Fluorescent-lamp formats

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For other types of lightbulb mounts and sockets, see Lightbulb sockets.

Since their introduction as a commercial product in 1939, many different types of fluorescent lamp have been introduced. Systematic nomenclature identifies mass-market lamps as to overall shape, power rating, length, color, and other electrical and illuminating characteristics.

Tube designations[edit]

Lamps are typically identified by a code such as FxxTy, where F is for fluorescent, the first number (xx) indicates either the power in watts or length in inches, the T indicates that the shape of the bulb is tubular, and the last number (y) is the diameter in eighths of an inch . Typical diameters are T12 or T38 (1 12 in or 38.1 mm) for residential lamps with magnetic ballasts, T8 or T26 (1 in) for commercial energy-saving lamps with electronic ballasts, and T5 or T16 (58 in) for very small lamps, which may even operate from a battery-powered device.

Fluorescent tube diameter designation comparison
Designation Tube diameter Extra
(in) (mm) Socket Notes
T2 14 approx. 7 WP4.5x8.5d Osram's Fluorescent Miniature (FM) tubes only

Sylvania Luxline Slim T2 Linear

T4 12 12.7 G5 bipin Slim lamps. Power ratings and lengths not standardized (and not the same) between different manufacturers
T5 T16 58 15.9 G5 bipin Original 4–13 W range from 1950s or earlier.[1]
T8 T26 1 25.4 G13 bipin/single pin/recessed double contact From the 1930s,[3] more common since the 1980s.[4]
T9 T29 1 18 28.6 G10q quadpin contact Circular (circline) fluorescent tubes only
T10 1 14 31.75 G13 bipin
T12 T38 1 12 G13 bipin/single pin/recessed double contact Also from the 1930s, not as efficient as new lamps.[5]
T17 2 18 Mogul bipin Large size for F90T17 (preheat) and F40T17/IS (instant start)
PG17 2 18 Recessed double contact General Electric's Power Groove tubes only


Cross section of a typical fluorescent lamp with and without a reflector

Some lamps have an internal opaque reflector. Coverage of the reflector ranges from 120° to 310° of the lamp's circumference. Often, a lamp is marked as a reflector lamp by adding the letter "R" in the model code, so a F##T## lamp with a reflector would be coded as "FR##T##". Very high output (VHO) lamps with reflectors may be coded as VHOR. No such designation exists for the amount of reflector coverage the lamp has.

Reflector lamps are used when light is only desired to be emitted in a single direction, or when an application requires the maximum amount of light. For example, these lamps can be used in tanning beds or in backlighting electronic displays. An internal reflector is more efficient than standard external reflectors. Another example is color matched aperture lights (with about 30° of opening) used in the food industry for robotic quality control inspection of cooked goods.

Aperture lamps have a clear break in the phosphor coating, typically of 30°, to concentrate light in one direction and provide higher brightness in the beam than can be achieved by uniform phosphor coatings. Aperture lamps include reflectors over the non-aperture area. Aperture lamps were commonly used in photocopiers in the 1960s and 1970s where a bank of fixed tubes was arranged to light up the image to be copied, but are rarely found nowadays. Aperture lamps can produce a concentrated beam of light suitable for edge-lit signs.[6]

Slimline lamps[edit]


High output/very high output lamps[edit]

High-output lamps are brighter and are driven at a higher electrical current, have different ends on the pins so they cannot be used in the wrong fixture, and are labeled F##T##HO, or F##T##VHO for very high output. Since about the early to mid-1950s to today, General Electric developed and improved the Power Groove lamp with the label F##PG17. These lamps are recognizable by their large diameter (21/8"), grooved tube shape and an R17d cap on each end.

Other tube shapes[edit]

U-shaped tubes are FB##T##, with the B meaning "bent". Most commonly, these have the same designations as linear tubes. Circular bulbs are FC##T#, with the outer diameter of the circle (not circumference or watts) in centimeters being the first number and the second number referring to the tube size.


Color is usually indicated by WW for warm white, EW for enhanced (neutral) white, CW for cool white (the most common), and DW for the bluish daylight white. BL is used for ultraviolet lamps commonly used in bug zappers. BLB is used for blacklight-blue lamps employing a woods glass envelope to filter out most visible light, commonly used in nightclubs. Other non-standard designations apply for plant lights or grow lights.

Philips and Osram use numeric color codes for the colors. On tri-phosphor and multi-phosphor tubes, the first digit indicates the color rendering index (CRI) of the lamp. If the first digit on a lamp says 8, then the CRI of that lamp will be approximately 85. The last two digits indicate the color temperature of the lamp in kelvins (K). For example, if the last two digits on a lamp say 41, that lamp's color temperature will be 4100 K, which is a common tri-phosphor cool white fluorescent lamp.

Halophosphate tubes
Numeric color code Color Approximate CRI Color temperature (K)
29 Warm white ~54 3000
35 White ~56 3500
33 Daylight/Cool White ~67 4300
25 Natural/Universal White ~75 4000
54 Tropical Daylight ~75 6500
Deluxe halophosphate tubes
Numeric color code Color Approximate CRI Color temperature (K)
27 Deluxe Extra Warm White ~95 2700
32 Deluxe Warm White ~85 3000
34 Deluxe White ~85 3850
79 Deluxe Natural ~93 3600
38 Deluxe Cool White/°Kolor-rite ~92 4000
55 Northlight/Colour Matching ~94 6500
Tri-phosphor tubes
Numeric color code Color Approximate CRI Color temperature (K)
827 Warm white ~85 2700
835 White ~85 3500
841 Cool white ~85 4100
850 Sunlight ~85 5000
865 Cool daylight ~85 6500
880 Skywhite ~85 8000
Multi-phosphor tubes
Numeric color code Color Approximate CRI Color temperature (K)
927 Warm white ~95 2700
941 Cool white ~95 4100
950 Sunlight ~98 5000
965 Cool daylight ~95 6500
Special purpose tubes
Numeric code Fluorescent

lamp type

05 Germicidal lamps No phosphors used at all,

using an envelope of fused quartz.

08 Black-light lamps
09 Sun-tanning lamps

Common tube ratings[edit]

This section lists the more common tube ratings for general lighting. Many more tube ratings exist, often country-specific. The Nominal Length may not exactly match any measured dimension of the tube. For some tube sizes, the nominal length (in feet) is the required spacing between centers of the lighting fixtures to create a continuous run, so the tubes are a little shorter than the nominal length.

Tube diameter in 18 in (3.175 mm) Nominal length Nominal power (W)
T5 6 in, 150 mm 4
T5 9 in, 225 mm 6
T5 12 in, 300 mm 8
T5 21 in, 525 mm 13
T8 18 in, 450 mm 14
T8 2 ft, 600 mm 18
T8 3 ft, 900 mm 30
T8 4 ft, 1200 mm 36
T8 5 ft, 1500 mm 58
T12 18 in, 450 mm 15
T12 2 ft, 600 mm 20
T12 4 ft, 1200 mm 40
T12 5 ft, 1500 mm 65, 80
T12 6 ft, 1800 mm 75, 85
T12 8 ft, 2400 mm 125

European energy-saving tubes[edit]

In the 1970s, Thorn Lighting introduced an energy-saving 8 ft retrofit tube in Europe. Designed to run on the existing 125 W (240 V) series ballast but with a different gas fill and operating voltage, the tube operated at only 100 W. Increased efficiency meant that the tube produced only 9% lumen reduction for a 20% power reduction.[7] This first energy-saving tube design remains a T12 tube even today. However, follow-on retrofit replacements for all the other original T12 tubes were T8, which helped with creating the required electrical characteristics and saving on the then new (and more expensive) polyphosphor/triphosphor coatings, and these were even more efficient. Note that because these tubes were all designed as retrofit tubes to be fitted in T12 fittings running on series ballasts on 220–240 V supplies, they could not be used in 120 V mains countries with inherently different control gear designs.

Type Diameter (in, mm) Nominal length (ft, m) Nominal power (W) Notes
T8 1.0, 25 2, 0.6 18 Retrofit replacement for 2 ft T12 20 W
T8 1.0, 25 4, 1.2 36 Retrofit replacement for 4 ft T12 40 W
T8 1.0, 25 5, 1.5 58 Retrofit replacement for 5 ft T12 65 W
T8 1.0, 25 6, 1.8 70 Retrofit replacement for 6 ft T12 75 W
T12 1.5, 38 8, 2.4 100 Retrofit replacement for 8 ft T12 125 W

By around 1980 (in the UK, at least), most new fluorescent fittings were designed to take only the newer, retrofit tubes (the lamp holders are designed not to take T12 tubes, except for 8 ft length). The earlier T12 halophosphate tubes still remained available as spares until 2012 but only fit in older fittings and some modern fittings (they often fit in fittings employing twist lock lamp holders even though the fittings themselves, were not electrically designed for them).

US energy-saving tubes[edit]

In the 1990s,[citation needed] various energy-saving tubes were introduced in the US, but unlike the T8 tubes introduced in Europe, they are not retrofits and require new matching ballasts to drive them. Running a T8 tube with a ballast for T12 will reduce lamp life and can increase energy consumption.[8] The tube type should always match the markings on the light fixture.

Type Diameter (in, mm) Nominal length (ft) Nominal power (W) Notes
T12 1.5, 38 2 17
T12 1.5, 38 4 34
T12 1.5, 38 5 40
T12 1.5, 38 8 59
T8 1.0, 25 4 25 Shoplite

T5 tubes[edit]

In the 1990s, longer T5 tubes were designed in Europe (making it to North America in the 2000s), in addition to the shorter ones (mentioned above) already in use worldwide.

Tube diameter is 58 in (15.875 mm) Length Nominal power (W) Notes
High-efficiency High output
T5 563 mm (22.2 in) 14 24 Fits within a 0.6 m modular unit
T5 863 mm (34.0 in) 21 39 Fits within a 0.9 m modular unit
T5 1,163 mm (45.8 in) 28 54 Fits within a 1.2 m modular unit
T5 1,463 mm (57.6 in) 35 80, 49 Fits within a 1.5 m modular unit

T5 lamps are approximately 40% smaller than T8 lamps and almost 60% smaller than T12 lamps. T5 lamps have a G5 base (bi-pin with 5 mm spacing).[9]

See also[edit]


  1. ^ Funke and Oranje, "Gas Discharge Lamps"; N.V Philips' Gloeilampenfabrieken (1951)
  2. ^ "EC&M: The T5 Fluorescent Lamp: Coming on Strong". 2003-09-01. Retrieved 2008-09-28. 
  3. ^ "Covington, E. J. The Story Behind This Account of Fluorescent Lamp Development". Retrieved 2008-09-28. 
  4. ^ "Lawrence Berkeley National Laboratory: T-8 lamp retrofits". Retrieved 2008-09-28. 
  5. ^ "Lawrence Berkeley National Laboratory: History and problems of T12 fluorescent lamps". Retrieved 2008-09-28. 
  6. ^ "Aperture lamps". Retrieved 2009-11-15. 
  7. ^ Thorn Lighting Technical Handbook
  8. ^ "Lighting Design Lab: http://www.lightingdesignlab.com/commercial/articles/Energy_Code.htm". 1995-11-01. Retrieved 2009-07-06. 
  9. ^ Lighting Research Center, "T5 Fluorescent Systems", [1], accessed 11-30-2009.