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 :
- Light (optics): some photons are sent to the finger, interacts some way with the skin, and we get a feedback, in general read with a camera using photodiodes (or any kind of photonic sensor).
- Electromagnetic field (capacitance, RF field...): some electrons, and by extension any electromagnetic field, interact with the skin and some feedback is obtained, using antennas or the like.
- Mechanical interactions (pressure): some forces is applied over some materials that give in return some information, in general an electrical answer (= a sensor, by definition). Pressure sensors, tactile sensors...
- Thermal: some heat (phonons) is exchanged with the skin, converted into some electrical signal thanks to pyroelectric material or thermistances.
- Ultrasound: an ultrasonic signal is sent, and an echo is obtained, which is converted into an electrical signal, thanks to piezoelectric materials.
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 exists : 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 :)