The best UV lights emit as little visible light as possible. For Ultraviolet (UV) light, shorter wavelengths have more energy and generally speaking, more materials will glow under shorter wavelength UV light. Ultraviolet light is generally considered to be from 400nm to under 290nm. 400nm is barely UV and is right on the line of the visible light spectrum. It will appear as a purple light. However, once you get into the short UV wavelengths (such as those under 300 nm) they have so much energy that they will burn your eyes & skin
(which is exactly why they are used in tanning machines).