Friends don't let friends use crappy unfiltered 390nm flashlights.
This is an educational post, directed towards folks new to the hobby. There has been discussion in our FB group lately about using cheapie 39Xnm flashlights for fluorescent minerals. I am passionate about this - 400/395/390nm LED flashlights are useless for our hobby.
First of all, you can't put a filter on a 390nm flashlight - it would block most of the light (most of the light from those LEDs is above 400nm, not UV at all).
Secondly, many (most?) minerals are not strikingly fluorescent under 390nm.
Third - if you attempt to take a pic with it, all you will get is a blue blob.
I understand that the only light you may have at the moment is a 390nm flashlight. Some would say it's better than nothing. I cannot present a serious argument to that. But I will say that I personally would not bother hunting for fluorescent minerals with it. Start shopping for a 365nm light now; be ready for your next outing.
Our goal is to educate folks about what is available to them - especially those new to the hobby. Very often we see folks who have developed an interest in the hobby but are stuck with a 395nm light. Some rock dealers still sell these lights to beginners, :-( and they can even be purchased at Home Depot. Although 365nm flashlights can be purchased for just a few $$$ more than a cheapie 390nm flashlight - but the problem is, they not available everywhere. Do yourself a favor - buy a UV flashlight today! The result will be more minerals found, better pics, and much more enjoyment in the hobby. It scares me to think of the new people who have been disillusioned by using one of these crappy flashlights when, for a few dollars more, they would truly see the beauty we all know is there.
Below I put together a set of pics of some commonly encountered fluorescent minerals. I put two lights next to each other illuminating a single specimen, with a divider to prevent light leakage from the two light as much as I could from side to side. On the left is a 390nm light, on the right is a 365nm light.
Judge for yourself.