Most countries is use some sort of coating, underprinting, or paper that glows when subjected to UV light. Stamps that exhibit this behavior are called tagged. Tagged stamps are used in automated sorting equipment by the various postal authorities.

Collecting this material requires the collector to use a mineral lamp or UV light source. But because these lamps are dangerous to your eyes never look directly into the lamp to see if it is turned on. Relatively short exposures to Ultra Violet (UV) light can cause very painful burns to the corneas of your eyes. Exposure to UV can cause burns so severe that the surface of your eyes will blister. If you think a hair in your eye is painful, wait until you give yourself a case of welders flash by blistering your eyes with a UV lamp. Wear glasses if possible when using your lamp to check the tagging on your stamps. The glass or plastic in the glasses will absorb a considerable amount of the reflected light. Limit the use of your lamp to the shortest period you can. Accomplish this by doing as much sorting and arranging of the stamps as possible before you light your lamp.

There is another reason for limiting the amount of time you have your lamp on. This is economic. The filter on your lamp has a set life time. It can be used with maximum effect for only so many hours and then it needs to be replaced. Therefore, plan ahead on how and when you will use your lamp to check the tagging on stamps. You will save money in the long run and possibly pain in the short.