If you want maximum effect from "black light" reactive items, 365nm is the proper wavelength for this.
The 400nm visible UV LED flashlights only appear brighter because they're illuminating the item with visible UV light, not because they're actually brighter.
A good UV flashlight will only activate fluorescent objects.
Also - standard UV LED torches and Flashlights in the 400nm range generally produce a strong 'UV beam' best suited to illuminate 'at a distance' and are not usually suited to close-up work owing to the higher visible-light 'interference' output, which tends to mask (or suffocate) the resulting fluorescence.
1. POOR QUALITY (Frequency Range: 395 - 405nm)
UV Torches operating in the wider 390nm - 405nm range generally use poor quality 'cheap' UV LEDs which produce far too much unwanted visible 'purple glare' (interference) and far too little in the way of a useful UV light.
A general rule-of-thumb is the more LEDs the torch has, the lower the Quality of UV produced.
Avoid torches with more than 9 UV LEDs.
2. GOOD QUALITY (Frequency Range: 380 - 390nm)
Torches outputting UV in the 380nm range perform much better and produce superior results, with a much reduced 'visible' (interference) output.
Such units will naturally be more expensive than the cheaper 390nm models.
3. HIGH QUALITY (Frequency Range: 365 - 375nm)
By stark contrast, the higher quality, more expensive torches producing the 365nm wavelength, produce a near pure UV output which is the optimum frequency for UV fluorescence. This means you will see far more detail and in a greater range of colours than any other wavelength. As these units generate less unwanted Interference light, they may naturally appear weaker or darker. This can be misleading and should not be misinterpreted.
General rule of thumb is that the more visible UV light you see a worse quality it will be.
Lots of visible purple light is not an indicator of UV quality and indeed it is usually the opposite.
ATTENTION - CAUTION
Danger !
Eye damage guaranteed from careless use. These LEDs are manufactured to be almost invisible to the naked eye. It is made for a specific purpose and should not be used by children or people unauthorised by you the tradesman. Invisible UV is particularly dangerous to the human eye because it cannot detect that damage is being done.
INFO here:
https://www.ledshoponline.com.au/ult...orch-australia