Seung-Hyuck Lee

A Behavioral Circuit Model of Active-Matrix Liquid Crystal Displays for Optical Response Simulation

We propose a behavioral circuit model to precisely predict optical responses of an active-matrix liquid crystal (LC) display (LCD) using a patterned vertical alignment (PVA) mode. To get more accurate simulation results, we propose two LC groups with different time constants for a pixel after observing the LCD pixels by using a high-speed camera. In addition, we include a time-delay concept into our behavioral model for brightening or rising transitions. We describe the behavior of the PVA-LCD by using the analog hardware description language Verilog-A. We simulate the PVA-LCD panel by importing the behavioral circuit model in the circuit simulator SmartSPICE. The simulation results of the transient optical responses show excellent matches with the measurement ones.

Behavioral Circuit Model of Active-Matrix Liquid Crystal Display With Charge-Shared Pixel Structure

This paper proposes a behavioral circuit model to precisely predict optical responses of an active-matrix liquid crystal (LC) display with charge-shared vertical alignment (CS-VA) mode. CS-VA mode consists of two subpixels in one pixel to mitigate the off-axis gamma distortion. We present a new method using overdrive voltages to obtain accurate capacitance versus voltage (C-V) characteristics in CS-VA mode. In addition, we present a new technique to obtain transmittance versus voltage (T-V) characteristics of the two subpixels by using a high-speed camera with image processing. In addition, we analyze the panel to obtain precise voltage levels after sharing charges. We describe the behavior of the CS-VA LC display (LCD) by using an analog hardware description language, Verilog-A, and we simulate the CS-VA LCD panel by importing the behavioral circuit model in a circuit simulator, Smart-SPICE. The simulation results of the transient optical responses show excellent matches with the measurement ones.

Behavioral Model of Patterned Vertical Alignment Pixel in Active-Matrix Liquid Crystal Displays

This paper proposes a more accurate behavioral circuit model to predict transient responses of an active-matrix liquid crystal display (AMLCD) with a patterned vertical alignment (PVA) mode. In the case of the PVA mode, we observed that simulation errors increase when the conventional circuit model is directly applied to overdriven static and dynamic data transitions. To overcome this issue, we investigate the actual behavior of the liquid crystal (LC) molecules in the PVA AMLCD pixel using a high-speed camera attached to a microscope. We discovered that the behavior of the LC molecules is significantly dependent on their proximity to the edges of transparent electrodes. Based on this investigation, we propose a first-order circuit model that has two LC groups with two different response characteristics. The proposed model is embedded in a circuit simulator using an analog hardware description language. Simulation results show that the transient optical responses are considerably more accurate than the conventional responses for any data transitions, including overdriven transitions.

Pixel Architecture for Low-Power Liquid Crystal Display Comprising Oxide and Ferroelectric Memory Thin Film Transistors

A new pixel architecture and driving scheme for a low-power liquid crystal display (LCD) with a low refresh-rate is proposed. The proposed pixel architecture comprises two oxide thin-film transistors (Ox-TFTs), two ferroelectric memory TFTs (Fe-MTFTs), and two capacitors. Both TFTs have the same indium gallium zinc oxide film as an active layer. The Fe-MTFT exhibits a hysteresis property owing to the ferroelectric gate insulator that enables it to operate as a memory. A prototype LCD with 6 × 5 pixels is fabricated in this letter. The fieldeffect mobility of Ox-TFTs is 11 cm2V-1s-1. The memory window of Fe-MTFT is 5 V with a gate-voltage sweep from -20 to 20 V. After programming the Fe-MTFTs, the fabricated LCD successfully operates at a 0.5-Hz refresh-rate.

A new precharging method without side effects for liquid crystal displays with insufficient charging time

In this paper, a new precharging method without any side effects is proposed to overcome image degradation caused by insufficient charging times. This work explains why the precharging method can compensate for short charging times. However, side effects of the precharging method in the form of horizontal line artifacts are addressed, wherein line artifact-compensating precharging (LCP) is presented to mitigate the side effects. Behavioral modeling is employed to investigate the side effects by estimating transient responses of a liquid crystal display. The LCP proves that it can dramatically reduce line artifacts caused by precharging because the brightness difference of adjacent pixels does not exceed 1.0.

Optimum display luminance dependence on ambient illuminance

Abstract. Viewing high luminance displays under low light conditions causes unbearable glare. On the other hand, viewing low luminance displays under bright light conditions reduces visibility. We investigate the comfortable display luminance range for several different conditions of ambient illuminance through a psychophysical experiment, which involved 30 subjects. The obtained upper (lower) limits of the comfortable zone were 516 (113), 574 (116), 612 (130), 664 (154), 737 (177), 790 (204), and 836 (246)  cd/m2, for ambient illuminances of 50, 100, 200, 500, 1000, 2000, and 5000 lx, respectively.

Physical Model of Pixels in Twisted Nematic Active-Matrix Liquid Crystal Displays

In this paper, we propose a new physical model to predict the transient responses of an active-matrix liquid crystal display (LCD) in a twisted nematic (TN) mode. For the behavioral model, three types of data are most important: 1) capacitance-voltage; 2) transmittance-voltage characteristics; and 3) time constant parameters a1 and a2. We extract them from the physical properties of TN LC molecules, such as dielectric anisotropy, birefringence, viscosity, cell gap, and elastic constant. We describe the behavior of the TN-LCD by using the analog hardware description language, Verilog-A. We simulate optically rising responses of the TN-LCD panel by importing the behavioral circuit model in the circuit simulator. The simulation results show that the transient optical responses show excellent matches with the measurement ones. In addition, we can observe how the behaviors of optical responses change with varying the physical parameters, such as cell gap, dielectric anisotropy, and elastic constant.

Memory-in-Pixel Circuit for Low-Power Liquid Crystal Displays Comprising Oxide Thin-Film Transistors

We propose a new memory-in-pixel (MIP) circuit with only oxide thin-film transistors (Ox-TFTs) for a low-power liquid crystal display with flicker-free feature. The proposed MIP circuit is composed of two new memory circuit units comprising two Ox-TFTs and a capacitor. The proposed memory circuit can modulate the threshold voltage via a simple driving scheme. When the threshold voltage is shifted properly, Ox-TFTs maintain the ON or OFF state by virtue of their extra low leakage current, which enables the MIP operation. By applying the black or white voltage to the pixel directly, flicker can be eliminated. We fabricated the MIP circuit and investigated the circuit performance depending on the capacitance and TFT sizes. Finally, we verified the feasibility of the proposed MIP circuit for low power operation.

Psychophysical research on switching between light emitting and reflecting modes of light adaptable display considering equal visibility

Abstract Viewing high-luminance displays such as liquid crystal displays or organic light emitting diode displays under low-light conditions causes an unbearable glare, while viewing them with low luminance under bright-light conditions reduces visibility. Recently, several research groups have reported light adaptable displays (LADs) to extend display visibility over a wide range of light conditions. Here, we present a psychophysical study on how to effectively utilize the LAD using two different display types for the first time. LAD features two switchable display types: light emitting mode (LEM) and light reflecting mode (LRM). To maintain visibility and prevent visual artifacts, we investigate when to switch modes between LEM and LRM. We conduct psychophysical experiments involving seventy subjects. They are asked to select illuminance levels under which they perceive that the LEM and LRM look the same depending on luminance of the LEM. We propose a mode-switching condition by combining our previous results on the comfortable luminance of LEM display according to the ambient illuminance and the selected illuminance results providing equal visibility of both LEM and LRM displays.

A Simple Overdrive Technology Using Behavioral Circuit Model for Liquid Crystal Displays

This paper proposes a new method to determine the overdrive (OD) values using a behavioral circuit model. Based on the accurate prediction of overdriven transitions in the behavioral circuit model, the proposed method enables us to obtain the OD values easily. The proposed method requires only three OD values to complete all the OD values. The estimated OD values agree with the measured OD values. The average OD deviation was only 1.60 over all data transitions.