Crystals 2021, 11, 1199. https://doi.org/10.3390/cryst11101199 www.mdpi.com/journal/crystals Article Optical Filter with Large Angular Dependence of Transmittance Using Liquid Crystal Devices Kohki Takatoh *, Masahiro Ito, Suguru Saito and Yuuta Takagi Department of Electrical Engineering, Faculty of Engineering, Sanyo-Onoda City University, 1-1-1 Daigaku-dori, Sanyo-Onoda, Yamaguchi 756-0884, Japan; m-ito@rs.socu.ac.jp (M.I.); F216025@ed.socu.ac.jp (S.S.); F215057@ed.socu.ac.jp (Y.T.) * Correspondence: takatoh@rs.socu.ac.jp; Tel.:+81-836-88-4544 Abstract: This study proposed a new type of optical device with variable transmittance based on the incident angle direction. These devices consist of two liquid crystal devices (LCDs) with a half- wave plate between them. Hybrid aligned nematic (HAN)-type guest-host (GH) LCDs or GH-LCDs with antiparallel alignment of high pretilt angles were used. The use of a half-wave plate allowed for the control of the p- and s-waves. Using these devices, a wide range of transmittances were obtained because no polarizer was used. The newly proposed LCDs have a wide range of applica- tions, including use on buildings, vehicles, and glasses. Keywords: liquid crystal; incident angle dependence; transmittance; HAN-LCD; hybrid aligned ne- matic; high pretilt LCD; half-wave plate; smart window; sunglasses; vehicle window 1. Introduction Light control is extremely important for observing objects correctly without stress. In particular, the light observed directly from the sky to the eyes can be a large hindrance to observing an object correctly. Under typical conditions, objects are observed in the front or lower direction. Therefore, optical filters whose transmittance is small in the upward direction and large in the front or downward directions should be valuable. Figure 1 shows the function of the optical filter with the incident angular dependence of the transmittance, which presents as a “louver” function. The louver can reduce the light only from the upward direction. Thin optical films with this function have a variety of applications, such as buildings, vehicles, and glasses. It can also be valuable if the trans- mittance is controlled by the electric field. This investigation used guest-host (GH) liquid crystal (LC) devices for these applications. GH-liquid crystal devices (LCDs) are devices that use dichroic dyes without polarizers. The dye molecules are aligned parallel to the molecular axis of the LC and absorb polarized light vibrating parallel to the molecular axis. By using the structure in which the dye molecules decline to the substrate plane, the incident light propagating along the dye molecules passes through the molecules with little absorption, and the light moving perpendicular to the molecular axis is absorbed more effectively. Two types of LCDs possess this structure. The first is the hybrid aligned nematic (HAN) and the other is the high pretilt angle LCDs. High pretilt angle LCDs are defined by alignment layers showing a high pretilt angle, and the directions of LC molec- ular declination on both substrates are opposite. The structures of these types of LCDs are shown in Figure 2a,b. Citation: Takatoh, K.; Ito, M.; Saito, S.; Takagi, Y. Optical Filter with Large Angular Dependence of Transmittance Using Liquid Crystal Devices. Crystals 2021, 11, 1199. https://doi.org/10.3390/cryst11101199 Academic Editor: Anatoliy V. Glushchenko Received: 7 September 2021 Accepted: 30 September 2021 Published: 3 October 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (http://crea- tivecommons.org/licenses/by/4.0/).