Holograms are a promising technology for realizing a natural three-dimensional display. While the holographic pattern records both the phase and intensity information of an object, the spatial light modulator, which is a device for reproducing it, is capable of modulating either phase or amplitude. Therefore, in order to realize a perfect holographic image, a complex hologram must be converted into a phase-only hologram. Currently, SLM resolution is 4k or less, but resolution needs to be increased to achieve better quality holograms. This will increase the computational complexity of the Fourier transform, which is needed for each iterate. Therefore, a way to create a phase-only-hologram with a minimum number of iterations is needed. It is known that in the iterative method, the initial predicted value greatly affects the result. In this study, we propose a method to make initial guess to obtain more accurate phase only hologram with fewer iteration times. To obtain the initial guess, the desired image was divided into several parts, and the phaseonly hologram corresponding to each was obtained and added again. We used hybrid-input-output iteration to maximize the effect of pre-iterated guess. When iterates the same number of times, the peak signal-to-noise ratio (PSNR) value obtained by using proposed pre-iterated guess was higher than using the random initial guess. In other words, the desired PSNR value can be reached with a smaller number of iterations by using proposed method. After 15 iterations, proposed initial guess have ~0.3dB higher PSNR value, which means one or two times of reduced iterations.
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.