T8 versus T5 Fluorescent 1. What do the alpha numeric codes T5 and T8 actually mean? Much has been made of the differences between T5 technology and T8 technology. I think it is wise to begin by discussing what T5 and T8 actually indicate. The letter ‘T’ in the lighting industry stands for “tubular.” This makes sense when one considers that fluorescent tubes are in fact tubular. The number directly following the letter “T” indicates the thickness or diameter of that particular tube in eighths of an inch. • T12 = twelve eighths of an inch in diameter or one and … Continue reading T8 versus T5 Fluorescent