Full-size fluorescent systems are among the most common and most efficient lamps in use. They are appropriate for general lighting in commercial, institutional, and industrial spaces with low to medium ceiling height. The introduction to the marketplace of high-intensity fluorescent lamps and fixtures also makes fluorescent systems a leading choice for areas with high ceilings (more than 15 feet)—the type of application that used to be the domain of high-intensity discharge (HID) light sources. (See the Purchasing Advisor topic Indirect Lighting.) How much energy a fluorescent lighting system uses depends on the efficiency of the lamps, ballasts, fixtures, and controls. To apply fluorescent lamps successfully, carefully consider lamp options—diameter, length, and phosphor blend—as well as the options for ballasts and fixtures. (See the Purchasing Advisor topics Fluorescent Ballasts and Lighting: HID Versus Fluorescent for High-Bay Lighting.)
There are several general characteristics as well as specific lamp types that differentiate fluorescent lamps.
All full-size fluorescent lamps differ by several characteristics, including lamp size, the phosphors used, and the starting method.
Lamp size. Fluorescent lamps range from 0.250 to 2.125 inches in diameter (Figure 1)—specified by "T" and the size in eighths of an inch: for example, a T12 lamp is 12/8 inch (1.5 inches) in diameter—and from 6 to 96 inches in length. Four-foot lamps are the most common length and thus the cheapest and easiest to buy and stock. Eight-foot lamps are slightly more efficient, but they break more easily and can be difficult to transport.
Phosphor type. Phosphors are the substances that coat the inside of fluorescent tubes and transform the ultraviolet light that is generated by an electric arc into visible light. The phosphor blend determines the color temperature and color rendering of the light emitted by the lamp. Halophosphors are the least expensive and lowest-quality phosphors. They are used in standard "cool white" and "warm white" commodity-grade T12 lamps. Rare-earth phosphors are more expensive, but they produce a higher-quality light and enable fluorescent lamps to maintain their light output over a longer period of time. Standard T8s use a blend of these two types, and high-performance T8s primarily or exclusively use rare-earth phosphors.
Starting method. Linear fluorescent lamps can be divided into "families" based on the three basic ways in which they are started: preheat start, instant start, and rapid start (Figure 2). Preheat start is sometimes also called "switch start," and there are a number of variations on rapid-start technology. The design of the ballast determines the starting mode used for any fluorescent lamp, but the lamp must be compatible with the ballast's starting mode to ensure proper operation.
Standard T8 fluorescent lamps offer better efficiency, lumen maintenance, color quality, fixture optics, and life-cycle costs than antiquated T12 systems. However, several other options now offer even better performance for most applications.
High-performance T8s. Fluorescent lighting technology has achieved new levels of efficiency, color quality, and longevity in a class of products called "high-performance T8s" (sometimes called "super T8s"). Most of these products carry a price premium, but they're typically more cost-effective replacements for T12s than standard T8s (Figure 3). In many cases, high-performance T8s can also cost-effectively replace standard T8s, potentially making the tens of millions of square feet of commercial space that use electronically ballasted T8 lighting systems ripe for another round of efficiency upgrades. Today's high-performance T8 lamp-and-ballast combinations can improve system performance by 70 to 81 percent over a T12 "energy-saver" lamp and magnetic ballast combination, and by 23 to 31 percent over their most common modern predecessor—the standard 700-series, rare-earth-phosphor T8 lamp and standard instant-start electronic ballast combination.
The Consortium for Energy Efficiency (CEE) has set specifications for high-performance T8 lamps to provide a voluntary national standard for lamp-and-ballast systems that energy service providers can use in their programs. The CEE specifications for a 4-foot, high-performance T8 lamp with a nominal wattage of 32 watts or less include the following key criteria:
Reduced-wattage and other T8s. Several other types of lamps have been evolving in parallel with high-performance T8s. One of these, reduced-wattage T8 lamps for which the CEE has developed a reduced-wattage specification, include the 28- and 30-watt reduced-light-output or energy-saver T8s—some of which operate almost as efficiently as the 32-watt high-performance T8. In some retrofit applications, they provide an easy way to harvest energy and demand savings because they don't require delamping or dealing with the expense of replacing ballasts or luminaires (Table 1). However, there are several limitations with their use (see "How to Make the Best Choice").
In addition to the reduced-wattage products, there are a number of "premium" 32-watt T8 products that offer extended lamp life or higher light output compared with standard 700- and 800-series CRI lamps. But none of these perform as well as high-performance T8s.
T5 lamps. T5 fluorescent lamps are only available in metric lengths and are therefore not a good retrofit option, but they can be an effective choice in new construction or major renovations. Their efficacy is similar to that of T8 lamps, but their smaller size affords better optical control. The T5 lamp is currently designed for operation only on high-frequency, rapid-start, or programmed rapid-start electronic ballasts. T5 lamps also offer high lumen maintenance, putting out as much as 97 percent of their original light output at 40 percent of rated life. And T5 lamps are designed for a high optimal operating temperature, which improves performance in enclosed fixtures and warm spaces.
High-output and very-high-output lamps. These lamps are available in several different diameters. They offer very high luminous intensities, which makes them good for outdoor signage applications, particularly those that are backlit through colored materials or that operate during the day, such as convenience store signs, billboards, and roadway signs. However, both types require special ballasts and are generally less efficient, more expensive, and not available in as many color temperatures as standard-output lamps.
Buyers and specifiers face challenges in sorting through the confusing array of lamps and ballasts entering the market. Lighting manufacturers are competing fiercely for their share of the new, replacement, and retrofit markets by introducing a stream of innovative high-performance and not-so-high-performance T8 products that promise energy reductions, lower maintenance costs, and greater versatility. Diligent scrutiny of manufacturer product literature, performance specifications, and the fine print is necessary to sort out and benchmark the relative performance of these competing products.
When comparing high-performance T8s among themselves or to similar lamps and ballasts that are not necessarily in the high-performance category, be sure to compare the various options on an equal footing. For lamps, compare:
When comparing performance among competing lamp-and-ballast systems, always reference the individual lamp and ballast criteria, and then perform a few key assessments:
When evaluating lamp options, also evaluate actual lighting needs to maximize savings—many spaces are overlit to some degree:
Keep in mind that T8 lamps labeled "energy-saver" and "reduced-wattage" can provide energy savings, but they suffer from some operational restrictions. Depending on the product, they aren't recommended where ambient temperatures fall below 60° Fahrenheit; in drafty locations; or for use with low-BF ballasts, rapid-start ballasts, dimming ballasts, or occupancy sensors. Some even require a brand-specific ballast to operate properly.
Another limitation is that people typically won't want to maintain two different types of lamp systems—one for unconditioned spaces or spaces where dimming might be used, and reduced-wattage systems for other uses. That's because it can be hard for maintenance crews to keep track of the different lamp types. As one lighting retrofit expert points out, it's difficult enough to keep track of where to install T8 and T12 lamps, which have different diameters, and even harder for workers to keep straight the multiple types of T8s, which look very much the same.
Fluorescent lamps are a mature technology, but manufacturers continue to make incremental improvements in efficiency and lamp life. The latest development is the use of mercury amalgams in full-size fluorescent lamps to reduce the sensitivity of lamp output to temperature change. In another trend, manufacturers are introducing lamps that carry a small penalty in efficacy to provide a larger increase in lamp life.