Broadband terahertz imaging of documents written with lead pencils

Far infrared transmission spectra of several graphite pencil leads on paper have been measured up to 2 THz using time-domain spectroscopy. The observation of the gradual absorption depending on the graphite proportion has been assessed for different pencils from hard to soft black-marking graphite leads. The resulting graphite transmittance is used to perform two-dimensional transmission terahertz imaging of written documents.