Many of us would not question the matter further until the time comes to replace the tube, when it becomes necessary to understand the criteria defining the type of fitting required. Unlike conventional bulbs, these tubes have an additional device, usually hidden away inside the enclosure, which is required to regulate the current supplied to the tube; this is termed a "ballast", and is another thing to consider when repair becomes necessary. Then, there are the size of the tube, the type of colour it emits, and the connectors it uses to be taken into account.
A tube is commonly identified by a sequence of letters and numbers, such as ‘F60T5’: Here, the 'F' designates 'fluorescent'; 60 is the wattage (the power of the tube); T describes the shape ('tubular'); 5 is the width (in multiples of an eighth of an inch).
So far so good, but the order and meaning of these abbreviations can vary in a sometimes not entirely predictable way. For example, the old imperial method of using fractions of an inch is sometimes superseded by a metric version, where the number represents the approximate width in millimetres. Thus, if you have a tube an inch wide, this might be called a 'T8' or a 'T26'.
Typical Tube Markings
To assist with deciphering these markings, the table below consists of a series of explanations of the letters commonly encountered.
The numbers are often accompanied by a designation of units (such as 'W' for watts), which helps to identify what they stand for. Therefore, look for an association of this kind to find out what the number might stand for. If there is no such explicit association, then the numbers are to be identifed by their position, as described above - for example, a number following a 'T' is commonly an indication of width of the tube.
Please note that whilst this guide is offered in good faith, please refer to the manufacturer if you require definitive information.
Tube Markings Table
|<number>||...|| What numbers refer to depends on their position in relation to the letters in the code; look for the letter codes in this table if you see: |
'C'<number>, see 'C'
'F'<number>, see 'F'
<number>'K', see 'K'
'T'<number>, see 'T'
<number>'W', see 'W'
Please also note what is stated under 'CRI', that a number can represent a concatenation of CRI index (see below) and "light temperature" in Kelvin (see below) - in other words, it represents the type of light emitted by the tube.
|B||Bent||In the world of fluorescent tubes, this can refer to a tube which is curved in some way by design (not to be confused with the way the contacts end up if you don't manage to get the tube out properly). This sometimes follows the 'F' letter designating 'fluorescent'.|
|...||Ballast||The ballast is an electronic component used in the tube to adjust the supply to help the tube "start", and to adjust the frequency of the alternating current. The type of technology used for the former determines how quickly the tube starts, and how long it lasts in terms of amount of times turned on; the latter reduces the "strobing" effect, and can also be used to dim fluorescent tubes remotely, by sending a signal to adjust the frequency.|
|...||Bipin/Bi-pin||The number of pins on the connector - sometimes also identified by size, as in: mini-bipin; medium-bipin etc. A G5 bipin has about a fifth-of-an-inch spacing (5mm), typically for T4 and T5 tubes; a G13 has a half-inch (12.5mm) spacing, typically for T8 and T12 tubes; a G20 'Mogul' is a rarer wider bipin found on General Electric's T17 range.|
|BL||Blue||The colour of light produced by the tube, in this case it normally refers to an "ultra-violet"-type light.|
|C||Circular||Denotes a circular-shaped tube, usually following the letter 'F'. 'FC' is often followed by a number indicating the diameter of the tube.|
|CCT||Correlated Colour Temperature||See entry for 'K' below.|
|CFL||Compact Fluorescent Lamp||This term refers to the diminutive tubes which are bent round into a compact unit to replace conventional bulbs, often referred to as "energy-saving".|
|<colours>||...||For details of light colour designations, see 'WW', 'EW', 'CW', 'DW', 'BL', 'CRI', 'K'.|
|CRI||Colour Rendition Index||This compares the lamp to a natural light source; if it renders colour perfectly, as if it were natural light, the index is 100; a lower index represents decreasingly accurate rendition. Somewhat confusingly, the CRI is sometimes joined with the light temperature value (see 'K') such that the first digit represents the CRI (divided by ten), and the next two numbers the 'K' temperature in hundreds of degrees.|
|CW||Cool White||The colour of light produced by the tube.|
|D||DESIGNER||Marketing term used by manufacturer Sylvania in some colour descriptions. May also indicate a 'Daylight' colour; see also 'DW'.|
|DW||Daylight White||The colour of light produced by the tube.|
|/ECO||ECOLOGIC||Marketing term used by manufacturer Sylvania.|
|F||Fluorescent||See also: 'C' (as in 'FC'); 'R' (as in 'FR');.|
|G5, G13, G20||...||See 'Bipin'.|
|HO||High Output||These lamps operate at a higher amperage (electrical current) than conventional tubes, and produce a brighter light. See also 'VHO' (very high output), and 'PG' (Power Groove). These types of lamps have different contacts so as to make them incompatible with their lower-current counterparts.|
|IS||Instant Start||The ballasts in these fittings uses a high voltage to start the lamp without pre-heating, hence a quicker startup, but also lower longevity if the lamp is turned on and off frequently. Characterised by a single-pin connection. See also 'RS'.|
|K||Kelvin||This indicates the "colour temperature" of the light. For practical purposes, the higher numeric range represents a "cooler" colour, with lower numbers corresponding to warmer light, over a typical range of 2000-6000K. See also 'CRI', which value is sometimes coalesced with the 'K' number.|
|KB||Kitchen & Bath||Used by General Electric for their Kitchen and Bath range, which the manufacturers claim makes food look "fresher and more appetising" ... .|
|...||Lumen/Lux||A unit of measurement of the amount of light energy emitted by a tube. 12.5 lumens would correspond roughly to the light of a candle. The term 'lux' is also sometimes used, being one lumen per square metre; it is thus a measure of light intensity.|
|O||Octron||Indicates 'Octron', a brand name used by manufacturer Sylvani, when used after 'F'.|
|P||PENTRON/PLUS/Pin||Marketing term used by manufacturer Sylvania for a range of intense-light lamps, as in 'FP'. PLUS is used as a suffix in some Sylvania colour descriptions. This letter is also used for 'Pin' by companies such as Philips, as in '/4P'. See also 'PG', 'PL'.|
|PG||Power Groove||A brand name used by General Electric to identify a range of higher output tubes, featuring a grooved bulb that increases effective arc length.|
|PL, PLC, PL-C, PLS etc.||Philips||Manufacturer Philips uses this as a prefix for its part numbers, denoting 'Philips Linear', being a straight tube, as in 'PLC' compact 'EnergySaver' brand bulbs.|
|R||Reflector||Typically, follows 'F', as in 'FR', or 'FHOR' (see 'HO' & 'F'); the reflector concentrates the light where needed. The 'R' may not refer to 'Reflector' - see 'RS'.|
|RS||Rapid Start||A type of ballast (see explanation in this table) which heats the lamp prior to firing the light; this results in a slight delay, but longer-lasting useage. RS bulbs require two contacts ("bipins") at each end. Compare with Instant Start (above).|
|SKU||Stock Keeping Unit||Not a tube characteristic as such, but a designation used by the manufacturer to identify stock.|
|/SS||SUPERSAVER||Marketing term used by manufacturer Sylvania.|
|T||Tubular|| The shape of the lamp as a whole. Often this is followed by the diameter of the tube, a number which may indicate multiples of an eighth of an inch (for example, a T8 is an inch wide), or it may be diameter in millimetres. If you do not have the tube in front of you, and are given the code T12, this might refer to a half-inch tube or a one-and-a-half-inch tube. However, there are a limited number of widths commonly encountered, so this might help to establish what is being referred to; for example, the following are common imperial widths: T2, T4, T5, T8, T9, T12, T17; |
whereas these are common metric diameters:
T16, T26, T29, T38.
|TL||Philips Efficient Tubular||Manufacturer Philips uses a 'TL' prefix for a range of energy-efficient tubes.|
|U||'U-shaped'||Used by General Electric and others.|
|VHO||Very High Output||See 'HO' for more details.|
|W||Watts||Usually preceded by a number indicating the power of the lamp, or its 'wattage'. The wattage is not always followed by 'W'; it may be the number appearing after the letter 'F'. To add extra mayhem to confusion, if it is a circular tube, the number following 'FC' is generally NOT a wattage, but a diameter.|
|WM||Watt Miser||Term used by General Electric for tubes that do not like spending power.|
|WW||Warm White||The colour of light produced by the tube.|
|XP/XPS||Extra Performance||Marketing term used by manufacturer Sylvania.|