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By John L. Fetters
September 2004 -
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The newest generation of lamps can reduce long-term operating costs of fluorescent lighting systems in institutional and commercial facilities. Making investments in these new lamps can improve the lighting and reduce both energy and maintenance cost.
Managers have so many lamp choices, however, that making the right choice can be confusing and can inhibit making any choice. Understanding where lamp technology has come from and where it is headed can help managers make the right decision.
Lamp manufacturers have changed the gas composition of T8 lamps to reduce the lamp power 2-4 watts (w) without affecting lamp life or color rendering index (CRI) characteristics and without substantially reducing the light output of standard T8 lamps. The lamp also has good lumen maintenance at 94 percent, only losing 6 percent of its light output in the first 40 percent of life, and it complies with the toxicity characteristic leaching procedure (TCLP).
But as with reduced-wattage T12 lamps, T8 lamps are more temperature sensitive than standard 32-w T8 lamps and do not function well below 60 degrees. They can operate on popular instant-start electronic ballasts or the new program-start ballasts but cannot be used on rapid-start or dimming ballasts.
These reduced-wattage, 4-foot T8 lamps are available in 28- and 30-w models and are designed to replace 34-w, reduced-wattage T12 lamps in fluorescent fixture retrofits or existing fixtures with standard 32-w T8 lamps. When the 28-w lamp is operated with a reduced-power ballast — one with a lower ballast factor (BF) — power use is 24 percent lower than a standard 32-w T8 reduced-power instant-start system with only a 6 percent reduction in lumens.
For this reason, reduced-wattage T8 lamps are a good solution for applications where a slight reduction in light levels is acceptable. Until production increases substantially, there will be a premium cost for the reduced-wattage lamps on the order of $0.50 per lamp, but this can be recovered easily from the energy savings, especially in areas where electricity costs are high.
Sometimes called super T8, these systems pair a high-lumen, extended-life T8 lamp with a reduced-power electronic ballast — one with a lower BF — to achieve a 15-20 percent lower input wattage than that of a standard T8 system. Managers can use existing reduced-power, instant-start electronic ballasts, or the lamps can be powered by new, program-start electronic ballast to obtain even longer life and cut maintenance costs.
Compared to existing systems that use 34-w T12 lamps and energy-efficient magnetic ballasts driving 34-w fluorescent lamps, super T8 systems can produce energy savings up to 40 percent. Converting existing lighting systems to high-performance lamps requires a ballast change, and this added investment of about $5 will take slightly longer to pay back than using the reduced-wattage lamps on existing ballasts. High-performance T8 lamps also cost about twice as much as standard T8 lamps.
The best way to identify a high-performance system is to look for lamps with an initial lumen rating of at least 3,100. Standard, 32-w T8 lamps are rated at 2,850-2,900 lumens. High-performance lamps have a barrier-coat design and a high lumen maintenance rating, in the mid-90s. The average rated lamp life is 24,000 hours at three hours per start vs. the 20,000-hour rating of standard T8 and 34-w T12 lamps.
Super T8 lamps by themselves do not save energy unless they are used with a low-BF, low-power electronic ballast. This combination provides the correct light level and reduces energy costs by using 15-20 percent less energy than standard T8 systems.
For example, a two-lamp fixture with super T8 lamps rated for 3,200 lumens each and operated on a reduced-power ballast with a BF of 0.78 will produce 4,992 lumens. Compare this with a fixture equipped with standard T8 lamps rated for 2,850 lumens and operated on a ballast with a BF of 0.88 that produces 5,016 lumens.
Both systems produce essentially the same output, but the high-performance system's per-fixture load is only 48 watts, compared to fixtures with standard T8 lamps that draw 56 watts — a difference of 15 percent. The power load of three-lamp fixtures with standard T8 lamps and electronic ballasts with BF of 0.88 is 87 watts, compared with three-lamp fixtures using super T8 lamps and low-BF — 0.78 — electronic ballasts that draw only 73 watts, a 16 percent savings.
These lamps are becoming popular in both retrofits and new construction, with a number of high-intensity-fluorescent (HIF) fixture manufacturers offering this as an alternative to T5 HO fixtures. When operated on dedicated program-start electronic ballasts, these lamps are rated with longer lamp life than standard systems — the average rated life is 30,000 hours versus 20,000 hours for rapid-start systems or 15,000 hours for instant-start systems — reducing maintenance re-lamping cost.
Not to be confused with older, miniature preheat fluorescent lamps of the same diameter nor with the line of long compact fluorescent lamps of the same diameter, these recently developed linear lamps come in two distinct and different families: standard, or high-efficiency; and high output (HO).
Lamps in the new T5 family are 5/8-inch in diameter and have miniature bi-pin bases. Physically, T5 lamps are intentionally not as long as T8 or T12 lamps because they are not intended for retrofit applications that use T8 lamps. Some people mistakenly think that the T5 must be the next in the progression after T12 and T8. They are about 21/2-3 inches shorter than standard 2-, 3-, 4-, and 5-foot T8 and T12 lamps, and they have different electrical characteristics, shown in the Table 1.
The main characteristic that distinguishes T5 lamps from other families of fluorescent lamps is that they are designed to peak in their lumen output at 95 degrees, compared to 77 degrees for T12 and T8 lamps. See Table 2.
This thermal characteristic provides higher light output in confined applications with little or no air circulation, and it provides more usable lumens per watt in indirect fixtures. But the high bulb-wall brightness creates discomfort glare and prevents their use in direct, low-ceiling applications.
T5 lamps are available in the three standard fluorescent color temperatures — cool at 4,100K; warm at 3,000K; and neutral at 3,500K. They also have a CRI greater than 80 and are rated for 20,000 hours at three hours per start.
Although the T5 HO lamp has a lower efficacy in lumens per watt than the standard T5, it has become more popular and is finding more applications than the standard T5. With a constant lumen output per millimeter of length, the standard T5 lamps are most suitable for coves and cornice applications — another confined application where the lamp's unique thermal characteristic has a distinct advantage.
Evidently, architects are not aware of these advantages for new construction, and building managers do not realize that new T5 strip fixtures can be used to relight coves.
Applications for T5 HO lamps include new slim-profile indirect fixtures where only 1 lamp per 4-foot section is required, replacing designs using two 4-foot T8 lamps per 4-foot section. Following the trend toward fluorescent, T5 HO lamps are also being used as HIF systems in high-ceiling applications, including gymnasiums and high-bay industrial fixtures. These applications are designed to take advantage of the smaller-diameter lamp. T5 lamps are appropriate for many other applications that require a high lumen-maintenance rating of 95 percent.
Managers should select amps to perform effectively in specific facility applications, with particular attention paid to lumen output, efficacy and life ratings. Using higher-efficacy fluorescent lamps will reduce energy costs, and longer life lamps will reduce labor cost. Sensible managers will make investments in new lamps to keep their lighting systems performing efficiently and effectively.
End users have discovered some disadvantages of T5 high-output (HO) lamps, and the use of these lamps has become controversial. This discussion includes the fact that T5 HO lamps take about two minutes to come up to full brightness after being switched on, which can be a factor when using aisle occupancy controls.
Other issues include the newness of the T5 HO technology compared with the more proven T8 technology. T5 HO lamps also are more expensive than T8 lamps and require more expensive program-start ballasts, and these lamps are not being manufactured in the United States.
For those managers considering HIF fixtures, there is an abundance of new designs, some which might have been rushed to market before complete testing under adverse conditions. T5 HO fixtures are narrower and tend to concentrate the heat of the lamps and ballasts. This heat has a history of causing T5 HO ballasts to fail prematurely.
Several ballast manufacturers have introduced 194-degree T5 HO ballasts, which can help reduce this problem, but manufacturers of these ballasts only provide a three-year warranty if the case temperature is 158-194 degrees.
— John L. Fetters
John L. Fetters is a certified lighting efficiency professional and president of Effective Lighting Solutions in Columbus, Ohio.