When you watched CSI, you must have thought that it would be nice to also look for traces of blood or other things using a black light. The so-called blacklight is soft UV radiation in the wavelength range of 300-400 nm and is used not only for interesting effects, but also for the just-mentioned detection of invisible traces under normal light or for verifying the authenticity of, for example, banknotes.
As you probably already guessed, you probably won't be able to create soft UV radiation at home with masking tape and a marker, that is, unless you have MacGyver written on the doorbell. Nevertheless, you can create an interesting gadget that will give your photos the same impression as if you were shooting real black light, by the way, see everything in the following video.
Either I have an extra clean toilet and the whole apartment or it doesn't work :)