Brightness Uniformity Research Articles

Design and fabrication of semi-transparent screen based on micro-patterns for direct-view type head-up display in automobiles

A semi-transparent screen with hemisphere micro-patterns was proposed and designed to enhance the brightness uniformity of the display image toward the driver for a direct-view type head-up display. The hemisphere micro-patterns were designed to consider the inclined angle of the windshield for efficient reflection and scattering toward to the driver. The density and radius of the hemisphere micro-patterns were adjusted as a function of position on the screen based on the geometrical calculation and analyzed by the commercial optical simulation tool based on a ray-tracing method. The designed hemisphere micro-patterns was fabricated by the thermal reflow method and evaluated to confirm the uniform illumination. From the results, the semi-transparent screen with variable micro-patterns shows the 91.9 % of brightness uniformity with the enhanced luminance compare to a screen without micro-patterns. A luminance of fabricated screen also shows good agreement with the simulation result to reflect the clear and bright driving information to the driver.

Defragmented image based autostereoscopic 3D displays with dynamic eye tracking

We studied defragmented image based autostereoscopic 3D displays with dynamic eye tracking. Specifically, we examined the impact of parallax barrier (PB) angular orientation on their image quality. The 3D display system required fine adjustment of PB angular orientation with respect to a display panel. This was critical for both image color balancing and minimizing image resolution mismatch between horizontal and vertical directions. For evaluating uniformity of image brightness, we applied optical ray tracing simulations. The simulations took effects of PB orientation misalignment into account. The simulation results were then compared with recorded experimental data. Our optimal simulated system produced significantly enhanced image uniformity at around sweet spots in viewing zones. However this was contradicted by real experimental results. We offer quantitative treatment of illuminance uniformity of view images to estimate misalignment of PB orientation, which could account for brightness non-uniformity observed experimentally.Our study also shows that slight imperfection in the adjustment of PB orientation due to practical restrictions of adjustment accuracy can induce substantial non-uniformity of view images’ brightness. We find that image brightness non-uniformity critically depends on misalignment of PB angular orientation, for example, as slight as ≤0.01° in our system. This reveals that reducing misalignment of PB angular orientation from the order of 10−2 to 10−3 degrees can greatly improve the brightness uniformity.

Enhanced facial texture illumination normalization for face recognition.

An uncontrolled lighting condition is one of the most critical challenges for practical face recognition applications. An enhanced facial texture illumination normalization method is put forward to resolve this challenge. An adaptive relighting algorithm is developed to improve the brightness uniformity of face images. Facial texture is extracted by using an illumination estimation difference algorithm. An anisotropic histogram-stretching algorithm is proposed to minimize the intraclass distance of facial skin and maximize the dynamic range of facial texture distribution. Compared with the existing methods, the proposed method can more effectively eliminate the redundant information of facial skin and illumination. Extensive experiments show that the proposed method has superior performance in normalizing illumination variation and enhancing facial texture features for illumination-insensitive face recognition.

Flat Panel Light Source with Lateral Gate Structure Based on SiC Nanowire Field Emitters.

A field-emission light source with high luminance, excellent luminance uniformity, and tunable luminance characteristics with a novel lateral-gate structure is demonstrated. The lateral-gate triode structure comprises SiC nanowire emitters on a Ag cathode electrode and a pair of Ag gate electrodes placed laterally on both sides of the cathode. The simple and cost-effective screen printing technique is employed to pattern the lateral-gates and cathode structure on soda lime glass. The area coverage of the screen-printed cathode and gates on the glass substrate (area: 6 × 8 cm2) is in the range of 2.04% – 4.74% depending on the set of cathode-gate electrodes on the substrate. The lateral-gate structure with its small area coverage exhibits a two-dimensional luminance pattern with high brightness and good luminance uniformity. A maximum luminance of 10952 cd/cm2 and a luminance uniformity of >90% can be achieved with a gate voltage of 500 V and an anode voltage of 4000 V, with an anode current of 1.44 mA and current leakage to the gate from the cathode of about 10%.

Acid-free sol-gel fabrication of glass thin films embedded with II-VI colloidal quantum dots

II-VI colloidal quantum dots (QDs) are ideal for optical sensors thanks to their high fluorescent brightness and good size uniformity. However, embedding colloidal QDs into a glass matrix with the standard sol-gel process leads to the QDs being damaged by the acid catalyst. Here, we report an acid-free sol-gel technique, which proves to be both simple and effective in fabricating silica glass thin films embedded with commercial II-VI colloidal QDs. Octadecylamine ligands are used as a bifunctional aid to not only stabilize the QDs in solution, but also assist the formation of the SiO2 gel. We demonstrate that high-quality QD-embedded glass thin films can be developed with this technique, and our fluorescent tests indicate that, except for a small blueshift in the emission spectrum, the QDs are very well preserved through the sol-gel process. This method offers a fast and low-cost path towards thin-film QD sensors with good mechanical and thermal stabilities, which are desirable for applications involving highly focused laser beams, such as ultrafast nanophotonics.

Parallax barrier engineering for image quality improvement in an autostereoscopic 3D display.

We present a image quality improvement in a parallax barrier (PB)-based multiview autostereoscopic 3D display system under a real-time tracking of positions of a viewer's eyes. The system presented exploits a parallax barrier engineered to offer significantly improved quality of three-dimensional images for a moving viewer without an eyewear under the dynamic eye tracking. The improved image quality includes enhanced uniformity of image brightness, reduced point crosstalk, and no pseudoscopic effects. We control the relative ratio between two parameters i.e., a pixel size and the aperture of a parallax barrier slit to improve uniformity of image brightness at a viewing zone. The eye tracking that monitors positions of a viewer's eyes enables pixel data control software to turn on only pixels for view images near the viewer's eyes (the other pixels turned off), thus reducing point crosstalk. The eye tracking combined software provides right images for the respective eyes, therefore producing no pseudoscopic effects at its zone boundaries. The viewing zone can be spanned over area larger than the central viewing zone offered by a conventional PB-based multiview autostereoscopic 3D display (no eye tracking). Our 3D display system also provides multiviews for motion parallax under eye tracking. More importantly, we demonstrate substantial reduction of point crosstalk of images at the viewing zone, its level being comparable to that of a commercialized eyewear-assisted 3D display system. The multiview autostereoscopic 3D display presented can greatly resolve the point crosstalk problem, which is one of the critical factors that make it difficult for previous technologies for a multiview autostereoscopic 3D display to replace an eyewear-assisted counterpart.

Simplified compact fisheye lens challenges and design

A simplified and compact F/2.8 fisheye lens system comprising only 6 lens elements with just one aspheric surface was successfully designed. 160° full field of view, and 11.6 mm lens total tracking length were intended. We further described the ultra-wide angle lens system design challenges and their overcome problems. Our lens system performance evaluation was done by ZEMAX optical design software. A perfect lateral color and axial chromatic aberration correction were achieved, and the relative illumination is above 85 % across the entire field of view with an admirable corrected field curvature along with fθ distortion less than 0.15 %. Our lens modulation transfer function produces a high-resolution projection with brightness uniformity for endoscopic, security, and monitoring applications.

立体发光LED灯片的光学性能研究

The luminescence performance characterized by luminous flux,color temperature,chromaticity coordinate of three-dimensional light emitting diodes( LEDs) has been investigated by using different phosphor coating methods. The LED die was directly coating with mixed phosphors and silicone which have the maximum total luminous flux. However,this encapsulation incurs luminous uneven problem of front and back which limit three-dimensional luminescence application. Therefore,this work attempts to use the silicon and spreading powder to change the light path on the basis of less loss of luminous flux. Experimental results indicate that the brightness uniformity can be improved when increasing silicon layer between LED die and phosphor,while spreading powder layer would induce significant light attenuation. Moreover,the three packing have similar light angle. Therefore,silicon layer provides an alternative packing solution for three-dimensional LED light application.

Approach of self-adaptive illumination for optical imaging systems.

An optical imaging signal is vulnerable to undesired features such as ambient light illumination and partial specular reflection from the target; the success of extracting target features from images depends largely on appropriate design of illumination. This paper presents an approach for self-adaptive illumination for optical imaging systems. The proposed illumination system projects a reference image to a target surface as an initial structured illumination, and then adjusts the projected image automatically to compensate the negative influences of undesired features. After this self-adaptive control process, undesired features would appear mostly invisible in the captured images. The signal-to-noise ratio would be improved dramatically well before subsequent image processing. In the validation experiments, several images with uniform brightness were offered as reference images; the captured images could achieve high brightness uniformity, even when the target surface was uneven or was illuminated by ambient light. In a further experiment of selective vessel illumination on a human palm, simulated vessel regions were selectively illuminated. Undesired features, like palm prints, almost disappeared in the images captured.

Optimizing Time-Multiplexing Auto-Stereoscopic Displays With a Genetic Algorithm

A figure-of-merit (FOM) of an auto-stereoscopic display system is introduced and adopted to characterize the system performance. This FOM takes into account of the ratio of the signal to the noise arising from the crosstalk from the adjacent channels as well as the brightness uniformity of viewing areas; hence, it is directly related to the glasses-free 3D viewing comfort. With a steadily improving FOM as a target, the genetic algorithm is applied to optimize the optical system, giving rise to substantially improved characteristics of an auto-stereoscopic display system. The numerical simulation is verified with an experiment of a multi-view auto-stereoscopic display unit. It is shown that the system can provide a high fidelity of the display effect with the crosstalk ratio being reduced from around 5% to nearly 1%, which is a very low value obtainable for an auto-stereoscopic system.

58.2: 55‐inch OLED TV using Optimal Driving Method for Large‐Size Panel based on InGaZnO TFTs

AbstractIn this paper, we provide the best way to drive large‐size OLED panel. This method compensates for variations in the on‐current and threshold voltage by extracting the TFT characteristics of each pixel of the panel. It also compensates the time‐dependent degradation characteristics of the TFT. From the extracted characteristics, corrected image‐data are generated that is composed gain and offset. By driving in the data which is corrected, it is possible to improve the uniformity of brightness and increase the life‐time of the panel. Consequently, 55‐inch OLED TV was manufactured and commercialized. For the commercialization of large‐size OLED TV, this driving method is very suitable and it will become an important.

36.4: Design and Calibration of 100‐Mpixel Multi‐Projection 3D Display with an Enhanced Image Quality

AbstractTo achieve an enhanced image quality in a 55‐inch, 100‐Mpixel multi‐projection 3D display for the medical and educational applications, the projector configuration design was improved, and the brightness uniformity compensation and real‐time calibration were performed. Maintaining the density of light‐field rays is important for an immersive natural 3D experiences, this 100‐Mpixel multi‐projection display was designed to have the same light‐field density as a 300‐Mpixel multi‐projection display, so we could acquire very smooth motion parallax without discontinuity in the viewing angle of 24°.

Multifunctional organic phototransistor-based nonvolatile memory achieved by UV/ozone treatment of the Ta₂O₅ gate dielectric.

An organic phototransistor (OPT) shows nonvolatile memory effect due to its novel optical writing and electrical erasing processes. In this work, we utilize an organic light-emitting diode (OLED) as the light source to investigate OPT-based memory (OPTM) performance. It is found that the OPTM can be used as either flash memory or write-once read-many-times memory by adjusting the properties of the Ta2O5 gate dielectric layer. UV/ozone treatment is applied to effectively change dielectric properties of the Ta2O5 film. The mechanisms for this are examined by X-ray photoelectron spectroscopy and capacitance-voltage measurement. It turns out that the densities of oxygen vacancies and defects in the first 1.8 nm Ta2O5 films near the Ta2O5/semiconductor interface are reduced. Furthermore, for the first time, we use this multifunctional OPTM, which unites the photosensitive and memory properties in one single device, as an optical feedback system to tune the brightness of the OLED. Our study suggests that these OPTMs have potential applications in tuning the brightness uniformity, improving the display quality and prolonging the lifetime of flat panel displays.

Optimisation and assessment of three modern touch screen tablet computers for clinical vision testing.

Technological advances have led to the development of powerful yet portable tablet computers whose touch-screen resolutions now permit the presentation of targets small enough to test the limits of normal visual acuity. Such devices have become ubiquitous in daily life and are moving into the clinical space. However, in order to produce clinically valid tests, it is important to identify the limits imposed by the screen characteristics, such as resolution, brightness uniformity, contrast linearity and the effect of viewing angle. Previously we have conducted such tests on the iPad 3. Here we extend our investigations to 2 other devices and outline a protocol for calibrating such screens, using standardised methods to measure the gamma function, warm up time, screen uniformity and the effects of viewing angle and screen reflections. We demonstrate that all three devices manifest typical gamma functions for voltage and luminance with warm up times of approximately 15 minutes. However, there were differences in homogeneity and reflectance among the displays. We suggest practical means to optimise quality of display for vision testing including screen calibration.

Improvement in Brightness Uniformity by Compensating for the Threshold Voltages of Both the Driving Thin-Film Transistor and the Organic Light-Emitting Diode for Active-Matrix Organic Light-Emitting Diode Displays

This paper proposes a novel pixel circuit design and driving method for active-matrix organic light-emitting diode (AM-OLED) displays that use low-temperature polycrystalline-silicon thin-film transistors (LTPS-TFTs) as driving element. The automatic integrated circuit modeling simulation program with integrated circuit emphasis (AIM-SPICE) simulator was used to verify that the proposed pixel circuit, which comprises five transistors and one capacitor, can supply uniform output current. The voltage programming method of the proposed pixel circuit comprises three periods: reset, compensation with data input, and emission periods. The simulated results reflected excellent performance. For instance, whenΔVTH=±0.33 V, the average error rate of the OLED current variation was low (<0.8%), and whenΔVTH_OLED=+0.33 V, the error rate of the OLED current variation was 4.7%. Moreover, when theI×R(current × resistance) drop voltage of a power line was 0.3 V, the error rate of the OLED current variation was 5.8%. The simulated results indicated that the proposed pixel circuit exhibits high immunity to the threshold voltage deviation of both the driving poly-Si TFTs and OLEDs, and simultaneously compensates for theI×Rdrop voltage of a power line.

Design of Integrated Light Guide Plate with Functional Structure of Enhanced Diffusion Length for Ultra-slim LED Backlight Unit

An integrated light guide plate (LGP) with a functional structure for a longer diffusion length to decrease the thickness was proposed and designed for the large area direct-type LED backlight unit (BLU). The integrated LGP is composed of a concave cone structure on the top surface and a rugged bottom surface. The light emitted from the LED sources can be reflected on the top surface and diffuse effectively inside the LGP and in the bottom air-gap, which is placed below the LGP to realize sufficient diffusion length even in the ultra-slim structure. After comparing the optical simulation results with the measured data from the fabricated hexagonal unit cell of an LGP, we verified our design concept and realized the integrated LGP. The final LGP for a 55-inch LED display shows a brightness uniformity of 90.5% with a reduced number of 81 LEDs and a thickness of 6 mm, a remarkably reduced value compared with previous direct-type large area LED backlight units.

A new poly-Si TFT compensation pixel circuit employing AC driving mode for AMOLED displays

This paper presents a new poly-Si pixel circuit employing AC driving mode for active matrix organic light-emitting diode (AMOLED) displays. The proposed pixel circuit, which consists of one driving thin-film transistor (TFT), three switching TFTs, and one storage capacitor, can effectively compensate for the threshold voltage variation in poly-Si and the OLED degradation. As there is no light emission, except for during the emitting period, and a small number of devices used in the proposed pixel circuit, a high contrast ratio and a high pixel aperture ratio can be easily achieved. Simulation results by SMART-SPICE software show that the non-uniformity of the OLED current for the proposed pixel circuit is significantly decreased (< 10%) with an average value of 2.63%, while that of the conventional 2T1C is 103%. Thus the brightness uniformity of AMOLED displays can be improved by using the proposed pixel circuit.

Thin LED light module using the inclined edge and inhomogeneous pattern reflection of a light guide plate

A low-power, high-brightness, thin, and flat LED light module was fabricated. LED arrays attached to the four walls of the lower part of a light guide plate (LGP) were used as the light source. The LGP was composed of a lower part with dot patterns on the bottom side, an intermediate section with inclined edges for the reduction of the bezel area, and a light-exiting plate. The bottom sides of the inclined edges and lower plate were coated with metal films for light reflection. The dot patterns on the bottom side of the lower plate were more concentrated in the center to obtain uniform brightness by inhomogeneous reflection. The dimensions of the fabricated LED light module were 100 mm (L) × 100 mm (W) × 10 mm (T). The average brightness and luminous uniformity of the LED light module were 12,155 cd/m2 and 96.7%, respectively.

Fabrication research on the sandwich layered cathode electrode for a triode field emission display prototype

Based on an effective screen-printing process, a novel sandwich layered cathode electrode was developed on a cathode faceplate. The ZnO electrode was sandwiched between an indium tin oxide (ITO) electrode layer and a silver electrode layer, and the carbon nanotube was prepared directly on the exposed ITO electrode layer surface. The cathode potential could be conducted to the carbon nanotube with the sandwich layered cathode electrode. Using the carbon nanotube as a field emitter, a triode field emission display prototype with a sandwich layered cathode electrode was fabricated, which possessed a better field emission characteristic, higher luminous brightness and better emission image luminance uniformity. The turn-on electric field was 1.88 V/μm and the measured maximum field emission current was 2273.6 μA at 3.19 V/μm. By the method of adjusting the field emission current, the electron-emitting uniform capacity of the carbon nanotube cold cathode could be modified, and the emission image luminance uniformity and the emission current stability of triode field emission display prototype was also be improved significantly. The emission current fluctuation of the sandwich layered cathode electrode type field emission display was less than 1.1%. Furthermore, the total manufacture cost of the triode field emission display prototype was low.

An AC Driving Pixel Circuit Compensating for TFTs Threshold-Voltage Shift and OLED Degradation for AMOLED

This paper presents an ac driving pixel circuit for active-matrix organic light-emitting diode (AMOLED) displays, which is composed of one driving thin-film transistor (TFT), three switching TFTs and one capacitor. The proposed pixel circuit can not only make OLED work at the ac driving mode, but also effectively compensate for the threshold-voltage shift of the driving TFT and the degradation of OLED. Simulation results show that the nonuniformity of the proposed pixel circuit is significantly reduced (<; 10%) with an average value of 4.2% compared with that of the conventional 2T1C pixel circuit, and thus, the brightness uniformity of AMOLED displays can be enhanced. Moreover, a high contrast ratio can be achieved by the proposed pixel circuit due to no light emitting except for the emission period, as well as a relatively high aperture ratio due to a small number of components.