Ink jet printing — Post Office project

The British Post Office uses a phosphorescent machine readable code to enable letters to be sorted automatically. The phosphorescent material is printed in the form of a dot pattern in two rows across the face of the envelope. When activated by passing the latter under an Ultra‐violet light the phospor dots will start to glow and will continue glowing for some time after leaving the activating light source. This afterglow is read by a photodetector and the code pattern is interpreted by the machine electronics which instructs the machine mechanics to place the letter in the correct sortation box.