What's The Difference Between a 365nm and a 395nm UV Flashlight?
First, an animation showing the difference between the UV flashlight we recommend (365nm filtered UV flashlight) versus unfiltered 395nm UV flashlights we see many folks carrying around at shows. So. much. visible purple light. You can barely see any fluorescent mineral. More on these crucial filters below.
A friend of ours bought a 385nm to 395nm 100 LED UV flashlight (presently being sold on Amazon: https://www.amazon.com/uvBeast-IntensityFlashlight-BlacklightCommercial/dp/B01CV24OGK).
It is advertised as having 100 high power LEDs and says - 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".
It's all marketing. It is not 18 watts; 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 we even doubt that.
For $30.00 it's not a total waste of money, but for folks interested in fluorescent minerals, there are better 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 fluorescent minerals, this ain't the light.
But, if you insist, please install a visible light blocking filter on it. That's why our buddy brought it over - so we could cut a piece of Wood's Glass to filter out all the bright blue/purple light the cheap 395nm LEDs generate. It took about 20 minutes on our lapidary wheel to round down a piece of Wood's glass to replace the plastic lens. The resulting light is almost acceptable for fluorescent minerals use (but we 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).
What are filters?
UV bandpass filters are optical filters designed to transmit ultraviolet (UV) light and block the visible light. Examples include wood's glass, ZWB2, Hoya. These filters are necessary in blocking the bright blue light emitted by UV lights. You can purchase specific sizes of these filters for your needs at a number of places: aliexpress, amazon.
Back to our demonstration:
We selected a bunch of rocks as a demonstration. Took pictures of them under his flashlight without the filter and then with the filter. We also took pictures with our filtered 365nm LED UV flashlight. An animation at the end of this post shows the differences very clearly.
Fourth pic: the rocks under 390nm filtered flashlight. Notable in this pic: the ruby a brighter red, the cerussite 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: Here is the animation again showing the difference in fluorescence between the two filtered lights 365nm and 390nm.
Moral of the story: A 395nm FILTERED UV flashlight is kind of OK. Not everything will be fluorescent compared to a 365nm UV flashlight, but it's better than nothing (just barely IMHO). UNFILTERED flashlights are a waste of money for fluorescent minerals.