The safest variant are tritium capsules, that contain a small amount of tritium of various colours, within a robust glass capsule. Tritium is one of those mildly radioactive compounds that can only emit up to alpha and beta rays, which are conveniently limited by the glass container. Radium emits a small amount of gamma rays, those can pierce through glass and iron. Now, phosphorus is the element that gave the name to the phosphorescence phoenomenon, so it is a relatively safe light-sensible coating that can have a small glow in the dark according to how much light it absorbed before, but in large amounts it isn’t good. Marco lodola used neon astethics to make these sulptures that are basically made of light in a dark room:

www.bing.com/images/search?q=marco+lodola&form=HD… I’ve seen some of these firsthand, they’re amazing, and rather large.