T5 Fluorescent Grow Lights

What are the differences in tubular fluorescent bulbs? Size and efficiency. The letter 'T' indicates the shape of the bulb (tubular) and the number after the 'T' represents the diameter of the bulb in eighths of an inch (A T5 bulb is 5/8 inch).

The T12 fluorescent bulb is the oldest and most common of the tubular bulbs. It is also the least efficient. Then came the T8 fluorescent bulb which will fit the same fixture as the T12 but uses roughly 20% less wattage to produce the same amount of light. Now the new T5 bulb is available which will NOT work in T12 and T8 bulb fixtures because it is shorter and has a different pin arrangement. 

T5 lamps are a popular energy-efficiency measure, due to their potential to cut energy use in lighting by more than 65%.