Fingerprint sensing techniques
Techniques de capture d'empreintes digitales


Many techniques to capture a live fingerprint exist, and sorting them is a challenge. I propose here to use physics, and particularly the kind of energy that is used to interact with the skin. A closer look shows that it is not only the skin that is involved, but very often the surrounding air, and particularly the differences between physical properties of air and skin.


The Mainguet classification of fingerprint sensing techniques is the following:

The situation is never so simple, and in some cases, there is a cascade of interactions between the primary interaction with the skin to the electrical signal that is required at the end for further processing. For instance, electro-optical reading starts with some electronic interaction, which is further converted into light, then into electrons using a photonic sensor (photodiode).


At least one exception exist: ink and paper. But it is difficult to sort this technique as there is no automatic conversion to an electrical signal at the end!


Ink

Ink and paper are the tried-and-true way to take fingerprints, but technology has found ways to eliminate smudges and ink stains.

L'encre et le papier a longtemps été la méthode traditionelle pour collecter les empreintes, mais la technologie a vite permis d'éliminer les tâches.

Fortunately, some inkless techniques exist :)

Fingerprinting Fingerprinting ink Fingerprinting inkless

Photon

Light
Optics
Electron

Capacitance
RF field
Impedance
Force

Mechanical
Pressure
Phonon

Thermal
Heat
Temperature
Sound

Ultrasound