black light.........
I like to think about a commercial I recall years ago. BASF. It was and is a German company. The ad had a line in it, "we don't make the products you use, we make the products you use better"
One group of such is brighteners. After WW II, paper manufactures started making paper with brighteners. Laundry detergents have them, nowadays. If you illuminate modern paper (after about 1947-48) it most likely has brighteners in it, and it will fluoresce. If you illuminate a page from a magazine from the 1920's, it won't fluoresce.
https://en.wikipedia.org/wiki/Optical_brightener