Achieving standard wide color gamut by tuning LED backlight and color filter spectrum in LCD

In order to achieve the standard red, green, and blue (sRGB) standard color gamut in color liquid crystal display and improve the image quality, the impact of the backlight and color filter spectrum on module's chroma was simulated and analyzed. The color gamut was enhanced by adjusting and optimizing the two parts of spectrums of LED backlight and color filter and by using red and green phosphor LED backlight to match the new color filter with an appropriate thickness. Experimental results show that: When the thickness of color filter is 2.2 µm, National Television System Committee color gamut increases from 65.3% to 74.9%, and sRGB matching rate enhances from 83.2% to 100%, achieving a full coverage of the sRGB standard color gamut, the transmittance of white light reaches 28.1%. Also, it is verified that shifting the peak position of the backlight and color filter spectrum to purification direction, as well as narrowing its half‐width can upgrade the color gamut. Meanwhile, the thicker the thickness of color filter is, the wider color gamut it has, based on the same pigment material.     

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.     

Color‐gamut expansion in CRTs

Abstract— This paper describes the architecture for a color picture tube (CPT), which enables a cost‐effective expansion of the color gamut. Besides a gamut expansion based on application of more‐saturated phosphors in the present three‐primary system, four‐ and five‐primary systems are described. Gun‐pitch modulation allows the application of a conventional electron gun for both a four‐ and five‐primary CRT. In these systems, the mask and matrix transmissions need to be reduced to maintain color purity. From this point of view, a five‐primary CRT is unrealistic. Two four‐primary CRTs are evaluated in more detail. But, here, the total anode current to generate white has to be increased substantially because of the reduced mask and matrix transmissions. Nonetheless, in this paper it is argued that with one of the four‐primary CRTs more‐saturated yellow colors can be displayed at 85% of the display luminance of a conventional system without loss of perceived sharpness.     

Thermal management of edge‐lit wide color gamut backlight for LCD using red laser diodes and cyan LEDs

The color gamut is one of the critical parameters that dictate the image quality of displays. The liquid crystal displays using white color light‐emitting diodes (LEDs) as the backlight, though having been widely employed recently, are not very satisfactory in terms of their color gamut because of the broad spectrum inherent to white LEDs. This prompted the authors to develop improved liquid crystal displays using an edge‐lit wide color gamut backlight that used red laser diodes and cyan LEDs. Generating laser beams with high color purity, the laser diodes are light sources with a significant effect on expanding the color gamut. However, laser diodes, red ones in particular, have unfavorable thermal characteristics. To cope with this shortcoming, the authors clearly defined the restrictive criteria for laying out two kinds of light source on the edge‐lit backlight and made a prototype 55‐type laser backlight for performance evaluation.     

Preferred and maximally acceptable color gamut for reproducing natural image content

Abstract— With the development of wide‐gamut display technology, the need is clear for understanding the required size and shape of color gamut from the viewers' perspective. To that end, experiments were conducted to explore color‐gamut requirements based on viewers' preferred level of chroma enhancement of standard‐gamut images. Chroma preferences were measured for multiple hues using single‐hue images, and a corresponding hue‐dependent preferred chroma enhancement was successfully applied to natural, multi‐hue images. The multi‐hue images showed overall success, though viewers indicated that reds could be decreased even further in colorfulness, and yellows could be increased, which may argue in favor of multi‐primary displays. Viewer preferences do vary within the population, primarily in overall chroma level, and the differences can be largely accounted for with a single parameter for chroma‐level adjustment that includes the preferred hue dependence. Image content dependencies were also found, but they remain too complex to model. The hue‐dependent chroma preference results can be applied to display design and color‐enhancement algorithms.     

Seventeen‐inch laser backlight in‐plane switching liquid crystal display with 8K, 120‐Hz driving,and BT.2020 color gamut

We have developed a 17‐inch laser backlight in‐plane switching liquid crystal display satisfying the main BT.2020 specifications, which are 8K, 120‐Hz driving, and a BT.2020 wide color gamut. The color gamut of the developed in‐plane switching liquid crystal display covers 98% of the BT.2020 wide color gamut, thanks to a laser backlight and appropriate color filters. The liquid crystal response time of 5 ms, which is sufficient for 120‐Hz driving, is achieved by adapting a faster in‐plane switching liquid crystal display, namely, the short‐range lurch control in‐plane switching liquid crystal display.     

Wide color gamut and high dynamic range displays using RGBW LCDs

An increasing number of studies on efficient implementation of vivid and realistic displays are being conducted as liquid crystal displays (LCDs) become widely used in TV applications. For vivid displays, the specifications such as wide color gamut (WCG) and high dynamic range (HDR) should be implemented in LCDs. However, the low transmittance rate of the WCG and the significant costs for the peak luminance capability of the HDR are major obstacles. Hence, an RGBW LCD, which is capable of increasing the transmission efficiency, may be a good platform to overcome these problems.In this paper, we estimate the perceived brightness effect of the WCG by using the Helmholtz–Kohlrausch (H–K) effect on RGB and RGBW LCDs. The simulation results showed that the RGBW LCD was more suitable for adopting the WCG than the RGB LCD in terms of the brightness balance of achromatic and chromatic colors, and the results were also confirmed by subjective tests. In addition, we propose an effective method to implement the HDR display based on the RGBW LCD. The data stretch, considering a local adaptation characteristic of a human visual system (HVS), greatly enhanced the details of the dark regions, and the local peak dimming using the white channel analysis and the white channel data itself increased the expressiveness of the peak luminance in irradiative or specular regions to 1500 nits.     

A phosphor sheet and a backlight system providing wider color gamut for LCDs

SrGa₂S₄:Eu and CaS:Eu phosphors are useful to provide a wider color gamut to LCDs since their emissions are narrower, but they easily react with moisture. The phosphor sheet containing them was developed. The moisture barrier layers on it can separate them from a humid atmosphere. The LCD with the phosphor sheet could realize the wider color gamut of 90% NTSC‐xy, while the color gamut of the original LCD with the white LEDs containing YAG:Ce phosphor was 71% NTSC‐xy. The improvement of the brightness of the LCD with the phosphor sheet was studied. Encapsulates with a greater curvature on blue LEDs used with the phosphor sheet can increase the brightness. The brightness of the LCD with the phosphor sheet and blue LEDs with the spherical encapsulates of which the curvature = about 0.7 was estimated to reach to 85% of that of the original LCD. The simpler phosphor sheet with the aluminum film edge seals was also developed. It was confirmed that it has sufficient reliability during storage and operation at high temperature and high humidity and there is no difference in its reliability between the edge area and the center area. The importance of the moisture barrier layers which protect the SrGa₂S₄:Eu and CaS:Eu phosphors was confirmed. The phosphor sheet with the moisture barrier layers had better reliability than that with the PET layers.     

Development of a dual‐lamp projector with higher brightness

Stationary projectors mainly used in system applications have recently gained a wider application range, including general presentations in halls and large conference rooms and being used, for example, in digital signage and for monitoring purposes. Consequently, they are required to meet new market demands for durability, reliability, and flexibility in installation in addition to their conventional basic performance such as especially high brightness. To achieve especially high brightness, a new optical system with two lamps is proposed; the profile of the cover glass that prevents the glass from scattering is made aspherical, the profile of the beam‐combining mirror is improved with its position optimized, and the size of the incident plane of the integrator rod is made larger (1.1 times). These measures resulted in an optical system of an even higher efficiency with 7000 lm, which is the highest in its class (according to a June 2010 investigation). Also, the arrangement of two parallel lamps completely eliminated the effect of heat passing from one lamp to the other, which helped secure durability, reliability, and flexibility in installation. Furthermore, the combined use of the unique non‐telecentric optical system, adopted from the conventional single‐lamp model, helps maintain the class‐highest contrast ratio.^1,2     

Relationship between mode‐boundary from surface color to fluorescent appearance and preferred gamut on wide‐gamut displays

Abstract— Color‐gamut design is a major concern in wide‐gamut displays. To determine a preferred gamut for displaying object color in natural scenes on a wide‐gamut display, subjective evaluations were conducted to investigate the preferred color and acceptable limit. Then, simple synthesized images were used to determine the mode boundary between surface color and fluorescent color appearance. It was found that (1) observers perceived the colors with high saturation and high lightness as fluorescent colors and (2) the fluorescent appearance decreased preference. The color‐mode index (CMI) was defined as an evaluation index of the color‐appearance mode so that the boundary between surface color and fluorescent color appearance was defined as CMI 100. Additionally, it was found that the CMI 100 loci could be interpreted as an optimal color loci. Then, it was clarified that the mode boundary and the preferred gamut were closely related and that the acceptable limit for L* was 1.1 times L* for CMI 100.     

White LEDs using the sharp β‐sialon: Eu phosphor and Mn‐doped red phosphor for wide‐color gamut display applications

The sharp β‐sialon (Si_6‐zAl_zO_zN_8‐z : 0 < z < 0.1):Eu green phosphor, combining with a blue LED and CaAlSiN₃:Eu red phosphor, is suitable for the wide‐color gamut white LEDs backlighting system, because of its sharp and asymmetric emission spectrum shape. However, the color gamut and the brightness of the aforementioned display is restricted because of the wide emission band of the CaAlSiN₃:Eu red phosphor. In this work, we used K₂SiF₆:Mn as an alternate red phosphor, which has a sharp emission spectrum. The display with the white LED using sharp β‐sialon:Eu and K₂SiF₆:Mn shows a wide‐color gamut, which covers the hole NTSC triangle. The use of K₂SiF₆:Mn enables to realize not only a wider color gamut but also a higher brightness of displays, compared with the use of CaAlSiN₃:Eu. Furthermore, it is confirmed that the white LED using sharp β‐sialon:Eu and K₂SiF₆:Mn is stable against temperature and also durable under the accelerated drive conditions.     

Power‐saving primary design for displays enclosing a target color gamut

Abstract— Display primaries are optimized for the trade‐off between the total primary power and color gamut under the requirement that a target color gamut is enclosed by the color gamut of the display. LED displays and HDTV color gamut are taken as examples. Compared to the display using a set of typical commercial RGB LEDs, it was found that a total optical (electrical) power of 23.6% (15.6%) can be saved for the display using optimal RGB LEDs. Although the size of the display color gamut is sacrificed, the color gamut of the display using optimal RGB LEDs still encloses the HDTV color gamut. The combined effect of the LED luminous efficiency and white‐point condition on the determination of the optimal LED wavelengths and bandwidths is also studied.     

High efficiency and ultra‐wide color gamut quantum dot LEDs for next generation displays

Colloidal quantum dot‐based hybrid light‐emitting diodes (QLEDs) have been demonstrated that exhibit quantum efficiencies (EQEs) >10% for all three fundamental colors red, green, and blue (21% EQE, 82 cd/A for green). This is the first report of a green QLED with EQE >20% and current efficiency >80 cd/A. The devices have the longest lifetimes reported in the literature (280k hrs) and extremely well‐tuned color fidelity. The narrow QLED emission spectra (full width at half maximum < 30 nm) and well‐controlled peak wavelengths generate a color gamut covering >170% of the National Television System Committee (NTSC) 1987 color space and ~90% of the Rec. 2020 color space. This color gamut is larger than that of OLED televisions in mass production and is the largest of all QLEDs reported. Additionally, these devices are completely fabricated using solution‐processing techniques. The extremely desirable properties of high efficiency, color tunability/fidelity, long lifetime, and low cost processing from solutions make QLED technology disruptive and will lead to next generation displays.     

The influence of unilateral acceleration on color‐gamut properties of a TFT‐LCD

Abstract— This letter describes experiments in which a TFT‐LCD was exposed to unilateral acceleration amplitudes of different temporal profiles and durations. The results show that the color gamut is extended by increasing the load amplitude. Following the loading, the color gamut recedes and gradually approaches the reference range.     

Invited Paper: UHP‐lamp systems for projection applications

Abstract— Projection systems have found widespread use in conference rooms and other professional applications during the last decade and are now entering the home‐TV market with considerable pace. Projectors as small as about one liter are nowadays able to deliver a screen flux of several thousand lumens and are, with a system efficacy of more than 10 lm/W, the most‐efficient display system realized today. Because such highly efficient projectors employ microdisplays as light valves, short‐arc lamps are a key component in realizing these properties. The introduction of the UHP‐lamp system by Philips in 1995 can be identified as one of the key enablers for the commercial success of projection systems. The ultra‐high‐performance (UHP) lamp concept features outstanding arc luminance, a well‐suited spectrum, long life, and excellent flux maintenance. For the first time, it combines a very‐high‐pressure mercury‐discharge lamp having an extremely short and stable arc length with a regenerative chemical cycle that keeps the discharge walls free from blackening, leading to lifetimes of over 10,000 hours. In this review, the most important aspects of the UHP concept that enabled its success in the projection market are described, followed by a discussion of some recent additions to the UHP‐product portfolio.     

Analysis of color volume of multi‐chromatic displays using gamut rings

There are claims that multi‐chromatic displays can achieve a wider color gamut by the use of additional highly saturated secondary color channels. However, there are other claims that these displays lose lightness and/or color saturation at brighter levels. These apparently divergent views have led to some controversy in the display industry and at standard setting organizations. This study examines the color gamut volume for a variety of simulated and measured multi‐chromatic (sometimes incorrectly referred to as “multi‐primary”) displays using combinations of white and/or secondary color channels, such as cyan, magenta, and yellow. Furthermore, a two‐dimensional gamut representation, referred to as “gamut rings,” is introduced to illustrate that the addition of nonprimary optical color channels to a trichromatic (RGB) display can result in a significant decrease in the chroma at higher lightness levels. The additional saturated color channels can increase the gamut volume only around their hues at darker levels. The results also confirm the validity of comparing the color light output and white light output for revealing the design trade‐offs between the high‐peak white and the color‐image brightness for multi‐chromatic displays.     

Micro‐cavity design of bottom‐emitting AMOLED with white OLED and RGBW color filters for 100% color gamut

Abstract— Two optical structures used for a bottom‐emitting white organic light‐emitting diode (OLED) is reported. An RGBW color system was employed because of its high efficiency. For red, green, and blue (RGB) subpixels, the cavity resonance was enhanced by the use of a dielectric mirror, and for the white (W) subpixel, the mirror was removed. The optical length of the cavities was controlled by two different ways: by the thickness of the dielectric filter on top of the mirror or by the angle of oblique emission. With both methods, active‐matrix OLEDs (AMOLEDs) that reproduced a color gamut exceeding 100% of the NTSC (National Television System Committee) standard were fabricated. More importantly, the transmission of a white OLED through R/G/B color filters was significantly higher (up to 50%) than that of a conventional structure not employing a mirror, while at the same time as the color gamut increased from ～75 to ～100% NTSC.     

Effects of display properties on perceived color‐gamut volume and preference

Abstract— The effect of varying the color gamut of an extended‐gamut LCD on color appearance and preference was measured psychometrically in two experiments at each of two separate laboratories over a representative set of 10 images each. The first experiment measured the effect of color gamut on appearance, and the effect on the appearance attribute colorfulness was shown to be relatively strong compared with other attributes as the volume of display color gamut is varied. Overall, colorfulness monotonically increased at constant sensitivity as the gamut area in xy chromaticities increased while tending to become less and less sensitive to increasing the gamut volumes in CIELAB and CIECAM02. In the second experiment, the overall preference indicated an optimal color gamut for the display gamut volume even though the results were shown to be highly scene dependent.     

Quantum dot phosphors free from hazardous elements: Current status and prospects for established materials and new ZnTe‐based alloys

Television broadcasts are moving to a digital video format based on ultra‐high‐definition (UHD). In addition to high resolution, such UHD displays require a wide color gamut. Quantum dots (QDs) have narrow and color‐tunable emission making these unique light sources for achieving a wide color gamut. Most progress in QDs has been made based on cadmium‐containing materials, as represented by CdSe. These systems have excellent performance in terms of their narrow‐band emission and high emission quantum yield; however, the toxicity of cadmium represents a barrier to practical applications of QD displays. Over the last decade, considerable efforts have been made to develop QDs that do not contain cadmium. Some established alternative materials include InP, CuInS₂, and lead halide perovskites. In addition, ZnTe‐based alloy QDs have recently been proposed as promising green and red phosphors. Narrow‐band and green emission (30 nm of full‐width at half‐maximum at a wavelength of 535 nm) has also been reported for Zn (Te, Se) alloy QDs. In this review article, we give a brief overview of progress in established cadmium‐free QDs and describe the current status and future challenges of new cadmium‐free QDs, ZnTe‐based alloy QDs.     

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.