A teardown of the newly launched OnePlus 6T reveals that the company is using an optical in-display fingerprint sensor. The camera that recognizes the fingerprint gets light from the AMOLED display that shines the light only in one direction (towards the camera). Overall, the implementation is well done and this could pave the way for in-display selfie cameras in future phones.
https://www.notebookcheck.net/OnePlus-6T-teardown-reveals-an-optical-in-display-fingerprint-sensor.358166.0.html