Black Light Discussion:

[nat]

"Is there anyone here who uses it?"

Yes

"What are some of the things that you can see on cards that aren't visible under normal light?"

Many post-WWII paper products (and, really, lots of other products) are made with optical brighteners. These chemicals make colors stand out more, but they also glow under a black light. Shine a black light on a white t-shirt and you'll see what I mean. If a card that purports to be from pre-WWII glows brightly under a black light, that's a strong sign that it's a fake. (Many things glow a little naturally, so you might also want to compare a suspect card with one you know to be genuine.) You might also be able to spot things that have been added to a card later (like re-coloring). Note, though, that not all newer materials have this stuff, so it's not a fool-proof test.

"Do you think that it's important to own one, especially for collectors who like to spend big money? "

I don't spend big money and I have one.

As for the specs, I don't know. Most of the little flashlight ones release a little visible light also. That's okay, but it makes it a bit harder to see the effect of the black light. Ideally you want it to be really dark.

[drcy]

How to Use a Black Light to Identify Reprints and Fakes of Antique Paper Collectibles

My favorite is the little LED flashlight shown in the article. But there are different styles that give off the same blacklight, so the style is just a personal preference. As long as they give off blacklight, they all work the same.

For more esoteric reading, shining a blacklight on a baseball card is an elemental form of spectroscopy, which is a highly advanced nuclear physics area. Spectroscopy in artifacts examination