Phosphor is used for postcode tagging dots and on the postage stamps of Great Britain from the 1960’s onwards. Is also present on stamps from Canada (older issues), Finland, Greece, Israel, Mexico (older issues) and the USA.
Mini UV Lamps
Stamps must be removed from protective mounts and are best viewed against a dark background in subdued light, complete darkness is not normally required.
You are looking for an afterglow as phosphor will absorb the light when you shine the light on the stamp and then re-emits it afterwards.
Shortwave UV can also be used to detect aniline inks, erased pen and fiscal cancellations, repairs to paper thins, added margins to imperforate stamps.
This is a skill that every collector of British stamps has to learn. It is an easy skill, and once learned, you wonder why you couldn’t ever do it before !