Recently I e-mailed SteriPen asking them if a bottle covered with Mylar reflective film would help the sterilization process. No answer yet. I think, based on direct observation, that we all know SOME UV rays pass thru Lexan & other clear plastics. Otherwise we would not see the purple light of the SteriPen's lamp. Correct?
So, reflecting that light back may help to kill bacteria & viruses "hiding" behind & within micro particles of debris. SteriPen requires swirling the water to give the UV rays a chance to kill any "hiding" microorganisms.
This is a good idea, but the problem is that we don't see UV light. What you're seeing is some visible light that the lamp emits, according to SteriPen, in order to let us know it is indeed working. So, reflecting the visible light back in won't change the sterilization. Remember, the primary action of the Pen is via UV-C light, and that wavelength is absorbed by plastic.
I'm curious how much UVA and UVB the Pen emits, as that might go through the plastic. (edit) Don't worry about that being a problem, though, I doubt a battery-powered lamp could emit enough energy to burn you, through a bottle full of water <img src="/forums/images/graemlins/grin.gif" alt="" />