T5 T8
T12 U-Bent

T5
T5 lamps are fluorescent lamps that are 5/8 of an inch in diameter. Differences in length and socket pin design versus conventional fluorescent lamps prevent any problems with electric circuits or human factors. T5 lamps have a diameter equal to 5 times an eighth of an inch or 5/8. These lamps are approximately 40% smaller than T8 lamps, which are one inch in diameter, and almost 60% smaller than T12 lamps which are 1 in diameter.

T8
T8 lamps are eight-eights and have an inch or one inch (2.54 cm), in diameter. T8 lamps use the medium bi-pin base, which allows T8 lamps to fit into the same luminaires as T12 lamps of the same length. T8 lamps achieve both improved color rendering and high efficacy by employing rare-earth phosphors, which are more expensive than halophosphates.

T12
A T12 lamp is twelve-eighths of an inch, or one-and-one-half inches (3.81 cm), in diameter. The phosphors used in standard T12 cool white and warm white lamps are halophosphates, which usually can provide either good color quality or high efficacy. Some types of T12 lamps also undergo this dual-coating process or a single thick rare-earth phosphor coating to achieve higher CRI values.

U-Bent
A U-Bend lamp is a type of fluorescent lamp that is shaped like the letter U and it is sometimes called a U-shape fluorescent lamp.