I'd guess two out of three first enquiries we get ask for 395nm. The reason is simple, the LED is cheaper and the torch looks brighter to the eye because you see more of that purple glow. So people think they are getting more for less.
The thing is, 395nm leaks a lot of visible violet light. For curing some resins or for a cheap glow-in-the-dark party torch, fine. But if the customer's real job is checking banknotes or finding pet stains, that visible purple actually fights against them. It washes out the faint fluorescence they are trying to see.
365nm costs more, the LED is dearer and the good ones need a filter glass on the front to cut the residual visible light. But it makes the fluorescent stuff pop while the torch itself looks dim. That dimness throws people off, I've had a customer email me sure that the sample was broken because it looked weak. It wasn't, it was doing exactly its job.
So my routine now is to ask what they are actually pointing the torch at. Money, documents, minerals, scorpions in a desert, urine on a carpet, leak dye in an aircon system. The answer almost always points one way or the other.
Quick aside, scorpion hunters are a whole world. They want 395nm usually, wide flood, long runtime, because scorpions glow like crazy and you are walking for hours. That is one of the few cases where the cheaper LED is genuinely the right call.
For currency and forensic type work it's nearly always 365nm with a proper ZWB2 filter. Resin curing is a bit of both, depends on the resin maker's spec sheet, and I tell people to check that sheet rather than trust me.
Where it goes wrong is when a buyer fixes the price first and the wavelength second. We had a batch of 2,000 go to a US distributor in 2022 spec'd at 395 to hit a price, and three months later they switched the reorder to 365 after their end users complained the bills did't fluoresce well. Lost everybody time. So now I push the wavelength conversation up front even if it slows the quote down.
If you are not sure, just say what you're shining it on and ask for both samples. We will send a 365 and a 395 of the same body so you can compare on your own desk. Cheaper than guessing.
