If you take Silly Putty and press it into your hand, then peel it off, you have a lovely impression of your palm print. That's what "GelSight" does digitally only at the 2-micron level. MIT researchers use an elastomer, derived from elastic polymer, with a thin layer of reflective paint as a sensor.
Impression of ink printed on paper (via MIT)
The paint closely conforms to any object pressed against it, which alters the reflective properties of the paint. Press the reflective side against a chicken feather, for example, and the system's camera captures a 3-D image of the intricate, microscopic structures of the feather. GelSight captures and displays surface features as small as 2 microns. The project started out as "robot skin" or e-skin, which attempts to give robots a sense of touch. They soon realized that this tool could also be used in criminal forensics, like ballistics and fingerprints. GelSight, about the size of a Coke can, is so sensitive it can display a three-dimensional image of the letter "T" in "Treasurer" at the bottom left of a $20 bill. I can hardly even read the word, much less the letter.
Eavesdropper
