Hello, I want to show my class how tonic water will fluoresce under UV light. My UV sources are a couple of 12V UV bulbs, as designed to "pimp out" (don't ask) a PC. Here's the main problem: these also put out some visible wavelength, purplish light. I'd like filter this visible light out, so my students don't think the fluorescence is somehow just a reflection of the emitted (visible) light. Can anyone suggest a material that would effectively render these lights truly "black?" Thanks in advance!