This is an awesome lamp for seeing how a lamp works under the phosphor coating - but it has to be used with caution!!

I remember when I frazzled my eyes with a 4W one a few years ago, absolute agony! The thing is the pain doesn't start for like a couple of hours after exposure so I thought I was fine having it on for 15 mins.

Actually it was made by Sankyo Denki in Japan, perhaps the best manufacturer of germicidal tubes! It is also not quartz, but ordinary soda-lime glass having a very low concentration of iron oxide. It is the iron impurity in soft glass which is responsible for its slight greenish colouration and UV-absorbing properties. But if you can make the glass without this impurity, it will transmit down to the UVC region. Since it is extremely difficult and rather expensive to make such pure glass, quartz is used instead by the cheap manufacturers of germicidal lamps but it is greatly inferior. The presesence of nitrogen in the air around a quartz tube causes it to degrade rapidly, and UV output falls. This does not happen to the soft glass tubes. The "flaws" you refer to in the glass are simply airlines and in no way reduce lamp performance - it is just a characteristic of Sankyo Denki's glassmaking process. Recently they stopped making their own glass however and buy it now from a Hungarian lampmaker.

Interesting! Thanks James!