Transmittance-control devices, such as a suspended particle device, electrochromic device, and dye-doped liquid crystal (LC) device, have been studied for a smart window, eyewear, and automotive applications. These devices require a high transmittance difference between the transparent and opaque states. Among the dye-doped LC devices, a dye-doped chiral-nematic LC (CNLC) cell has been widely used for transmittance-control devices. However, the colors of cells are different between the homogeneously aligned and CNLC cell. In this study, we demonstrated a systematic approach to find optimum dye concentrations for black color in a dye-doped CNLC cell. We took its transmission spectrum into account in the numerical calculation to realize the black color in a dye-doped CNLC cell. Through the iterative method, we could optimize the concentration of each single dye for realizing the black color. We confirmed that a dye-doped CNLC cell designed by considering transmission spectrum of it could provide the black color in the CIE 1931 color space.
Liquid crystal (LC) devices have been used for smart window and see-through display applications. Especially, LC devices which can be used to control the haze value have been studied for smart window applications. LC devices with the polymer structure, such as polymer-dispersed and polymer-stabilized LC cells, can be used to control the haze value. However, for wider applications, it is urgent to overcome disadvantages, such as the high operating voltage, low transmittance in the transparent state, and narrow viewing angle because of the index mismatch between the LC and polymer structure. In this paper, we introduce LC devices based on the electro-hydrodynamic effect. They can provide a high haze in the translucent state because of the turbulence caused by the electro-hydrodynamic effect. They can provide a high transmittance in the transparent state and wide viewing angle because it does not contain any polymer structure. We believe that LC devices based on the electro-hydrodynamic effect can be an excellent candidate for smart window applications.
We introduce an electrically switchable two-dimensional liquid crystal (LC) phase grating device for window display applications. The device consists of the top and bottom substrates with crossed interdigitated electrodes and vertically aligned LCs sandwiched between the two substrates. The device, switchable between the transparent and translucent states by applying an electric field, can provide high haze by the strong diffraction effect with little dependence on the azimuth angle owing to a large spatial phase difference. This device exhibits outstanding features, such as a low operating voltage, high transmittance, and wide viewing angle in the transparent state and a high haze in the translucent state. In addition, the LC device can provide sub-millisecond switching between the transparent and translucent states with the use of an overdrive scheme and a vertical trigger pulse.
Recently, see-through displays have been attracted much attention as next-generation displays. There are two basic
technologies by which we can realize a see-through display: organic light-emitting diodes (OLEDs) and liquid crystal
(LC) displays. The pixel structure of a see-through display includes a transparent window area through which the
background image can be seen. Therefore, background images are always seen along with the displayed image. In
addition, a see-through display using OLEDs cannot provide the black color. As a result, a see-thorough display exhibits
poor visibility. This inevitable problem can be solved by placing a light shutter at the back of a see-through display.
Light shutter technology can be divided into two types; light absorption and light scattering. Light shutter based on light
absorption can be used to control the transmittance, but it cannot block the object behind the display panel completely.
Light shutters based on light scattering can be used to control the haze, but it cannot provide black color. To realize a
high-visibility see-through display, we need a light shutter by which we can control haze and transmittance
simultaneously. In this talk we would like to introduce technologies for LC light shutters by which we can block the
background image and provide black color by utilizing light scattering and absorption effects simultaneously.
See-through displays have got high attention as one of the next generation display devices. Especially, see-through
displays that use organic light-emitting diodes (OLEDs) and liquid crystal displays (LCDs) have been actively studied.
However, a see-through display using OLEDs cannot provide black color because of their see-through area. Although a
see-through display using LCDs can provide black color with crossed polarizers, it cannot block the background. This
inevitable problem can be solved by placing a light shutter at the back of a see-through display. To maintain the
transparent or opaque state, an electric field must be applied to a light shutter. To achieve low power consumption, a
bistable light shutter using polymer-stabilized cholesteric liquid crystals (CLC) has been proposed. It is switchable
between the translucent and transparent states only. Therefore, it cannot provide black color. Moreover, it cannot block
the background perfectly because of poor performance in the translucent state. In this work we will introduce a bistable
light shutter using dye-doped CLCs. To improve the electro-optic characteristics in the opaque state, we employed a
crossed electrode structure instead of a parallel one. We will demonstrate that the light shutter can exhibit stable bistable
operation between the transparent homeotropic and opaque focal-conic states thanks to polymer stabilization.
Recently, a transparent display has got much attention as one of the next generation display devices. Especially, active studies on a transparent display using organic light-emitting diodes (OLEDs) are in progress. However, since it is not possible to obtain black color using a transparent OLED, it suffers from poor visibility. This inevitable problem can be solved by using a light shutter. Light shutter technology can be divided into two types; light absorption and scattering. However, a light shutter based on light absorption cannot block the background image perfectly and a light shutter based on light scattering cannot provide black color. In this work we demonstrate a light shutter using two liquid crystal (LC) layers, a light absorption layer and a light scattering layer. To realize a light absorption layer and a light scattering layer, we use the planar state of a dye-doped chiral nematic LC (CNLC) cell and the focal-conic state of a long-pitch CNLC cell, respectively. The proposed light shutter device can block the background image perfectly and show black color. We expect that the proposed light shutter can increase the visibility of a transparent display.
Recently, active studies on a transparent organic light-emitting diode (OLED) are in progress as a next generation display. However, since it is not possible to obtain a dark state using a transparent OLED, it exhibits poor visibility. This inevitable problem can be solved by placing a light shutter behind a transparent OLED display. In this paper, we propose a light shutter using dye-doped liquid crystals (LCs) whose Bragg reflection wavelength is chosen to be infrared by controlling the pitch of cholesteric liquid crystals (ChLCs). The proposed light shutter is switchable between the dark planar state and the transparent homeotropic state. The proposed light shutter has the advantages of the high transmittance, low operation voltage, and easy fabrication process compared with previous light shutter devices using liquid crystals. It is expected that the proposed light shutter can be applied to realize high visibility transparent OLEDs and emerging smart windows.
Cholesteric liquid crystals (CLCs) have been used for a reflective display because of their reflective nature in the planar state. In a reflective display, the planar and the focal-conic states are used for the bright state and the dark state, respectively. In this paper we introduce a long-pitch CLC device, in which a selective wavelength of the reflected light is shifted to infrared (IR) wavelengths by controlling the pitch. The planar state of a long-pitch CLC device is transparent over the entire visible wavelengths in the field-off state. Omni-directional achromatic reflection through light scattering in the focal-conic state can be achieved without a polarizer. Compared to conventional CLC cells that reflect the visible light in the planar state, a long-pitch CLC device has a longer pitch, of which the operating voltage for switching between the two state is much lower so that achromatic reflective displays and light shutters with low power consumption can be realized using long-pitch CLC devices. By coupling with a reflector, the light efficiency of a longpitch CLC cell in the focal-conic state can be enhanced, by which higher brightness can be obtained for application to reflective displays. A dye-doped long-pitch CLC device can be placed behind a transparent organic light-emitting diode display for use as a light shutter to block the ambient light.