gasketed floodlighting luminaries use precisely located specular reflectors and cover lenses to attain their desired beam patterns. Any dirt accumulation on the reflector and cover glass of a narrow beam floodlight will tend to widen the beam spread, thus reducing the maximum candlepower. Therefore, in this case, the depreciation in footcandle intensity of the main beam of a dirty luminaire is more important than the depreciation in total light output.
Under comparable conditions, an enclosed floodlight has a higher maintained efficiency that an open unit because neither the reflector nor the lamp receive as much dirt accumulation as an open fixture. For these types of fixtures, a maintenance factor from 0.65 to 0.95 is usually appropriate. However, if the floodlights are cleaned infrequently or the lamps replaced only on burnout, the use of a lower maintenance factor is advised.
Expanding the formula. Based on the discussion above it's possible to derive a more detailed, and therefore more accurate, lumen method equation as follows:
where E is average illuminance in footcandles, N is number of luminaires used in the space, L is total lamp lumens produced from the lamps in the luminaire, CU is coefficient of utilization, LLF is light loss factor (described above), and A is area of room in square feet.
The coefficient of utilization CU, mentioned above, is derived from a number of factors, including the following:
- Type of luminaire. The level of efficiency and distribution pattern are important factors.
- Reflectance of the room surfaces. The higher the reflectance factor of the ceilings, walls and floors, the greater the percentage of the lamp lumens that will reach the workplane. Cleaner room surfaces offer higher reflectances.
- Mounting height of the luminaires. The greater the height, the larger the corresponding area of the wall surface, which in turn absorbs light from the luminaires.
- Area of the room. The larger the room, the greater the number of luminaires needed. However, the light output from each luminaire overlaps the output of adjacent luminaires, thus raising the total light level. In addition, there is less wall surface per unit of area to absorb the light.
- Proportions of the room. The dimensions of a room directly affect the CU.
Accurately accounting for dirt depreciation in new lighting designs offers the opportunity to reduce the number of fixtures required to achieve the target maintained light level, thus reducing initial and operating costs of a fixture for the owner. In retrofit situations, this offers the opportunity to select components that produce less light output while saving more energy.
Sidebar: Study Reveals New Energy-Saving Opportunities
In 1999, The interNational Association of Lighting Management Companies (NALMCO) completed a three-year, EPA-funded study of luminaire (lighting fixture) dirt depreciation. Analysis of the results indicates that existing light loss factors, related to dirt and dust buildup on fixture surfaces, used by the lighting community for more than 50 years overestimate the extent of light loss, particularly in cleaner spaces such as offices, schools and retail spaces.
The study indicates that in a typical commercial indoor environment, the extent of dirt depreciation on light fixtures isn't as high as the accepted norms used in the past. In the time frame of a two- to three-year cleaning cycle, for example, the new findings recommend allowing for about a 10% loss for many areas, as opposed to a 20% loss cited in present lighting standards.
The controlled LDD field study included more than 200 sites at office, retail, and school facilities in the United States, and looked at four popular recessed fluorescent lighting fixture types: 2×4 lensed, 2×4 louvered, 2×2 louvered, and 2×4 air exhaust louvered. Based on some estimates, these fixtures collectively represented about 90% percent of recessed fixtures in operation in the United States in 1995. The split between lensed and louvered fixtures in the study was about 50-50.
The Figure above contrasts the new LDD function with lensed and louvered fixtures in clean conditions. It assumes better-than-average air filtration and some generated or ambient dirt. At 18 months, the LDD factor is 0.92 versus 0.84-0.85 using the traditional IESNA procedure. And at 36 months, the LDD factor is 0.89 versus 0.75-0.80. Lensed and louvered fixtures show virtually identical dirt depreciation, and variable operating hours per year have negligible effect, according to the study.
This LDD data can significantly affect lighting design of commercial facilities where fluorescent, flat-bottomed, and recessed or ceiling-mounted fixtures are installed.
“Test results indicate that in very clean locations, about 8% to 10% fewer fixtures are required to provide a specific light level, compared to using design calculations with earlier LDD values,” says Norma Frank, CLMC, chair of the IESNA Maintenance Committee and vice president of Colorado Lighting. “Renovation projects in older facilities would result in the order of 15-20% fewer fixtures if this new data is utilized.”
The LDD study results have been incorporated into a new IESNA Recommended Practice on Maintenance (RP-36) and will be published in the IES Handbook.
Craig DiLouie, Lighting Controls Association
Sidebar: A New Way of Identifying LDD
Although the procedure outlined in the main text of this article is accurate and accepted as an established practice in the IES Handbook, a newer, simpler procedure for determining Luminaire Dirt Depreciation (LDD) can be found in IESNA/NALMCO RP-36-03, Recommended Practice for Planned Indoor Lighting Maintenance.
The LDD factor for indoor environments in this document is determined through a five-step procedure that first characterizes the operating environment in one of three categories (clean, moderate, and dirty). You then place the luminaire into one of two groups (open/unventilated or all other). The next step is to determine the luminaire's CIE classification (direct, semi-direct, general diffuse, semi-indirect, or indirect). Then determine a letter assignment according to the luminaire's working environment and CIE classification. The last step is to select the appropriate LDD factor from a set of curves published in this document.
This new procedure will most likely be included in the next revision of the IES Handbook.