Did any try to paint UV cathode? Well, need to explain myself - I would like to use UV cathode to light up UV tubing and coils but don't want that blue light from UV.. Is it possible to paint the cathode tube cover with some bright color (orange) to get rid of this blue color?
No, unless you have access to some fancy materials (and if you did then you wouldn't be asking). Paint will block UV.
Zero is correct. However, it is possible to buy 395 nm LEDs, which have zero visible light output but should cause an extremely strong fluorescence in anything even mildly UV-reactive. You can also get LEDs with wavelengths even shorter than 395 nm (i.e., even more UV) but I'm not sure how short you can go and still get visible fluorescence. EDIT: Warning: If you do follow the course I have suggested, please be aware that UV LEDs are actually mildly dangerous, because they can be quite powerful but your pupils will not constrict to block the invisible light. You would want to install them in locations where they didn't shine directly through your case windows. It wouldn't even be completely foolish to wear UV-blocking sunglasses while testing and working with the LEDs.
