LED backlighting for LCD HDTV

Light‐emitting diodes (LEDs) are used today for backlighting of small displays such as PDAs and mobile phones. We show in this paper that a new LED technology can be used for high‐demanding display‐backlighting applications such as LCD HDTV. Using this new type of emitter, called a Luxeon? Power Light Source, a brightness higher than an edge‐lit CCFL backlight can be achieved, while compared to a direct‐lit CCFL backlight the thickness is lower and the uniformity is better. With on‐going improvements in LED performance over the coming years, LED backlights will reach and even outperform the brightness performance of direct‐lit backlights while maintaining the benefits of edge‐lit solutions at even higher brightness levels.     

LED‐backlight feedback control system with integrated amorphous‐silicon color sensor on an LCD panel

Abstract— Thin‐film‐transistor liquid‐crystal displays (TFT‐LCDs) have the largest market share of all digital flat‐panel displays. An LCD backlighting system employing a three‐color red‐green‐blue light‐emitting diode (RGB‐LED) array is very attractive, considering its wide color gamut, tunable white point, high dimming ratio, long lifetime, and environmental compatibility. But the high‐intensity LED has problems with thermal stability and degradation of brightness over time. Color and white luminance levels are not stable over a wide range of temperature due to inherent long‐term aging characteristics. In order to minimize color point and brightness differences over time, optical feedback control is the key technology for any LED‐backlight system. In this paper, the feasibility of an optical color‐sensing feedback system for an LED backlight by integrating the amorphous‐silicon (a‐Si) color sensor onto the LCD panel will be presented. To minimize the photoconductivity degradation of a‐Si, a new laser exposure treatment has been applied. The integrated color‐sensor optical‐feedback‐controlled LED‐backlight system minimized the color variation to less than 0.008 Δu'v' (CIE1976) compared to 0.025 for an open‐loop system over the temperature range of 42–76°C.     

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.     

Physical‐based photon mapping for an LED backlight lighting simulator

Abstract— Liquid‐crystal displays (LCDs) have become increasingly popular due to their lower price and larger sizes. In particular, backlights having an RGB LED source have recently attracted attention, because they have a wider color gamut, higher luminance, and lower power consumption. However, even when the backlight area is uniformly covered with light modules based on arrays of individual LEDs, this does not ensure a uniform chromaticity and luminance over the backlight panel, thereby stressing the need for lighting simulation of the backlight. Accordingly, this paper proposes an effective lighting simulator to predict the chromaticity and luminance distribution of an LED backlight panel for an LCD. First, the spectrum‐based photons are all initially generated using a random function with a constraint satisfying the spectral power distribution of the actual LED light sources, while their emitting directions are determined based on a pre‐calculated probability using a random variable angle. The optical characteristics of the inner sheets in the LCD backlight structure are then modeled using the wavelength and incident angle to predict the next direction of each photon based on the reflection and transmittance at an intersection. All the photons that reach the unit area of the outward panel are gathered to shape their spectral power distribution, then converted to CIEXYZ values and multiplied with a color‐matching function. Finally, a realistic image visualization of these CIEXYZ values is achieved through standardized device characterization using the sRGB mode. Experiments confirm that the proposed spectrum‐based photon mapping can effectively predict the chromaticity and luminance distribution of an LED backlight panel, providing a good lighting simulation of an LED backlight before manufacturing the LCD.     

Review Paper: Diffractive backlight technologies for mobile applications

Abstract— Liquid‐crystal‐display backlight units have developed in their conventional configuration into very efficient and uniform components that allow the display to present a high‐quality image to the user. Developing the backlight unit itself further faces a challenge of diminishing returns to the investment in innovation. A system‐level redesign is required for the entire display module, and diffractive alternatives to the backlight design can allow a more‐energy‐efficient design for the display. This review concentrates on small‐to‐medium displays because diffractive backlight studies have also centered in this class of displays. The state of the art of backlight design is summarized and the motivation for energy‐efficient system design is outlined. The theoretical basis of diffractive backlights is given, and key research studies in the area of diffractive backlights are reviewed. Finally, a discussion on the performance and future outlook of diffractive backlights completes the paper.     

120 Hz low cross-talk stereoscopic display with intelligent LED backlight enabled by multi-dimensional controlling IC

This paper proposes to employ multi-dimensional controller for driving LED backlight scanning in a 120 Hz LCD for overcoming the hold-type characteristic of an LCD in time-multiplexed stereoscopic displays. A synchronization signal circuit is developed to connect the time scheme of the vertical synchronization for reducing scanning time. The general strategy is to integrate 3D controller and all relatively small-signal electronic functions into one ASIC to minimize the total number of the components. The display panel, LED backlight scanning, and shutter glass signals could be adjusted by vertical synchronization and modulation to obtain stereoscopic images. Each row of LED in a backlight module is controlled by multi-dimensional data registration and synchronization control circuits for LED backlight scanning to flash in bright or dark. LED backlight scanning stereoscopic display incorporated with shutter glasses is provided to realize stereoscopic images even viewed in a liquid crystal display. The eye shutter signal is alternately switched from the left eye to the right eye with 120 Hz of LCD Vertical synchronization (V-sync). This kind of low cross-talk shutter glasses stereoscopic display with an intelligent multiplexing control of LED backlight scanning has low cross-talk below 1% through a liquid crystal shutter glasses.     

A novel diffractive backlight concept for mobile displays

Abstract— Power‐efficiency demands on mobile communications device displays have become severe with the emergence of full‐video‐capable cellular phones and mobile telephony services such as third‐generation (3G) networks. The display is the main culprit for power consumption in the mobile‐phone user interface and the backlight unit (BLU) of commonly used active‐matrix liquid‐crystal displays (AMLCDs) is the main power drain in the display. One way of reducing the power dissipation of a mobile liquid‐crystal display is to efficiently distribute and outcouple the light available in the backlight unit to direct the primary wavelength bands in a spectrum‐specific fashion through the respective color subpixels. This paper describes a diffractive‐optics approach for a novel backlight unit to realize this goal. A model grating structure was fabricated and the distribution of outcoupled light was studied. The results verify that the new BLU concept based on an array of spectrum‐specific gratings is feasible.     

Color performance of an MVA‐LCD using an LED backlight

Abstract— The color performance, including color gamut, color shift, and gamma curve, of a multi‐domain vertical‐alignment (MVA) liquid‐crystal display (LCD) using an LED backlight are calculated quantitatively. Simulation results indicate that an LED backlight exhibits better angular color uniformity and smaller color shifts than a CCFL backlight. Color gamut can be further widened and color shift reduced when using a color‐sequential RGB‐LED backlight without color filters, while the angular‐dependent gamma curves are less influenced using different backlights. The obtained quantitative results are useful for optimizing the color performance and color management of high‐end LCD monitors and LCD TVs.     

LED backlight system with double‐prism pattern

Abstract— A LED backlight system with a double‐prism pattern for use in mobile phones to achieve thin and high luminance LED backlight systems is proposed. The double‐prism pattern is formed on the light guide of the proposed LED backlight system and simultaneously exhibited two optical functions: shifting of the light from the direction of the guided light toward the radiated light and controlling the directivity of the radiated light. Therefore, using the double‐prism pattern eliminates two prism sheets and a diffusive sheet, which are indispensable optical elements to exhibit the optical function that controls the directivity of light in conventional LED backlight systems. Consequently, the thickness of the proposed LED backlight system is reduced to 0.75 mm compared to that of the conventional system. A luminance of 3115 nits and a full‐width half maximum of 35° for radiated light, which are comparable to conventional LED backlight systems, were obtained.     

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.     

Novel direct‐view LED backlight unit with an aspheric microlens array and a rough‐texture sheet

Abstract— This study proposes a novel direct‐view light‐emitting‐diode (LED) backlight unit with a high‐fill‐factor aspheric microlens array and a rough‐texture sheet. An aspheric microlens array and a rough‐texture sheet made from polysiloxine were used as the grating element and optical diffuser, respectively, which increases light‐extraction efficiency and improves luminance uniformity. The specific aspheric microlens‐array mold was fabricated by using a heating encapsulated air process based on a glass wafer. This microlens array has the features of high fill factor and square‐foot boundary with a continuous surface‐relief profile. This unique out‐of‐plane surface profile creates a square light pattern with uniform luminance, and thus composes a uniform large‐sized light pattern exactly in accordance with the layout of the LED array. The rough‐texture sheet, which can scatter light uniformly, was formed by fine‐grit‐sandpaper molding. Experimental results show that by using an aspheric microlens array and rough‐texture sheet reported here as the backlight diffusing components is highly effective in improving light uniformity at a wide viewing angle. An increase in illuminance by more than 10% was achieved in comparison with commercial backlight modules. Low cost in fabricating the aspheric microlens array and rough‐texture sheet is anticipated due to the simplicity of the process.     

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.     

Backlight unit with double‐surface light emission using a single micro‐structured lightguide plate

Abstract— A novel illumination system for a liquid‐crystal‐display (LCD) module used in a dual‐display cellular phone has been developed. A double‐surface light‐emitting backlight uses a single light‐guide plate to illuminate both LCDs. A single lightguide, two prism sheets, and four light‐emitting diodes (LED) were used in the new structure, compared with ten components and two sets of light sources with six LEDs in the current backlight. The thickness and power consumption of the new backlight were reduced by a factor of 0.59 and 0.67, respectively.     

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.     

Design and fabrication of a micropatterned polydimethylsiloxane (PDMS) light‐guide plate for sheet‐less LCD backlight unit

Abstract— A polydimethylsiloxane (PDMS) light‐guide plate (LGP) having micropatterns with an inverse‐trapezoidal cross section was developed for a sheet‐less LCD backlight unit (BLU). The micropatterned PDMS LGP was fabricated by backside 3‐D diffuser lithography followed by two consecutive PDMS replication processes: photoresist‐to‐PDMS and PDMS‐to‐PDMS replications. The fabricated LGP showed an average luminance of 2878 nits and a uniformity of 73.3% in a 2‐in. backlight module with four side‐view 0.85‐cd LEDs. It also could feasibly be applied to a light source for flexible displays owing to the flexible characteristic of the PDMS itself.     

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.     

LED‐backlit computer screens influence our biological clock and keep us more awake

Abstract— In commodity LC flat‐panel displays, the traditional CCFL backlight is being replaced more and more by LEDs. The typical spectrum of LED‐lit displays shows a significantly higher amount of blue light in the range around 464 nm. Blue light in this wavelength area suppresses the melatonin level in humans and thus effects the biological clock. Our hypothesis was that the amount of blue light reaching the human eye from a LED‐backlit display is sufficient to have a significant effect on the biological clock. The results of clinical user studies comparing the effects of LED‐ vs. CCFL‐backlit displays on humans, resulting from the emitted amount of 464‐nm light, will be presented. It was found that the LED‐backlit display causes significant suppression of melatonin, which effects the biological clock of the test persons, indicating the necessity for displays with a controllable 464‐nm emission. A technical concept for a display with such functionality will be presented.     

A 23‐in. full‐panel‐resolution autostereoscopic LCD with a novel directional backlight system

Abstract— An autostereoscopic display that shows stereoscopic images with full‐panel resolution has been developed,¹ but it has a problem in terms of unit size. To resolve this problem, a new directional backlight system was developed, and it was applied to a prototype autostereoscopic LCD. The backlight system has two light sources — one for the right eye and the another for the left eye — and an elliptically shaped mirror that controls the direction of light from the light sources. The LCD uses a field‐sequential method which re‐writes an image for one eye and one for the other eye at a frame rate of 120 Hz, and the light sources alternately blink in synchronization with each frame so that the LCD shows full‐panel‐resolution stereoscopic images without flicker. In this paper, the new backlight system is described. The backlight system is effective for large screen such as 23 in. on the diagonal. By using this backlight system, the prototype LCD achieved practible unit size, brightness over the entire screen, and cross‐talk.     

Scanning backlight for motion‐blur reduction on mobile displays

Abstract— Small‐form‐factor liquid‐crystal displays (LCDs) are mainly used in mobile applications (e.g., mobile phones, PDAs, and portable game consoles) but also in digital still cameras, video cameras, automotive applications, etc. Like all active‐matrix LCDs, mobile displays suffer from motion blur caused by the sample‐and‐hold effect. One option for improving the motion portrayal on active‐matrix LCDs is the use of a scanning backlight, which results in an imaging behavior similar to the one present in impulsive displays. In this paper, the realization of a scanning backlight for mobile displays is reported. This employs a backlight with seven individually lit segments for reducing the motion blur. Results of perception experiments performed with two identical displays confirm the benefit of using this technology. Optimal driving conditions result in a major improvement in motion portrayal on mobile LCDs.     

A directly addressed monolithic LED array as a projection source

Abstract— Light‐emitting‐diode arrays have been fabricated and demonstrated by several groups in a variety of applications in which structured illumination is desired, such as direct‐view displays and fluorescence detection. State‐of‐the‐art LEDs can achieve a high optical power density, exceeding 40 W/cm². This allows the use of such LEDs, if patterned into an array, as a light and pattern source for projection displays. As a proof of principle, a projection source using a monolithically integrated patterned LED array at a power density of 0.69 W/cm², a source brightness of 21,000,000 cd/m², and a projected brightness of 100,000 cd/m² is reported. It is shown that an improved version of such a display would allow significantly better power consumption and overall system efficiency than permitted by current projection‐display concepts.