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Concealed
information (top) becomes visible by absorbing the infrared
fluorescence emitted by the masking ink (bottom) |
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Infrared
fluorescence emitted by the concealed ink is sufficient
to penetrate the masking ink when illuminated by intense
visible light. The weak emission is integrated until visible |
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Revealing
concealed or masked information
Concealed information on a document may
be revealed by examining the document under visible or near
infrared light. A number of effects may be exploited:-
- If the masking ink is
transparent to near infra red light and the underlying ink
is opaque, the underlying information will be revealed using
either incident or transmitted infra red light and a suitable
infrared high pass filter.
- If the masking ink is
slightly transparent to visible light and the underlying
ink is photoluminescent, the masked information may be revealed
by illuminating the document with intense visible light,
some of which will penetrate the masking ink to induce the
underlying ink to fluoresce. Although very weak, the fluorescence
signal may be integrated to produce a strong legible image.
- If the masking ink is
fluorescent and the underlying ink absorbs the fluorescence
the underlying information may be revealed by examining
the document under intense visible light.
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