In-display fingerprint scanners have been a trendy way to unlock your phone, where the fingerprint sensor is located under the screen. It’s generally possible only in amoled displays, but some brands have come through a technology to use those sensors in an IPS panel, but they are not yet commercially available.
Optical fingerprint scanner
This sensor takes a 2D image of the fingerprint by lighting up the device at the portion of the sensor and stores it on your device and when you try to unlock the phone, it checks if the image which is saved is same as the one which is trying to unlock the device. So, if the image is similar, it unlocks the device. It does not work some times when your finger is dirty or oily.
Ultrasonic fingerprint scanner
ultrasonic fingerprint sensors use very high-frequency sound. These waves are used to map out the details of the user’s fingerprint. When you try to unlock the device, the screen lights up and checks the pulse’s pressure. If it matches, then the device is unlocked. It is more secure than optical fingerprint scanner, and even costlier. It works even though your finger is oily. You can now see these sensors only in some high-end flagships.
Techstuff 
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