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.

General Lamp Characteristics

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.

• Preheat-start lamps. Preheat-start lamps are, in general, relatively short (6 to 36 inches) and typically use low-cost, low-performance phosphors. However, new versions with good color rendering are available. Preheat starting degrades lamp electrodes more rapidly than other starting methods, so preheat-start lamps have relatively short lifetimes. They are typically used only with magnetic or resistive ballasts. Users seeking to maximize energy efficiency should avoid preheat-start lamps when possible.
• Instant-start lamps.Instant-start lamps operate with the most efficient type of ballast, but the ballast yields the shortest lamp life in most applications. Instant-start lamps should be used with caution in spaces controlled by occupancy sensors. If the average operating time per start will be significantly less than three hours, lamp life will be short and some type of rapid-start lamp would be a better choice. Eight-foot instant-start lamps are widely used by supermarkets and mass-merchandisers.
• Rapid-start lamps.The newest version of a rapid-start ballast is the programmed-start ballast, also known as programmed rapid-start ballast. In almost all cases, these ballasts maximize lamp life but carry some penalty in efficiency. They are the best choice in applications where lights will frequently be turned on and off.

Types of Fluorescent Lamps

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:

• Produces 3,100 or more initial lumens and maintains at least 2,900 mean lumens
• Achieves 94 percent lumen maintenance
• Provides a color rendering index (CRI, or how well a light source renders colors) of 81 or higher
• Achieves a rated life of 24,000 hours or more at 3 hours per start on a rapid-start or programmed rapid-start ballast

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:

• Mean lamp lumens. Fluorescent lamp light output falls over time, which makes comparisons based on initial lumens misleading. Comparing by the "mean lamp lumens" that are listed in lamp catalogs is a better alternative. This value is the lumen output of a lamp after it has operated for 40 percent of its rated life.
• Color rendering index. Measured on a scale of 0 to 100, CRI describes the ability of a light source to render a sample of eight standard colors relative to a reference source. A CRI of 100 means that the source renders the eight standard colors in exactly the same way that the reference light source renders them. CRI is an average value, so it will not describe how a light source renders a specific color. Generally speaking, however, high-CRI light sources render colors better than low-CRI sources. Most T8 products have CRIs in the 70s or 80s; T5 lamps offer CRIs in the 80s. A CRI of 80 or greater is considered by the industry to provide excellent color rendering.
• Color temperature. Measured in kelvins (K), this value indicates the sense of warmth or coolness that a light source gives to a space. The lower the color temperature, the warmer the light appears. Temperatures below 3,500 K are generally considered warm; those above 4,000 K are considered cool. Fluorescent lamps generally range from 3,000 K to about 4,100 K, although 2,700-, 5,000-, and 6,500-K temperatures are also available. (Daylight typically ranges from 5,000 to 10,000 K.) In some cases, light output varies with color temperature within the same lamp series. For example, the Sylvania XPS 4100K produces 45 mean lumens more than its 3500K and 3000K versions.
• Lifetime, under a given duty cycle and ballast type. Lifetime comparisons are meaningless if the duty cycles and ballast types aren't identical. For example, Philips has made a major effort to reach 30,000-hour lifetimes with its Advantage product line at 3 hours per start on any ballast. But in the low-wattage energy-saver lamps, the three major U.S. companies have become cagier—one manufacturer reports lamp life using 12 hours per start to maintain the "20,000-hour" rating of its 30-watt product. This is misleading—the truth is that the longer any fluorescent lamp remains on, the longer its life will be.

When comparing performance among competing lamp-and-ballast systems, always reference the individual lamp and ballast criteria, and then perform a few key assessments:

• Pair up lamps and ballasts that are optimized to work as a system.
• Mix and match different lamp and ballast combinations to provide desired lumen levels and acceptable lamp lifetimes. Note that light levels will increase when you substitute normal ballast factor (BF) high-performance T8 ballasts for normal-BF standard T8 ballasts. To maintain the same light levels and achieve greater energy savings, the high-performance replacement ballast should have a lower BF.
• Compare systems rated for operation at the same ambient temperature (for example, in open-air or enclosed fixtures). Otherwise, light output and power consumption might differ. Some ballast literature is especially helpful in that it provides watt ratings for both open-air and enclosed lamps.

When evaluating lamp options, also evaluate actual lighting needs to maximize savings—many spaces are overlit to some degree:

• Highly overlit spaces. Many older spaces are highly overlit by current standards. Standard design light levels for many applications, such as offices and classrooms, have been reduced from the 200 to 150 foot-candles deemed acceptable in the 1960s down to 75 foot-candles by the 1990s, and then to 50 foot-candles in recent years. These kinds of spaces represent energy-saving gold mines, particularly using delamping strategies and upgrading to high-performance T8 lamps and ballasts that have the optimal BF for the space.
• Slightly overlit spaces. In many cases, you can design energy-saving retrofits to reduce light levels in spaces that are slightly overlit. This is an application where 30- or 28-watt reduced-wattage or energy-saver lamps might be a good choice when paired with a high-performance T8 ballast, saving an additional watt or two per lamp. Alternatively, you can save more energy by delamping the fixtures and retrofitting them with 32-watt high-performance T8 lamps paired with ballasts that have the appropriate BF to achieve the desired lighting level.
• Properly lit spaces. Replacing standard T8 lamps with 32-watt, high-performance T8 lamps and low-BF ballasts can deliver savings of at least 6 watts per lamp while maintaining existing light levels. Replacing T12s with high-performance T8s, lamp-for-lamp, can save even more.

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.