Comparison of 365nm Flashlight to 390nm Flashlight
A friend of mine bought a 385nm to 395nm 100 LED flashlight (presently being sold on Amazon: https://www.amazon.com/uvBeast-Intensity-Flashlight-Blacklight-Commercial/dp/B01CV24OGK). It is advertised as having 100 high power LEDs and says - I quote "uvBeast - LATEST HIGHER RADIANT Intensity FLOOD EFFECT UV Flashlight Blacklight - 100 LED - Mega 18w 30ft UV Beam - Best for Commercial/Domestic Use Works Even in Ambient Light - USA Stock - UK Design".
To this I say phooey. All lies. It is not 18 watts; have no idea what they are talking about but even being extremely generous it is at best 4 watts of UVA. More likely around/less than 2 watts. It may consume 18 watts of power to generate all the blue light, but I even doubt that.
For $43.00 it's not a total waste of money, but for serious mineral enthusiasts there are better (but more expensive) options. If you're looking for scorpions, cat urine, scanning your hotel room, etc this would probably be just fine. But if you're using it for mineral fluorescence this ain't the light.
But, If you insist, please install a visible light blocking filter on it. That's why my buddy brought it over - so we could cut a piece of Wood's Glass to filter out all the objectionable blue/purple light the cheap 395nm LEDs generate. It took about 20 minutes on my lapidary wheel to round down a piece of Wood's glass to replace the cheap plastic lens. The resulting light is almost acceptable for mineral use (but I still highly recommend you invest in a serious 365nm flashlight). (BTW - Wood's glass for this light is actually better than a Hoya filter because the UV is almost in the visible region and Hoya doesn't pass it very well).
I selected a bunch of rocks as a demonstration. Took pictures of them under his flashlight without the filter and then with the filter. I also took pictures with my filtered 365nm LED flashlight. An animation at the end of this post shows the differences very clearly.
First pic: Index to the rocks, white light.
Second pic: the rocks under shortwave UV. Some will appear overexposed - very difficult to take a bunch of different intensity rocks and photograph them all at once, but I wanted to provide this as a reference. Note that the ruby is not SW fluorescent, the cerrusite and fluorite barely fluorescent. Terlingua calcite is blue. Afghan sodalite is white. Aragonite is violet/pink.
Third pic: the rocks under 365nm filtered flashlight. Notable in this pic: the ruby a dull red, the cerrusite and fluorite are nicely fluorescent. Terlingua calcite is pink. Afghan sodalite is orange. Aragonite is hot pink. Willemite is green with a dull glow from the calcite. Both the Franklin willemite and the Puttapa willemite are nicely fluorescent.
Fourth pic: the rocks under 390nm filtered flashlight. Notable in this pic: the ruby a brighter red, the cerrusite and fluorite are somewhat fluorescent, but dimmer. Terlingua calcite is blue (this is very interesting, the shift back to blue). Afghan sodalite is still orange. Aragonite is hot pink but slightly duller. Franklin willemite is not fluorescent, Puttapa willemite dimmer.
Fifth pic: the rocks under unfiltered 390nm flashlight: If this picture doesn't convince you of the need for a filter, nothing will. All of these pictures were taken under identical conditions, exposure settings, and RAW development settings. The blue cast in this picture shows you why all those pictures on Ebay look blue. Useless for minerals. The blue overwhelms all the color and changes tints on everything.
Sixth pic: an animation showing the difference in fluorescence between the two filtered lights - 365nm and 390nm.
Moral of the story: 395nm FILTERED flashlights are kind of OK. Not everything will be fluorescent compared to 365nm flashlights, but it's better than nothing (just barely IMHO). UNFILTERED flashlights are a waste of money for the fluorescent mineral hobbyist.