UV and Potter

The Harry Potter stamps from England carry a little secret. When they are held under a UV-lamp, some funny effects light up. A nice gimmick, but how does this work? Let's dive into that.
Already in 1959 luminescence was used to accelerate the sorting of the post. As successor of the graphit line sorting experiments were done in England with phosphorus. Phosphorus, a poisonous substance but not very harmful when applied in small amounts, was applied in lines over the stamps. After strong illumination the phosphorus stayed glowing for a short time, and by this a machine could determine the position of the stamp and thereby position the letter with the stamp on the lower left side, so the stamp could be canceled by an automatic stamping machine.
Other countries also used this technology. Some American stamps light up in red or green, and some from Australia also in red, but most phosphorus stamps show a yellowish-white light when they are shine upon by ultraviolet light. Some countries experimented with addition of phosphorus in other ways. Germany tried to add phosphorus to the paper during the production, which makes that stamps printed on this paper light up at both the front- and backside. Later coating of the paper was seen as the best method. These stamps only light up at the frontside. Gradually there was a change to the cheaper and less dangerous fluor. Fluorescence works in the same way as phosphorescence. Shine upon by ultraviolet light fluor lights up in white or yellow, but when you switch off the light the luminescence stops immediately, while phospor has a slight after-glow.
UV light is a sort of light that is just outside the violet colour, and that can not be seen by the human eye. The wavelength of light is expressed in nanometers (nm). One nanometer is 1 billionst of a meter = 0.000 000 001 meter. For UV light the wavelength is between 100 and 420 nm.

For philatelic use the wavelength around 365 nm is the most common, although there are some countries with phosphorus stamps that require a shorter wavelength (Great Britain, Yugoslavia, Israel and the USA). The nice characteristic is that the UV-radiation causes a reaction in the phosphorus and fluor that generates the glowing. Radiation with a wavelength shorter than 100 nm is known as Röntgen radiation, which passes through many substances, but not through our bones.
For the Potter issue phosphorus with a long wavelength is used and under a normal UV lamp the secret message can be seen. On the Hagrid motorcycle stamp the headlight of the Royal Enfield is on and the moon shines through the trees. Further Voldemort leaves behind a trail of light. On the left with, on the right without UV-lighting:

The complete series under UV-light

The complete series in normal light

Hans de Kloet

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