Sunlite 30177-su f28t8/sp841 28w fluorescent t8 bulb, medium bi-pin, cool white the fluorescent 28 watt 4,100k cool white t8 high performance straight tube features a medium bi-pin base, provides a light output of 2,400 lumens at a color rendering index (cri) of 83 and can last an average of 20,000 hours. These lamp utilize a single and double coat phosphor system. Series lamps have an increased cri due to the use of tri-phosphors (rare earth phosphors). Tri-phosphors also increase the lumens/watt output of the lamp. These series lamps offer superior quality of lighting, increased energy efficiencies, and extend life.