Detecting Counterfeit Money, Part IV: Intaglio Printing
We mentioned previously the intaglio printing process that creates fine lines around the portraits on genuine dollars. Now we get to see some details on the lines. Below is a magnification of Benjamin Franklin's engraved portrait on a $100 bill.
The oval shaped area containing the portrait and the fine-line patterns behind the portrait are made using intaglio printing, enabling very high resolution images and also “ridges”, essentially ink-filled grooves produced by very heavy (something on the order of 20 tons) pressure. The individual “ridges” in the paper can actually be felt by running a fingernail across the lines.
Your typical counterfeit bill will not fully duplicate the level of detail: it would likely have smudged or scratched fine lines and a smooth to the touch surface; overall, looking somewhat like in the picture below.
High-end modern copiers are getting better at reproducing the visual effect of fine semi-circular lines you see running around the face, but without intaglio printing presses cannot duplicate the raised-ink feel of genuine dollars. A smooth surface or noticeable breaks in the fine lines within the portrait are sure signs of a forgery.