The visualization tag has been applied to software related questions as well as questions involving light. Given that we already have a software tag, all software-related visualization questions are covered by that. The other two questions that use visualization for non-software reasons could work though I would personally like to see these two questions be tagged with something along the lines of light or electromagnetic-spectrum or electromagnetic-radiation.
(Personally, I'd remove "visualization" from the invisible ink question.)
I just edited everything under visualization so there are no more questions there.