The code on any fluorescent lamp will start with an F. This simply indicates that it is fluorescent rather than incandescent, like traditional light bulbs.
The letter F will be followed by a number that denotes the lamp's wattage, just like the one you would find on an incandescent bulb.
Next comes a letter that stands for the lamp's shape. T, for "tubular," is by far the most common because tubes are part of the basic structure of most fluorescent lamps.
The first part of the code on a fluorescent lamp is capped off with one more number; this one indicates the lamp's diameter. Fluorescent lamps are usually quite narrow and their diameter is measured in eighths of an inch. A lamp with a 1-inch diameter will be marked with an 8. A lamp with a 1.5-inch diameter will be marked with a 12.
You'll see a representation of the color of the lamp's light. The higher a lamp's temperature, the cooler its color; the lower the temperature, the warmer the light's color will be. This is often represented by a number that states the lamp's temperature in degrees Kelvin. Alternatively, you may see the letters CW or WW, which stand for "cool white" or "warm white," respectively.