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 guide on
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 guides on Fluorescent Ballasts and HID Versus Fluorescent for High-Bay Lighting.)
What are the options?
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.
Figure 1: Lamp codes
Each part of the lamp designation code conveys information. The first number in a lamp's designation usually indicates its nominal wattage for rapid-start lamps and its nominal tube diameter for instant-start lamps.
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 a small penalty in efficiency. They are the best choice in applications where lights will frequently be turned on and off.
Figure 2: Fluorescent lamp families
This schematic shows the fluorescent lamp families and their relationships to the three starting methods (circle sizes do not represent market share). Most lamps are only compatible with one starting method; the exceptions are the popular high-performance T8s, which can be rapid- or instant-started, and some rapid-start lamps that can be preheat-started.
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.
Figure 3: Life-cycle cost comparison of high-performance T8 versus generic T8 systems
High-performance T8 lamps and ballasts can cost significantly more than their standard cousins, but the difference can be made up through higher efficiencies and longer lamp life. The overall life-cycle cost for a high-performance T8 system (A) is about $2.40 less per lamp per year than for a generic T8 system (B), or a 10 percent savings.
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").
Table 1: The T8 family tree
Linear T8 fluorescent lamps are available with a wide variety of characteristics. Super T8 lamps offer the highest output and the best color quality.
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.