fluorescent light bulbs


In these entries, the word “lamp” means a fluorescent light bulb. It does not mean a fixture (the thing with lamp holder sockets).


To turn on a fluorescent lamp, a high-voltage pulse of electricity 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. Otherwise the lamp burns out in about a second.

The ballast provides the initial surge and then later limits the current. In most of the 20ᵗʰ century, ballasts were black metal boxes in the lamp housing, their insides constructed something like an electrical 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. That movement creates a 120-Hz hum. Magnetic ballasts, not the lamps themselves, are the source of the hum formerly associated with fluorescents.

Transistors made possible another type of ballast, the electronic ballast, which uses about 40% less energy than a magnetic ballast. An electronic ballast does not hum. It can raise the frequency of the current going to the lamp to over 20,000 Hz, above the range of human hearing, which also eliminates the flicker which annoys many people, and increases the light output. An electrician can retrofit some older fixtures with an electronic ballast.

By 2001, more electronic ballasts were being sold in the United States than magnetic ones. The Energy Policy Act of 2005 forbade the production of magnetic ballasts after 1 January 2009, although replacements will still be available for residential use only.

Ballasts are usually made so that a single ballast powers all the lamps in the fixture, typically two, though as many as four.

There are three main types of ballast: pre-heat, rapid start, and instant start. Electronic ballasts programmed start) In general, each type of ballast corresponds to a type of fluorescent lamp.

The type of ballast a lamp needs is related to how it starts. filament. much like the filament in an incandescent bulb. When the lamp is started, the filaments heat, vaporizing liquid mercury within the tube. two connections at each end pre heat requiring a starter) or rapid start.

In an instant start lamp, only one connection is required at each end of the tube. (Some T8 medium bipin lamps can be used with an instant start ballast. When they are, the fixture shorts the two pins together to become a single pin.) The pulse of electricity that starts the lamp is at a much higher voltage than it is in a rapid start lamp. How high depends on the length of the lamp.

An important characteristic of a ballast is its ballast factor. The ballast factor is the fraction of the initial lumens that the lamp provides if it is powered through that ballast, compared to the lumens the same lamp would produce powered through a reference ballast. The ballast factor is not simply a characteristic of the ballast, since it also depends on the attached lamps. Ballast factors range from 0.70 to 1.20.

Brightness and life

graph of brightness of a T8 lamp as time passes

Decline in brightness for a high quality T8 lamp.

Brightness is measured in lumens.

A new fluorescent lamp may not reach maximum brightness for several hours. A lamp's initial lumens rating is measured after the lamp has burned for 100 hours.

After an initial steep decline of a few percent, brightness declines slowly. At 40% of their rated life, most lamps will retain at least 90% of their initial rated brightness.

Brightness is affected by the ballast factor and by temperature. T8 and T12 get brighter up to an ambient temperature of about @ degrees, and grow progressively dimmer at higher temperatures. T5 lamps reach their peak luminosity at about @ degrees. Remember that the temperature within a fixture may be much higher than that of the room air.

Compared to incandescent bulbs, fluorescent lamps are long-lived. Many are rated at 20,000 or even 30,000 hours of life (which is about 3½ years). 

graph of lamp mortality

Manufacturer's ratings of fluorescent lamp life are based on operating groups of lamps in a cycle of 3 hours on and 20 minutes off until all the lamps fail. The rated life is the time by which half the bulbs have failed. A lamp left on continuously will probably outlast its rated life. The more often a fluorescent is switched on and off, the shorter its life and the faster it dims. Nonetheless, when the cost of electricity is balanced against the cost of a lamp, it is cheaper to turn fluorescent lighting off if you will not need it for 15 minutes or more. Special, life-extending, ballasts are available for lamps that will be switched on and off frequently.



Usually, Special ballasts are available Some compact fluroecents come with such ballasts buitilin. 


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 an old-fashioned 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 (CCT) and color rendering index (CRI). Many other color designations are in use.

Verbal descriptions Name (abbreviation) CCT CRI
(correlated color temperatures
below 3200 kelvin)
Incandescent Fluorescent (IF) 2750 89
Deluxe warm white (WWX) 2900 82
Warm white (WW) 3000 52
(3200 kelvin to 4000 kelvin)
White (W) 3450 57
Natural white (N) 3600 86
(correlated color temperatures
above 4000 kelvin)
Deluxe cool white (CWX) 4100 89
Lite white (LW) 4150 48
Cool white (CW) 4200 62
Daylight (D) 6300 76
Deluxe daylight (DX) 6500 88
Octron Skywhite (Sylvania) 8000 88

In the 1990s manufacturers began to add a numeric code describing a lamp's color to the string of characters identifying the model, replacing the "CW", "WWX", etc designations. In this code, "70" means a CRI between 70 and 79; "80" means a CRI between 80 and 89, and so on. Further, the zero can be replaced with the two digits that are the thousands and hundreds places of the lamp's CCT. So, for example, "741" would mean the lamp has a CRI in the 70s and a CCT of 4100. Some add the prefix "RE" to indicate rare earth phosphors have been used.

Special colors are available for some purposes, such as lighting aquariums, growing plants, killing germs, tanning starlets, and making fluorescent substances fluoresce (the latter three emit significant amounts of ultraviolet light).

Shapes, sizes and bases

The most common form of fluorescent lamp is a straight tube. The diameter of the tube is described in eighths of an inch, as it is for incandescent lamps, so a fluorescent lamp 1 inch in diameter (eight eighths) is a T8. Sizes range from T2 to T17.

A 5-watt, 25-watt, or 300-watt incandescent bulb can be screwed into an Edison socket and they will all work as they are supposed to. One socket; any wattage. That is not true of fluorescent lamps, because the lamp must match the ballast in the fixture. Different lengths are associated with different wattages; for example, 4-foot bulbs were 40-watts (at one time).

The nominal lengths of T12 and T8 lamps are not measured on the lamp, but the fixture: it is the distance between the inner faces of the lamp holder sockets. The real length of the lamp is a fraction of an inch shorter.

The nominal length of a T5 lamp, however, is almost 2 inches longer than the real end-to-end length of the bulb.


At the ends of the tube are bases that connect to the sockets in the fixture. Most bases have two pins and are called bi-pin bases. They come in several sizes, but the most common in homes is the medium bi-pin, found on T12 and T8 tubular lamps. A smaller version, the miniature bipin, is found on T5 lamps.

photo of the end of a tubular lamp showing a single pin base

Single pin bases are found on long (8 feet, for example) tubular T12 and T8 instant-start lamps. They are rarely found in homes. Recessed double contact bases are also largely commercial and industrial.

Mogul bi-pin bases are rare. They are found on 60″ T17 preheat lamps, usually of 90 watts, but 82 watts in energy-efficient versions. They are also used on 40-watt instant start lamps: 48″ T12's and 60″ T17.

The lamp code

photo of the end of a tubular lamp showing the lamp code


F: a fluorescent lamp. The F may be followed by other letters. “FO”, for example, identifies a high-output lamp.

32: wattage 

T8: has a tubular body 8/8ths of a inch in diameter, that is 1 inch in diameter.


TL841: a color designation. The "8" indicates a CRI in the 80's and the 41 a CCT of 4100. Older abbreviations, such as "CW" for "Cool White", may appear in this position. For their meanings see Color, above.


ALTO: a proprietary designation. Other manufacturers have designations with similar meanings, e.g., "ECO".

Rapid start lamps


Medium bi-pin base T12 (1½ inch in diameter) lamps once came in 24″, 36″ and 48″ lengths, of 30, 25, and 40 watts respectively. They did not require a starter. As a result of the Energy Policy Act of 1992, the full-wattage 48″ 40-watt T12 CW (cool white, once the most widely used fluorescent), D, WW and WWX lamps may no longer be legally made or imported, among other bulbs. Instead, lower wattage 34-watt replacements were to be used. Full-wattage bulbs may only be made if the maker makes a more energy-efficient bulb by using a more expensive mixture of phosphors.

The 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 T8 bulbs used, which are more efficient. T8 bulbs cannot be used with ballasts designed for T12 bulbs!

T12 lamps are essentially obsolescent.

Typical T12 Straight Bulbs, all Medium BiPin

Watts Initial
15 14
18 15
24 20
36 25 1925-2050
36 30 1900-2250
48 25
48 34 1930-2800
48 40 1980-3300

T8 rapid start

introduced in the U.S. in 1981 and are now the usual choice in new fixtures. An individual T8 lamp usually gives off less light than a T12 lamp the same length and color, but, oddly enough, in a fixture they usually give as much light. One reason is that the larger T12 lamp gets in the way of its own light, and another that the T8 lamp is probably lower wattage and runs cooler.

Typical T8 Straight Bulbs, all Medium BiPin
Watts Lumens
18 15 860
24 17 1325 - 1400
36 25 2125 - 2250
30 2200
48 32 2850 - 3100


T5, rapid start

The diameter of T5 tubes, 5/8ths of an inch, is too small to accommodate a medium bipin base. A miniature bipin base is used instead.

Typical T5 Straight Bulbs, all Mini BiPin
Wattage Lumens
6 4 130
9 6 270
12 8 380
21 13 960
34 39 3500
46 28 2900
54 5000


Special shapes

U-shaped lamps

drawing of U-shaped fluorescent tube

sized by the distance from center to center of the bases, and the distance from the bases to the outside edge of the bend. In T12, T8  6 inch leg spacing. T8, 1 5/8 inc leg spacing


photo of circular fluorescent tube

Fluorescent lamps in the form of circles are called Circline lamps. Those with a T9 tube diameter are available with outside diameters of 6½″, 8″, 12″, or 16″. They use a special 4-pin connector. Circline lamps are also made in T5 versions, with diameters of 8.85″ or 30 centimeters (11.77″).

Compact fluorescents

into two categories. with and without their own ballast.

Compact fluorescents without a built-in ballast

photo of compact fluorescent bulb on card

Never put 2-pin compact fluorescent bulb in a 4-pin socket even if it fits!

Compact fluorescents with a built-in ballast


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.


CFLs compared with incandescents

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      


NEMA ANSI C82.1-1985. American National Standard for Ballasts for Fluorescent Lamps.
ANSI C82.11-1993, High-Frequency Fluorescent Lamp Ballasts.


The major manufacturers provide extremely informative web sites, for example: www.sylvania.com/, www.gelighting.com/na/, and www.phillips.com. They also offer catalogs of their entire line:


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