Compact Touchless Fingerprint Reader Based on Digital Variable- Focus Liquid Lens C. W. Tsai* a , P. J. Wang b and J. A. Yeh c a Lustrous Electric-Optics, Ltd. Inc., b Department of Power Mechanical Engineering, c Institute of Nanoengineering and Microsystems, National Tsing Hua University, 101, Guangfu Road Sec. 2, Hsinchu 30013, Taiwan ABSTRACT Identity certification in the cyberworld has always been troublesome if critical information and financial transaction must be processed. Biometric identification is the most effective measure to circumvent the identity issues in mobile devices. Due to bulky and pricy optical design, conventional optical fingerprint readers have been discarded for mobile applications. In this paper, a digital variable-focus liquid lens was adopted for capture of a floating finger via fast focus- plane scanning. Only putting a finger in front of a camera could fulfill the fingerprint ID process. This prototyped fingerprint reader scans multiple focal planes from 30 mm to 15 mm in 0.2 second. Through multiple images at various focuses, one of the images is chosen for extraction of fingerprint minutiae used for identity certification. In the optical design, a digital liquid lens atop a webcam with a fixed-focus lens module is to fast-scan a floating finger at preset focus planes. The distance, rolling angle and pitching angle of the finger are stored for crucial parameters during the match process of fingerprint minutiae. This innovative compact touchless fingerprint reader could be packed into a minute size of 9.8*9.8*5 (mm) after the optical design and multiple focus-plane scan function are optimized. Keywords: Fingerprint reader, Digital Liquid Lens, Variable-focus, focus-plane scanning 1. INTRODUCTION Biometric technology is recently promoted to replace the conventional password in online person authentication as the internet frauds in identity theft have grown exponentially [1]. Compared with other biometric technologies, fingerprint recognition is widely applied because of its convenience and high success rate. For example, TouchID of Apple’s iPhone 5S used the capacitive touch sensor to read user’s fingerprint. But, the optical touch fingerprint readers, known for high reliability and wide acceptance in entrance security systems, have become obsolete in mobile applications because of the difficult in miniaturization of systems. In fingerprint recognition, fingerprint capture area is crucial for extraction of sufficient fingerprint minutiae in later comparison of data. Therefore, design a long optical path distance is necessary for the optical touch fingerprint reader to acquire the large fingerprint area in the principle of fundamental optics theory. In addition, physical contact between fingers and reader results potential problems such as fingerprint deformation over-applied forces, smear image from wet or oily fingers, dull and dusty fingers, and residual fingerprint imprints from previous users. To circumvent those problems, a handful of touchless fingerprint readers have been devised [2-3]. But, most of the touchless fingerprint readers have to rely on a holder to fix the finger position for a good fingerprint image. Without the holder design, the reader will generate high recognition rejection because the rolling angle and pitching angle of the finger would give distorted 2D image plus variable image magnification at non-constant object distance. As a result, miniaturization of the previous touchless fingerprint readers would be nearly impractical. To achieve the goals in miniaturization and touchless structure, we proposed to adapt a digital variable-focus liquid lens for fast-scan at pre-elected focal planes with algorithms in calculation of the distance, rolling angle and pitching angle of the targeted finger. In this paper, the compact touchless fingerprint reader consists of a compact camera module, a digital liquid lens and a LED illumination source with optimized optics design. In the prototype device, a commercial webcam module was used to capture the fingerprint image. The digital liquid lens atop the webcam module is designed for fast scan multiple focal planes from 30 mm to 15 mm in 0.2 second, consecutively. The fingerprint recognition software based on NIST open source was developed to extract the fingerprint minutiae, enroll the minutiae and identify the fingers. Meanwhile, optical design of the lenses module with digital liquid lens for a compact touchless fingerprint reader was completed for miniaturization into a size of 9.8*9.8*5 (mm).