Full-Size Fluorescent Lamps
What Are the Options?
How to Make the Best Choice
What's on the Horizon?
Full-size fluorescent systems are among the most common, and most efficient, lamps in use. They are most 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. To apply fluorescent lamps successfully, carefully consider options for lamps (diameter, length, and phosphor blend), as well as for ballasts and fixtures.
| Some Fluorescent Lighting Terms |
| Ballast. An electrical device designed to control the current delivered to a fluorescent or HID lamp. Most ballasts also convert the line voltage into the proper voltage and waveform to start and operate the lamp. Color rendering index (CRI). 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. CRIs for T8 fluorescents range from 52 to 91, with most products having 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. Describes the apparent color of a light source and the sense of warmth or coolness that it will give to a space. The lower the color temperature, the warmer the light appears. Temperatures below 3,500 kelvins (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.) Efficacy. The light output of a lamp divided by the input power, expressed in lumens per watt (lm/W). Mean lumens. Fluorescent lamp light output falls over time, which makes comparisons based on initial lumens misleading. Comparing by “mean lumens,” as listed in lamp catalogs, is a better alternative. Mean lumens indicate the lumen output of a lamp after it has operated for 40 percent of its rated life. Phosphors. These are substances that coat the inside of fluorescent tubes and transform ultraviolet light 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. |
What Are the Options?
Fluorescent lamps differ primarily in their size, type of phosphor coating, fill gas, and starting method. By varying these four characteristics, manufacturers create lamps that span a wide range of wattages, light outputs, colors, and lifetimes. Diameters range from 0.250 to 2.125 inches (in.) and lengths from 6 to 96 in. Nominal power consumption (without ballasts) ranges from under 20 W to over 200 W, and light output also spans an order of magnitude from under 1,500 lm to over 15,000 lm. Lamp-only efficacy covers a smaller range: from 60 to 100 lm/W. As shown in Figure 1, lamps are designated primarily by their wattage or length and tube diameter.
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Figure 1: Lamp codes Each part of the lamp designation code conveys information. The first number in a lamp’s designation usually – but not always – indicates its nominal wattage for rapid-start lamps and its nominal tube diameter for instant-start lamps. |
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 called switch start, and there are a number of variations of rapid start. The design of the ballast determines the starting mode used for any fluorescent lamp, but the lamp must be compatible with the starting mode of the ballast.
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Figure 2: Fluorescent lamp families This schematic shows the fluorescent lamp families and their relationships to the three starting methods (circles’ 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. The so-called energy-saving krypton-filled lamps are available in nearly all categories. Source: E Source |
Preheat-start lamps. Preheat-start lamps are, in general, relatively short lamps (6 to 36 in. long) 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 rapid-start or programmed rapid-start lamps would be a better choice. Eight-ft 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.
How to Make the Best Choice
For most general lighting applications, these are the best choices:
T8 lamps. T8s offer better efficiency, lumen maintenance, color quality, fixture optics, and life-cycle costs than T12 systems. The most efficient kind of T8 is the higher-lumen type, also commonly called high-performance or super T8 (Table 1). Higher-lumen T8 lamps can be installed to cost-effectively upgrade lower-quality T8s. “Energy-saver” and “reduced-wattage” T8s can also provide energy savings, but they have several shortcomings: They can only operate in spaces where temperatures are kept at a minimum of 60° Fahrenheit, they produce less light than full-wattage lamps, and they are not dimmable with current ballast technology.
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Table 1: The T8 family tree Linear T8 fluorescent lamps are available with a wide variety of characteristics. The "higher-lumen" T8 lamps offer the highest output and the best color quality. |
T5 lamps. T5 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. T5 lamps are also designed for a high optimal operating temperature, which improves performance in enclosed fixtures and warm spaces.
Four-foot lamps. This is 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.
Standard light output. Standard-output lamps are generally more efficient and less costly than high output (HO) and very high output (VHO) systems, and they are available with a wider range of color temperatures.
CRI in the 80s. Light sources with a CRI in the 80s will provide sufficient color rendering for most purposes and are far superior to the old cool white halophosphor lamps. Fluorescent lamps with CRIs in the 90s are available, but they carry a substantial penalty in efficacy.
Correlated color temperature (CCT) of 3,500 K to 4,100 K. A CCT of 3,500 K is a good middle ground that can blend acceptably with warmer incandescents; 4,100-K lamps may blend better with the cooler daylight and HID sources.
What's on the Horizon?
Fluorescent lamps are a mature technology, but manufacturers continue to make incremental improvements in efficiency and lamp life. The latest development, just entering the market, is the use of mercury amalgams in full-size fluorescent lamps to reduce the sensitivity of lamp output to temperature change.
Copyright © 2006 E Source Companies LLC
