The Secret Service and the Department of Homeland Security Science and Technology Directorate helped create the Digital Ink Library, a searchable database containing the electronic version of the International Ink Library's collection of ink samples.

Ink of different types creates unique color bands, thus forming a "fingerprint." Analysts working on questioned documents compare the unknown sample's bands to known samples to find a match. The software automatically selects the known samples that most closely match the ink of interest.

Source: The Economic Times