Review Paper: Multi‐primary‐color displays: The latest technologies and their benefits

Abstract— Multi‐primary‐color (MPC) display technology is one of the fastest emerging research areas in recent years. Wide‐color‐gamut display devices have been required for visually sufficient and/or accurate color reproduction. It is well known that MPC displays can reproduce accurate colors with high efficiency. In addition, not only the image‐quality improvement but some other performance of display devices is also required for display devices. This paper reviews achievements in MPC display technologies and focuses on the benefits of MPC displays: power‐savings and high resolution.     

The luminance resolution characteristics of multi‐primary‐color display

Abstract— In this paper, the resolution characteristics of multi‐primary‐color (MPC) display systems are analyzed. That four‐primary‐color (4PC) displays can increase the effective resolution for achromatic images in the luminance domain by a factor of two as compared to conventional RGB‐based displays with MPC‐specialized subpixel rendering, which is proposed in this paper, is demonstrated. Five‐ and six‐primary‐color (5PC and 6PC) display systems can reproduce denser luminance data than conventional RGB‐based display systems and solve a problem of MPC displays, viz. an increase of production costs and a decrease in the aperture ratio caused by increasing the number of subpixels in one pixel. This is an essential advantage of MPC display systems, which is related to the combination of the proposed color‐filter architecture and image processing. Thus, a completely new advantage of MPC display systems, in addition to their well‐known capabilities of color reproduction and power saving, is proposed.     

Wide gamut LCD using locally dimmable four‐primary‐color LED backlight

This paper proposes a wide gamut LCD using locally dimmable four‐primary‐color (4PC) LED backlight. Although the color gamut of LCDs has been improved in recent years, it is insufficient to reproduce all the colors in the real world. The objective of this paper is to propose a wide gamut LCD that reproduces all the colors in the real world while keeping the cost increases to a minimum. We evaluated the color gamut reproduced by LEDs of multiple primary colors and selected cyan as the optimal color to be added to the three primary colors to reproduce all the colors in the real world. Therefore, we designed an LED backlight consisting of an additional only‐cyan LED with three‐primary‐color LEDs and developed a prototype LCD with 4PC LED backlight. Furthermore, we developed a local dimming algorithm for the 4PC LED backlight. As a result, we confirmed that the prototype LCD with the 4PC LED backlight is able to cover almost all the colors in the real world and also able to display natural images with highly saturated colors by local dimming.     

Multi‐primary design of spectrally accurate displays

Abstract— Spectral color reproduction overcomes some inherent problems of colorimetric reproduction. An implementation of a spectral display for surface color reproduction, capable of reproducing a desired spectrum for each pixel, based on multi‐primary projection technology, is presented. A light source with a spectrum identical to that of the illumination is filtered by a positive linear combination of several color filters, which reproduces the reflectance spectra. The spectra of the color filters are tailored to span the space of possible surface spectra. Various methods for choosing the color filters vis‐à‐vis the required performance are discussed in detail. Soft‐proofing application is examined as a test case for the concept.     

A field‐sequential‐color display with a local‐primary‐desaturation backlight scheme

Abstract— Field‐sequential‐color technology eliminates the need for color filters in liquid‐crystal displays (LCDs) and results in significant power savings and higher resolution. But the LCD suffers from color breakup, which degrades image quality and limits practical applications. By controlling the backlight temporally and spatially, a so‐called local‐primary‐desaturation (LPD) backlight scheme was developed and implemented in a 180‐Hz optically compensated bend (OCB) mode LCD equipped with a backlight consisting of a matrix of light‐emitting diodes (LEDs). It restores image quality by suppressing color breakup and saves power because it has no color filter and uses local dimming. A perceptual experiment was implemented for verification, and the results showed that a field‐sequential‐color display with a local‐primary‐desaturation backlight reduced the color breakup from very annoying to not annoying and even invisible.     

Multi‐color LC/BL algorithm in field‐sequential‐color LCD for color‐breakup suppression

Abstract— The proposed liquid‐crystal and backlight (LC/BL) algorithm presents the dynamic field‐sequential‐color (D‐FSC) algorithm to reduce the color‐breakup (CBU) effect without greatly increasing the subframe rate. The D‐FSC algorithm can intelligently select one adequate color sequence from multiple color sequences according to the image data. In other words, the scope of CBU suppression of the proposed LC/BL algorithm is more extensive than other conventional FSCs. Simulation results show that the CBU suppression can be improved substantially by the proposed evaluation equation.     

MUTED: Multi‐user 3‐D display

Abstract— Research described in this paper encompasses the design and building of glasses‐free (autostereoscopic) displays that utilize a direct‐view liquid‐crystal display whose backlight is provided by a projector and novel steering optics. This is controlled by the output of a multi‐user head‐position tracker. As the displays employ spatial multiplexing on a liquid‐crystal‐display screen, they are inherently 2‐D/3‐D switchable with 2‐D being achieved by simply displaying the same image in the left and right channels. Two prototypes are described in this paper; one incorporating a holographic projector and the other a conventional LCOS projector. The LCOS projector version addresses the limitations of brightness, cross‐talk, banding in the images, and laser stability that occur in the holographic projector version. The future development is considered and a comparison between the prototypes and with other 3‐D displays is given.     

Avoiding on‐screen metamerism in N‐primary displays

Abstract— The present paper describes a method for using more than three primaries in an additive‐primary display. The method ensures that each tristimulus specification can be produced in no more than one way, even if a non‐singular filter (i.e., one that does not reduce the dimensionality of color‐matching space) is interposed between the screen and the viewer. Starting with N primaries, the method uses only three at a time, but these may be composites — fixed linear combinations of the original N. As further insurance against on‐screen metamerism, a criterion on the primary spectra, based on the Binet‐Cauchy theorem, ensures that a triad of primaries keeps its right/left‐handed chromaticity ordering when a filter is interposed.     

Multi‐layered flat‐surface micro‐optical components directly molded on an LCD panel

Abstract— We have developed a process to fabricate optical components, such as a lens, prism, or diffuser, directly on to a glass substrate. Processes include precision mastering by diamond cutting and multi‐layer photopolymer (2P) molding to realize flat surfaces and the integration of multiple components with an alignment within a few micrometers.     

Color‐breakup evaluation of spatio‐temporal color displays with two‐ and three‐color fields

Abstract— Spatio‐temporal color displays have higher transmission and resolution than conventional LCDs, but suffer from color breakup. In this paper, a 120‐Hz display with two‐color filters and two‐color fields is described and the amount of color breakup is compared with that of a 180‐Hz full‐color‐sequential display with no color filters and three‐color fields. The results indicate that color breakup in a color‐filterless display is annoying, whereas it is just visible in displays with two‐color filters even though the refresh rate is much lower.     

Color‐conversion method for a multi‐primary display to reduce power consumption and conversion time

Abstract— A color‐conversion method for a light‐emitting multi‐primary‐color display is proposed. While amulti‐primary‐color display uses four or more primary colors to reproduce a wide color gamut, multiple sets of primary‐color signals are needed to reproduce one color. Therefore, linear programming, which results in low power consumption, was adopted to uniquely determine the set of primary‐color signals. Although a highly accurate color conversion was achieved by using linear programming with low power consumption, it requires a very long time to convert colors of high‐resolution images. Therefore, by categorizing the color conversion of linear programming as a classification problem, colors are converted by using the decision‐tree method, which is a classification method. As a result, color conversion with high accuracy, low power consumption, and short conversion time was achieved.     

Flexible color LCD panel driven by low‐voltage‐operation organic TFT

Abstract— A flexible color LCD panel driven by organic TFTs (OTFTs) was successfully demonstrated. A pentacene OTFT with an anodized Ta₂O₅ gate insulator, which can be operated at low voltage, was developed. In order to improve the electrical performance of the OTFT, the gate insulator was surface treated by processes such as O₂ plasma, UV light irradiation, and hexamethyldisilane treatments. The fabricated OTFT exhibited a mobility of 0.3 cm²/V‐sec and a current on/off ratio of 10⁷ with a low operating drain voltage of ?5 V. A fast‐response‐time flexible ferroelectric LCD, which contains polymer networks and walls, was integrated with the OTFTs by using a lamination and a printing technique. As a result, color images were achieved on the fabricated panel by using a field‐sequential‐color method at a low driving voltage of less than 15 V_pp.     

Electron‐beam curing of color filters for flexible‐display applications

Abstract— Color filters spin‐coated on plastic and glass substrates have been cured by electron‐beam radiation instead of by the conventional thermal‐heating method. The electron‐beam curing of the color filters has many advantages over the thermal curing method. Electron‐beam curing is, in principle, a non‐thermal method where low‐temperature (<100°C) curing of color filters on plastic substrates can be realized for the manufacturing process of flexible display panels. A color‐filter resist having a 1.5‐μm thickness was spin‐coated on plastic (polycarbonate) and glass (corning 1737) substrates. The effect of the electron‐beam radiation conditions, such as electron‐beam energy (0.3–1.0 keV), radiation dosage (10–200 kGy), and ambient oxygen has been characterized. The degree of curing was analyzed by using the characteristic absorption peaks at 808 and 1405 cm^?1 in the FT‐IR spectrum. These two peaks originate from the carbon double bonds (>C=C<) of the multi‐functional acrylate monomer which exist in the color‐filter resist. By electron‐beam radiation, the spin‐coated color filter can be effectively polymerized at g (glass transition temperature) of the plastic substrates. The electron beam can solve the problems of the conventional thermal curing method, such as thermal deformation of a plastic substrate and difficulty in achieving dimensional control of a color‐filter pattern due to a large coefficient of thermal expansion (20–70 ppm/°C) compared to that of a glass substrate.     

A novel spectrum‐sequential display design with a wide color gamut and reduced color breakup

Abstract— The advantage of RGB color‐sequential displays is that they have no color filters, but the disadvantage is that they need to run at high refresh rates (> >180 Hz) to prevent flicker and color breakup. An alternative color‐sequential display, which can operate at relatively low refresh rates (～ 100 Hz) without disturbing color breakup or flicker, has been developed. The display has two color filters per pixel (cyan and magenta) on the LCD panel and the backlight can generate two types of spectra (blue‐green and green‐red), which results in a wide gamut four‐primary display, effectively. One part of the paper describes the color reproduction, including color‐filter design, gamut mapping, and multi‐primary conversion. The other part deals with the reduced perception of color breakup on the novel spectrum‐sequential display compared to conventional color‐sequential displays.     

A fast‐switching OCB‐mode LCD for high‐quality display applications

Abstract— Optically compensated bend (OCB) mode is a promising technology, due to its wide range of viewing angles without gray‐scale inversion or color shift, fast response, high contrast ratio, and wide temperature range. This paper summarizes the fundamental characteristics of OCB mode and discusses the development of field‐sequential‐color displays and 3‐D displays for future high‐quality display applications.     

Super‐reflective color LCD with PDLC technology

Abstract— A novel reflective color LCD without polarizers has been developed using a PDLC film and a retro‐reflector. Bright color images including moving images are achievable with ambient light. This novel LCD will enable the new application area of electronic paper.     

Advanced monochrome gray‐scale TFT‐LCD using diamond‐like‐carbon/ion‐beam alignment technology

Abstract— A novel liquid‐crystal alignment method, diamond‐like carbon and ion beam alignment (DLC/IB) technology, was announced at the 2001 SID Symposium. And since December 2001, a new‐generation ion‐beam machine has been placed into the manufacturing line of IDTech. DLC/IB technology is mainly used for medical displays, which require a monochrome high‐density and super‐uniform display. We report on the latest developments of these advanced monochrome displays.     

Single‐layered multi‐color electrowetting display by using ink‐jet‐printing technology and fluid‐motion prediction with simulation

Abstract— This paper describes a single‐layered multi‐color electrowetting display (EWD) by using ink‐jet‐printing (IJP) technology and comparing different pattern electrodes with the use of the numerical investigations of ANSYS FLUENT®. This work consists of two parts: the first describes the design of implementing a single‐layered multi‐color EWD and the second demonstrates the application of ANSYS FLUENT® simulation in different pattern electrodes settings on the proposed EWD. The single‐layered multi‐color EW device was evaluated by using various colored oils without adopting a color filter. The single‐layered multi‐color EWD at a driving voltage of 25 V can achieve a maximum aperture ratio and reflectivity of 80% and 38.5%, respectively. The colored saturation of R, G, B oils can increase to 50% (NTSC: 13.3–27.8%). In addition, a radiate electrode at the required viewable area condition of 85% and force 5 * F^k, which results in ink stable contraction and a shorter response time of 50% (radiate vs. square), was proposed. The experimental results and simulation demonstrate that ink‐jet‐printing (IJP) technology along with the use of radiate electrodes can result in a single‐layered multi‐color EWD with a shorter response time.     

Extensible multi‐primary control sequences

Abstract— In a display with more than three primaries (called a multi‐primary display), a color can be expressed as multiple combinations (called control sequences) of primaries. This paper presents an algorithm for assigning control sequences that preserves current assignments when further primaries are added. These control sequences are cal led extensible. It is shown that the gamut of any number of primaries is a zonohedron, which can be dissected into parallelepipeds. Control sequences are assigned within each parallelepiped. The current parallelepipeds remain when more primaries are added, so the current assignments are preserved. Multi‐primary displays can also cause unwanted metamerism and make continuous color scales appear discontinuous. The algorithm avoids these problems. When viewed through natural filters, such as yellowed ocular lenses, multi‐primary displays can sometimes make two different colors appear identical. If the primaries satisfy the Binet—Cauchy criterion, which is always the case when all primaries are monochromatic, then these spurious matches are avoided.     

Exponential Stability for Multi‐Area Power Systems with Time Delays Under Load Frequency Controller Failures

This paper investigates the exponential stability problem for a class of multi‐area power systems with time delays under load frequency controller failures (LFCFs). For describing the phenomenon of LFCFs, the considered multi‐area power system is rewritten as a switched system with multiple time delays. By adopting the switching technique, the exponential stability conditions for multi‐area power systems are developed when the controller failure frequency and the unavailability ratio of the controller are restricted. Finally, one example is given to show the applicability of the proposed method.