Page 15 - Frank Walton GB Queen Elizabeth II Wildings
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Graphite and Phosphors
The post office commenced the trial of automated letter facing machines in Southampton on 19 December 1957.
The equipment used needed to be able to detect the position of the stamp on the envelope, so the stamps had
to have a feature of some sort that could be read automatically.
Stamps appeared with graphite lines printed on the back. The
graphite conducted electricity, which allowed it to be detected.
A single line on 2d stamps to allow the
Two lines on all values other than 2d detection of printed paper rate mail
Graphite lines proved not to be as effective as desired, so phosphor bands that could be detected under ultraviolet
light were developed. Three different formulae were used.
Green phosphor Blue phosphor Violet phosphor
Lettalite B1 Lettalite B2 Lettalite B3
Very long green afterglow Long blue afterglow Short violet afterglow
Reacts to ultraviolet 200-300nm Reacts to ultraviolet 200-400nm Reacts to ultraviolet 254nm
