New application of liquid crystal lens of active polarized filter for micro camera Giichi Shibuya, * Nobuyuki Okuzawa, and Mitsuo Hayashi Department Devices Development Center, Technology Group, TDK Corporation, 160 Miyazawa, Minami-Alps City, Yamanashi, 400-0495, JAPAN * gshibuya@jp.tdk.com Abstract: Liquid-crystal (LC) lens with low-voltage (3.5V) driving is reported with the experimental results of lens power, wavefront aberration, storage test and also the imaging test. Optical quality can be estimated by interference pattern under two polarizer plates set with the crossed Nichol position, and the optical quality is certified by the measurement of wavefront aberration. Durable stability of over 1000 hours under continuous driving in high temperature (85 ) environment is also verified and obtained less-damaged interference patterns. Finally a new application of active polarized filter for micro camera with focus control function is reported with sufficient quality of images. 2010 Optical Society of America OCIS codes: (230.3720) Liquid-crystal devices; (220.3630) Lenses; (230.2090) Electro-optical devices References and links 1. S. Sato, Liquid-crystal lens-cells with variable focal length, Jpn. J. Appl. Phys. 18(9), 1679-1684 (1979) 2. M. Ye, B. Wang, and S. Sato, Liquid-crystal lens with a focal length that is variable in a wide range, Appl. Opt. 43(35), 6407-6412 (2004) 3. M. Ye, B. Wang, M. Uchida, S. Yanase, S. Takahashi, M. Yamaguchi, and S. Sato, Low-Voltage-Driving Liquid Crystal Lens, Jpn. J. Appl. Phys. Vol.49, 100204 (2010) 4. M. Ye, B. Wang, M. Yamaguchi, and S. Sato, Reducing Driving Voltages for Liquid Crystal Lens Using Weakly Conductive Thin Film, Jpn. J. Appl. Phys. 47(6), 4597-4599 (2008) 5. M. Ye, B. Wang, and S. Sato, Effects of Dielectric Constant of Glass Substrates on Properties of Liquid Crystal Lens, IEEE Photon. Technol. Lett. 19(17), 1295-1297 (2007) 6. M. Ye, B. Wang, M. Kawamura, and S. Sato, Image Formation Using Liquid Crystal Lens, Jpn. J. Appl. Phys. 46(10A), 6776-6777 (2007) 7. T. Takahashi, M. Ye, and S. Sato, Wavefront Aberrations of a Liquid Crystal Lens with Focal Length Variable from Negative to Positive Values, Jpn. J. Appl. Phys. 46(5A), 2926-2931 (2007) 8. M. Born and E. Wolf, Principle of Optics (Pergamon Press, Oxford, 1975) 9. T. Galstian, P. Clark, and S. Venkatraman, LensVector Tunable Liquid Crystal Lens, EDOM-Express 97, 2011/7/22 EDOM Technology Co.Ltd. http://www.edom.com.tw/en/index.jsp?m=techview&id=1845 1. Introduction Liquid crystal (LC) lens with variable focal length have been proposed since 1979 and developed for the long years. The early structure of the LC lens had the spherical surface with transparent glass material and LC was filled inside with the transparent electrode [1]. Discrete electrical circuit was also prepared to apply alternative voltage to the LC layer. Then, after 21 st century, various micro cameras were more widely spread with simple handling and also the advanced LC lens become more unique for their style and the electrode. Recent LC lens have several kinds of structures of Fresnel, diffractive, in addition to the conventional spherical ones, and also their type of electrode has varied to concentric, orbicular and novel
gradient resistivity one to apply the radial distributed voltage level. In such trend, LC lens proposed by Sato s team (Akita University) [2] has very simple structure of sealed LC between two plane glasses. And its electrode has very simple circular separated pattern that we can manufacture and supply with lower cost for mass production. On the other hand, the gross output of the cell phone in the global market has achieved over 1.4 billion on 2010 and still increased every year. And about 1 billion of them have single camera and also 200 million have 2 cameras on the outside and the inside. Almost all cameras have smaller aperture of under 2mm which is the acceptable size for the LC lens, and their focus control functions are implemented by mechanical method like voice coil motor (VCM), piezoelectric device and also the manual moving switch with macro and infinite focal positions. However, this market has also the severe competition of the selling prices and all suppliers must be suffered to provide more low cost for materials and manufacturing. (Described values are reported by Mercury Research Corporation.) We have manufactured the smaller size and the lower driving voltage (3.5V) of the LC lens based on the structure by Sato s team, and installed on the micro camera module. Then several optical characteristics and storage reliability was evaluated and discussed. Moreover, we achieved and proposed the new type of polarized filter device with focus control function for the example of new application of the LC lens. 2. Structure and sample preparation Figure 1(a) shows the structure of LC lens manufactured. Nematic LC material was sealed with the thickness of 30μm between two pieces of glass substrate of 0.3mm thickness. Polarized film was set on the light incident side with the optical vibrating surface parallel to the direction of LC orientation. On the inside surface of the glass, the indium tin oxide (ITO) transparent electrode was patterned into circular shape with the diameter of 2.0mm. And polyimide layers rubbed in the direction of diagonal lines are covered on the ITO surfaces. Thin films of insulator (based on SiO 2 ) and high-resistivity material (based on ZnO) are sandwiched between the ITO and the polyimide at the only side of the circular (e.g. opposite to the flat) electrode. This contribution of high-resistivity layer to rearrange the distribution of the electric charges to be more gradual decreases the LC driving voltage once over 100 V into less than 5 V. This accomplishment is very important to make the LC lens possible to apply for mobile instruments like cell phones, rapidly [3] [4]. Figure 1(b) shows the outlines of transparent electrodes. Electrode 1 of the top part has the circular hole and electrode 2 has the circular shape inside the electrode 1. Electrode COM of the bottom part has a plane flat electrode for common. AC voltages of V 1 and V 2 applied independently to the electrode 1 and 2 make both characteristics of convex and concave lens [5]. Such unique functions of controllable lens power and polarity simply with small numbers of electrode channels are one of the good advantages of LC lens with Sato s investigations. The diameter of the effective lens aperture can be decided by that of the circle electrodes 1 and 2 which were designed 2.0mm for our target application of the micro camera. Figure 1(c) shows the photos of manufactured LC lens cell. Glasses of the lens cell have rectangular shape to pull out the electrode terminals and their dimensions are 6 x 8.5 mm. Polymer resin for sealing LC material is formed with semi-square shape between 2 glass plates with the beads of 30μm diameter mixed for the role of the spacer. LC material is filled inside of the dam with the thickness of the spacer and sealed with the polymer resin. The dimension of the inner side area of the dam is 4.6mm which is 2.3 times larger from the lens aperture size. Naturally, the down sizing of the lens device is desirable for practical use. We also have checked the lens size was enough for desirable optical performance. Flexible Printed Circuit (FPC) is connected at the end of the glass cell to apply the AC voltages. The number of terminals is 3 on each LC lens and prepared FPC has 6 wires to consider the case of 2 LC lenses attached. Polarized film is also attached at the beam incident side and the achieved outline thickness of the LC cell is 0.6mm. Transparence of the polarizer is about
44%. Moreover, in this study, the new device with Twisted Nematic LC material, widely applied for the display use, is attached at the beam incident side and the new effect of the rotation of polarization plane is observed. direction of incident beam (a) electrode 2(inner circle) electrode 1(outer circle) electrode COM(plain) polarizer glass insulater high resistivity film polyimide liquid crystal polyimide glass 6.0mm Diameter 2.0mm Sealer LC Cell 6.0mm Liquid Crystal (b) electrode 1 electrode COM electrode 2 Conductive Material (c) Fig. 1. Schematic structure of LC lens. (a) LC cell structure. (b) Patterned ITO electrode. (c) Photos of manufactured LC cell. 3. Evaluation by interference pattern For the purpose of simple evaluation of the LC lens, the observation of interference patterns from the ordinary and the extraordinary lights has been proposed [2], therefore, we also apply the same method in this study. LC lens sample is set between two pieces of polarized plates aligned with crossed Nichol position. The rubbing direction of the LC lens is set 45 from each polarizer and the collimated green laser beam is irradiated into them. Under this condition, when the AC voltages are applied to the LC lens, only the extraordinary light can be changed its wavefront into convex or concave spherical surface accompanied with the applied voltage level by the continuous distribution of the refractive index. While the ordinary light can be received the plane wavefront by the uniformity of refractive index. These spherical and plane optical wavefront transmitted from the LC lens interfere each other and make light and dark contrast of concentric stripes of the intensity. We can evaluate the lens power and the optical quality based on the wavefront aberration by the analysis of the pitch of the contrast [2]. Wavelength of the light source is 532nm which is almost the center of the visible light wave. Patterned circular electrode and controlled 2 channels of the applied voltages could perform convex and concave lens characteristics and the lens power varies with the root mean
square of the applied voltages under the fixed frequency [5]. In this study, however, we have found out that similar characteristic curve is obtained under the fixed AC voltage level and the variable frequency of the applied signal. Figure 2 shows the obtained characteristic curves. The applied voltages onto the outer and the inner area of the circular pattern, V 1 and V 2, are set to 3.5V rms, 1.3V rms at the convex mode and 1.3V rms, 2.5V rms at the concave mode, respectively. Such lower driving voltages are achieved by the role of the preceding effect of high resistivity thin film described on Figure 1(a). Then convex and concave lens powers and the wavefront error levels from the ideal spherical curve under the variable applied frequency are described together Figure 2. Indicated line curves are approximated from the phase difference of the interference fringe patterns into the second order parabolic curve. The wavefront error level is evaluated by the minimum difference of the root mean square from the ideal parabolic curve, and the value is described with the unit of λ rms. The evaluated wavefront error is effective to indicate the level of wavefront aberration of the LC lens systems. The lower error level means the higher quality of the optical characteristics of the lens. The obtained maximum powers are 17[1/m] and -8[1/m], when the positive and the negative sign mean the convex and the concave lens characteristics, respectively. Interference fringes of the higher convex power range tend to gather to the outer area of the lens aperture and the wavefront error also increases. This behavior can be explained by the dominated non-rotated LC molecular at the center of the circular electrode affected by the electrical field with the increased signal frequency. When we develop the LC lens for practical use, we had better to keep the limit level of the wavefront aberration and use within the safety range of the wavefront error under the designed driving frequency. In this study, we have decided the upper limit level of the wavefront error to 0.1λ rms, and according to this limit value, the available frequency is under 200Hz in the case of convex lens and under 130Hz in the case of concave lens. The obtained lens power at each condition is 8[1/m] and -5[1/m], respectively. Both control of voltage and frequency are found to be necessary to maintain higher quality images for LC lens driving. 4. Result of the reliability test for storage When we use the LC lens for electrical devices, we have to keep their high reliability against the long time operation and storage. Therefore, we have implemented the storage tests under high temperature and also high temperature and humidity conditions with our trial samples. In order to investigate the degradation mode under the operating condition, LC lens samples were maintained with two kinds of the environment of 85 dry and 60 95% RH(Relative Humidity) under the AC voltage application of 3.5V rms onto the electrodes of V 1 and V 2, continuously. Figure 3 shows the optimal frequency (defined by the frequency which could obtain the same lens power as the initial state) calculated from the interference fringe patterns at each storage time. In the case of high temperature condition, no degradation of the interference pattern was observed until 1000 hours storage, however, 1500 hours storage made the fringe pattern distorted and unable to evaluate the lens power. On the other hand, high temperature and humidity condition storage could keep the initial characteristic curve until 250 hours, and then the curve was shifted toward the low frequency side under the 500 hours storage. The amount of the frequency shift was about 60Hz from the initial state and the optimal frequency was changed from initial 70Hz to 13Hz after 500 hours storage. The obtained lens power after storage was 7.3[1/m] which was almost the same as initial value.
Fig. 2. Interference patterns and characteristic curves of LC lens at various values of applied AC frequency. Fig. 3. Result of the storage test under LC lens driving. As a result, the degradation mode was found to be different between high temperature condition and high temperature/humidity condition storage. Under the high temperature and humidity condition, no degradation of the interference fringe pattern was obtained, however, the optimal frequency was changed first. This case means the characteristic of the high resistivity film discussed in section 2. was changed by the storage, and the distribution of the
electrical field in the LC material was supposed to be affected by the electrical charge moved internally [4]. The direct factor of the characteristic change of the high resistivity(hr) film still cannot distinguished, however, if the chemical component change caused by the moisture penetration from the sealing material to the HR film was dominant, the improvement of the sealing material and process will be able to prevent them completely. 5. Evaluation of focus control from images Image formation of the LC lens with the C-MOS digital camera (f=16mm) were reported [6]. In this study, we have implemented the auto focus control with the LC lens set in front of the micro camera module with under 1 mm distance. Figure 4(a) shows the experimental setup of imaging system including the LC lens and the camera module. Polarized film with the same transmission plane as the rubbing direction of the LC lens was set in front of the LC cell. Three kinds of objects were set at near, middle and far spot with the distance of 0.1, 0.28 and 1.1m from the LC cell, respectively. Then applied voltage for LC cell was controlled to keep the maximum contrast level of the object image with the inside area of the red square indicated. The F number of the camera module was 2.8 and the number of pixels of the image sensor was 5,000,000(5M). Figure 4(b) shows the images on near focus controlled with the indicated applied voltages, V 1 and V 2, for the convex lens mode. Obtained lens power of this condition was 6.0[1/m], approximately. Figure 4(c) shows the images on far focus controlled with the applied voltages set for the concave lens mode. Obtained lens power of this condition was - 4.0[1/m], approximately. These results show that proper focus control for near and far distance with the LC lens was achieved with sufficient optical quality. Current focus control was implemented to vary the distance between the image sensor and the lens systems, however, several essential problems of decenter, lens skew, acoustic noise or damages of physical shock like drop down were also existed. LC lens can resolve all of these troubles and carry the new age s advanced system of focus control. (a) far object near object middle object LC Lens polarizer camera module 0.28m 0.1m 1.1m (b) (c) (V1,V2)=(3.5,1.7)Vrms Lens Power : 6.0 [1/m] (V1,V2)=(1.6,2.5)Vrms Lens Power : -4.0 [1/m]
Fig. 4. (a) Experimental setup of imaging system including LC lens and camera module. (b) Images on near focus. (c) Images on far focus. 6. Evaluation of Wavefront Aberration When the LC lens applies for the imaging lens devices, sufficient optical quality must be maintained for each purpose. Wavefront aberration is one of the good index to judge the optical quality of lens, and the measured result by Fizeau interferometer (Zygo DVD-400, wavelength is 405nm) was reported [7]. LC lens power and polarity are not fixed but varied with the applied voltages, therefore, they had prepared the optical power compensation unit inserted to recollimate the light lays exited from the LC lens with various lens powers. Then compensated plane optical wavefront was reflected by the mirror and make interference pattern with the pre-incident light, and the wavefront aberration was evaluated from the obtained pattern. Therefore we also measured the wavefront aberrations of our manufactured LC lens samples with the same equipment including the power compensator. Measured range of the optical power was -3~+6[1/m] which was decided by the tunable range of the compensator. Frequency of the driving signal was fixed to the optimal value and its voltage was sweeped to control the lens power. 3 kinds of Seidel s aberrations and the total summation with the root mean square was described in Figure 5 as a measured result of two samples manufactured under the same condition. The broken line on (a), indicates the value of 0.07λ rms, which is the Marechal s criterion defined as the limit value for all kinds of wavefront aberrations to keep the high optical quality. The broken lines on (b),(c), and (d), indicate the limitations from Rayleigh s rule corresponded for third-order spherical, coma and astigmatic aberrations, and each limit value is ±0.94, 0.6 and 0.35λ, respectively [8]. This evaluation results show that four kinds of aberrations are lower than the limit values for measured lens power range, and can be ensured that optical quality of the LC lens seems good enough for the application of camera and other imaging systems. Under the roughly discussion, LC lens power shows ±6~8[1/m] and the corresponded F-number is about 70 which are not so higher power compared with the whole systems value of 2.8. Therefore, the aberration levels seem to be remained also smaller compared with another high-power lens. Actually the power of the LC lens must be expected more higher, however, now we had better to make good use for the unique characteristics of variable lens power and polarity with the only small voltages applied for the technical contribution to the market. For the next step, we have to combine the LC lens with another lens system and achieved to make the overall wavefront aberrations to be compensated each other to maintain their high optical qualities for whole range of the available lens power. 7. Evaluation of LC Polarization Filter LC lens is one of the birefringent devices, therefore, the lens effect can be obtained with the only extraordinary light, whereas no lens effect can be shown with the ordinary light which receives the only uniform refractive index from the LC material. According to this physical property, composite lens structure with two pieces of LC cells laminated with orthogonal rubbing direction was reported. It could be achieved the independent characteristics of the polarization for the application of imaging [6][9]. On the other hand, however, more positive application of the polarization characteristics of that should be also discussed for the further conception. Then we have discussed the advanced polarization filter for camera with focus control function by the polarized LC lens and the new optical rotator combined. The new polarization filter has the advanced Twisted Nematic LC device shown in Fig. 6(a) which was set to the incident side of the polarized LC lens. This structure was designed to apply the optical active (rotative) characteristics of the LC material. When the AC voltage
was applied on the plane transparent electrode, the polarization plane could be rotated with the same mechanism as current LC display. This device also has no mechanical structure, same as the preceding function of focus control and possible to supply the advantages of easy handling, reliability and also the ecological effect for both production and consumption. Sum. [λrms] Coma [λ] 0.1 (a).wavefront Aberration(Summation by r.m.s.) 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0-4.0-2.0 0.0 2.0 4.0 6.0 8.0 Lens Power[1/m] 1.0 0.8 0.6 0.4 0.2 0.0 (c).wavefront Aberration(Coma Aberration) -4.0-2.0 0.0 2.0 4.0 6.0 8.0 Lens Power[1/m] SA [λ] AS [λ] (b).wavefront Aberration(Spherical Aberration) 1.0 0.8 0.6 0.4 0.2 0.0-0.2-0.4-0.6-0.8-1.0-4.0-2.0 0.0 2.0 4.0 6.0 8.0 Lens Power[1/m] (d).wavefront Aberration(Astigmatism) 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00-4.0-2.0 0.0 2.0 4.0 6.0 8.0 Lens Power[1/m] Fig. 5. (a) Summation of wavefront aberration as a function of lens power. (b) Spherical aberration as a function of lens power. (c) Coma aberration as a function of lens power. (d) Astigmatic aberration as a function of lens power. Figure 6 shows the image taken by the new polarization filter and the camera module described on Fig. 4(a). Polarized filters for cameras show several optical effects to make original images more sharp, enhancement of the color especially for the landscapes of the sky, prevention of the stray reflection from the windows or glossy surface and contrasting the object from the background. However, images monitored on the LC displays for televisions or personal computers cannot be detected sometimes because of its linearly polarized characteristic of the transmitted light. When such trouble occurs, we have to rotate the polarized filter itself by manual and search good position to obtain the desirable images. This new type of filter can control the polarized plane electrically and easy to set on the current micro camera. Figure 6(b) shows the image with no voltage applied on the electrode of the optical rotator shown in Fig. 6(a). In this case, the vibrating surface of the polarizer was set parallel with that of the irradiated light from the display, but, it rotates about 90 degrees at the entrance position of the LC lens by the effect of preceding optical rotator and then almost all components of the light from the display cannot be transmitted through the polarizer set in front of the LC lens. Therefore, though the images were existed on the display, only dark color could be visible with no images on it. Then the AC voltage was applied on the optical rotator with the level of over 2.0V rms, images on the LC display turns to be visible gradually, and when it arrives over 2.5V rms, images clearly appeared with the same quality as no filter. The threshold level of the driving voltage of used LC material was 2.32V rms, and the molecular of the LC rotated to align to the parallel direction of the optical axis with the electric field under the voltage application of 2.5V rms. Then the effect of the optical rotation was disappeared and the images on the monitor became visible with the same optical directions of the vibrating surface between the polarizer and the light from the LC display.
Figure 6(c) shows the same image as (b) with the voltage application of 3.0 V rms on the preceding optical rotator cell. Image on PC s display turned to be visible by the effect of the rotation of polarized plane. Moreover, as shown with the image on the mirror at the bottom of the right side, light ray through the blind and the view of the window became more clear than (b) by the same effect of the polarizer. This new device is also available to change the focal length with the same manner of voltage application and proper to use for the micro camera instead of the current mechanical voice coil motor (VCM) or other actuators. (a) electrode 3 electrode 4 glass polyimid liquid crystal polyimid glass (b) (c) Fig. 6. Images of LC polarized filter. (a) Schematic Structure of new LC device. (b) 0.0V applied between electrode 3 and 4. (c) 3.0V applied between electrode 3 and 4. 8. Conclusions LC lens with only flat plate and simple circular electrode was experimentally discussed for practical use. The optical characteristics evaluated from the interference fringe pattern, the wavefront aberrations and the image formation were described, and also its reliability was confirmed by the storage test. A new type of active LC polarized filter with the function of focus control was also manufactured and verified. LC lens is now under developing and also has several challenges of response speed, F No. from lens power and brightness, development of manufacturing process for low cost and also more downsizing. On the other hand, nonmechanical controls for focusing or rotating of polarized plane are practically possible and expected for several progressive new applications with their unique characteristics.
9. Acknowledgments We feel very grateful to the members of Akita Industrial Technology Center to offer the instruction of the assembly of interference fringe detector, evaluation of the pattern, interferometer and its technical support for operating.