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To start a fluorescent lamp, a high-voltage pulse must be sent through it. Once the lamp lights, the stuff inside the tube becomes a good conductor of electricity–too good–and then some way of preventing too much current from flowing through the bulb must be provided.
Providing the initial surge and the later limiting the current is done by the ballast, a black box in the lamp housing. Originally ballasts were constructed something like a transformer, of many thin sheets of special steels. Such ballasts are called magnetic ballasts. Unfortunately the sheets move a very small amount as alternating current passes through the ballast, and thst movement creates hum. Magnetic ballasts, not the lamps themselves, are the source of the hum associated with fluorescents.
A newer type of ballast, the electronic ballast, though more expensive, does not hum and uses less energy than a magnetic ballast. In addition, an electronic ballast can raise the frequency of the current going to the lamp, which eliminates the flicker which annoys some people. Old fixtures can be retrofitted with electronic ballasts.
A new fluorescent lamp may not reach maximum brightness for several hours. After that its brightness slowly declines. A four-foot, 40-watt cool white tube, the typical home workshop light, has an initial brightness of 3250 lumens. Six thousand hours later (40% of its rated life of 15,000 hours) its brightness will have declined to 2960 lumens, on average.
The visible light given off by a fluorescent comes from a powdery coating of phosphors on the inside of the tube, somewhat like the phosphors on the inside of the face of a color TV tube. The color of the light given off by the tube is manipulated by changing the mixture of phosphors. The color balance is described by terms like those below, shown with their correlated color temperature and color rendering index.
| Name (abbreviation) | CCT | CRI | |
|---|---|---|---|
|
"Warm" (correlated color temperatures below 3200 kelvin) |
|||
| Incandescent Fluorescent (IF) | 2750 | 89 | |
| Deluxe warm white (WWX) | 2950 | 74 | |
| Warm white (WW) | 3000 | 52 | |
|
In-between (3200 kelvin to 4000 kelvin) |
|||
| White (W) | 3450 | 57 | |
| Natural white (N) | 3600 | 86 | |
"Cool" (correlated color temperatures above 4000 kelvin) |
|||
| Deluxe cool white | 4100 | 89 | |
| Cool white (CW) | 4200 | 62 | |
| Daylight (D) | 6300 | 76 | |
Some older fluorescent fixtures and those under 20 watts often require starters, which are the small aluminum cans in a socket near one end of the lamp. When its starter fails the lamp won't ignite, but starters are easily replaceable. Twisting a starter less than a quarter turn frees it from its socket. Starter types are numbered, with the prefix FS (fluorescent starter); the designation can be found stamped on the end of the can. The type must match the wattage of the lamp.
| Starter | Wattages served |
|---|---|
| FS-2 | 14, 15, 20 |
| FS-4 | 13, 30, 40 |
| FS-12 | 32 |
| FS-25 | 22, 25 |
| FS-40/400 | 40 |
| FS-85 | 90, 100 |
Most fluorescent fixtures installed today are either rapid-start or instant-start types that do not require starters. Some low-wattage desk fixtures require a person to be the starter by holding down a push button until the lamp ignites.
The more often a fluorescent is switched on and off, the shorter its life and the faster it dims. Manufacturer's ratings of lamp life are based on operating the lamp for 3 hours each time it is started.
The most common form of fluorescent lamp is a tube with two pins on each end. The diameter of the tube is described in eighths of an inch, as it is for incandescent lamps, so a fluorescent lamp 1½ inches in diameter is a T-12. The 2-pin bases (called bi-pin bases) come in several sizes, but by far the most common in homes is the medium bi-pin.
Medium bi-pin base lamps that are 1½ inch in diameter (T-12) don't require a starter. They once came in 24″, 36″ and 48″ lengths, of 30, 25, and 40 watts respectively. As a result of the Energy Policy Act of 1992, the 48″ 40-watt T-12 CW (Cool white, once the most widely used fluorescent), D, WW and WWX may no longer be legally made or imported, among other bulbs. 32-watt replacements are available, but 32- and 34-watt bulbs should not be used with ballasts made before 1979 (life is halved).
Alternatively, the ballast may be changed and 32-watt T-8 (1 inch in diameter) bulbs used, which are more efficient. The new ballast may be magnetic or, better, electronic. T-8 bulbs cannot be used with ballasts designed for T-12 bulbs!
Pre-heat style lamps require starters. Lamps 15″ long are 14-watt and 18″ lamps are 15-watt; they may be either T-8 or T-12. The 12-inch, 13-watt lamps are made only in T-8 and 24″, 20-watt bulbs only as T-12.
Miniature bi-pin sockets are used in lanterns, mobile home and automobile fixtures, and similar settings. Lamps made to fit these sockets are 5/8 inch in diameter and 6, 9, 12 or 21 inches long, and the wattage will be 4, 6, 8 or 13 watts respectively. They need a starter, but this usually takes the form of a push button.
Mogul bi-pin bases are rare. They are found on 60″ T-17 preheat lamps, usually of 90 watts, but 82 watts in energy-efficient versions. They are also used on 40-watt instant start lamps, 48″ T-12's and 60″ T-17.
Lamps in the form of circles (Circline is a trademark for a popular version) are 8″ or 12″ in diameter and have a special 4-pin connector.
Compact fluorescents were designed to overcome householders' resistance to fluorescent lights, with the goal of saving large amounts of electricity. Compact fluorescents are no more efficient than their tubular cousins, but they screw into medium screw sockets and their light has a more pleasing color than that of most tubular fluorescents. Some have electronic ballasts.
The table below compares the typical light outputs in lumens of typical medium screw base incandescent bulbs (without reflectors) with that of compact fluorescents.
Inside Frosted Incandescent |
Compact Fluorescent |
||||
|---|---|---|---|---|---|
| wattage | life (hrs) | initial lumens | lumens | life (hrs) | wattage |
| 15 | 2500 | 125 | 575 | 10,000 | 11 |
| 25 | 2500 | 190 | 720 | 9000 | 15 |
| 40 | 1000 | 505 | 900 | 10,000 | 16 |
| 60 | 870 | 1000 | 1100 | 10,000 | 18 |
| 75 | 750 | 1190 | 1200 | 10,000 | 20 |
| 100 | 750 | 1750 | 1500 | 10,000 | 23 |
| 150 | 750 | 2850 | 1650 | 10,000 | 27 |
| 300 | 750 | 6360 | |||
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Last revised: 23 September 2003.