Flourescent lamp code photos (C) InspectApedia.comFluorescent Lamp Codes
Types, codes, connector bases for fluorescent bulbs

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Here we describe the different types of fluorescent lamps or "bulbs" by size, shape, connector, wattage, color temperature and other features.

This article series explains how to diagnose the causes of flickering, dim, or dead fluorescent lights at or in buildings.

Watch out: flickering or dimming lights might indicate a dangerous condition risking a building fire or an electrical shock. If the simple bulb or starter repairs we describe here don't cure the flickering, switch off the bad-acting light fixture, leave it off, circuit and ask for help from a licensed electrician.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Identify Fluorescent Lamp Types / Bulb Codes

T-8 fluorescent bulbs, 3 brands, all 36", all will work in the light fixture shown here (C) Daniel Friedman at InspectApedia.comPhoto: Packaging for T8 fluorescent bulbs. The property owners whose lights I was repairing had kept the original packaging for two generations of GE brand T8, 36-inch fluorescent bulbs that were used in various fixtures in the home.

The Sylvania T8 bulb shown by the bottom-packaging is a perfect replacement.

Keeping the original bulb package (or writing down what type of bulb was installed) means that even if you have the bad luck to break a bulb and your client throws away your carefully-kept bulb base, you can still identify the proper replacement lamp.

Some of bulb specifications below are redundant: for example if you know that it's a T5 bulb you know that it is a 5/8" (15.8mm) diameter lamp with a G5 bi-pin connector.

However there are some substitutions that will work: You can not install a shorter bulb into the existing fluorescent light fixture than the original, but you may find a bulb that is

In U.S., Canadian, European and some other communities you'll pay a small recycling fee - an environmental handling charge - that covers the safe-handling and disposal of these older bulbs, some of which contain mercury.

Types of Fluorescent Bulbs: code, diameter, socket type

Bulb Type Code
Tube Diameter Socket Type Comments
T2 1/4" - 7mm WP4.5×8.5d  
T4 1/2" 0 12.7 mm G5 Bi-Pin  
T5 / T16   G5 Bi-Pin

Smaller, light-weight bulbs, architectural & high bay applications, produce more light per fixture than T8s.

Uses miniature Bi-Pin base, or G5 base, pin spacing 5 mm

T8 / T26

1" - 25.4 mm


G13 bi pin / single pin / recessed double contact

Higher efficiency replacement for traditional 40W T12 lamps, 32 Watts, vary in color temperature.

T8s replace T12s, give about the same light output at 60% of the T12 bulb size.

Uses Medium Bi-Pin base, or G13 base, pin spacing 13 mm

T8 LED Replacements 1" - 25.4 mm G13 bi pin / single pin / recessed double contact

LED linear T8 Tubes can replace conventional T8 lamps IF an electronic T8 ballast is used


T9 / T29 1 1/8" - 28.6 mm G10 quad pin Circular fluorescent 4-pin connector

1 1/4" - 31.75 mm


G13 bi pin

High-wattage, (40W) high-lumen alternative to T12

Uses Standard F40T12 magnetic or electronic ballast

Uses Medium Bi-Pin base, or G13 base, pin spacing 13 mm

Some versions provide a 4-pin connector at one end of bulb, specialty, e.g. germicidal fluorescent

T12 / T38

1 1/2" - 38.1 mm


G13 bi pin / single pin / recessed double contact

Older, traditional design, US EPACT mandates phase-out in many applications, replaced by T5, T8, CFL

T12 bulbs used electromagnetic induction to create light; replacements use newer electronic ballasts.

Uses Medium Bi-Pin base, pin spacing 13 mm.

Alternative T12 bulbs using a single center pin single connect to an Fa6 base.

The single pin is 6mm in diameter.

T17 / PG17 2 1/8" - 54 mm Mogul bi pin or recessed double contact

GE Power-Groove™ and other brands

Connect to a Mobul Bi-Pin G20 base.

Some models connect to an R17d base, recessed connectors, pin spacing 17mm

CFL Compact fluorescents 1/2" - 5/8" Edison base screw-type bulb / lamp Tube doubles back, curls, bends to produce a bulb that replaces traditional incandescent lamps

Notes to the table:

T8 and T12 Fluorescent lamps fit in to the same connector base (C) InspectApedia.comOur sketch illustrates that on both older T12 fluorescents and T8 fluorescent lamps the bulb's connecting pin dimensions and spacing are identical even though the T8 is a signifcantly-smaller-diameter bulb.

Photo Guide to Fluorescent Bulbs by Connector / Base

These photos show the different types of connectors, pins, or bulb bases for various fluorescent lamps. The illustrations are not to scale. For bulb dimensions see the table above starting at FLUORESCENT LIGHT BULB CODES & TYPES

[Click to enlarge any image]

T2 Fluorescent lamp (C)

Above: T2 Fluorescent bulb, 8 watt, from Bulbrite®

T4 Fluorescent bulb properties at

Above: T4 Fluorescent bulb, 28 watt, from PL&T®, pin spacing 4 mm.

GE F13 T5 Bulb at

Above: T5 Fluorescent bulb, 13 watt, from GE®, pin spacing, 5 mm. Connects to miniature G5 base.

T8 fluorescent bulb base and pin connectors (C)

Above: T8 Fluorescent bulb, 32 watt, pin spacing 13mm, sold in a wide range of color temperature and light output in lumens, straight or U-shaped.

Connects to a medium Bi-Pin G13 base.

T9 circular fluorescent bulb base connector 4 pin at

Above, the 4-pin connector segment of a T9 circular fluorescent, 32-watt.

T10 fluorescent lamp example at

Above: T10 fluorescent lamp, high output 40-Watt, connects to a medium Bi-Pin G13 base.

F96T12 center pin fluorescent bulb from Philips at

Above: T12, center pin, F96T12/DX/ALTO fluorescent bulb from Philips, 75 Watt, connects to a single pin Fa6 base.

R17d-base-T12-LED replacement lamp for fluorescent fixtures at

Above: a T12 LED replacement bulb for T12 fluorescents using a type R17D base connector.

T17 fluorescent lamp from GE, using G20 Mobul bi-pin base at

Above: a T17 fluorescent lamp produced by GE. This 180-Watt T17 lamp connects to a Mobul Bi-Pin (G20) base.

Flouorescent Lamp Color Temperatures

Color temperature of lights - demo © D Friedman at Color temperature of lights - demo © D Friedman at Color temperature of lights - demo © D Friedman at

The color temperature of fluorescent lamps is adjusted by the manufacturer by changing the chemistry of the phosphorescent coating lining the inside of the lamp tube.

Above in order we show four CFL edison base (screw-in type) fluorescent bulbs of different color temperatures. We kept the camera on the same "automatic" white balance setting for all four bulb color photographs.

The accuracy of color rendition of different lights and the CRI scale are explained at Color Temperature and CRI for Indoor Lights.

Flourescent lamp code photos (C)

These colors are not exactly what your eye will see, nor do they consider the effects of the colors of surroundings, but you can see the color temperature differences among these light bulbs.

The applications for these bulb color temperatures (corresponding to order of the photographs above) are:

Most fluorescent lamps such as the USHIO bulb shown just above include the bulb's color temperature (5000K in this example) right next to lamp's WATT data.

Exampes of fluorescent lamp color temperatures



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