A UV Flashlight Buyers Guide
UV light is by definition invisible light. Both UV & IR (infrared) light cannot be seen by the Human eye.
Why then can you see a purple glow from a UV light? The answer is simple. UV LED lights emit a very narrow bandwidth of light but peak at a specific wavelength (in our case, 385 nm). They do however emit a small amount of light energy on a side of that peak which is in the visible range.
See graph below:

The best UV lights emit as little visible light as possible. Shorter wavelengths have more energy and more materials will glow under shorter wavelength UV light. Extremely short UV wavelengths (such as those under 300 nm) have so much energy they will burn your eyes & skin (which is exactly why they are used in tanning machines). So, in short, if you see an inexpensive UV light for sale that emits an very bright blue/purple light, you'd know they are using inexpensive 400 nm LEDs and you'd certainly be wasting your money buying it. Good UV lights will emit some visible purple/violet light, but it should never appear to be very bright or blue.

• Entry control at events (pass mark on people's hands)
• Leak detection using fluorescent dyes (automotive, etc)
• Curing UV adhesives
• Detecting repairs in pottery, china, etc
• Scorpion/insect hunting
• Law enforcement (detecting UV strip or insignia on ID)
• Product QA (detect authentic strips on product)
• Counterfeit bill detection
• Electronics (test sensors, etc)
• Lab/science experiments