Abstract
This work showcases the breakthrough achieved with the 2117 pixels per inch (PPI) Liquid Crystal Display (LCD) in developing high-resolution Virtual Reality (VR) displays. This technology significantly enhances the dynamic range and reduces the screen door effect of VR displays. The paper discusses the challenges and potential solutions in achieving LCDs with over 2000 PPI, including pixel aperture ratio design, LC efficiency improvements, and overall transmittance. Additionally, it proposes using micro-LED backlighting and low-power solutions in high-resolution designs to maintain image quality.
1.1 Introduction
In recent years, the demand for display quality in Virtual Reality (VR) headsets has increased, leading to continuous improvements in specifications such as resolution, color gamut, and response speed. To reduce the “screen door effect,” the resolution of VR displays has increased from 500 to 600 PPI in 2017 to over 2000 PPI. The “screen door effect” in VR is a visual illusion where users perceive a noticeable gap between pixels or subpixels on the VR display, creating a grid-like pattern that reduces immersion and visual quality. However, the highest commercially available resolution on the market is currently around 1200 PPI.
To meet the demand for higher PPI displays, we propose the first 4K VR LCD technology exceeding 2000 PPI. While many micro-organic light-emitting diode (OLED) displays with higher PPI (3000 to 5000 PPI) have emerged in the market, their panel sizes are relatively small. Using optical mechanical systems to magnify these panels may lead to a relatively low number of pixels per degree (PPD) visible to the human eye. Currently, the PPD specification on the market is about 20. Moreover, increasing the optical power of the lens will also increase its aberration. Thus, we choose to increase the panel size and resolution as an alternative solution.
In contrast, our proposed 2117 PPI 4K VR LCD has a higher PPD specification of approximately 40, providing better image quality and less distortion due to its moderate panel size. Additionally, the use of high-resolution micro-LED backlighting improves contrast, color, and viewing angles, delivering a high-quality experience comparable to micro-OLED.
This paper discusses the challenges related to high-resolution VR LCDs, including the design and specifications of LCDs above 1000 PPI, which differ significantly from traditional mobile phones. Such high-resolution displays require various designs to increase the pixel aperture ratio while improving liquid crystal (LC) efficiency and increasing overall transmittance. Furthermore, we propose methods to reduce panel power consumption and improve backlight efficiency, aiming to maintain image quality in designs exceeding 2000 PPI while providing customers with lower-power components.
2. Structure and Design
2.1 Achieving High-Resolution VR under Process Constraints
Liquid Crystal Displays (LCDs) consist of a layered structure made up of two polarizing panels and a layer of liquid crystal solution. A thin glass substrate with a calibration layer is sandwiched in between the LC to ensure their correct orientation. Color LCDs typically use color filters to determine the RGB color composition of pixels. The backlight source located behind the panel illuminates the LC, while thin-film transistors (TFTs) or similar electronic components individually control the voltage of each pixel, allowing precise control of the LC’s behavior, adjusting the brightness and color of the pixels. Electronic controllers and drive circuits interpret input signals and drive transistors to generate the desired image, making liquid crystal display technology an indispensable technology for modern displays, TVs, and mobile devices.
As pixel density increases to over 2000 PPI, it becomes increasingly difficult to manufacture lines and spaces using traditional micro-lithography equipment. For example, in a 2117 PPI striped RGB display, the subpixel size is 4.0μm×12.0μm, with a line/space width of approximately 1μm. However, the current process capability of micro-lithography equipment may not be sufficient to achieve such fine resolutions. To address this issue, we have adopted a subpixel rendering design and algorithm, increasing the subpixel size to 6.0μm×12.0μm, as shown in Figure 1. Comparing Figures 1(a) and 1(b), it is evident that Figure 1(b) has a larger pixel spacing (width) in the horizontal direction.
Width/Space=1.5/1.65.
Thus, in terms of lateral dimensions, the width and spacing of the original components can be larger, exceeding the production limit of 1μm. This allows us to achieve the desired line width/spacing and produce ultra-high-resolution LCDs at 2117 PPI.
(a) Achieving the limit of striped RGB line width/space; (b) Subpixel rendering design with larger line width/space.
2.2 TFT Design Enhancing Aperture Ratio
Aperture ratio is an important indicator of LCDs as it affects the brightness of the display. As pixel density increases, the aperture size significantly decreases due to the larger volume of traditional TFTs used in LCDs. For example, in a 2117 PPI display, the TFTs used occupy a significant portion of the pixel area, resulting in a very small aperture. To solve this problem, we developed smaller TFTs through optimized design and fine-tuning of the process. The 2117 PPI pixels using this new TFT maintain a considerable aperture ratio while ensuring appropriate characteristics and reliability. Figures 2(a) and 2(b) demonstrate the impact of different-sized TFT devices on pixel aperture ratio.
Figure 2(b) shows that the pixel aperture ratio using the new TFT design is approximately 19%, more than double that of the traditional design shown in Figure 2(a), which has an aperture ratio of only 9%.
2.3 Enhancing Efficiency within Small Pixels
Our new concept involves employing a special indium tin oxide (ITO) electrode profile and morphology design to improve the electric field in non-aperture areas, thereby better aligning the LC. This results in a 70% increase in LC efficiency, with a corresponding improvement in brightness and contrast. Figures 3(a) and 3(b) illustrate the differences between traditional and new ITO electrode designs.
(a) Traditional pixel design and (b) Special ITO electrode profile and morphology design.
Figure 3(a) shows that if the deviation lines of the LC enter the aperture area, their effectiveness decreases. The design of the ITO electrode and the terrain around the aperture area affects the distribution of deviation lines. A major factor is the PLN via, which serves as a bridge for the Pixel ITO electrode and causes uneven electric field distribution, resulting in deviation lines. PLN is an organic material used in the TFT manufacturing process for planarizing the terrain. The distribution of dark lines is influenced by the relative position between the end of the common (COM) ITO slit and the PLN via. The COM electrode is an electrode used as a reference voltage in the circuit. It is typically connected to ground potential, hence referred to as the ground electrode. The end of the slit can be placed at the bottom of the via to enhance LC efficiency, but during processing, deeper PLN vias may not expose the ITO well. Directly filling the PLN via can extend the ITO slit downwards, but this is a complex process. The crab leg design at the end of the slit can fine-tune the position of the dark lines, reducing the impact of process variations. To achieve better transmittance, we must consider the impact of PLN vias and ITO electrode design on the distribution of LC dark lines.
2.4 Chromaticity Angle Uniformity
Misalignment between the TFT and color filter (CF) substrates can lead to significant color shifts when viewed at large angles, especially in high PPI pixels. Generally, there is a deviation of 1 to 3μm between the TFT and CF substrates. The greater the deviation, the more pronounced the color shift. To address this issue, it is necessary to enhance the design of the light shielding layer (M1), which can effectively improve this phenomenon. Figure 4(a) shows the situation without a metal shielding layer, where green light leakage can be observed at wide angles, leading to color deviation. Figure 4(b) incorporates a layer of M1 as an optical shielding layer, effectively eliminating color deviation at wide viewing angles.
(a) Without metal shielding and (b) With metal shielding.
2.5 Low Power Solutions
As the pixel density of high-resolution panels continues to increase, the power consumption of panel driving has also significantly increased, making it unsuitable for wearable devices. Data demultiplexers (also known as H drivers) account for a large portion of power loss. In this section, we will introduce methods to reduce H driver power consumption. To address this issue, we have adopted a dual-voltage H driver/V driver design, allowing the selection of appropriate operating voltages based on the specific requirements of the driving circuit. The so-called “V driver” refers to the gate panel design in TFT technology. This approach effectively reduces panel driving power consumption and improves mass production capability. Figure 5(a) illustrates the traditional method, where both the V driver and H driver are powered by a single voltage, leading to excessive power consumption of the H driver. In contrast, Figure 5(b) uses different voltage drivers based on varying voltage requirements, thus saving power.
(a) Traditional method using VH/VL signals and (b) New method using multiple VH/VL signals.
As the resolution of LCD screens increases to 4K, the driving frequency of the H driver also sharply increases. This not only increases the power consumption of the panel but can also reduce reliability due to device overheating. To solve this problem, we have adopted low-power driving technology, halving the driving frequency of the H driver and display signal. Therefore, a more reliable and energy-efficient panel can be achieved. Comparing Figures 6(a) and 6(b), it is evident that the driving mechanism in Figure 6(b) operates at a lower frequency, thus reducing the operating frequency of the V driver by 50%, leading to a decrease in power consumption.
(a) Traditional driving and (b) Low-power driving.
Due to the high PPI display limiting the available circuit area, the circuit design is compact, resulting in high parasitic capacitance, leading to poor driving capability and high power consumption. The highest power consumption occurs during the loading process of the H driver. To overcome this challenge, we have adopted several low-load designs, such as metal replacement and control line sharing. Both designs effectively reduce parasitic capacitance, allowing the 2117 PPI VR display to achieve a high refresh rate without consuming excessive energy. Figure 7(a) utilizes a metal replacement method to select a low-load wiring configuration that reduces impedance. Figure 7(b) employs a control line sharing method to avoid metal crossing. Both methods effectively reduce load and are implemented in our display.
(a) Metal replacement method to reduce capacitance and (b) Control line sharing method to avoid metal crossing.
2.6 High Dynamic Range Mini LED Backlight
2.6.1.1 Improving Light Leakage Phenomena
To achieve high dynamic range (HDR) in LCDs using mini LED backlighting, a wide color gamut is essential for accurately reproducing the real world in high PPI VR head-mounted displays (HMDs). In this study, we use the 2117 PPI VR display as the evaluation object. As shown in Figure 8, due to insufficient width in the red and green regions, the color gamut of the VR display only reaches 89% of DCI-P3.
Color gamut of VR displays using mini LED backlighting.
Figure 9 shows the spectra of each pure color channel (red, green, blue) of the LCD. It is clear that red and green pure colors leak light in the blue band, affecting the performance of these pure colors.
Spectra of each pure color channel (red, green, blue) of the LCD.
By carefully considering the LCD panel, we can simulate and adjust the intensity of light leakage in the red and green pure colors in the blue light band to appropriate levels. As shown in Figure 9, the light leakage in the red and green regions has been significantly improved, allowing the color gamut to exceed 97% of DCI-P3, as shown in Table 1. Based on this phenomenon, we adopted high-gamut RGB color resistance in the new 2117 PPI VR HMD and optimized the manufacturing process. Furthermore, our mini LED backlight is equipped with quantum dot (QD) films to enhance light conversion efficiency and expand the red and green regions, achieving a higher DCI-P3 color gamut.
Table 1
Color gamut before and after light leakage improvement.
Improved Light Leakage – DCI-P3 (%)
Before Improvement – 89
After Improvement – 97
2.6.2 High-Efficiency Backlight Source of Mini LED + QD
By appropriately matching the spectra of mini LEDs, QDs, and color filters, the efficiency of direct backlighting using mini LEDs and QDs can be optimized. As shown in Figure 10, compared to general edge-lit backlighting, the full width at half maximum (FWHM) of each color of mini LED backlighting is narrower, resulting in higher color purity and transmittance, meeting the standards of the National Television System Committee (NTSC). NTSC is a color television standard primarily used in North America and some other regions, defining the color encoding system for analog television broadcasting and display.
Comparison of spectra of mini LED + QD and edge-lit backlighting.
The black and gray lines in Figure 10 compare the spectra of mini LED + QD and edge-lit backlighting. R, G, and B represent the spectra of color filters. It is evident that mini LED + QD performs better in the green and red light spectra, with narrower FWHM and higher light conversion efficiency. Combined with the spectra of color filters, it achieves a wider color gamut and purer chromaticity.
2.6.3 High Dynamic Zones
Another key point to achieve HDR is the number of dimming zones. In our VR mini LED backlight device, a single LED driver IC is used, resulting in 1024 dimming zones. As shown in Figure 11, it brings more fine details at the bright-dark junction. This enhances the brightness and darkness of the image, thereby improving contrast and image quality.
VR mini-LED backlight with 1024 dimming zones brings more detail at bright-dark junctions.
2.7 Ultra-Thin Backlight
In practical applications, the lightweight design of VR devices is an important aspect to consider. To this end, our VR mini LED backlight adopts a special optical structure film with zero outer diameter design for light mixing. The thickness of the backlight module can be controlled to below 0.9 mm. The backlight structure is illustrated in Figure 12.
Using a zero outer diameter design, the thickness of the backlight module can be controlled to below 0.9 mm.
3. Prototypes and Specifications
The specifications of the ultra-high-resolution VR LCD screen are shown in Figure 13. This LCD panel can provide high-quality 4K x 4K resolution images with 2117 PPI and approximately 40 PPD. The slim VR display system uses Pancake optical systems to reduce the size of the VR headset while providing high clarity and fully immersive experiences.
Pancake lens optical technology in VR refers to a compact and lightweight optical design used in VR headsets. These lenses are thin, with a wide field of view (FOV) and minimal distortion, significantly enhancing the comfort and immersion of the VR experience.
Specifications of the 2.56-inch VR display.
4. Comparing VR Display Technologies
Comparing various display technologies in VR applications reveals their distinct advantages and disadvantages. Micro OLEDs (especially silicon-based OLEDs) have the potential to deliver high-resolution VR experiences, but their cost poses a challenge. While VR LCDs have issues with color contrast and pixel size, they excel in providing a wider field of view. The adoption of mini LED backlight technology has significantly improved the performance of LCDs, making them more competitive compared to micro OLEDs in terms of response time, contrast, brightness, and lifespan.
However, the choice of LCD is primarily due to its ability to achieve larger panel sizes, thus providing an immersive experience. Micro OLEDs and mini LEDs are smaller in size, primarily targeting the high-end market, such as Apple’s Vision Pro, but they come at a higher cost. On the other hand, LCDs remain a more economical choice, targeting a broader mainstream audience. In summary, each display technology has its unique advantages and limitations, suitable for different market segments within the VR display industry (Table 2).
Table 2
Comparison of VR Display Technologies.
Display Technology
LCD
Advantages: Wider field of view, color contrast, suitable for mainstream users, moderately priced
Disadvantages: Response time and color pixel size limitations
Mini LED Backlight LCD
Advantages: Larger field of view, improved color, contrast, and brightness. Longer lifespan than micro OLED.
Disadvantages: Thicker modules compared to other displays
Micro OLED (Wafer-based)
Advantages: Huge potential for high-resolution VR. Compact, suitable for high-end applications
Disadvantages: High cost, only suitable for high-end markets like Vision Pro, small panel sizes
Complex optical designs.
Limited production capacity for small-sized panels suitable for high-end applications.
5. Conclusion
In this paper, we discussed several challenges related to high-resolution VR LCDs. We explored how subpixel rendering helps overcome manufacturing constraints, enabling VR LCDs to achieve 2117 PPI. Furthermore, we improved the LC efficiency of small pixel LCDs and enhanced their light transmittance. Additionally, we discussed methods to reduce overall power consumption in driving. To achieve HDR and improve the contrast and color of high-resolution LCDs, we adopted mini LED backlight technology. We also introduced the concept of higher partitioning to enhance the image quality of high-resolution VR displays.
Finally, we are proud to announce the industry’s first display with approximately 40 PPD, providing good visual detail and VR immersion. This breakthrough in display technology represents a significant advancement in the development of VR applications, and we are excited to see its impact on the industry.
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