Gadgets use one of three technologies for reading fingerprints: optical, capacitive or ultrasonic. The optical fingerprint reader is the oldest. Essentially, a miniature camera is used for finger recognition, which is often illuminated by small LEDs or a phone screen.
Unfortunately, these sensors are easy to fool. A good quality photo will mislead the device. Today, this method of reading fingerprints has remained only on the most budget smartphones.
The capacitive fingerprint sensor uses a grid of tiny capacitors that store electricity. The accumulated charge will only change at the point where the fingerprint protrusions touch. Thousands of capacitors then work to display your fingerprint pattern.
The third and most advanced form of fingerprint recognition is the ultrasound scanner. This technology is more reliable, not so easy to fool, and much faster (increases the speed of fingerprint recognition). As with medical ultrasound scanners, an ultrasonic pulse is transmitted to the finger and measured by a transducer. Bats, whales and dolphins use ultrasound to understand the shape of their surroundings. Sensors on a smartphone use ultrasound to understand the 3D shape of the ridges in a fingerprint. It works even through the phone screen.