Electrowetting is a phenomenon that controls the surface tension of droplets by electricity and changes the wettability of the droplets. There are many applications of electrowetting, such as tunable liquid lenses, electronic paper and 3D display. Response time of electrowetting applications is important for them, but the relationship between response time and the physical parameters for electrowetting operation has not been deeply investigated. Therefore, we have investigated the effects of physical properties such as viscosity, interfacial tension and substrate roughness on the response time in AC electrowetting and found the optimal conditions for fast electrowetting. Also, an electrowetting circular lens was fabricated based on the optimal conditions and compared to a conventional electrowetting circular lens for response time. The experiment was conducted on a 0.4 mm thick aluminum plate with a 1μm thin parylene C film deposited with a 50 nm Teflon coating. The experiment results showed that the fastest response time is obtained at 5 mPa∙s conducting liquid (water-glycerol mixture) with 0 wt % SDS (sodium dodecyl sulfate) on default aluminum plate (RMS roughness 270 nm). Through this experiment, it was possible to control the spreading response pattern of electrowetting from under-damped response to over-damped response by changing the conditions of viscosity of conducting liquid, surface tension between two immiscible liquids, and substrate roughness. Also, a critical damping response was implemented using a hardware method by applying the optimum condition without voltage shape variation technology.
In this study, we propose light field 3D endoscope using the electro-wetting lens array. Compared to conventional light field endoscope technology, the electro-wetting micro lens array are not only switchable between 2D and 3D, but also adjusts the focal length to capture the varying images and control the diopter sufficiently fast (ms). The electro-wetting lens array has diameter 2.4mm and diopter -20D ~ 28D with 40ms of response time, which is an appropriate to get an endoscopic image. We also compare with light field 3D endoscope using a fixed focus lens array and our proposed light field 3D endoscope under the same condition. To achieve the electro-wetting lens array, parylene C layer are deposited on the silicon through hole substrate. In this study, we focus on the electro-wetting lens array fabrication and feasibility of a light field 3D system based on the electro-wetting micro lens array, accordingly we do not assemble the whole system in the real endoscope. Although it is performed on the optical stage, we successfully captured a light field images of several objects and reproduce a 3D image. Hereafter research, we will apply extended depth-of-field algorithm in our technology to improve the 3D image resolution and depth of field.
These days micro lens array is used in various fields such as fiber coupling, laser collimation, imaging and sensor system and beam homogenizer, etc. One of important thing in using micro lens array is, choice of its pitch. Especially imaging systems like integral imaging or light-field camera, pitch of micro lens array defines the system property and thus it could limit the variability of the system. There are already researches about lens array using liquid, and droplet control by electrowetting. This paper reports the result of combining them, the liquid lens array that could vary its pitch by electrowetting.
Since lens array is a repeated system, realization of a small part of lens array is enough to show its property. The lens array is composed of nine (3 by 3) liquid droplets on flat surface. On substrate, 11 line electrodes are patterned along vertical and horizontal direction respectively. The width of line electrodes is 300um and interval is 200um. Each droplet is positioned to contain three electrode lines for both of vertical and horizontal direction. So there is one remaining electrode line in each of outermost side for both direction. In original state the voltage is applied to inner electrodes. When voltage of outermost electrodes are turned on, eight outermost droplets move to outer side, thereby increasing pitch of lens array. The original pitch was 1.5mm and it increased to 2.5mm after electrodes of voltage applied is changed.
Electrowetting lens is a promising technique for non-mechanical vari-focal lens, because of fast response time, wide expressible diopter, and etc. Although electrowetting related papers are actively published, no one did not clearly define the relationship among electrowetting parameters, especially in AC driven case. Analysis for AC voltage driving is needed because AC electrowetting has many advantages like low hysteresis and short settling time. In this experiment we confirmed that the response time depends on aperture size and applied voltage. Response time measurement for lens aperture of 200-1000um and applied voltage of 0-70V with 1kHz frequency was conducted. Experimental data was compared with simulation result by COMSOL Multiphysics program with the same condition, and they correspond with each other well. As voltage increases, the overshoot height becomes higher, so it has longer oscillation and settling time. On the other hand if aperture size decreases, the surface tension of lens wall could be delivered effectively to the center region of meniscus, so it has less oscillation and shorter settling time. The result was that in 500um aperture no more than 30V should be applied to ensure 1ms response time. In 200um aperture, the voltage limit is disappeared.
This study describes easy fabrication method for micro-lens array which has desired focal length in such a way that
without the use of reflow technique. The process includes conventional lithographic process only which can be
compatible with general semiconductor process. As constituent material, Negative photo-resist SU-8 with its developer
PGMEA is used. Two main phenomena during lithography process are adjusted to expand the volume of the PR. During
UV exposure, hardening proceeds from the top of the PR. Just after first exposure, using this property, very thin
membrane on the top of the surface of the PR can be formed by short time exposure. In the development process,
unexposed area of the PR is removed by chemical reaction with developer which causes the volume expansion if the
unexposed area is covered with thin cured film. This method is to form the lens as the molecules in the volume are not
easily escaped from the covered region. The thickness of the thin film depends on the exposure dose of 2mJ cm-2 μm-1
which determines the degree of expansion. The symmetrical volume expansion creates the membrane of lens shape and
the focal length is directly related with second exposure dose. An extended research of affecting the change of the focal
length of lens using volume expansion method by changing any other elements is discussed. This process can achieve a
focal length selective for the applications of micro-optics.
In this paper, we present a depth enhancing technique for integral imaging (II) system using a varifocal lens array. Expressible depth range of II is restricted in a specific region. If the image gets out of the region, displayed image becomes distorted and broken. The center of the region which called central depth plane (CDP) is defined by the focal length of lens array. In our experiment, liquid lens array is used for II system instead of ordinary solid lens array. The focal length of lens array varies depending on the applied voltage across. As a result, the proposed II system enables control of the location of image planes electrically. With this depth plane controllable system, time multiplexed II system is implemented. For this purpose, two objects of different positions and appropriate voltage level for each object are chosen. In display panel, elemental images for each object are alternately displayed with high frame rate and appropriate voltage levels are applied to the liquid lens array. Because the time period between two sequences is very short, both objects are seems to appear simultaneously. Hence the depth range of the constructed image is enhanced.
Generally, volumetric 3D display panel produce volume-filling three dimensional images. This paper discusses a volumetric 3D display based on periodical point light sources(PLSs) construction using a multi focal lens array(MFLA). The voxel of discrete 3D images is formed in the air via construction of point light source emitted by multi focal lens array. This system consists of a parallel beam, a spatial light modulator(SLM), a lens array, and a polarizing filter. The multi focal lens array is made with UV adhesive polymer droplet control using a dispersing machine. The MFLA consists of 20x20 circular lens array. Each lens aperture of the MFLA shows 300um on average. The polarizing filter is placed after the SLM and the MFLA to set in phase mostly mode. By the point spread function, the PLSs of the system are located by the focal length of each lens of the MFLA. It can also provide the moving parallax and relatively high resolution. However it has a limit of viewing angle and crosstalk by a property of each lens. In our experiment, we present the letter ‘C’, ‘O’, ‘DE’ and ball’s surface with the different depth location. It could be seen clearly that when CCD camera is moved to its position following as transverse axis of the display system. From our result, we expect that varifocal lens like EWOD and LC-lens can be applied for real time volumetric 3D display system.
Electrowetting has been widely studied for various optical applications such as optical switch, sensor, prism, and display. In this study, vari-focal liquid lens array is developed using electrowetting principle to construct integral 3-dimensional imaging. The electrowetting principle that changes the surface tension by applying voltage has several advantages to realize active optical device such as fast response time, low electrical consumption, and no mechanical moving parts. Two immiscible liquids that are water and oil are used for forming lens. By applying a voltage to the water, the focal length of the lens could be tuned as changing contact angle of water. The fabricated electrowetting vari-focal liquid lens array has 1mm diameter spherical lens shape that has 1.6mm distance between each lens. The number of lenses on the panel is 23x23 and the focal length of the lens array is simultaneously tuned from -125 to 110 diopters depending on the applied voltage. The fabricated lens array is implemented to integral 3-dimensional imaging. A 3D object is reconstructed by fabricated liquid lens array with 23x23 elemental images that are generated by 3D max tools. When liquid lens array is tuned as convex state. From vari-focal liquid lens array implemented integral imaging system, we expect that depth enhanced integral imaging can be realized in the near future.
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