Many people are aware that quartz is 96-98% transparent to UV-C and that normal glass reduces its transmission by approximately 50%. This is why U.V. Sterilisers have a quartz sleeve between the water and the bulb.
What many people are not aware of is that conventional UV sterilisers produce the UV radiation from a conventional glass tube therefore half of the intensity is lost before it even leaves the tube.
The D-D Professional UV Sterilisers feature a pure quartz bulb as well as a quartz sleeve so 98% of the UV-C produced is utilised.