Starting in the late 1950's, countries around the world began to mechanize their mail sorting, in order to deal with the tremendous volume of mail going through the postal system. Automatic facer cancelling machines used optical recognition to locate the stamps on the envelopes, so that they could be properly cancelled. Some of the the first experiments in optical recognition, were conducted by the British Post Office, and made use of black lines printed on the back of the stamps in graphite, underneath the gum. These "graphite lines" did not work properly when the mail was damp, which in the UK was often. So, they were overprinted with phosphorescent bands in addition to the graphite lines, and then later with just the bands only.
After this, in the 1960's most industrialized countries began to "tag" the stamps with a phosphorescent compound in a similar way, either by overprinting the stamps with bands, or by coating the entire surface of the stamp. Generally, where different configurations of bands are found for a particular country or issue, this generally corresponds to different postage rates. So, for example, a Great Britain stamp tagged with 3 phosphor bands is usually a stamp used to pay the local first class postage rate, whereas a stamp with one single phosphor band is usually a make-up stamp, or a stamp for a lesser class, like third class mail or printed matter.
Generally, these phosphorescent compounds only react to short-wave ultraviolet light. Countries that used compounds like this include the United States and Great Britain. It should be noted that shortwave UV is extremely dangerous to the eyes, and any shortwave UV light should only be used with a protective shield to protect the eyes, and even then you should never look directly at the light. An exception to this general rule is Canada, which employed a taggant compound that reacts to the safer, long-wave, UV light, or black light as it is more commonly known.
Stamps tagged with this compound generally show bluish white bands or creamy yellow bands running down the face of the stamp, and these are usually visible even without the use of the UV lamp, and appear as yellowish bands, or clear, matte bands on the face of the stamp.
One key characteristic of phosphorescent tagging is that it will continue to glow very briefly after the light source has been switched off, and this glow will progressively fade. The stamps of Great Britain from the 1953-67 Wilding Issue come in 3 different types of phosphorescent chemical: green, blue and violet, depending on the length of the afterglow. The first phosphors that glowed a green colour, has an afterglow of approximately 5 seconds. The blue phosphors glow for 8-10 seconds, while the violet phosphors glow for only 2-3 seconds after the light is switched off. All Great Britain stamps tagged and issued after 1965 have the violet phosphor.
The pictures below show the appearance of the tagging under long-wave UV, and the faint afterglow that results when the light is switched off:
In the above picture, you can see the yellowish cream tagging bar running down the middle of the pair, and the bluish white tagging bars running down the sides of the 20c Prairies definitive on the right.
Here, if you look to the right, you can catch a glimpse of the afterglow on the 20c Prairies stamp that is left in the tagging when the UV light is switched off. It is difficult to capture, as it fades so quickly. But here, you can clearly see it.