I took a photo a few years back of the back of a US five dollar bill with my camera with an infrared filter attached at the end. The purple five disappeared, as we can see below
https://imgur.com/a/gSRXdNF
Now, I posted this online a few days ago somewhere because I thought it was interesting. I figured other people would, too. Some people may have been intrigued, but a lot of the comments were basically telling me this was basic color theory and not all that different from children's spy glasses. That didn't sit right with me, but I couldn't explain it, because I am no expert in any of this.
So this evening I took an image of a five with a different camera - one that has been converted for infrared photography - no filter. The five was purple in that image, but then, after discussing with a friend, it hit me. The image I took way back when was outside in sunlight. I could take a photo in similar conditions tomorrow with my new camera and I suspect it would behave similarly but my friend came up with an idea that worked exactly as he theorized.
I went into my laundry room, positioned my camera, the bill and two two remote controls(for their infrared light). I shut the lights off and took a long exposure clicking the remotes to introduce only infrared light(or as good I could get without being in an actual dark room), and here we go:
https://imgur.com/rS7J4pv
the five disappears but also two white strips appear. This was expected, but I have never been able to see this until now. Every denomination 5 and higher has these strips, too(but I don't think any have a purple 5 that disappears) so I'll be able to get images of those also.
And lastly, I took a photo of a fake five dollar bill I have to see how it doesn't appear any different:
https://imgur.com/OQjwgSu
Anyways, this was actually a kind of fun experiment but I am glad it worked exactly as my friend thought it might. I can also stop going crazy now because I have exactly the answer I was looking for but didn't know enough about to say.
Hot damn! Some day soon i'll image all of the notes with this method and we'll see!