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add ultrasonic diagram

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@ -61,6 +61,11 @@ Conveniently they also don't require a light source under them to work, although
Ultrasonic fingerprint sensors use sound to create a detailed 3D representation of your fingerprint using ultrasound waves (sound waves with a frequency greater than 20khz). It's a similar concept to what's used to map the ocean floor: sound is emitted from transducers and bounces off your skin before being picked up by tiny microphones. By measuring the time it takes for the sound to reach the microphones, your phone can create a detailed map of the ridges and valleys in your finger.
<figure>
![Diagram showing the transducers emitting ultrasound waves and reflecting off the finger tissue, bouncing back and then being picked up by microphones](../assets/images/biometrics-explained/ultrasonic-diagram.jpg)
<figcaption>source: <a href="https://www.researchgate.net/publication/285770473_Piezoelectric_Micromachined_Ultrasonic_Transducers_for_Fingerprint_Sensing">Yipeng Lu</a></figcaption>
</figure>
This is the most expensive type of sensor but it produces the most accurate readings with the highest reliability. It doesn't matter if your finger is a bit dirty or wet, it will still work unlike optical or capacitive sensors.
Ultrasonic sensors can't be fooled by 2D images.