Lamp manufacturers have adopted a common system for designating most of the fluorescent lamps available on the market. This system was intended to standardize lamp codes. However, some lamp designations are still manufacturer specific.

All fluorescent lamp designations begin with the letter "F" which simply stands for "fluorescent." The "F" is followed by a number that has two meanings depending on the mode in which the lamp operates. Fluorescent lamps that operate in the Preheat or Rapid Start mode use this number to indicate the nominal or rated wattage of the lamp. In this example, the "40" indicates that this lamp is rated at 40 watts.

Fluorescent lamps that operate in the Instant Start mode are commonly known as "Slimline" lamps. The number following the "F" for these lamps is used to indicate the length of the lamp in inches.

This can be rather confusing when reading the designation off the lamp or using a lamp catalog. Fortunately, most lamp manufacturers also include supplemental information such as "Slimline", "Instant Start" or "Rapid Start."

The next two or three digits indicate the bulb type. In our example, the bulb is tubular and is 12/8 inches in diameter. Other lamp types include T-5, T-8, T-10, and T-17.

U-Bent fluorescent lamps have the designation FB. These lamps also have a U3 or U6 code indicating whether the ends are 3-inches or 6-inches apart.