Identity certification in the cyberworld has always been troublesome if critical .... was mounted on a commercial 2MP webcam module with a LED illustrator aside ...
Compact Touchless Fingerprint Reader Based on Digital VariableFocus Liquid Lens C. W. Tsai*a, P. J. Wangb and J. A. Yehc a Lustrous Electric-Optics, Ltd. Inc., b Department of Power Mechanical Engineering, cInstitute 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 focusplane 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).
2. OPERATIONAL PRINCIPLE Operational flow of the current touchless fingerprint reader is illustrated in Fig. 1. At the beginning of identity certification, the reader has to determine if there is any target finger by analyzing the video image streams. After acknowledge the existence of the target finger, the LED illuminator turns on to enhance the fingerprint image contrast and increase the light intensity for small shutter duration of camera module. Then, the digital liquid lens starts to scan multiple focal planes by changing its diopters together with camera capturing synchronously a series of pictures at preselected focal planes. The reader determines the best one in the acquired finger images by analyzing the acquired images. Hence, the targeted picture is processed sequentially via contrast enhancement, binarization, and etc. And, the fingerprint minutiae are calculated. Finial, the reader conducts the enrollment and identification process to confirm the target fingerprint. Scan at multiple focal planes is rudimentary in the current touchless fingerprint reader. It extends the image capture range to the operational range in use and maintains a narrow depth of field (DOF) characteristics. In the touchless fingerprint readers reported elsewhere, a tradeoff exists between wide DOF and narrow DOF of the camera [4]. To achieve high minutiae match accuracy, a narrow DOF feature is necessary for capture of the fingerprint with the same image magnification. But, a narrow DOF results in focus difficulty because the target finger must be at exact location and standstill. Scan at multiple focal planes also circumvents the problem in locating the position of a floating finger. The fingerprint picture captured by the reader will store the focal plane distance. The value would adjust the image magnification factor during match of fingerprint minutiae. By analyzing the image sharpness distribution variance at each captured picture, the rolling angle and pitching angle of the finger can be calculated. The location of finger minutiae should be re-adjusted to remove the influences by rolling and pitching. With the help of scan at multiple focal planes, the touchless fingerprint reader needs neither a glass plane nor a holder to acquire the fingerprint. Therefore, the size of the proposed fingerprint reader can be adequately miniaturized without complex and impractical design but an extra adapted digital liquid lens. Moreover, this reader can detect the fake 2D fingerprint picture or forged 2D fingerprint model duplicated from the latent fingerprint on the plane surface by the same image processes.
Sensing if a finger in front of reader by analyzing the image stream
YES, then DO the followings. 1. LED lighting 2. Liquid lens actuation and capturing a series images at different distance
Select an image with the best quality
Process the image and capture the minutiae
Enrollment and identification Fig. 1. Operation flow of touchless fingerprint reader based on digital liquid lens
A photo picture of the prototype of touchless fingerprint reader was demonstrated in Fig. 2(a). A digital liquid lens was mounted on a commercial 2MP webcam module with a LED illustrator aside for light source. A fingerprint recognition software based on NIST open source was developed to execute the fingerprint minutiae extraction, enrollment and identification. The fingerprint reader scans multiple focal planes (7 consecutive pictures) from 30 mm to 15 mm in 0.2 second immediately after sensing a finger in front of webcam module. A target fingerprint image captured by the prototyped reader is shown in Fig. 2(b). The fingerprint image is extracted with more than 40 effective minutiae as shown in Fig. 2(c).
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Fig. 2. (a) Photo picture of prototype of touchless fingerprint reader; (b) Processed fingerprint picture captured by the reader in (a); (c) B/W photo showing the corresponding minutiae.
3. DIGITAL VARIABLE-FOCUS LIQUID LENS Digital variable-focus liquid lens is a key component to complete a fast focus-plane scanning in near consecutive distances. Main advantages are as follows; 1) fast response time, 2) wide close-up shooting range, 3) over-damped operation, 4) miniature size. In conventional voice coil motor (VCM) widely applied for the auto-focusing in compact camera module, it is almost impossible to fulfill the above stringent requirements because of small actuation distance. The digital liquid lens is mainly realized by the electrostatic principle that the liquid-liquid interface movement actuated by the electric forces would balance at the edge of electrode [5]. The schematic illustration of digital liquid lens actuated by the concentric rings is shown in Fig. 3(a) with top and cross-sectional view [6]. A curved liquid-liquid interface composed of two non-conductive immiscible liquid (silicone oil mixture 50 and alcohol mixture 80) works as an optical lens to bend the incoming light rays [7]. The curvature of the curved liquid-liquid interface change under the action of an electric force generated on the tri-phase line after electric field was applied in the vicinity of tri-phase line. To complete the digital operation, the AC electrode arrangement is that one electrode is connected to the alcohol mixture and the corresponding electrodes are those concentric rings covered dielectric layer and patterned Teflon layer on the driving substrate. So the tri-phase line of the oil mixture droplet will move to the edge of ith concentric ring while the driving voltage was applied into the first ring to the ith ring. The digital liquid lens has better operation characteristics than analogy operation of the conventional liquid lens [8]. No extra centering design is required to maintain the optical axis during operation. Its accuracy in diopter change only depends on the volume control of oil mixture and size accuracy of driving electrode. The relationship between the amplitude of driving voltage and the diopter change was broken. Therefore, response time of the digital liquid lens is fast and stable because of controllable characteristics. The over-driving voltage is a useful and convenient method to fast up the response without losing the focal length change accuracy. Better driving voltage arrangement would be that one driving voltage with higher amplitude only be applied to switch the diopter stage in a short time. After the target diopter was achieved, the driving voltage was turned to lower amplitude for a loner life time. In the analog operation of liquid lens, this over-driving technology cannot be applied to fast up the response time because the amplitude of driving voltage also determines the focal length change. A digital liquid lens with a flexible print circuit (FPC) lead is shown in Fig. 3(b). The lens has seven presets of focal length from 36.1 mm to 15.8 mm. The mechanical dimensions are ψ19 mm in diameter and 3.8 mm in thickness. Figure 3(c) shows the measured result of the radius variance of oil droplet while the driving voltage adds into the next
rings every 32 ms apart. The response time is smaller than 6ms while driving voltage is at 30 Vrms of 1 kHz in squarewave.
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(c) Fig. 3. (a) Schematic illustration of digital liquid lens [6]; (b) photo picture of digital liquid lens; (c) Measured radius variances of oil mixture droplet versus actuation time.
4. OPTICAL SYSTEM DESIGN The compact touchless fingerprint reader is required to fit in size of 9.8*9.8*5 mm. Figure 4 is final result in optical design for the lens module with pre-selected three object distances, 30 mm, 20 mm and 10 mm. Red arrow in Fig. 4 points out the location of curved liquid-liquid interface of the digital liquid lens; and, it shows the variance of this curvature of liquid-liquid interface. The optical architecture is a digital liquid lens plus a conventional lens module based on a three solid lenses design. The liquid lens and the solid lenses would be designed to be separated modules in consideration of production yield and bench tests in optical image quality. Only green light was considered because the green light will generate higher ridge-valley image contrast than other color light [9]. The total optical track length (TTL) is 3.45 mm. Image format, pixel size, diagonal field of view and F/# are 1/9”, 1.4um, 60 degree, 2.0, respectively. The calculated MTF of the lens module for compact fingerprint reader is illustrated at Fig. 5. The image quality can fulfill the requirements of standard commercial fingerprint readers. Only the outside field of the lens module at 10 mm of object distance, the closet shooting distance, shows lower MTF values. Figure 6 and Figure 7 collect the analysis result of the tolerance influences of the gap between cover glass and driving substrate for digital liquid lens and the gap between digital liquid lens and solid lens module. Simulated MTF results in Fig. 6 show that the assembly tolerance can be a larger value around 0.1 mm. Even though the gap deviation is larger than 0.1 mm, the image quality still keep the same level of deign values in all conditions of object distance. Gap between cover glass and driving substrate is another main assembly deviation source for the digital liquid lens. Figure 7
shows the corresponding influences on MTF results. Image quality also can keep invariance even the gap deviation is large. Only MTF of some edge field angles show noticeable influences at the 10 mm of object distance.
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Figure 4. Optical simulation result of compact touchless fingerprint reader; objection distances at (a) 30 mm, (b). 20 mm, (c) 10 mm with red arrows point to the surface of liquid-liquid interface of digital liquid lens.
(a) (b) (c) Figure 5. MTF simulation results of compact touchless fingerprint reader; objection distances at (a) 30 mm, (b) 20 mm, (c) 10 mm.
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Figure 6. Tolerance analysis results of gap between digital liquid lens and lens module for compact touchless fingerprint reader; objection distances at (a) 30 mm, (b) 20 mm, (c) 10 mm.
(a) (b) (c) Figure 7. Tolerance analysis results of gap between cover and driving substrate of digital liquid lens for compact touchless fingerprint reader; objection distances at (a) 30 mm, (b) 20 mm, (c) 10 mm.
5. CONCLUSIONS A compact touchless fingerprint reader has been developed and demonstrated with preliminary measured performance on a prototype module in this paper. The scan at multiple focus-planes technology implemented with the digital liquid lens can acquire fingerprints in an acceptable operational range. Also, the focal distance, rolling angle and pitching angle of the floating fingers with magnification factor are calculated for match of fingerprint minutiae. Therefore, the compact size of reader is validated in the mobile applications via touchless fingerprint reader technology. Based on the optical design of lenses module, the compact touchless fingerprint reader can be manufactured in the size of 9.8*9.8*5 mm.
ACKNOWLEDGEMENT The authors would like to thank the fabrication help of Instrument Technology Research Center, National Applied Research Laboratories, Taiwan.
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