normal glass works fine!
No you want borosilicate or quartz.
soda lime stops most uv.
That’s why I have to take reptiles outside for good uv exposure.
window glass is soda lime
hmm anybody know what kind of glass the UCL’s are?
Probably soda-lime, according to Wikipedia, which says that most float glass is soda-lime.
This random website indicates that for 365nm and up soda-lime glass is not that bad (EDIT, not that good either): http://www.sinclairmfg.com/datasheets/optical3.html
Borofloat seems to be better than soda-lime or the borosilicate glass shown on the link above: http://www.us.schott.com/borofloat/english/attribute/optical/index.html
There is not much light coming out from 365nm, but somehow those invisible light still hurts my eyes.
I hope you’re joking. Even though you can’t “see” UV light, it can still do serious damage to your eyes. Just like staring at the sun does damage to your eyes.
and worse because your pupils don't narrow down and you're not aware what goes wrong. (Except for the disturbing sting on your retina 
)
I am totally aware of it. I only tried like 15 secs seconds each time, already feel kind of uneasy. I have totally stopped using it for good.
Too bad they don’t offer this in flashlight sizes.
http://www.bhphotovideo.com/c/product/189951-REG/Rosco_120336507495_Permacolor_UV_Woods.html
Ok. I misread your original post and assumed you were staring down the reflector 
.
I wouldn’t want one of my fellow BLFers to go blind! :cowboy_hat_face:
Stop looking into uv leds!
the visible light isn’t dangerous.the uv you can’t see is what’s hurting you.
What you can’t see will hurt you. When I point my 365nm light at the palm of my hand, there isn’t much visible light but I feel heat.
Use glasses people! Uvex amber or even uvex clear. Available on amazon and elsewhere.
Uvex sct-orange if using high power 400nm leds. Led wavelengths bleed out to the sides somewhat. The marked wavelength (like 365nm or 400nm) is the center peak. Need to cover more than just the center peak.
Simon, can we get this LED with 3 mode driver in an S2+ 18350 version? I was going to build that but if you have it already I will just buy it!
They do! The problem is yours, your flashlight is not big enough 
That is definitely Rule #1 for flashlight owner. I am referring the reflected beam. Even it is projected 3-5m distance away.
I ever used it with my Oakley sunglasses 
but it blocked out too much light. A clear UV glass seem to be better choice.
reflector is not suitable ,the led may be not in the centra
What about using the plastic (bow tie) LED centering disks? I find they work well with reflectors that fit XML. Does it focus properly?
Do you sell singles (not 5 pack) of this LED? On a 16mm board perhaps?