Conventional stereoscopic display glasses often lead to viewer discomfort and resultingly, there has been continued research on autostereoscopic displays that obviate the need for wearing glasses. Using the multi-view method is one of the most common approaches to autostereoscopic display design, with two types of multi-view 3D displays: parallax barrier type and lenticular lens type. The lenticular lens uses a small, rugged vertical type lens, which allows an image to be divided into left and right images. The lenticular lens has the merit in that a higher optical transmittance can be obtained because there is no optical barrier. However, since a conventional lenticular lens is solid, its shape is fixed, and when a lenticular lens is mounted on a flat panel display, only a 3D image can be implemented, as a 2D image is difficult to implement. In keeping with this, autostereoscopic displays, which can convert 2D and 3D images, have emerged as one of the next generation display technologies. Therefore, various studies on the fabrication of tunable lenticular lenses using liquid crystals, polymer membranes, and liquid lenses have been carried out to develop autostereoscopic displays capable of 2D/3D conversion. Among them, liquid lenticular lenses using the electrowetting phenomenon have a high optical transmittance as well as a driving speed which is much faster than the other two methods (liquid crystal and polymer membrane). Because they have a low driving voltage, liquid lenticular lenses are also suitable for use in mobile devices that are sensitive to power consumption. The electrowetting liquid lenticular lens consists of two immiscible liquids, one corresponding to a conductive liquid, and the other a nonconductive liquid. The interface between the two immiscible liquids forms a lens. If the contact angle of the conductive liquid located on the electrode is changed by controlling the applied voltage, the radius of curvature of the lens can be changed. As the radius of curvature of the lens changes, the focal length also changes. Therefore, 2D images can be viewed in the plane lens state, and 3D images can be viewed in the convex lens state. However, since liquid lenticular lenses using the electrowetting phenomenon, which was previously proposed, all exhibited the concave lens state in the initial state due to planoconvex structure, it was necessary to apply an additional voltage to form the plane lens state. This initial concave lens state is a step that should be omitted in order to lower the driving voltage. Therefore, in this research, a new biconvex structure is proposed in which the initial lens state is a plane lens.
The fabrication of the electrowetting lenticular lens is based on microelectromechanical system (MEMS) technology. First, for the reproducible production of the chamber, a hot embossing process using a metal slave mold was adopted. The metal slave mold used in this process was prepared by electro-plating nickel after etching a (100) silicon wafer with KOH. After the completed metal slave mold was placed on a polymer substrate of PMMA, heat and pressure were applied through a pressing machine to produce a chamber of the same shape. The lens pitch in the fabricated chamber was 412.68 μ m and the slope of the partition wall was 54.7 degrees. The upper part of the partition wall was designed to have a width of 30 μ m. A 2-inch PMMA lenticular lens chamber was fabricated by this method. Since the cross-sectional view of the fabricated chamber had an inverted trapezoidal shape, additional measures were required to fabricate the chamber with a biconvex structure. Therefore, ethoxylated trimethylolpropane triacrylate (ETPTA) was uniformly dosed into the chamber and cured with ultraviolet rays to form a curved shape. Next, a gold electrode was deposited using a thermal evaporation process, and a parylene C dielectric layer was deposited using a chemical vapor deposition (CVD) process. Under atmospheric conditions, nonconductive liquid chloronaphthalene (CN) was injected into the chamber through a micro syringe. After that, a chamber filled with nonconductive liquid was placed in a water tank, and sealed with ITO coated glass. At this point, a polycarbonate (PC) gasket was placed between the chamber and the ITO glass to maintain the gap, and double-sided sticky tape and UV adhesive (NOA 63 and NOA 81) were used for the sealing process.
In this research, to improve the characteristics of an electrowetting lenticular lens with planoconvex structure, the fabrication method and characteristics of the newly developed electrowetting lens with biconvex structure through ETPTA were examined. Since the newly fabricated electrowetting lenticular lens had a biconvex structure, it was possible to obtain a high dioptric power even with a relatively small voltage applied, since light refraction occurs twice in total. Also, since it was no longer necessary to match the refractive index of the nonconductive liquid with the refractive index of the chamber material, it was also possible to use a nonconductive liquid with a higher refractive index than that of a conventional planoconvex structure. The nonconductive liquid with the high refractive index also helped the electrowetting lenticular lens array to achieve high dioptric power. In turn, this improved power makes it possible to make the distance from the optical center of the lens to the image (display) shorter. This means that the thickness of the chamber can be reduced accordingly. By reducing the thickness of the chamber, the ratio of the crosstalk to the field of view is also reduced, enabling a viewer to watch clearer 3D images than before. In addition, when the amount of nonconductive liquid was quantified and dosed into the chamber, the lower part of the lenticular lens could function as a convex lens, and the upper part could serve as a concave lens. If the specific radius of curvature condition is satisfied, the effect is the same as a plane lens. This research used geometric optics and mathematical analysis to discover the conditions under which a biconvex electrowetting lenticular lens can show a plane lens state when no voltage is applied. As an electrowetting lenticular lens has the same effect as a plane lens even when the voltage is not applied, the driving voltage can be lower than that of a conventional planoconvex lenticular lens. Because the planoconvex structure electrowetting lenticular lens showed a concave lens state in the initial state when no voltage was applied, it was necessary to apply an additional voltage to make the plane lens state.
To demonstrate and analyze the various advantages of an electrowetting lenticular lens with biconvex structure as mentioned above, the following were compared with an electrowetting lenticular lens with planoconvex structure: response time measurement, viewing angle and crosstalk measurement, and image test.