Photoacoustic (PA) imaging for the examination of altered documents in forensic science has been investigated by an innovative system with a nanosecond pulsed laser, a microphone, and a handmade sample holder. The PA signals from lines written by black pens on paper were successfully detected without coupling media such as gels, and the signal amplitude was found to depend on the type of writing pen. The original contents of the document changed by the addition of a line and of the obliterated document were clearly observed by the PA imaging. The nondestructive PA measurement and imaging have been demonstrated to be a high-potential technique to discriminate writing inks and to examine altered documents.
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