A photographic technique for assessing the viewing‐angle performance of liquid‐crystal displays

Abstract— Liquid‐crystal displays (LCDs) have notable variation in luminance and perceived contrast as a function of the angle from which they are viewed. Though this is an important performance issue for LCDs, most evaluation techniques for assessing this variation have been limited to laboratory settings. This study demonstrates the use of a photographic technique for such an evaluation. The technique is based on an actively cooled charge‐coupled‐device (CCD) detector in combination with a macro lens covering a circular angular range (θ) of ±42.5°. The camera was used to evaluate the luminance and perceived contrast properties of an LCD. Uniform field images corresponding to 17 equally spaced gray‐scale values in the digital driving level (DDL) range of the display system were acquired. The 12‐bit gray‐scale digital images produced by the camera were converted to luminance units (cd/m²) via the measured luminance vs. DDL response function of the camera. The changes in perceived contrast as a function of viewing angle were derived from the Barten model of the gray‐scale response of the human‐visual system using the methods proposed by the AAPM TG18 Report. The results of this photographic technique were compared to measurements acquired from a similar display using a Fourier‐optics‐based luminance meter. The results of the two methods generally agreed to within 5%. The photographic methods used were found to be accurate and robust for in‐field assessment of the angular response of LCDs over the FOV of the camera.     

Progress in viewing‐angle properties of liquid‐crystal displays

Abstract— Viewing‐angle dependences of the contrast ratio and color shift of LCDs have been radically improved as evidenced by the increasing application of LCDs in high‐quality television. This paper describes the concept of optical compensation and the fundamental characteristics of the viewing‐angle property for various LC modes.     

Controling color of liquid‐crystal displays

Abstract— Two newly derived characterization models for a liquid‐crystal (LC) display have been tested for five LC‐based displays. Data measured from a series of test colors indicated that all LC‐based displays showed similar characteristics, including an S‐shaped tone curve and poor channel chromaticity constancy. Because they include a hyperbolic function in their definition, the models do not have analytical inverses, and so iterative mathematical techniques are applied. It was shown that a new characterization model based on a hyperbolic function fits the tone curve very accurately with only four coefficients per channel for any type of LCD. In addition, it was also shown that the first derivative of the function provides a means of accurate correction of the chromaticity variation.     

Comb‐on‐plane switching electrodes for liquid‐crystal displays

Abstract— Although the common twisted‐nematic liquid‐crystal displays (TN‐LCD) has excellent contrast and low color dispersion, it suffers from small viewing angle when driven into the homeotropic state. Among the many techniques proposed, in‐plane switching (IPS) has been quite effective in improving viewing angle. However, there may be difficulty in adopting conventional IPS to higher‐definition displays because it suffers from limited storage capacitance and reduced transmittance. A new comb‐on‐plane switching (COPS) electrode design is proposed. Compared to conventional IPS, COPS allows for lower switching voltage and offers advantages including naturally scalable storage capacitance, wide viewing angle with TN‐like high transmittance, and low color dispersion.     

A single‐cell‐gap transflective liquid‐crystal display using different pretilt angles in transmissive and reflective regions

Abstract— A single‐cel l‐gap transflective liquid‐crystal display with two types of liquid‐crystal alignment based on an in‐plane‐switching structure is proposed. The transmissive region is almost homeotropically aligned with the rubbed surfaces at parallel directions while the reflective region has a homeotropic liquid‐crystal alignment. For every driving voltage for a positive‐dielectric‐anisotropy nematic liquid crystal, the effective cell‐retardation value in the transmissive region becomes larger than that in the reflective region because of optical compensation film which is generated by low‐pretilt‐angle liquid crystal in the transmissive region. Under the optimization of the liquid‐crystal cell and alignment used in the transmissive and reflective areas, the transmissive and reflective parts have similar gamma curves. An identical response time in both the transmissive and reflective regions and a desirable viewing angle for personal portable displays can also be obtained.     

Fast‐response liquid‐crystal displays using crossed fringe fields

Abstract— A fast‐response and wide‐view liquid‐crystal display (LCD) using the crossed fringe‐field‐switching (CFFS) mode is proposed, where the fringe‐field electrodes exist on both the top and bottom substrates. The bottom fringe field is used to turn on the LC directors and the top fringe field is used to assist in the LC decay process. The decay time is reduced by ～2× compared to that of the conventional FFS mode between the full bright and dark states, and more than a 2× improvement is obtained for other gray‐scale transitions. This CFFS mode also preserves the wide‐view characteristics as the conventional FFS mode. Its applications to LCD TVs and monitors for reducing image blur are addressed.     

A viewing‐angle‐controllable blue‐phase liquid‐crystal display

Abstract— A viewing‐angle‐controllable liquid‐crystal display (LCD) is proposed. When the device is only driven by an in‐plane electric field, it exhibits a wide‐viewing‐angle (WVA) mode. And it exhibits narrow‐viewing‐angle (NVA) mode when it is driven by a vertical electric field as well as an in‐plane electric field. In this manner, the viewing angle of the device can be controlled from 100° to 30°. The device exhibits a good viewing‐angle‐controlling characteristic and high transmittance.     

Review of operating principle and performance of polarizer‐free reflective liquid‐crystal displays

Abstract— In this paper the operational principle and performance of guest‐host, liquid‐crystal/polymer‐composite scattering, and cholesteric liquid‐crystal reflective displays are reviewed. These displays do not use polarizers and have the advantage of providing high reflectance and compatibility with flexible plastic substrates.     

Accurate determination of pretilt angle in twisted‐nematic liquid‐crystal cells by using a rotating analyzer

Abstract— A new optical method for determining the pretilt angle ψ⁰, particularly on twisted‐nematic (TN) liquid‐crystal (LC) cells, is proposed. ψ⁰ was rapidly determined with good reproducibility on a TN‐LC cell by using a rotating analyzer optical system, a twist angle Φ, the azimuth of the director at a substrate, ϕ⁰, and the retardation Δnd as known values. The thickness d was also determined simultaneously with ψ⁰. ψ⁰ and d were determined within minutes. ψ⁰ was previously determined to be in the range of 0.1° with a standard deviation of 0.01°; this was obtained by repeating the measurement 50 times. The principle behind the determination and the experimental set up are described in detail.     

Pre‐tilt angle and cell‐gap measurement of vertically aligned non‐twisted liquid‐crystal displays

Abstract— A measurement method for the determination of the cell parameters of vertically aligned nematic LCOS devices has been developed. It provides the values for the pre‐tilt angle and the cell thickness in a reliable way, without the need for spectroscopic instruments. The method uses oblique incidence to separate the determination of pre‐tilt angle and cell thickness from each other and thus enhance the measurement accuracy. As a bonus, the measurement system consists only of simple optical components and does not need costly instruments.     

Performance of liquid‐crystal displays for fire‐service thermal‐imaging cameras

Abstract— As use of handheld thermal‐imaging cameras (TICs) becomes more prevalent in the first‐responder community, it is important that standard test metrics be available to characterize imaging performance. A key performance consideration is the quality of the image presented on the TIC display. This paper focuses on TICs that use liquid‐crystal displays to render an image for the user. Current research on TIC performance for first‐responder applications makes use of trained observers and/or composite‐video‐output‐signal measurements. Trained observer tests are subjective and composite video output tests do not evaluate the performance of the complete imaging system. A non‐destructive objective method was developed that tests the performance of the entire thermal‐imaging system, from the infrared sensor to the display. A thermal target was used to correlate the measured thermal imager composite video output signal with the luminance of the display. A well‐characterized charge‐coupled‐device (CCD) camera and digital recording device were used to measure the display luminance. An electro‐optical transfer function was determined that directly relates the composite video output signal to the luminance of the display, providing a realistic characterization of system performance.     

A continuous‐viewing‐angle‐controllable liquid‐crystal display using a blue‐phase liquid crystal

Abstract— A continuous‐viewing‐angle‐controllable liquid‐crystal display (LCD) using a blue‐phase liquid crystal is proposed. To realize both wide‐viewing‐angle (WVA) mode and narrow‐viewing‐angle (NVA) mode with a single liquid‐crystal panel, each pixel is divided into a main pixel and a subpixel. The main pixel is for displaying images in both modes. The subpixel is for displaying images in WVA mode and controlling the viewing angle in NVA mode. The device exhibits a good viewing‐angle‐controlling characteristic and high transmittance.     

Luminescent dichroic‐dye‐doped cholesteric liquid‐crystal displays

Abstract— LCDs based on a luminescent dichroic‐dye‐doped non‐absorbing cholesteric LC with positive dielectric anisotropy is proposed. In the initial state, the orientation of the dye molecules provides effective light absorption and irradiation. By applying an electric field to the cell, the absorption and thus the luminescence is absent. A two‐color luminescence could be achieved by sandwiching two cells: the upper cell consists of a cholesteric LC with two dyes (sensitizer and emitter) and is used with an applied voltage (active cell); the lower cell consists of a cholesteric LC doped with one dye and works without applying a voltage (passive cell). The performance characteristics of luminescent dye‐doped cholesteric‐LCDs were investigated.     

Liquid‐crystal displays using printed plastic TFT backplanes

Abstract— In this paper, we present results from a new liquid crystal over plastic printed thin‐film‐transistor (TFT) display. The display demonstrator shows that the processing incompatibilities between the plastic TFT backplane and the liquid‐crystal materials can be addressed to make a stable twisted‐nematic structure. New fabrication processes such as the photo‐alignment of liquid crystals have made it possible to create a new generation of displays, which pave the way towards fully integrated plastic liquid‐crystal‐display technologies.     

Technologies toward flexible liquid‐crystal displays

Abstract— Several leading technologies for flexible liquid‐crystal displays have been developed recently at ERSO. The roll‐to‐roll compatible techniques, polymer‐added liquid crystal, have been applied on a film‐like substrate. A flexible black‐and‐white cholesteric liquid‐crystal display was also implemented by photo‐induced phase separation. Color filters placed on a plastic substrate by a low‐temperature manufacturing process was successfully fabricated. A novel design of a wide‐viewing‐angle color plastic LCD was also proposed.     

A multistable smectic‐A liquid‐crystal device with low threshold field

Abstract— A multistable liquid‐crystal device has been realized in this study; its smectic‐A LC is oriented by in‐plane and out‐of‐plane electric fields with various amplitudes. This paper introduces improvements made to electrode structure and hybrid surface treatments that have enhanced SmA LC device performance. The driving voltage of the hybrid‐aligned cell has been reduced to 20% of the vertical‐aligned one, and light leakage has been suppressed by out‐of‐plane electric‐field‐strength modulation. The many advantages of this multi‐stabilized LC device could be applied to memory devices and to flexible displays.     

How to reduce light leakage and clipping in local‐dimming liquid‐crystal displays

Abstract— In conventional LCDs, the backlight is set to maximum luminance regardless of the image. For dark scenes, this approach causes light leakage and power waste. Especially, light leakage in dark scenes degrades the contrast ratio of LCDs; to circumvent this problem, local‐dimming systems have been proposed. In these systems, the LED backlight is divided into several local blocks and the backlight luminance of each local block is controlled individually, and pixel values are adjusted simultaneously according to the luminance profile of the dimmed backlight. In this paper, a method of determining the LED backlight luminance of each local block depending on the image is proposed; this method significantly improves the image quality of LCDs. First, we introduce methods of quantifying light‐leakage at dark gray levels and clipping at bright gray levels. Then, the proposed method to determine the dimming duty, which controls the LED backlight luminance by compromising between these two measures, was derived. The proposed algorithm preserves the original image with little clipping distortion and effectively reduces light leakage.     

Novel application of ink‐jet‐printing technology in multi‐domain alignment liquid‐crystal displays

Abstract— Based on the drop‐on‐demand characteristics of ink‐jet printing, the multi‐domain alignment liquid‐crystal display (LCD) could be achieved by using patterned polyimide materials. These polyimide ink locations with different alignment procedures could be defined in a single pixel, depending on the designer 's setting. In this paper, we combined the electro‐optical design, polyimide ink formulation, and ink‐jetting technology to demonstrate the application of multi‐domain alignment liquid‐crystal display manufactory. The first one was a multi‐domain vertical‐alignment LCD. After the horizontal alignment material pattern on the vertical alignment film, the viewing angle would reach 150° without compensation film. The second one was a single‐cell‐gap transflective LCD within integrating the horizontal alignment in the transmissive region and hybrid alignment in the reflective one in the same pixel. In addition, this transflective LCD was also demonstrated in the form of a 2.4‐in. 170‐ppi prototype.     

Analysis of the dependence of optical response time of liquid‐crystal displays on the viewing direction

Abstract— The optical response time of liquid‐crystal displays (LCDs) has recently been observed to be dependent on the viewing direction. For the vertically aligned LC mode, response time increased as the viewing angle increased when the final state is the zero gray level of the minimum luminance. This change in response time is analyzed to relate to the deformation of the normalized luminance curve of LCDs for different viewing directions. The dependency trends of the optical response time for the oblique direction can be estimated from the temporal luminance measurement data along the normal direction and the normalized luminance curve for oblique viewing directions.     

Cell‐gap and pretilt‐angle determination of a vertically aligned reflective liquid‐crystal display

Abstract— A simple method for determining the cell gap and pretilt angle of a vertically aligned reflective liquid‐crystal display has been developed. By extrapolating the phase‐retardation curve of the VA—LC cell in the high‐voltage regime, we can determine the cell gap. With this value for the cell gap and the low‐voltage part of the phase‐retardation curve, we can determine the pretilt angle.