Inks on a document that are identical in colour but have different chemical compositions may provide evidence of an alteration when examined in the near infrared region of the spectrum. Although undetectable with the naked eye, a near infrared image is easily converted into visible one, using an infrared sensitive video imaging system such as the VSC5000 or VSC4^PLUS. Two techniques may be used.

The first is to look for differences in the near infrared light reflected or transmitted by the two inks. The document is illuminated with either incident or transmitted broadband infrared illumination and examined through a range of long wave pass filters with increasing thresholds between 650nm and 1000nm. Any difference in the spectrum of the transmitted or reflected light will become evident under at least one of the filters.

The second is to look for differences in the photoluminescence or fluorescence emitted by inks and, again, this is normally observed in the red to near infrared region of the spectrum. This fluorescence may be generated by illumination with intense visible light that has been filtered free of near infrared. As before, an examination of the document is conducted through a range of long wave pass filters with increasing thresholds between 650nm and 1000nm. Any difference in the spectrum of the fluorescence will become evident under at least one of the filters.

Both the VSC5000 and the VSC4^PLUS provide the examiner with the sensitive infrared imaging system and the light sources and filter sets to perform both types of examination. Image integration is also provided to enhance the fluorescent images which are often of very low intensity.

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