Bulb Types: The bulb shape and size of a fluorescent lamp are expressed by means of a code consisting of the letter "T" (which designates that the bulb is "tubular" in shape) followed by a number which expreses the diameter of the bulb in eighths of an inch. They vary in diameter from T-2 (1/4 inch) to T-12 (1 1/2 inches). In nominal overall length, fluorescent lamps range from 6 to 96 inches, which is always measured from back of lampholder to back of lampholder. For example, the actual overall length of the 40 watt rapid start T-12, 48 inch lamp is 47 3/4 inches. Circline lamps, which are circular, are available in four sizes: 6 1/2 inches, 8 inches, 12 inches and 16 inches outside diameter. There are also U-shaped fluorescent types (curvalume) with T-8 and T-12 bulbs. The width of U shaped types are measured for the distance between the ends. The overall length is measured from the face of the bases to the outside of the glass bend.
Base Types: For preheat and rapid start fluorescents, four electrical contacts are required, two at each end of the lamp. This is accomplished in the standard line of lamps by the use of a miniature bipin for T-8 and T-12 bulbs and mogul bipin for T-17 bulbs. In circline lamps, the cathodes are connected to a four-pin base located between the junction of the two ends of the lamp. High Output and Very High Output lamps have recessed double ontact (RDC) type bases. Slimline (Instant Start) lamps require only two electrical contacts, one at each lamp end and have single pin bases.
All compact fluorescent lamp bases shown have 2 or 4 pins. The 2 pin lamps are designed for preheat operation, with lamps having an internal starter. The 4 pin lamps are dimmable and are designed for electronic ballast operation; these lamps have no internal starter.