All Sterilight UV systems include a quartz sleeve, which surrounds the lamp. The quartz material used in R-Can's quartz sleeves are a 100% fused silica which lend to the sleeve's high transmittance. A high transmittance is important as the UV light must travel through the sleeve to get to the water.    NOTE: There are a variety of quartz sleeve designs, depending on the system that they are being used in. The QS-287D is a domed quartz sleeve which is used in the Copper™ series only. All replacement sleeves are shipped with two o-rings.