Letters and postcards fed into an automated mail processing plant are illuminated with ultraviolet light. The reaction of the luminescent features of the stamps on this illumination is used to position the mail items such that the stamps can be cancelled, and that the significant parts of the address such as postcodes may be read and the mail be sorted accordingly.
The luminescent features of the stamps are generally invisible or barely visible to the human eye in normal illumination. They can, however, be identified under ultraviolet light similar to the way it is done in the postal machinery. In general, fluorescent features can be identified with UV light of a longer wavelength than needed for phosphorescent features (see below).
The luminescent substance ("taggant") can be printed over the whole surface of the stamp, the main design, the margins only, single bands or bars or other patterns, or can be added the paper itself.
The tagging pattern can also be varied to enable sorting of mail according to the service class.