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How to Identify What Fluorescent Light Bulb You Have in 4 Easy Steps

If you’re looking to replace a fluorescent bulb then much of the information you will need to order a new one is on the tube itself. Information will be printed on one end of the bulb and will include:

  • Name of the company which makes the tube
  • The model name of the tube
  • The wattage
  • The colour temperature

For compact fluorescent light bulbs (CFLs), the information will be printed on the plastic fitting.

If, for any reason, this information isn’t available, there are other ways you can identify what type of bulb it is.

1. Shape


CFLs come in a number of shapes, the most common being twin tubes, quads, triples, spirals, F type lamps, 2D lamps and circular lamps. All of these lamps use the same technology as linear fluorescent tubes but are wrapped into shapes which make them more suitable for smaller spaces. You will usually be able to find all the details you need to source a replacement bulb on the plastic base of the lamp.


This brief guide will help you to identify which type of lamp you have:

  • Single turn lamps are also known as PL-S lamps and the brand names for these are GE Biax S, Osram Dulux S and, Sylvania Lynx S.
  • Long single turn lamps are also known as PL-L lamps and the brand names for these are GE Biax L, Osram Dulux L and Sylvania Lynx L.
  • Double turn lamps are also known as PL-C lamps and the brand name for these are GE Biax D, Osram Dulux D and Sylvania Lynx D. PL-C is the brand name for double turn tubes manufactured by Philips.
  • Triple turn lamps are also known as PL-T lamps and the brand name for these are GE Biax T, Osram Dulux T and Sylvania Lynx T. These lamps have three individual tubes which are curved so they can be fitted into small spaces such as table lamps. The have four pins which means they can be operated with high-frequency ballasts, which are more modern ballasts when compared to low-frequency choke and starter fittings.
  • Spiral lamps feature one tube which has been compressed into a spiral shape and is commonly found in homes, it’s a popular replacement for incandescent bulbs for applications such as 240w table lamps.
  • F lamps resemble the prongs of the letter ‘F’: they have two side-by-side tubes and four pins. These lamps are manufactured by Osram and Radium.
  • Circular lamps are, as you’d expect, fluorescent tubes moulded into a circular shape and are available in T5 and T9 diameters. They are usually used for commercial purposes in ceilings and wall mounts in retail, offices and hospitality applications.
  • 2D lamps serve a similar purpose to circular lamps, and are a single tube curved a D shape which looks like two letter ‘Ds’ stacked on top of each other. These are also known as PL-Q lamps or a compact fluorescent square.


The shape of a tube is self-explanatory as they are linear in length, with two metal pins on each end. With fluorescent tubes, the length and width of the tubes differ and the dimensions you need will depend on the size and wattage of the fitting.

2. Length and width

The length of CFL bulbs vary slightly and is dependent on the type of bulb. You will very rarely need to know the length of a CFL bulb to be able to replace it as the wattage and number of pins are much more important.

Length for fluorescent tubes is generally based on the diameter of the tube, as tubes with a smaller diameter tend to be smaller in length.

The most common fluorescent tubes have the following diameters:

  • T5 tubes measure 16mm in diameter
  • T8 tubes have a diameter of 26mm
  • T12 have the largest diameter of 38mm

The length varies for each size of bulb, standard T8s are available in 2ft, 3ft, 4ft, 5ft and 6ft; standard T12s are available in 2ft, 4ft, 5ft, 6ft and 8ft. Standard T5s are smaller and are available in 6, 9, 12 and 21 inches. It’s important to note that specialist sizes are available in T8 tubes, so if you have a non-standard size then let us know as we may have a suitable bulb for you.

When you’re buying a replacement bulb, you’ll usually be able to identify the length by the wattage (more on this below). In some cases, you may need to measure the tube to find the length, and if that’s the case then measure the entire tube from end-to-end, including the metal pins on each end.

3. Wattage

CFL bulbs have the wattage printed on the plastic base and on linear tubes it’s written on the tube itself. If you’re looking to upgrade incandescent or halogen bulbs to CFLs or a CFL to an LED then our wattage comparison table will help you make the right choice. If you’re unsure of the wattage you need, then this table will help you choose which wattage is right for your needs – as a basic rule the higher the wattage, the brighter the bulb.

Fluorescent tubes will have the wattage printed on the tube itself, if a tube doesn’t, then when you speak to LampShopOnline we will be able to help you pinpoint the wattage you need. Generally, the longer and thicker the tube, the higher the wattage.

You can replace T12 tubes with T8 bulbs, but as the wattage will be different, (a typical wattage for a 4ft T12 bulb is 40W, compared to 36W for a T8 bulb of the same length) in most cases you will need to update the ballast too, so it can support the new, reduced wattage. However, this isn’t always the case as some control gear will accept a slightly lower wattage bulb without problems. It’s important to check though if you do use an incompatible tube and ballast combination, this can cause the ballast and/or the tube to wear out quickly.

4. Colour temperature

The colour temperature of a CFL and a linear tube will also be printed on the plastic base or on the bulb itself. The colour temperature will either be written as a word e.g. ‘cool white’ or as the corresponding number, which for cool white would be 4000k. If there is no number or wording on the lamp, you can find out more about the difference in colour temperatures in our article Full-Spectrum Fluorescent Lights: Everything You Need to Know, which will help you discern the right colour temperature for your space.

If you’re not sure what type of bulb you have, you can always call the LampShopOnline on 0113 887 6270 and we will help you identify it.

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Published 2016/08/04