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.     

Energy‐efficient liquid‐crystal displays (e2‐LCDs) using a photonic‐crystal backlight system

Abstract— Cholesteric liquid crystals automatically form one‐dimensional photonic crystals. For a photonic crystal in which light‐emitting moieties are embedded, unique properties such as microcavity effects and simultaneous light emission and light reflection can be expected. Three primary‐color photonic‐crystal films were prepared based on cholesteric liquid crystal in which fluorescent dye is incorporated. Microcavity effects, i.e., emission enhancement and spectrum narrowing, were observed. Two types of demonstration liquid‐crystal displays (LCDs) were fabricated using the prepared photonic‐crystal films in a backlight system. One is an area‐color LCD in which a single photonic‐crystal layer is used for each color pixel and the other is a full‐color TFT‐LCD in which three stacked photonic‐crystal layers are used as light‐conversion layers. The area‐color LCD was excited by using 365‐nm UV light, and the full‐color TFT‐LCD was excited by using 470‐nm blue LED light. Because of the photonic crystal's unique features that allow it to work as light‐emitting and light‐reflecting layers simultaneously, both LCDs demonstrate clear readable images even under strong ambient light, such as direct‐sunlight conditions, under which conventional displays including LCDs and OLED displays cannot demonstrate clear images. In particular, an area‐color LCD, which eliminated color filters, gives clear images under bright ambient light conditions even without backlight illumination. This fact suggests that a backlight system using novel photonic‐crystal layers will be suitable for energy‐efficient LCDs (e²‐LCDs), especially for displays designed for outdoor usage.     

Fast full‐color reflective display via photoluminescent enhancement

Reflective displays are advantageous in applications requiring low power or daylight readability. However, there are no low‐cost reflective technologies capable of displaying bright colors. By employing photoluminescence to more efficiently use ambient light, we created a prototype display that provides bright, full color in a simple, low‐cost architecture. This prototype includes a novel electrokinetic shutter, a layer that incorporates patterned luminescent red, green, and blue sub‐pixel elements, and a novel optical out‐coupling scheme. The luminescent elements convert otherwise‐wasted portions of the incident spectrum to light in the desired color band, resulting in improved color saturation and lightness. This prototype provides a color gamut that is superior to competing reflective display technologies that utilize color filters in single‐layer side‐by‐side sub‐pixel architectures. The current prototype is capable of switching in <0.5 s; future displays based on an alternative electro‐optic shutter technology should achieve video rate operation. A transflective version of this technology has also been prototyped. The transflective version utilizes its backlight with a power efficiency that is at least three times that of a conventional liquid crystal display. These photoluminescence‐based technologies enable a host of applications ranging from low‐power mobile products and retail pricing signage to daylight readable signage for outdoor advertising segments.     

High‐luminance 1.8‐mm‐pixel‐pitch CNT‐FED for ubiquitous color character displays

Abstract— A high‐luminance 1.8‐mm‐pixel‐pitch CNT‐FED for color character displays has been developed. The display panel has 32 × 256 color pixels, and the subpixel size is 0.6 × 1.8 mm. The display panel can provide good visibility when installed even in outdoor locations. The power consumption is low enough for the display to be battery driven. The practical application is the display of important messages regarding the evacuation from disaster areas, even under emergent no‐power conditions similar to the messages on vending machines.     

Improving content visibility for high‐ambient‐illumination viewable display and energy‐saving display

Abstract— Nowadays, low‐contrast viewing of LC displays (LCDs) occurs very often, which includes the viewing of mobile LCDs at high ambient illumination and the viewing of LCDs at low‐power mode. These cases result in low‐content visibility and low contrast, leading to an unpleasant viewing experience. In this paper, a technique to improve the perceived contrast and visibility of images at low‐contrast viewing conditions is proposed. The proposed approach enhances image brightness with content and ambient adaptive image brightening and highlights visual parts and boundaries with non‐photorealistic rendering. The proposed technique enables longer battery life for mobile LC devices and makes mobile LC devices viewable at high ambient illumination. It also enables TVs with extreme low‐power consumption and smart‐grid responsive TVs.     

A novel pixel memory using integrated voltage‐loss‐compensation (VLC) circuit for ultra‐low‐power TFT‐LCDs

Abstract— A novel pixel memory using an integrated voltage‐loss‐compensation (VLC) circuit has been proposed for ultra‐low‐power TFT‐LCDs, which can increase the number of gray‐scale levels for a single subpixel using an analog voltage gray‐scale technique. The new pixel with a VLC circuit is integrated under a small reflective electrode in a high‐transmissive aperture‐ratio (39%) 3.17‐in. HVGA transflective panel by using a standard low‐temperature‐polysilicon process based on 1.5‐μm rules. No additional process steps are required. The VLC circuit in each pixel enables simultaneous refresh with a very small change in voltage, resulting in a two‐orders‐of‐magnitude reduction in circuit power for a 64‐color image display. The advanced transflective TFT‐LCD using the newly proposed pixel can display high‐quality multi‐color images anytime and anywhere, due to its low power consumption and good outdoor readability.     

Adapting softcopy color reproduction to ambient illumination

Abstract— The perceived colors of an image seen on a self‐luminous display are affected by ambient illumination. The ambient light reflected from the display faceplate is mixed with the image‐forming light emitted by the display. In addition to this direct physical effect of viewing flare, ambient illumination causes perceptual changes by affecting the adaptation state of the viewer's visual system. This paper first discusses these effects and how they can be compensated, outlining a display system able to adjust its output based on prevailing lighting conditions. The emphasis is on compensating for the perceptual effects of viewing conditions by means of color‐appearance modeling. The effects of varying the degree of chromatic adaptation parameter D and the surround compensation parameters c and N_c of the CIECAM97s color‐appearance model were studied in psychophysical experiments. In these memory‐based paired comparison experiments, the observers judged the appearance of images shown on an LCD under three different ambient‐illumination conditions. The dependence of the optimal parameter values on the level of ambient illumination was evident. The results of the final experiment, using a category scaling technique, showed the benefit of using the color‐appearance model with the optimized parameters in compensating for the perceptual changes caused by varying ambient illumination.     

Design conditions for attractive reality in mobile‐type 3‐D display

Abstract— An autostereoscopic 3‐D display suitable for the mobile environment is prototyped and evaluated. First, the required conditions for a 3‐D display in a mobile environment are considered, and the three major requirements are clarified: small size, viewing‐position flexibility, and application support. An application of a mobile‐type 3‐D display should be different from that of a large‐sized 3‐D display because a mobile‐type 3‐D display cannot realize the feeling of immersion while large‐sized 3‐D displays can realize it easily. From this assumption, it is considered that it is important to realize the feeling to handle a 3‐D image. Three types of 3‐D displays are developed to satisfy these requirements. They are subjectively evaluated to confirm their attractiveness. Results of the tests show that intuitive interaction can increase the reality of the 3‐D image in the sense of unity and also can improve the solidity and depth impression of the 3‐D image.     

Contrast‐ratio analysis of sunlight‐readable color LCDs for outdoor applications

Abstract— This paper presents contrast‐ratio data measured from high‐brightness TFT color LCDs under various ambient illumination levels encountered in outdoor environments. In these measurements, several LCD front polarizers with anti‐glare (AG) and anti‐reflective (AR) coatings have been used. The measured contrast‐ratio data are compared and the impact due to the AG and AR coatings is briefly discussed. The test equipment that simulates different cases of outdoor illumination is also presented.     

Dynamic adaptation model and equal‐whiteness CCT curves for the choice of display reference white

Abstract— A dynamic adaptation model (DAM), based on the variations in the luminance levels under the same chromaticity viewing conditions and equal‐whiteness correlated‐color‐temperature (CCT) curves (EWCCs) derived by using the proposed DAM, is proposed. The proposed model was obtained by the transformation of the test colors for high luminance into the corresponding colors for low luminance. In the proposed model, the optimal coefficients are obtained from the corresponding color data from Breneman's experiments. From the experimental results, it was confirmed that the chromaticity errors between the predicted colors by the proposed model and the corresponding colors of Breneman's experiments are 0.004 in u′v′ chromaticity coordinates. The prediction performance of the proposed model is excellent because this error is the threshold value that distinguishes two adjacent color patches. Equal‐whiteness CCT curves (EWCs) are also proposed using the proposed DAM. By using the proposed EWCCs, an analysis of the difference between the selected video‐display‐unit reference‐white CCT values can be made. Additionally, the proposed EWCCs can be used as the theoretical standard which determines the reference white of the color video display units.     

Predicting the viewing direction performance of e‐paper displays with front light under ambient lighting conditions

The inherent advantages of reflective e‐paper displays (EPD) – wide viewing direction range and excellent readability even under bright sunlight – can be extended into low‐light conditions by the addition of an integrated lighting unit (ILU) such as a front light. A methodology for predicting the viewing direction dependence of ambient contrast and color from display measurements and illumination models is here applied to reflective EPD where the ILU is switched on. The predictions can be used to optimize task lighting. The ILU improves the indoor performance of EPD without compromising their superior daylight performance.     

Time‐multiplexed tiled projection system with improved pixel and spatial resolution

We propose two methods to increase the pixel and spatial resolution of Digital Micromirror Devices (DMD)‐based projectors by utilizing the large bandwidth provided by their high pattern rates. By varying the intensity of the illumination for each binary pattern displayed on the DMD, the time required to display an 8‐bit grayscale image can be reduced by up to factor 32 compared to using constant illumination and binary pulse‐width modulation (BPWM). The high image rate projection is then spatially separated by either using a galvanometer scanner or sequentially illuminating the DMD from different directions, thus creating multiple independently addressable projections which are then tiled to form a larger, higher resolution image.     

Multi‐primary‐color LCD: Its characteristics and extended applications

Abstract— The development of multi‐primary‐color (MPC) display systems is one of the big paradigm shifts in recent display technologies and induces new potentials of display devices. The development of MPC display systems for different goals is briefly reviewed. Especially, by employing MPC systems, it is possible to reproduce the real material colors faithfully and efficiently. For signal processing, MPC systems have a big advantage in the so‐called color‐reproduction redundancy. A number of applications can be derived from this characteristic, such as improving the viewing‐angle dependency issue and power savings. On the other hand, MPC systems have a typical trade‐off versus RGB‐standardized input signals, especially for reproducing bright green. New algorithms to moderate this trade‐off on MPC systems by employing color‐reproduction redundancy are proposed. The goal of our algorithms is to maintain the compatibility with RGB‐based input signals though the initial display design so that the characteristics of MPC systems are not changed or lost. These algorithms indicate that MPC display systems are applicable not only for a specifically limited objective but also for other applications, e.g., TV broadcasting.     

Polarization‐independent liquid‐crystal‐etalon modulator

Abstract— The experimental demonstration of a polarization‐independent high‐transmission field‐sequential liquid‐crystal‐etalon modulator for portable projection‐display application is presented. Polarization independence allows for high transmission efficiency for laser illumination sources using the polarization diversity method of speckle reduction. These devices can also be considered for LED illumination sources; however, the spectral width of current LEDs does not allow for high efficiency, especially for the green channel. These devices demonstrate millisecond switching needed for field‐sequential‐color generation.     

A novel field‐sequential blue‐phase‐mode AMLCD

Abstract— Through the realization of a blue‐phase‐mode (hereinafter, the operational mode of liquid crystal having a blue phase is referred to as a blue‐phase mode), a display using an improved field‐sequential method was confirmed to be capable of display at a frame rate of 180 fps (field frequency of 540 Hz) or higher. Under this condition, an image without annoyance caused by color breakup was obtained. Moreover, a novel field‐sequential AMLCD integrated with a scan driver by combining the liquid‐crystal‐display (LCD) technology using blue phase and oxide‐semiconductor technology has been developed.     

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.     

A high‐contrast front‐projection display system optimizing the projected light‐angle range

Abstract— The problem with front‐projection displays is that the screen contrast ratio decreases under bright‐ambient conditions. To overcome this problem, the design of a special screen, composed of the diffuser whose diffusing property shows top‐hat characteristics and a sawtooth reflector, is proposed. The screen diffuses the incident image light arriving at a projection‐angle range that is a lower‐angle range than the viewing‐angle range, and reflects the ambient light out of the viewing‐angle range. In this paper, the projection‐angle range and the viewing‐angle range was optimized to improve the contrast ratio of a front‐projection display. As a result, a special screen with the above‐mentioned diffusing property was realized, and a high‐quality front‐projection display with a high contrast ratio, even in a bright room, was achieved.     

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.     

OLED lifetime issues from a mobile‐phone‐industry point of view

Abstract— Lifetime issues have been a hot topic throughout the history of OLEDs. The rapid development of lifetimes since 2002 has enabled OLED displays to become acceptable for mobile phones. The lifetime requirements of 30,000 hours expressed by the representatives of mobile‐phone‐terminal makers were felt to be unrealistic to be obtained in 2003, since the lifetime of the blue color was below 1000 hours. Today, 5 years later, lifetimes of AMOLED panels are over 50,000 hours. OLED displays are suffering from a burn‐in effect due to limited lifetime. After 2003, it was understood by the panel and terminal makers that instead of lifetime, burn‐in sensitivity became the limiting factor from an AMOLED‐panel usability point of view. The burn‐in effect becomes visible at 2–3% luminance degradation levels between adjacent pixels. To take this effect into account in mobile‐phone applications, the lifetime needs to be increased from 30,000 to 60,000 hours, and suitable algorithms need to be used for the display of the terminal. There is also pressure to double the peak luminance values used in the terminals in order to improve the performance of the screen in outdoor environments. The roles of the material developers, panel makers, and terminal makers are reviewed in this paper from a lifetime perspective.     

Nano‐emissive display technology for large‐area HDTV

Abstract— Using nano‐emissive display (NED) technology, Motorola labs has successfully developed 5‐in. full‐color display prototypes. Carbon‐nanotube‐based field‐emission displays with a pixel size of 0.726 mm for a 42‐in. HDTV exhibit video image quality comparable to CRT displays and demonstrate a luminance of 350 cd/m². These novel low‐drive‐voltage NEDs take advantage of selective growth of CNTs to obtain the desired electron‐emission performance while maintaining inexpensive manufacturing due to a simple self‐focusing and self‐regulating planar structure. Improved video image quality and color purity are achieved with very low power consumption and without the need for an expensive focusing grid.