has anyone else heard of this?

I have multiple cats and a couple times in the past week someone has been urinating outside the box. As much as I try to watch them 24/7....I have no clue who it is. I have my suspicions of 2 of the cats, and if I can't figure it out they are both just going to go to the Vet anyway but today I was looking around online and came across something on humanesociety.org that I found interesting.

This is copied from the site.

"The case of multiple cats: A whodunnit

If you only have one cat, it's obvious who's misbehaving. But what if you have more than one? You need to do some detective work.

It's a process of elimination, which means you will isolate one cat at a time to see if the inappropriate behavior stops while he's in isolation. This method isn’t foolproof, however, because if the culprit's behavior is stress-induced (see below), it may not occur if isolating him has removed him from the source of stress.

Another method is adding fluorescent dye to the cats' food (one cat at a time). The dye will glow in the cat's urine when a black light is held over it. You have your culprit."

Here is the link to the entire article. http://www.humanesociety.org/animals/cats/tips/solving_litter_box_problems.html

Anyway in what I have copied above, it suggests adding fluorescent dye to the cat food so that when they urinate outside the box, the urine will glow under a blacklight.

The questions I have-

1. Is this safe? It doesn't seem safe? I don't know that I would feel right trying this without talking to a vet first because it sounds odd to me, so I was wondering if anyone has heard of this and if they know if it is/isn't safe.

2. Doesn't all cat urine glow under a blacklight?