If you buy a cheapie from people who just sell all sorts of torches and don’t know anything about mineral fluorescence you will almost certainly have problems. That applies to shops like Bunnings and most Outdoor/Camping outlets as well as most internet sites (Bunnings sell a cheap unfiltered UV torch). We meet lots of people at shows who bought cheapies but then find they have to buy a proper UV torch to get proper fluorescence.
Cheap UV torches are mostly unfiltered and often use LEDs around 390nm which is not suitable for mineral fluorescence. Without a filter, they emit visible light as well as UV, which distorts and can overpower the fluorescence colours, making everything blue or purple-tinted (see picture below). They usually come with batteries that ridiculously overstate their capacity. 18650 batteries offering more than about 3700mAh will be fakes. Genuine Li-Ion 18650 batteries weigh around 45-48gm. Fakes weigh around 35gm or even less and can be potentially dangerous. They don’t run for long before they need recharging. Detailed information about fakes here.
