How (and Why) to Use Ultraviolet Lighting
by Kathleen Wunderly
Most "stamps that glow" were issued since the 1950's, when automated mail equipment became increasingly common in the United States and in other parts of the industrialized world. Because a human being is no longer looking at a piece of mail and making sure it has correct postage before canceling it, there has to be some way for the postal equipment to determine if a letter is properly franked.
This is done by adding a taggant - a phosphorescent or fluorescent material -- to the stamp, either in the paper or in the ink. When a tagged stamp passes under the optical reader in an automated mail sorter, the equipment "sees" the tagging and signals the machine to turn the envelope in the correct direction and then to cancel the stamp.
Collectors can see the tagging by using an shortwave or longwave ultraviolet light. Fluorescent tagging is visible under longwave UV light , and does not leave an afterglow. It will glow only while it is under the light. Phosphorescent tagging glows under shortwave UV light, and does have an afterglow for a short time after the lighting is removed. (Shortwave UV light also is used in philately to detect different kinds of paper, and to detect some types of fraudulent changes to stamps, such as added perforation teeth and or filled-in thins on stamps.)
Tagging may have been added in strips or blocks on a stamp, or over the entire stamp as a finish coating, or added to the fiber of the paper. There are several books that deal with the detailed history of tagged stamps, and the Scott U.S. Specialized Catalogue, for one, specifies tagging details for U.S. stamps.
UV lights are available from scientific supply stores as well as philatelic dealers. They are not inexpensive, but they can add an exciting and interesting new angle to your hobby research. Warning: Do not look into the bulb of a UV lamp! It is very damaging to eyesight. If you're not sure if the bulb is working or not, test it by shining it on a stamp that you know is tagged. Do not test the bulb by looking at it.