Fluorescent-lamp formats

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Fluorescent lamps in various embodiments

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]

In the United States and Canada, 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 wattage rating or the diameter in eighths of an inch (sometimes in millimeters, rounded-up to the nearest millimeter). Typical diameters are T12 or T38 (1+12 in or 38 mm) for residential lamps with magnetic ballasts, T8 or T26 (1 in or 25 mm) for commercial energy-saving lamps with electronic ballasts, and T5 or T16 (58 in or 16 mm) 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 WP4.5×8.5d 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]
Two newer ranges high efficiency (HE) 14–35 W, and high output (HO) 24–80 W introduced in the 1990s[2]
T8 T26 1 25.4 G13 bipin/single pin/recessed double contact From the 1930s,[3] more common format since the 1980s.[4]
T9 T29 1+18 28.6 G10q quadpin contact Circular fluorescent tubes only
T10 1+14 31.75 G13 bipin
T12 T38 1+12 38.1 G13 bipin/single pin/recessed double contact Also from the 1930s, not as efficient as new lamps.[5]
T17 2+18 54 G20 Mogul bipin Large size for F90T17 (preheat) and F40T17/IS (instant start)
PG17 2+18 54 R17d Recessed double contact General Electric's Power Groove tubes only

For T2–T12 and T17, the number indicates the tube diameter in 18 inches, e.g. T2 → 28 in and T17 → 178 in. Whereas for T16 and T26–T38, the number indicates the approximate tube diameter in millimeters.


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.

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.

Single pin lamps[edit]

Single pin lamps (Also generically called "Slimline" in the United States) operate on an instant start ballast or (In 220-240 V) with a series choke without a starter.

High output/very high output lamps[edit]

High-output lamps are brighter and are driven at a higher electric current, have different ends on the pins so they cannot be used in the wrong fixture. Since about the early to mid-1950s to today, General Electric developed and improved the Power Groove lamp. These lamps are recognizable by their large diameter (2+18 in or 54 mm), grooved tube shape and an R17d cap on each end.


Color is usually indicated by WW for warm white, EW for enhanced (neutral) white, CW for cool white, 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 Wood's 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 ≈52 3000
35 White ≈56 3500
33 Cool White ≈66 4300
25 Natural/Universal White ≈75 4000
54 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
840 Cool white ≈85 4000
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 (152 mm) 4
T5 9 in (229 mm) 6
T5 12 in (305 mm) 8
T5 21 in (533 mm) 13
T8 14 in (356 mm) 14,15
T8 2 ft (610 mm) 18
T8 3 ft (914 mm) 30
T8 4 ft (1,219 mm) 36
T8 5 ft (1,524 mm) 58
T8 6 ft (1,829 mm) 70
T12 14–14.5 in (356–368 mm) 14,15
T12 16.5 in (419 mm) 15
T12 2 ft (610 mm) 20
T12 4 ft (1,219 mm) 40
T12 5 ft (1,524 mm) 65, 80
T12 6 ft (1,829 mm) 75, 85
T12 8 ft (2,438 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.[6] 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

Around 1980 (in the UK, at least), some 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. They fit in older fittings and some modern fittings that employ twist lock lamp holders, even though the modern fittings were not electrically designed for them.

US energy-saving tubes[edit]

In the 1990s,[citation needed] various energy-saving tubes were introduced in North America, but unlike the T8 tubes introduced in Europe, the T8s are not retrofits and require new matching ballasts to drive them, while some T12s can use the older ballasts. Running a T8 tube with a ballast for T12 will reduce lamp life and can increase energy consumption.[7] Conversely, a T12 tube on a T8 ballast will usually draw too much power and so may burn out the ballast, unless it is within the range that the particular model of electronic ballast can compensate for. 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. Like the European modular furniture, display cabinets, ceiling tile grids, etc. they were designed for, these are based on multiples of the 300 mm (11.8 in) "metric foot" instead of the 12 in (305 mm) imperial foot, but are all 37 mm (1.5 in) shorter to allow space for the lampholder connections within the 300 mm modular units, and for much easier insertion into and removal from troffer lights within the grid.

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

The T5 diameter is nearly 40% smaller than T8 lamps and almost 60% smaller than T12 lamps. T5 lamps have a G5 base (bi-pin with 5 mm spacing), even for high-output (HO and VHO) tubes.[8]

See also[edit]


  1. ^ Funke and Oranje, "Gas Discharge Lamps"; N.V Philips' Gloeilampenfabrieken (1951)
  2. ^ "The T5 Fluorescent Lamp: Coming on Strong". 1 September 2003. Retrieved 20 February 2020.
  3. ^ "Covington, E. J. The Story Behind This Account of Fluorescent Lamp Development". Archived from the original on 24 March 2007. Retrieved 28 September 2008.
  4. ^ "Lawrence Berkeley National Laboratory: T-8 lamp retrofits". Archived from the original on 16 September 2008. Retrieved 28 September 2008.
  5. ^ "Lawrence Berkeley National Laboratory: History and problems of T12 fluorescent lamps". Archived from the original on 16 September 2008. Retrieved 28 September 2008.
  6. ^ Thorn Lighting Technical Handbook
  7. ^ "Energy Codes". 1 November 1995. Archived from the original on 22 May 2011.
  8. ^ Lighting Research Center, "T5 Fluorescent Systems", [1], accessed 11-30-2009.