Stamp (Welsh, English & Song Thrush)

In ordinary, visible light all these stamps look like they are printed on the same adhesive-backed paper. Put this collection under the UV tube though, and you’ll see that the Welsh and English standard Queen’s head stamps fluoresce a brilliant blue-white. The special edition Song thrush stamp on the other hand remains its original cream colour.

Some stamps, especially those from the US, are printed with fluorescent or phosphorescent inks to aid in the automatic sorting of different classes of mail. In this instance, however, it’s the paper, not the ink, which fluoresces. These fluorescent papers have been brightened during manufacturing by the addition of an optical brightener; a dye that absorbs light in the ultraviolet region of the electromagnetic spectrum and re-emits it in the blue region. These optical brighteners are also found in household detergents like washing powder; practically all white clothes now fluoresce a vibrant blue under UV light. 

The 31 pence Song Thrush stamp was introduced by the Royal Mail in 1998 as part of a campaign for biodiversity that aimed to draw attention to threatened species native to the British Isles; the Song thrush or Turdus philomelos is still widespread but is in decline, especially since agricultural changes on farmland have reduced its food supply and nest sites.