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Abstract
We propose a focus-tunable double-convex (DCX) lens based on a non-ionic PVC (nPVC) gel to be used at close conjugates. The proposed lens is composed of an nPVC gel and two plates with electrodes. Each plate has a hole whose boundary and inner part are pasted with an electrode (anode) and has another ring shaped electrode (cathode) whose center point is the same as the hole’s center. The gel is sandwiched between an upper plate and a lower plate, and it is bulged inward between the holes of two plates by applied pressure from the plates (double-convex lens shape). The lens’s focal length changed from 3 mm to 24.5 mm with applied voltages from 0 V to 400 V. We also observed that the proposed lens’s field-of-view decreased from 121.9 ° to 41.9 ° according to the applied voltages. The proposed lens brings additional benefit for users with higher transmittance (over 94%).
© 2017 Optical Society of America
1. Introduction
Generally, there are several types of convex lenses, with the two basic ones being the plano-convex (PCX) lens and the double convex (DCX) lens. The plano-convex (PCX) lens and double convex (DCX) lens have their own optimized operating area. The PCX lenses are compatible to near infinity conjugate, whereas the DCX lenses are widely used for image relay applications and imaging [1–3]. The DCX lenses have been applied to various imaging systems (telescopes, monocular, microscopes, binoculars, cameras, projectors, and etc.) in the form of varifocal lenses whose focus changes as the focal length changes [4].
Recently, few research studies have been conducted to minimize the size of varifocal DCX lenses based on various actuation mechanisms. Agarwal et al. developed a fluidic focus-tunable DCX lens whose curvature can be controlled by an external fluidic infusion pump [5]. Choi et al. presented another type of a fluidic DCX lens in which oil pressure is controlled by magnets and a voice coil motor [6]. However, it is not easy to apply these fluidic focus-tunable DCX lenses to many application devices because they have some problems such as leakage of liquid, distortion of lens shape by gravity, and complex external systems (heaters, pumps, and other mechanical structures). In order to reduce or prevent the above-mentioned problems, electrically focus-tunable DCX lenses, which are composed of the fluids and electroactive polymers (EAPs), were developed [7,8]. Carpi et al. introduced a bioinspired DCX lens using EAPs and a fluid [7]. Shian et al. presented an asymmetrical double-convex lens system based on a dielectric elastomer actuator membrane and a clear liquid [8]. These lenses do not need complex external systems, but they have still problems like leakage of liquid and distortion of lens shape by gravity.
In order to make a tiny, thin, and lightweight focus-tunable lens without any leakage of liquid and disturbance by gravity, an electrically focus-tunable meniscus lens was developed based on an electro-active polymer [9]. Although electroactive polymers allow varifocal lenses to be thin and lightweight, pre-strain needs to be applied to the EAP to obtain a large deformation of the lens. Another drawback is that stretchable, highly transparent, and electrically conductive electrodes are needed to operate the EAP. The non-ionic poly (vinyl chloride) (nPVC) gel is a good material for thin, lightweight, and electrically focus-tunable lenses because it can be actuated without the aid of stretchable and highly transparent electrodes [10–14]. Although previous lenses based on nPVC gels have a large variation of focal length, silent operation, and low power consumption, an ITO (Indium Tin Oxide) coated material (for example, ITO glass) is attached to the one side of the lens. It not only disturbs the double convex shape of the lens, but also reduces the optical transmittance of the lens module.
In this paper, we propose a new electrically focus-tunable DCX lens, which is composed of an nPVC gel and two plates with electrodes, without any stretchable or transparent electrodes and additional actuation mechanisms. The nPVC gel can be deformed by an electroactive adhesive force under applied input voltages (electric-induced-deformation) [15]. The plates with holes in the center are in contact with each other on the top and bottom surfaces of the nPVC gel, and the pressure is applied in the thickness direction to form a DCX lens. The proposed focus-tunable DCX lens has a thin, lightweight, low power consumption and silent operation.
2. Double convex focus-tunable lens
2.1 Structure of the proposed DCX lens
In this section, we introduce a new structure for a miniature and focus-tunable double convex (DCX) lens based on an nPVC gel, as shown in Fig. 1. The proposed DCX lens is composed of an upper plate, a non-ionic PVC gel, and a lower plate, as shown in Fig. 1(a). The nPVC gel is sandwiched between the upper plate and the lower plate. Each plate has two annular electrodes; one is an electrode (Ael) in the center of the plate and the other is an electrode (Cel) that keeps its distance from Ael. Figure 1(b) shows the cross-sectional view of the proposed DCX lens. The flat non-ionic PVC gel is compressed by the upper and lower substrates with the annular electrode forming a convex shape on both sides of the nPVC gel. By doing this, we can achieve the proposed DCX lens (Fig. 1(b)) whose diameter is 1.5 mm.
Fig. 1 The structure of a proposed DCX lens. (a) An expanded view of the proposed DCX lens and the structure of two plates’ electrodes, (b) a cross-sectional view of the proposed DCX lens.
2.2 Fabrication process for nPVC gel
In order to make a gel type lens material, we used a Poly(vinyl chloride) (PVC) (Sigma-Aldrich, Mn = ~99,000, CAS: 9002-86-2) powder, tetrahydrofuran (THF) (Sigma-Aldrich, 99.9%, CAS: 109-99-9), and a dibutyl adipate (DBA) (Sigma-Aldrich, CAS: 105-99-7) plasticizer. Initially, the PVC powder was dissolved in the THF, and then the solution was poured into the methanol, little by little, to be precipitated. After that, the precipitated PVC powder was filtered and dried. As shown in Fig. 2, the purified PVC was obtained by repeating this precipitation process three times. Then, the purified PVC resin and the DBA plasticizer were also added to the THF solvent and stirred for 4 hours with 300 rpm. The mixed PVC-DBA solution was drop-cast in a flat Teflon dish and then dried to remove the THF for three days at room temperature. Finally, the remained THF in the PVC gel was fully evaporated in a vacuum oven for 1 day at room temperature. As a result, we obtained a transparent non-ionic PVC gel with 1 mm thickness. The refractive index of the nPVC gel was evaluated by an Abbe refractometer (ATAGO NAR-4T Solid, Kirkland, WA, USA). The refractive index of the nPVC gel was 1.447.
2.3 Operating principle
Figure 3 shows the operating principle of the proposed DCX lens. Figure 3(a) is the initial state of the proposed DCX lens when there is no voltage input. When an electric field is applied to the lens, the DBA plasticizer molecules in the nPVC gel are quickly charged and polarized. This facilitates the molecular dipole rotation of PVC chain segments in the PVC gel network [15]. As a result, the applied electric field makes the polarized DBA and PVC chains move toward the anode, as shown in Fig. 3(b). This phenomenon is known as the electric-field-induced deformation [15]. This electric-field-induced deformation causes the change of the proposed gel lens’s curvature. Therefore, the proposed double-convex nPVC gel lens can alter its focal length based on the applied input voltages.
Fig. 3 Operating principle of the proposed DCX lens based on nPVC gel. (a) The initial state and (b) the activated state of the proposed DCX lens.
3. Experiments and results
In this section, we show that the proposed DCX lens has potential to be used for optical devices by investigating its optical transmittance, focal length, FOV, and the response time. First, we measured the optical transmittance of the proposed DCX lens by using an ultraviolet–visible (UV-Vis) spectrophotometer (HP 8452, HP, USA) and compared the optical transmittance of the proposed DCX lens with that of the conventional nPVC gel based PCX lens. We plotted two transmittances of both the nPVC gel based PCX lens and the proposed DCX lens for easy comparison. The optical transmittance of the proposed DCX lens is much higher (T ≈94.18% at 550 nm) than the nPVC gel based PCX lens (T ≈88.08% at 550 nm). This is because the proposed DCX lens does not need transparent electrodes (for example, ITO). This means that the proposed DCX lens has a lower optical noise when compared with the nPVC gel based PCX lens with ITO glass (Fig. 4).
We measured the focal length of the proposed DCX lens. Figure 5 shows the custom-built optical layout (an experimental environment) consisting of a collimated light (CPS532, Thorlabs, Inc., USA), the proposed DCX lens, and a viewing screen. The collimated light (532 nm, 4.5 mW), which is used for incident rays, was focused on the viewing screen based on the curvature of the proposed DCX lens. The focal length was quantitatively measured as the distance between the proposed DCX lens and the viewing screen where the sharpest and the smallest point was projected. We measured the focal length of the proposed DCX lens with an incremental increase in input voltage from 0 V to 400 V at 50 V intervals. During measuring the distance, there can be errors. Therefore, we measured the distance 10 times and computed the standard error and mean value of the measured data. When the input voltage was applied to the proposed DCX lens, each radius of curvature of the proposed DCX lens has increased by the electric-field-induced deformation. As a result, the lens’s focal length has decreased. For this experiment, an initial compressed pressure was applied by pressing the upper plate with a hole onto the nPVC gel lens to create a focal length of the initial focal length of the proposed DCX lens is set as 3 mm at 0 V.
Fig. 5 An experimental environment to measure the focal length of the proposed DCX lens according to the input voltages.
Figure 6 shows the result of the proposed DCX lens’s focal lengths as a function of input voltages. The focal length variation of the proposed DCX lens is confirmed to be about 21.5 mm (from 3 mm to 24.5 mm) upon electrical activation from 0 V to 400 V. The applied current to the proposed DCX lens is measured as 30 μA at a maximum voltage (400 V). Therefore, the maximum power consumption is about only 12 mW.
We made an add-on DCX varifocal lens, which is composed of a top cover, an upper plate, an nPVC gel, a lower plate, and a housing. We also attached the add-on lens to the front of a smart phone camera lens, as shown in Fig. 7(a). We obtained images from the smart phone to calculate the FOV of the add-on lens. The radius of curvature of the add-on lens increased as we increased the electric potential difference between the anode and cathode. The lens’s FOV may became narrower with an increase in the radius of its curvature. In this paper, we measured the FOV of the add-on lens when we changed the input voltage. Figure 7(b) shows the experimental environment for observing FOV, which consisted of an add-on lens, a smart phone, and graph paper. We applied the voltages to the add-on lens from 0 V to 400 V at 50 V intervals. The image including grid patterns was observed by the add-on lens, and then the observed image was conveyed to the smart phone. The diameter of the observed images from the smart phone was calculated as a function of the number of the gradations on the circular images. The add-on lens’s FOV (θ) can be calculated by
Fig. 7 An experimental environment to confirm the FOV (see Visualization 1). (a) An expanded view of an add-on lens and its assembled 3D model, (b) cross-sectional view of the experimental environment for FOV, and (c) front view of the graph paper.
In Eq. (1), D is the diameter of the observed image and f is the focal length of the add-on lens. Figure 7(c) shows a cross-sectional view of the experimental environment for FOV and the front view of the graph paper. In Fig. 7(c), θinit, finit, and Dinit are the initial FOVs of the add-on lens, the initial focal length of the add-on lens, and the initial diameter of the observed image, respectively. The initial FOV of the add-on lens depends on the initial focal length of the add-on lens and the initial diameter of the observed image. When the external voltage was applied to the add-on lens, the radius of the lens’s curvature was proportional to its focal length. The diameter of the observed image gradually decreased based on the increase of the applied voltages.
Figure 8 shows the experimental results, which are the variation of the add-on lens’s FOV based on the applied electric fields. Figure 8(a) shows the images obtained from the smartphone with the add-on lens. The FOVs of the add-on lens were calculated by Eq. (1) and were plotted in Fig. 8(b). The result shows that the FOV is in inverse proportional to the focal length. We confirmed that the FOV decreases when the focal length of the proposed DCX lens increases. As a result, the FOV of the add-on lens changed from 121.9 ° to 41.9 ° when the input voltages were applied from 0 V to 400 V. We demonstrate the possibility that the proposed DCX lens can easily change its field-of-view according to an external voltage.
Fig. 8 Experimental results of the FOV. (a) Images obtained from smartphone with the add-on lens based on applied voltages, (b) observed variable field-of-view versus applied voltages.
To show the response of the proposed lens, we installed an experimental environment as shown in Fig. 9(a). The focus of the microscope was fixed. We provided step input to the proposed lens and then we captured the target image as video recording at 25fps. After that, we observed the time when the deformation of the proposed lens is done. Figure 9(b) shows the response time as a function of the voltage. Figure 9(c) and 9(d) show the time-lapse images of a character target. The response time of the lens in the on or off state of the electrical activation increased with the applied voltage. In particular, the two response times upon turning on (400 V) and off (0 V) of the applied voltage for the adaptive gel lenses were approximately 0.8 s and 0.84 s, respectively.
Fig. 9 An experimental environment and results of the response time. (a) An experimental environment for measuring the response time of the proposed lens by applied voltages, (b) Response time as a function of electric field, and the time-lapse images of a character target formed through the proposed DCX lens at (c) the on state (0 V → 400 V) and (d) the off state (400 V → 0 V). The response times at the on state and at the off state for the proposed DCX lens are 0.8 s and 0.84 s, respectively.
4. Conclusion
We have proposed an electrically focus-tunable DCX lens that has a large variation of focal length, high optical transmittance, and wide FOV variation. The nPVC gel was sandwiched between two plates and formed into a DCX lens shape based on applied pressure. The curvature of the proposed DCX lens could be changed by input voltages. The proposed DCX lens has a higher optical transmittance (about 94%) than the previous nPVC based PCX lenses. We verified that the focal length variation of the proposed DCX lens was about 21.5 mm (3 mm ~24.5 mm) when the input voltages were applied from 0 V to 400 V. Moreover, we showed that the FOV variation of the proposed DCX lens was about 80 ° (from 121.9 ° to 41.9 °). We have demonstrated that the proposed DCX lens has a high transparency, a low operating voltage, a large focal length variation, and a wide FOV range. We expect that the proposed DCX lens can be widely used for image relay and imaging objects at close conjugates. To make the nPVC gel lens, an initial compressed pressure was applied by pressing the upper plate onto the nPVC gel. The applied force is related to the initial focal point and the focal length variation in the proposed lens. Therefore, we are investigating focal length’s variation of the proposed gel lens and we will analyze the initial force effect.
We prepared additional two collimated lasers which have other wave lengths (650 nm and 450nm) to check the chromatic effects of the proposed gel lens. Each laser represents the primary colors of light (red (650nm), green (532nm), and blue (450 nm)). We measured the focal length of the proposed lens with changing light sources. Figure 10 shows the results of the proposed lens’s focal length as a function of input voltage according to the wave lengths of the collimated lasers. We observed that there is a little difference among focal lengths according to the wave lengths of the collimated lasers. It means that the proposed PVC gel lens has a little different refractive index for different wavelengths of light. We think that the chromatic effect of the proposed lens is not so sensitive. Although the chromatic effect of the proposed PVC gel lens may not be so critical, we will minimize the chromatic effect. We are currently studying a new method for minimizing the chromatic effect.
Funding
Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3059588). This work was supported by the Technology Innovation Program (10077367, Development of a film-type transparent /stretchable 3D touch sensor /haptic actuator combined module and advanced UI/UX) was funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).
Acknowledgments
We gratefully acknowledge the helpful technical support of the UV-Vis spectroscopy from Ms. Jeong-Hyun Ryu of KOREATECH.
References and links
1. H. Ren, S. Xu, Y. Liu, and S.-T. Wu, “A plano-convex/biconvex microlens array based on self-assembled photocurable polymer droplets,” J. Mater. Chem. C Mater. Opt. Electron. Devices 1(44), 7453–7458 (2013). [CrossRef]
2. Thorlabs, “Selecting the Proper Lens,” https://www.thorlabs.com/tutorials.cfm?tabID=ba49b425-f85b-4549-8c1a-f111ddbb9099
3. EKSMA OPTICS, “BK7 Biconvex lenses,” http://eksmaoptics.com/optical-components/lenses/bk7-biconvex-lenses/
4. BYJU’S, “Biconvex lens,” http://byjus.com/physics/biconvex-lens/
5. M. Agarwal, R. A. Gunasekaran, P. Coane, and K. Varahramyan, “Polymer-based variable focal length microlens system,” J. Micromech. Microeng. 14(12), 1665–1673 (2004). [CrossRef]
6. H. Choi, D. S. Han, and Y. H. Won, “Adaptive double-sided fluidic lens of polydimethylsiloxane membranes of matching thickness,” Opt. Lett. 36(23), 4701–4703 (2011). [CrossRef] [PubMed]
7. F. Carpi, G. Frediani, S. Turco, and D. De Rossi, “Bioinspired Tunable Lens with Muscle-Like Electroactive Elastomers,” Adv. Funct. Mater. 21(21), 4152–4158 (2011). [CrossRef]
8. S. Shian, R. M. Diebold, and D. R. Clarke, “Tunable lenses using transparent dielectric elastomer actuators,” Opt. Express 21(7), 8669–8676 (2013). [CrossRef] [PubMed]
9. S. I. Son, D. Pugal, T. Hwang, H. R. Choi, J. C. Koo, Y. Lee, K. Kim, and J.-D. Nam, “Electromechanically driven variable-focus lens based on transparent dielectric elastomer,” Appl. Opt. 51(15), 2987–2996 (2012). [CrossRef] [PubMed]
10. T. Hirai, T. Ogiwara, K. Fujii, T. Ueki, K. Kinoshita, and M. Takasaki, “Electrically active artificial pupil showing amoeba-like pseudopodial deformation,” Adv. Mater. 21(28), 2886–2888 (2009). [CrossRef]
11. S.-Y. Kim, M. Yeo, E.-J. Shin, W.-H. Park, J.-S. Jang, B.-U. Nam, and J. W. Bae, “Fabrication and evaluation of variable focus and large deformation plano-convex microlens based on non-ionic poly(vinyl chloride) / dibutyl adipate gels,” Smart Mater. Struct. 24(11), 115006 (2015). [CrossRef]
12. J. W. Bae, M. Yeo, E.-J. Shin, W.-H. Park, J. E. Lee, B.-U. Nam, and S.-Y. Kim, “Eco-friendly plasticized poly(vinyl chloride)–acetyl tributyl citrate gels for varifocal lens,” RSC Advances 5(115), 94919–94925 (2015). [CrossRef]
13. M. Xu, B. Jin, R. He, and H. Ren, “Adaptive lenticular microlens array based on voltage-induced waves at the surface of polyvinyl chloride/dibutyl phthalate gels,” Opt. Express 24(8), 8142–8148 (2016). [CrossRef] [PubMed]
14. J. W. Bae, E.-J. Shin, J. Jeong, D.-S. Choi, J. E. Lee, B. U. Nam, L. Lin, and S.-Y. Kim, “High-Performance PVC Gel for Adaptive Micro-Lenses with Variable Focal Length,” Sci. Rep. 7(1), 2068 (2017). [CrossRef] [PubMed]
15. M. Ali, T. Ueki, D. Tsurumi, and T. Hirai, “Influence of plasticizer content on the transition of electromechanical behavior of PVC gel actuator,” Langmuir 27(12), 7902–7908 (2011). [CrossRef] [PubMed]
