Fluorescent Ink & Stamp

Fluorescent inks are often used in ‘security printing’ on banknotes and postage stamps to prevent forgery, tampering and counterfeiting, and to help with the identification of different classes of mail. One of the stamps on this envelope (the one that features the side profile of a woman derived from a 5th century Roman coin) fluoresces a very weak greenish-yellow under ultraviolet light. 

The envelope also features a fluorescent orange tag that runs just underneath the stamps. In the same way as a barcode on a tin of baked beans, this combination of vertical lines of varying heights and groupings codifies information, in this instance, about the date, time and location of the letter's posting. The use of fluorescent inks allows for the automatic sorting of post by machines, as under UV light this barcode can be read clearly without being obscured or confused by  other markings that may be on the envelope.