What is the difference between T5, T8 and T12 lamps? Fluorescent lamps, or fluorescent linear tubes as they are also known, are categorized according to their wattage, shape and diameter. The “T” in T5 indicates the bulb is tubular shaped, while the “5” denotes that it is five eighths of an inch in diameter.Click to see full answer. Simply so, how do I know which fluorescent light bulb to buy?This will reveal a lot about your current fluorescent tube and usually indicate if the bulb is T8 or T12. If no markings are available, the size in diameter of the tube is the easiest way to determine the type you have installed. T8 tubes are 1-inch in diameter and T12 tubes are 1 1/2 -inch.Also, what are the different types of fluorescent tubes? The two most common types are 40-watt, 4-foot (1.2-meter) lamps, and 75-watt, 8-foot (2.4-meter) lamps. Now, tubular fluorescent technology has improved. New products such as T8 and T5 are much more energy efficiency than T12 (T12 – 57 lumens/watt; T8 – 92 lumens/watt; T5 – 103 lumens/watt). Just so, are all fluorescent tubes the same? 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.How do I know what light bulb I have? Determine much light you need Instead of watts, look for the number of lumens, the measure of how much light a bulb produces rather than how much energy it uses. You’ll usually also see a watt equivalent, but to be sure, look for the lumens.