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.

Fingerprinting Fingerprinting ink Fingerprinting inkless

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 :)

The Mainguet classification of fingerprint sensing techniques

Photon: Light, Optics

FTIR: frustrated total internal reflection

Reflection with sweep

Reflection with direct sensor contact

Transmission through the skin

Reflection Touchless

Optical Coherence Tomography OCT

Electron: Capacitance RF field Impedance

Passive capacitance

Active capacitance RF field

Electrochemical

Electro-optical

Impedance

Force Pressure Mechanical

Conductive membrane
on CMOS, TFT

Micro-electromechanical
(MEMS, NEMS) switches

Piezo-electric nanowires

Phonon: Thermal, Heat, Temperature

Temperature, thermal mass

Sound, Ultrasound

Ultrasound