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The Optics of Fingerprint Sensors
By Courtney Krafczyk, Rebecca Holmes, Michelle Victora, Jia Jun Wong, and Sheldon Scot Schlie, University of Illinois at Urbana Champaign, Illinois, USA 

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Optical fingerprint sensors use frustrated total internal reflection to distinguish the ridges and valleys in a fingerprint. A source illuminates a high refractive index material at a steep angle. When a finger is pressed against the material, the resulting bright and dark regions are captured with a camera and used to identify a person.