TheCodeBreakers
Munich censorship station, and an undamaged one in the censorship
station on an upper floor of the Hamburg post office. It examined
suspected letters on an assembly-line basis by ingeniously exploiting
some principles of physics to make the invisible ink glow. It first exposed
the paper to ultraviolet light. This pumped energy into chemicals of the
ink, boosting their electrons out of their normal orbits into higher ones.
The chemical was then in a metastable state. The heat from a source of
infrared then nudged the electrons from their higher orbits back into
their regular ones. As they did so, the substance would give up, in the
form of visible light, the energy that it had absorbed from the ultraviolet.
Since this phenomenon will occur for nearly all substances, even
common salt, though some will "naturally shine more brightly than
others, the Germans had a system that would develop a good many inks.
annaabi.ee 
