How does a phone fingerprint scanner work?

[soldar]

I have started using a Samsung Galaxy S7 Edge and the main button has a fingerprint scanner which allows unlocking the phone. I am curious as to how it works and I did some searching and all I could find is that it is by ultrasound.

I do not understand how an ultrasound transducer can read fingerprints. Does it really read fingerprints? I mean the prints from the skin as taken by the Gestapo. Or is it more of an ultrasonic signature of the entire finger? Even then it seems magical.

So, how do they do it?

[MosherIV]

Your googlefoo is weak young one

https://thenextweb.com/plugged/2019/07/23/how-does-in-screen-fingerprint-scanner-work/

https://www.androidauthority.com/how-do-ultrasonic-fingerprint-scanners-work-666053/

[soldar]

Your googlefoo is weak young one

https://thenextweb.com/plugged/2019/07/23/how-does-in-screen-fingerprint-scanner-work/

https://www.androidauthority.com/how-do-ultrasonic-fingerprint-scanners-work-666053/

Yes, I had seen those pages but I was hoping for something more detailed.

In other words, the ultrasonic method creates a 3D map of a digit, as compared to the flat image of the optical approach.

Great. How do I build one?

[MosherIV]

Great. How do I build one?

With a LOT of time, money, equipment, learning, trial, error, even more time and money.

I worked in ultrasonic ndt. The analogue front end electronics is very difficult.
That was only single beam and sinlge point through material.

The finger print application sounds like array sensor - 50x more complexity than single beam.
The software behind array sensor was way beyond me 

The resolution to get finger print ridges is also very very high, so very difficult.

[tom66]

I had a theory that the ultrasound fingerprint scanner is not doing anything more fancy than a frequency sweep, recording the intensity for given bins and matching this up against a known model.

And this method is not very secure but then people want on-screen fingerprint scanning for some reason.

[Zero999]

I found this patent, after a bit of Googling.
https://patents.google.com/patent/US8601876B2/en