The clear tube that protects a UV lamp is called a sleeve and is made of quartz. The UV clear quartz sleeve is an essential part of any UV water filtration system. This barrier can gradually become coated with mineral deposits and other contaminants, but its' job is to be a barrier between the UV lamp and the water flowing past. Regularly cleaning the UV quartz sleeve of your UV water purifier is important because contaminant buildup on the sleeve can hinder UV transmission and reduce overall system effectiveness. Thorough cleaning and maintenance of the quartz sleeve is recommended every 12 months (best to at the same time you're replacing the UV lamp), and the sleeve should be replaced every 24-36 months. Depending on the quality of your water, you may need to pull out the quartz sleeve and clean it every few months.

http://www.fudongquartz.com/fused-quartz-tuebs/clear-quartz-sleeve/