Ultrasonic fingerprint sensing
Capture d'empreintes digitales à ultrasons
Each time the sound speed varies, for instance at an interface between two materials,
a reflection/transmission occurs (the same as light). An ultrasonic wave is sent, and an echo is received
later, depending on the sound speed. One problem is the multiple echoes.
Ultra-sound fingerprint reading was not common in the past,
and required quite a big device with mechanical parts, and was quite expensive.
Moreover, it took a few seconds to grab an image,
and was not suited for large production volumes at low cost.
Its main advantage is the possibility to read the derma, the sub-surface of the skin,
rather than only the surface.
Several companies are working to make small solid-state ultrasonic sensors,
and production started with UltraScan/Qualcomm in 2016.
In 2017, there is a big hype because ultrasound sensors may solve the "in-display" problem
for smartphone makers, as they want to propose an edge-to-edge displays, and moreover, it looks
more natural to put your fingerprint directly on the screen. (Fingerprint sensors on the back
are less expensive, but this position is not natural although some people like that).