A preview of some of the most interesting papers appearing in the December 2007 issue of the Journal of the SID. 
To obtain access to these articles on-line, please go to www.sid.org

Edited by Aris Silzars

Locally pixel-compensated backlight dimming on LED-backlit LCD TV

Hanfeng Chen
Junho Sung
Taehyeun Ha
Yungjun Park

Samsung Electronics Co.

Abstract — The pixel brightness of an LCD panel perceived by a user is the product of the backlight brightness and the panel transmittance. In conventional LCD panels, the backlight brightness is constant and always at peak luminance. This design suffers from light leakage and power waste problems at dark scenes. This paper presents a new LCD system, which uses locally pixel-compensated backlight dimming (PCBD). The proposed method combines backlight control and pixel processing for reducing light leakage and power consumption while keeping the image at the original brightness. Backlight luminance is dimmed locally in the dark-image region, and pixel values are compensated synchronously according to the luminance profile of dimmed backlight. By reducing the light leakage, a static contrast of over 20,000:1 has been achieved on a large-sized LCD panel with the proposed PCBD method. No obvious artifacts have been noticed as well. The power consumption of the panel can also be greatly reduced, depending on various video content. The PCBD method could be widely used for developing state-of-the-art LCD panels with LED backlights.

The proposed PCBD method separately dims the LED backlight at the bright and dark image areas and locally compensates the pixel values according to the luminance profile of the dimmed backlight to maintain visual brightness at the same level. Compared with previous methods, this paper proposes a more-reasonable LED luminance intensity calculation method which takes the major gray levels, small bright objects, and noise pixels into consideration.



FIGURE 4 — Structure of the locally pixel compensated backlight dimming algorithm.


Two approaches to derive LED driving signals for high-dynamic-range LCD backlights

Feng Li
Xiaofan Feng
Ibrahim Sezan
Scott Daly

Rochester Institute of Technology

Abstract — The LED-array backlight technique dramatically enhances the dynamic range of an LCD and hence extends its ability to present images with high reality. This is achieved by modulating LEDs individually, thus providing an area-adaptive backlight for the display. The spatial overlap of light from the LED (crosstalk) occurs due to the diffusion screen placed between the backlight and LCD layer. However, the crosstalk is not only a blessing for supplying high brightness but is also a curse for causing potential artifacts, making the derivation of an LED driving signal a challenging task. This paper formulates the problem into two mathematical models: an iterative de-convolution approach and a linear optimization approach. Algorithms for solving these two models are provided. The first approach provides instantaneous and satisfactory results except for high-intensity highlights in the image. The linear optimization method conquers this drawback, but requires much more computation, possibly requiring preprocessing of the target, and also introduces undesired artifacts. These two approaches are extensively evaluated by building an image database composed of 161 high-dynamic-range images.

To obtain a crosstalk matrix for LEDs, the central LED in the LED array is lit and the luminous values in all LED positions are measured. These values are then normalized to make the central value unity. The resultant 2-D array is taken as the crosstalk matrix of LEDs as shown in Fig. 2.

The generation of an LED backlight can be modeled as a convolution process:

g = h x f,


where g, h, and f are the constructed backlight, crosstalk matrix, and driving signals of LEDs, respectively.



FIGURE 2 — (a) 2-D view and (b) 3-D view of a crosstalk matrix for LEDs.


Image-display algorithms for high- and low-dynamic-range display devices

Erik Reinhard
Timo Kunkel
Yoann Marion
Jonathan Brouillat
Rémi Cozot
Kadi Bouatouch

University of Bristol

Abstract — With interest in high-dynamic-range imaging mounting, techniques for displaying such images on conventional display devices are gaining in importance. Conversely, high-dynamic-range display hardware is creating the need for display algorithms that prepare images for such displays. In this paper, the current state of the art in dynamic-range reduction and expansion is reviewed, and in particular the theoretical and practical need to structure tone reproduction as a combination of a forward and a reverse pass is passed.

It is reasonable to anticipate that the variety in display capabilities will increase. Some displays will have a much higher dynamic range than others, whereas differences in mean luminance will also increase due to a greater variety in backlighting technology. As a result, the burden on general-purpose display algorithms will change. High-dynamic-range (HDR) image acquisition has already created the need for tone-reproduction operators, which reduce the dynamic range of images prior to display. The advent of HDR display devices creates a need for tone-reproduction operators to expand the dynamic range of images such that traditional 8-bit images can be displayed appropriately on HDR displays.



FIGURE 1 — With conventional photography some parts of the scene may be under- or over-exposed. Capture of this scene with nine exposures, and assemblage of these into one HDR image followed by tone reproduction, affords the result shown.


Volumetric three-dimensional up-conversion display medium

Jung-Hyun Cho
Michael Bass
Hans P. Jenssen

University of Central Florida

Abstract — Several rare-earth-doped fluoride crystals that are excited to emit visible light by sequential two-photon absorption have been investigated as display-medium candidates for static volumetric three-dimensional displays. Dispersion of powders of these materials in a refractive-index-matched polymer is reported because such a medium may result in a scalable display. The scattering problem in such a medium is greatly reduced by index-matching the polymer to the crystalline particles. An index-matching condition that optimizes the performance is identified.

Static volumetric 3-D displays can be classified by how their volume pixels (voxels) are activated and what kind of display medium is used. In our work, we mainly consider a 3-D display with a passive-display medium based on a rare-earth-doped material. Its voxels can be activated by sequential absorption of light from two invisible near-infrared laser beams, as shown in Fig. 1. The excitation process can be summarized into three steps: (1) The first absorption of near-infrared light populates the intermediate energy level of ions in the medium, (2) a second absorption of a different wavelength of infrared light at the intersection point of the two beams excites the ions in the intermediate level to the upper level, and (3) ions in the upper level then decay to the ground state while radiating visible light.



FIGURE 1 — Schematic diagram of static volume 3-D display. A two-frequency two-step up-conversion process is diagrammed on the left. A voxel is sketched in which visible light is generated in the display medium by two different-frequency IR laser beams intersecting in the voxel.


Applications of augmented-vision head-mounted systems in vision rehabilitation

Eli Peli
Gang Luo
Alex Bowers
Noa Rensing

Schepens Eye Research Institute,Harvard School of Medicine

Abstract — Vision loss typically affects either the wide peripheral vision (important for mobility) or central vision (important for seeing details). Conventional optical visual aids usually recover the lost visual function, but at a high cost for the remaining visual function. A novel concept of vision-multiplexing using augmented-vision head-mounted display systems to address vision loss has been developed. Two applications are discussed in this paper. In the first, minified edge images from a head-mounted video camera are presented on a see-through display providing visual field expansion for those with peripheral vision loss, while still enabling the full resolution of the residual central vision to be maintained. The concept has been applied in daytime and nighttime devices. A series of studies suggested that the system could help with visual search, obstacle avoidance, and nighttime mobility. Subjects were positive in their ratings of device cosmetics and ergonomics. The second application is for those with central-vision loss. Using an on-axis aligned camera and display system, central visibility is enhanced with 1:1 scale edge images, while still enabling the wide field of the unimpaired peripheral vision to be maintained. The registration error of the system was found to be low in laboratory testing.

A number of designs and experimental iterations served to refine the carrier lens size and the shape of the frame, including facilities for adjustments to ensure the frame fits securely and comfortably on people with various facial dimensions. The current generation of HMD weighs approximately 110 g and provides a field of view of 16° (H) by 12° (V) (Fig. 3), which represents a substantial improvement in cosmetics and ergonomics (reduced weight and better fit).


FIGURE 3 — The current generation of the augmented-vision HMD visual field expander for patients with tunnel vision. The wide-angle image captured by the video camera is processed by the controller to provide edge-contour images of the scene. The edge images are displayed on the see-through display providing an expanded view. Once an object is detected via the minified cartoon, it can be examined with full resolution and color through the transparent display.


A carbon-nanotube field-emission display with simple electron-beam trajectory control

Kenneth A. Dean, Bernard F. Coll,
Emmett Howard, Michael R. Johnson,
Hao Li, Larry Marshbanks,
Larry Dworsky

Motorola, Inc.

Abstract — A unique gated cathode structure for a carbon-nanotube-based field-emission display has been designed and built. This structure optimizes the electron-beam profiles to assure a good color gamut and high anode efficiency without requiring specific focusing electrodes or structure. A computer simulation, written to analyze and improve the device design, shows good correlation with the experimental data and helps predict design margins. A full-color frit-sealed display built with this approach demonstrates an excellent color gamut of the phosphor, and the model predicts avenues for further color-gamut improvements.

The actual subpixel structure is shown in Fig. 3, as viewed through the red and green phosphors of the anode. This design contains 209 pads of nanotubes arranged with 11 pads in each of 19 trenches. In this layout, the gate electrode resides on the X-directed side of the pixels only on the outer left and right edges to bus the electrode lines together. The X-directed field contribution on the end nanotube pads was minimized by spacing this edge bus line approximately three times the distance from the pad as the gate electrode is spaced in the Y-direction.



FIGURE 3 — Cathode subpixel arrangement underneath the anode. The anode black surround is black, the underlying lines are the gate electrode and the dots are pads of nanotubes.


Development of field-emission displays

Shigeo Itoh, Mitsuru Tanaka,
Takeshi Tonegwa, Masateru Taniguchi,
Kazuyoshi Otsu, Takahiro Niiyama,
Kiyoshi Tamura, Mamoru Namikawa,
Yasuyuki Naito, Yuji Obara,
Masaki Toriumi, Manabu Kitada,
Yasuyuki Takeya, Kiyoyuki Deguchi,
Satoru Kawata, Yoshitaka Sato,
Fumiaki Kataoka, Hitoshi Toki,
Kyoko Sakurada, Tatsuo Yamaura

Futaba Corp.

Abstract — FEDs are one of the attractive flat-panel displays that realize high-quality motion images and low power consumption. FEDs are constructed by using three elemental technologies: micro- or nano-fabrication technology of emitters, opto-electronic semiconductor technology of anode patterns, and vacuum-packaging technology. Each of the three elemental technologies is essential to realize FEDs. The present status of each three technologies, especially the improvement of Spindt-type field emitters, the trend of flat vacuum packages, and development of phosphors for FEDs is described in this paper.

A 3-in. color FED produced for automobile-engine monitors is now in the marketplace. The same type of color FED is shown in Fig. 6. The screen size is 30 x 70 mm, the pixel count is 184 (H) x 80 (V) x RGB, the brightness is 600 cd/m2, and the power consumption is 4 W.



FIGURE 6 — Spindt-type 3-in. color FED.


Modeling motion-induced color artifacts from the temporal step response

Kees Teunissen
Xiaohua Li
Lin Chai
Ingrid Heynderickx

Philips Consumer Electronics

Abstract — LCD motion blur is a well-known phenomenon, and a lot of research is attributed to characterize and improve it. Until recently, most studies were focused on explaining the effects visible in black-and-white patterns, and hence color effects were ignored. However, when a colored pattern is moving over a colored background, an additional motion-induced artifact becomes visible, which is referred to as chromatic aberration. To describe this phenomenon, our model to characterize the appearance of moving achromatic patterns is extended in such a way that it now calculates the apparent image from the temporal step response of the individual primary colors. The results of a perception experiment indicate that there is a good correspondence between the apparent image predicted with the model and the actual image perceived during motion.

It is well known that temporal characteristics of the display are gray-level dependent, which means that different gray-level transitions per subpixel will lead to different responses in the primary colors. When the human eye is tracking an object with different initial and targetlevels per primary color, the temporal integration of the light at the retina depends on the transmissive state of the liquid-crystal cell, in each sub-pixel, when the eye passes it.



FIGURE 1 — Illustration of luminance step transitions and the apparent images, perceived during motion, when a gray block moves on a black background from left to right in horizontal direction with a speed of 12 ppf; (a) the normalized temporal luminance characteristics of the white (R + G + B) and (b) the normalized temporal luminance characteristics of the R, G, and B subpixel separately.


Fabrication of low-temperature-polysilicon thin-film transistors on flexible substrates using excimer-laser crystallization

Yong-Hoon Kim
Won-Keun Kim
Jeong-In Han
Dae-Gyu Moon

Korea Electronics TechnologyInstitute

Abstract — Low-temperature-polysilicon thin-film transistors (LTPS TFTs) were fabricated on polymer substrates using sputtered amorphous-Si (a-Si) films and excimer-laser crystallization. The in-film argon concentration of a-Si films was minimized as low as 1.6% by using an argon/helium gas mixture as the sputtering gas. By employing XeCl excimer-laser crystallization, poly-Si films were successfully fabricated on polymer substrates with an average grain size of 400 nm. With a four-mask process, a poly-Si TFT was fabricated with a fully self-aligned top-gate structure, and the pMOS TFT device showed a field-effect mobility of 63.6 cm2/V-sec, ON/OFF ratio of 105, and threshold voltage of –1.5 V.

In order to form a source/drain contact region, ion shower doping was carried out to incorporate boron ions. The doping temperature and acceleration voltage were at room temperature and 10 kV, respectively. After ion shower doping, laser activation was carried out with an energy density of 160–320 mJ/cm2. The sheet resistances of laser-activated poly-Si films were 0.47 kΩ/•. In Fig. 5, the transfer characteristic of fabricated pMOS TFT is shown. The gate voltage was swept from +5 to –20 V and the drain-to-source voltage was –1 V. The pMOS TFT showed a field-effect mobility of 63.6 cm2/V-sec, ON/OFF ratio of 105, and threshold voltage of –1.5 V.



FIGURE 5 — Transfer characteristics of a pMOS LTPS TFT fabricated on a polymer substrate. The gate voltage was swept from +5 to –20 V and the drain-to-source voltage was –1 V. Inset figure shows an optical microscope image of fabricated TFT device.


LCD backlights, light sources, and flat fluorescent lamps

J. L. Park
Sungkyoo Lim

Dankook University

Abstract — Backlights are indispensable for the operation of LCDs. Light sources or lamps are the core components of backlights. There are many types of light sources for backlights such as cold-cathode fluorescent lamps (CCFLs), external electrode fluorescent lamps (EEFLs), hot-cathode fluorescent lamps (HCFLs), flat fluorescent lamps (FFLs), and light-emitting diodes (LEDs). Recently, FFLs are becoming one the most interesting light sources for LCD-TV backlights. Channel profiles and the structure of FFLs in more detail are discussed. The channel profile of FFLs with maximum brightness uniformity was designed, fabricated, and characterized. The FFL backlight demonstrated 10,500 nits and the total power consumption was 110 W at room temperature.

An FFL is composed of two sheets of glass. The channels should be formed in the upper glass sheet and the flat bottom glass should be sealed with the formed glass inside of which is coated with white phosphor and filled with Ar gas and a small amount of mercury. The production ability, lifetime, and luminance efficiency of FFLs depend on the channel profile formed by using the molds. Figure 12 shows an example of an FFL channel profile.



FIGURE 12 — An example of an FFL channel.


Discharge characteristics and low-voltage driving of high-Xe-content PDPs

Tomokazu Shiga
Shigeo Mikoshiba
Gerrit Oversluizen

University of Electro-Communications

Abstract — High-Xe-content PDPs attain improved luminous efficiency, but with sacrifices of higher sustain and address voltages and slower discharge build-up. By examining PDPs with 3.5–100%-Xe content, it was revealed that space-charge priming as well as wall-charge accumulation are effective in obtaining low-voltage and high-speed operation. In addition, it was found that the effectiveness is emphasized for higher-Xe-pressure PDPs. In this respect, erase addressing is more favorable than write addressing, especially for high-Xe-pressure PDPs. The formative time lag of the discharge and diffusion/drift of the space charges are shorter for high Xe content. In this respect, high-Xe-content PDPs have a potential for high-speed addressing, if driven adequately. The use of space-charge priming, however, is limited by the duration between the priming and scan pulses. Accumulation of wall charges is limited by ignition of a self-erase discharge with which all the wall charges are dissipated. Although the highest efficiency and luminance are attained with a 100%-Xe panel, the optimum Xe gas content, considering the sustain pulse voltage and drive voltage margin, would be 70% Xe + Ne.

Figure 1 shows the luminous efficiency as functions of Vs and f for a Ne + Xe (10%) panel. There is a peak in the luminous efficiency with respect to Vs for each frequency. When Vs is low, the discharge current build-up is slow and the Xe atoms are not efficiently excited. As Vs is increased, the efficiency increases. A further increase of Vs, however, results in reduction of efficiency due to VUV and phosphor saturation under high luminance. The maximum efficiency, therefore, is determined by a compromise of stable discharges and avoiding losses by plasma saturation, ion-heating, or non-optimal electron temperature.



FIGURE 1 — Luminous efficiency vs. sustain pulse voltage, Ne + Xe (10%). Sustain pulse duty. T1/T0 was 5%.


An averaging pixel structure using microcrystalline-silicon films prepared at high temperature for AMOLED displays

Arc'hanmael Gaillard
Régis Rogel
Samuel Crand
Tayeb Mohammed-Brahim
Philippe Le Roy
Christophe Prat

Université de Rennes

Abstract — A new voltage-addressed pixel using a multiple drive distribution has been developed to improve, in a simple way, the brightness uniformity of active-matrix organic light-emitting-diode (AMOLED) displays. Moreover, circuits were realized using microcrystalline-silicon (μc-Si) films prepared at 600°C using a standard low-pressure CVD system. The developed p-channel TFTs exhibit a field-effect mobility close to 6 cm2/V-sec. The experimental results show that the proposed spatial distribution of driving TFTs improves the uniformity of current levels, in contrast to the conventional two-TFT pixel structure. Backplane performances have been compared using circuits based on μc-Si and furnace-annealed polysilicon materials. Finally, this technology has been used to make an AMOLED demonstration unit using a top-emission OLED structure. Thus, by combining both an μc-Si active-layer and a current-averaging driver, an unsophisticated solution is provided to solve the inter-pixel non-uniformity issue.

To demonstrate the operation of the pixel circuit, we have fabricated a small-sized display. A monochromatic (green color) demonstration unit is realized to reveal more pixel-to-pixel uniformity. Figure 11 shows a photograph of the world's first μc-Si TFT-driven AMOLED display realized using a furnace-crystallized silicon technique. The pixel has a top-emissive aperture ratio of 71% achieved thanks to reflective chromium regions. DC signals are supplied on power lines [VDD (10 V) and common cathode electrode (0 V)] while pulsed voltages were continuously applied to all scan lines (–20 V) and data lines.



FIGURE 11 — A display image of a monochromatic green top-light-emitting AMOLED realized on a glass-substrate integrating proposed active pixel circuit. The fill factor is about 70%.