Who will become the protagonist in many technology industries? Some people say that the next technology will be the era of "Display Central", I believe it will be recognized by many people; the most important reason is that the display is under the technical blessing, and people are increasingly demanding visual needs, and almost All electronic products have display applications, making flat-panel displays an indispensable protagonist in the technology industry. This paper takes the liquid crystal display technology as the main axis, and talks about the technical development of flat panel display and the research direction in the laboratory.
LCD monitors are increasingly demanding color
In the past, most of the research focusing on color performance was mainly in the art and design industries. In contrast, engineers with backgrounds in electronics, electrical engineering, etc., are simply “unclearâ€, but the long-term development of display panel technology Underneath, the display industry has a further need for "color." Because the image signal is converted by the display device, the picture or image is easy to produce color distortion, and the engineering technology covered by the color performance of the display device is quite high. Therefore, “color performance†will become one of the technologies that show the industry and focus on future development.
In the color reproduction technology used in the display device, the main purpose is to convert the device signal at the input end into a signal of the output device through a series of color calculations and processing processes under the visual system and environmental conditions, so that the output image is output. The color of the image is the same as the color of the input image, which is the color reproduction technology of the display device. However, in terms of color formation and performance, since color is formed by the combination of light source, object, and human visual experience, and the same object and different light sources are illuminated (reflected), the color and actual color produced by the display device will be The difference. Therefore, in order to achieve color reproduction technology in display devices, parameters such as light source, object, and human visual perception must be considered.
In the past, The InternaTIonal Commission on Illumina TIon (CIE) has defined a series of color-related definition criteria and expression spaces based on the human vision system's response to twilight, each with its own characteristics and applicability. For example, the CIE 1931 XYZ color space, where X, Y, and Z are tristimulus values, describes the response of the human visual system to incident light. Because this space is the true response of the visual system, if the visual system has the same response to different environments and incident light of the object, that is, the same X, Y, and Z values, it is considered to have the same color. Therefore, if the relevant input signal in the display device is converted to a color space that is independent of the instrument characteristics like the XYZ color space, it will facilitate the calculation of color reproduction.
Display color reproduction steps and future research areas
Profile Description: The most basic method for constructing a set of color characteristics is to look up the table and directly map each group of digits on the instrument to the associated tristimulus value. This method establishes the color characteristics of the file. The file is the most correct and complete, but the memory space is very impressive. For example, with a set of colors (8, 8, 8), the color feature narrative file has 256 x 256 x 256 groups. The project is vast, and generally try to adopt other methods to approach. The methods for constructing the color characteristic narrative file include an analytical model (analyTIcal model), a regression model (Regression), and a comparison table interpolation model (LUT + interpolaTIon). For example, the interpolated model of the comparison table is the look-up table method combined with the interpolation method. First, find out the remaining data of several sets of data to use the interpolation method to approach, so that a lot of time can be saved.
Color Appearance Model: Changes in the surrounding environment can affect the perception of color in the human visual system. In order to converge environmental factors, there are many modules, such as CIELAB, CIELUV, Hunt, Nayatani, etc. model. The color appearance model is a model that considers the influence of the environment (light source) on color and is used to calculate the influence of different environments (light sources) on color perception. For example, the same observer, if observing the same object illuminated by different light sources, may produce different color perceptions if possible. In addition, if we consider the influence of the psychological layer on color perception, because the human body has the ability to adapt to the surrounding environment, the same object under different light sources may also be distinguished as a very similar color due to the adaptability of the visual system. Therefore, the effect of the environment (light source) on color perception is quite complicated. Based on this complexity, if the color appearance mode is used, the tristimulus value contributed by the ambient light source in the visual system is divided, and then the environment-independent tristimulus remains. Value, for processing operations.
As mentioned earlier, CIE has a lot of color space, and each has its own characteristics and applicability. Among them, space such as CIE 1971 LUV, CIE 1971 LAB, etc., because the parameters of the ambient light source have been separated from the defined parameters, these Space is not only a color space, but also a usable color appearance model, and LCH space is also a color space of this type, which is clear (L), hue (C), and chroma (H). Therefore, if the role of the ambient light source is to be considered, the XYZ tristimulus value can be converted to the LCH model for processing.
Gamut mapping: The range of colors that can be exhibited in a display device is called the "gamut" and is also considered the color rendering capability of the display device. However, the color gamut of each display device is not the same. If the color of the input end cannot be displayed at the output end, serious distortion will inevitably occur; therefore, the gamut correspondence becomes a very important step in the color reproduction process. In addition, why, in conjunction with the color gamut of each display device, there are several common methods for processing the gamut corresponding, such as: Keep hue, Minimize lightness shift, Adjust chromaticity (Move) Chroma) and other methods.
Improve the development bottleneck of traditional liquid crystal display technology, improve system color gamut and saturation
At present, the market demand for flat panel displays is expanding, and various types of display technologies are also booming, which are differentiated by the size of mass production, although LCD display applications are the largest in the display market; however, along with other display technologies, such as plasma TV, Organic light-emitting diode panels, or field-type displays, etc., each have advantages over liquid crystal displays, such as: self-illumination, fast response, contrast, color saturation, flexibility, etc., making LCD Display technology has suffered a considerable amount of impact in the flat panel display industry. Therefore, in order to ensure that the liquid crystal display technology can continue to have the current competitive advantage, it is necessary to continuously invest more manpower and research and development costs for the performance, display quality and price of the liquid crystal display technology.
For example, Field-Sequential Full-Color (FS-FC) technology, for example, can not only improve the development bottleneck of traditional liquid crystal display technology, but also improve system color gamut and saturation, and reduce material cost. Even more can greatly improve the electro-optical conversion performance of the display panel by about 40%, in response to the current global requirements for green products. Therefore, the University of Transportation's display research technology is also funded by the FS-FC LCD research technology, including high-performance integrated panels, under the auspices of the Ministry of Economics, the "High-definition image quality system panel key components and technology research" and the panel industry. The architecture of the light source uses LED as the light source and combines a high-reflectivity optical cavity to form a high-efficiency direct-type backlight module.
On the other hand, since the general display panel mostly uses a color filter with a lower luminous efficiency, the optical efficiency of the display panel is less than 8%, or the polarizing conversion efficiency of the polarizer is low. In the introduction of the control circuit with R, G, B as the main light source and the color sequential technology, the light mixing effect is further achieved, and the operating voltage can be lowered and the optical efficiency can be improved without using the color filter. To make the display panel have better color performance characteristics.
Color filter can be removed with RGB-LED as the main backlight to enhance brightness
In the display panel, the polarization conversion efficiency and high-efficiency backlight module development technology can be combined with the development concept of sub-wavelength grating. Analysis of the traditional liquid crystal display using CCFL as the backlight, it can be known that when the backlight provides 8500 nits of brightness, the actual light output brightness is about 800 nits after the loss of each component. The same is the light output of 800nits. For this high-efficiency backlight module, you only need to provide 1650nits. In other words, the display of this high-efficiency backlight module frame can only provide 1650 nits of backlight, which is equivalent to the brightness of 8500 nits provided by the conventional backlight. In addition, since the biggest advantage of this design is that the color filter is removed to increase the light efficiency by three times, even with the conventional polarizer, the light efficiency can be achieved by about 40%; in addition, if the RGB-LED is used as the main backlight, only 2650nits is required to achieve the brightness of 8500nits provided by an equivalent conventional backlight.
The sub-wavelength grating can define the nano-grating pattern by the technique of electron beam direct writing, and the sample is fabricated by the semiconductor process. The experimental results show that the polarization conversion efficiency is 1.7 times that of the conventional LCD polarizer. This method adopts the technology of electron beam direct writing to define the grating, which will be time-consuming and costly in the production of large size. Therefore, we propose a method for fabricating large-sized sub-wavelength grating polarizer by nano-imprint technology. . The key to its technology lies in the "precision fabrication of the structure of the mold." In addition, the promotion of laser stereotypes is also expected to become a new way of making molds. The master mold is defined by the laser-engraving technique, and the mold is made by the injection molding method. For large-sized mold cores, the laser engraving speed is fast and inexpensive, so this method is expected to quickly produce a nano-scale master mold.
At present, the nano laser read/write head technology can reach the line width of <100nm. If it can be applied to the laser engraving technology, it can not only quickly produce the grating condition that the nano grating mother satisfies the high spectral efficiency (grating period = 200nm) ). After the grating mold is made, it must be reprinted into a silica gel (PDMS) mold, and then subjected to nano-imprinting and etching to complete the fabrication of the sub-wavelength grating polarizer.
Color separation effect, left and right display, delicate image quality
At the time when the primary color light source of the display device is displayed, it can be positioned at the performance time of the color field; the light of three consecutive color field times is incident on the human eye, and after the visual system is effective, it is sufficient to form a color. image. The ideal image forming condition is that under the respective pixel light stimuli in the three image color fields included in the color image, the image is projected to the same position corresponding to each pixel on the retina, and the color information of each pixel will be Reproduced visually and completely.
If the three image color fields contained in a color image are projected on different positions on the retina and are visually recognized by the visual system, the observer will separate the color field from the misaligned image, which is called the color separation (CBU) phenomenon. And because the CBU usually forms a ribbon arrangement at the edge of the object in the image, like a rainbow stripe, the CBU is also called the Rainbow Effect. Basically, the color separation phenomenon not only reduces the perceived quality, but also reports that it is easy to cause an uncomfortable feeling such as dizziness after watching the FS-FC type display for a long time. Therefore, the potential color separation effect of liquid crystal displays will be the primary goal of FS-FC LCD improvement.
Laboratory research and actual verification process
In the laboratory stage, 5.6 å‹ light guide plate was used for early verification and development, and the LED power and brightness conversion capability was 378 nits/W. Comparing the measurement results with the analysis charts provided by the LuxeonTM website, the conversion capabilities of the two are similar. Therefore, it can be inferred from this that if the design is extended to a 37-inch backlight module, the conversion capability is about 40 nits/W. In other words, when the backlight module is designed to provide 2800 nits of light at 37 ( (greater than the 2650 nits mentioned above, to ensure better performance than the conventional CCFL backlight module provides 8500 nits), the required LED power is 70 W. In addition to the power consumption of the circuit, the power after summing is about 88W.
The light distribution measurement results and photos of R, G, and B alone and mixed white light show that the uniformity is good and the brightness is stable. The results show that the brightness is very stable after 20 minutes of lighting, although it is hot. The brightness is reduced, but the decline is only 4%, which is not perceived by the human eye. The measured viewing angle is 35 degrees, which is wider than the specification requirement of 30 degrees; the color gamut distribution is also close to NTSC, far exceeding the current CCFL's 72% NTSC color gamut.
Save power consumption and reduce heat to increase the competitiveness of display products
The high-efficiency RGB LED array backlight module architecture of “color-sequence method instead of color filter†and “sub-wavelength grating polarizer replaces traditional polarizer†can achieve 37å‹ backlight module power consumption less than 90W. The experimental verification and analysis of the RGB-LED color-sequence method show that in addition to low power consumption, the efficiency of the backlight module is greatly improved by more than three times, and the image with extremely high contrast (>10,000) can also be passed through the backlight. And the processing of the input image is presented, and the verification results of other performances are also superior to the existing specifications. In addition, in the case of sub-wavelength grating polarizers, it can replace the experiment of the conventional polarizing plate, except that it has been verified that the polarization conversion efficiency can be improved by 1.7 times.
As far as the overall application advantages are concerned, the light efficiency of the backlight module can be improved by more than 5 times. The high-efficiency backlight module is a display component having both high light efficiency and low electric power consumption, which can further improve the existing characteristics of the flat panel display and play a flat surface. The advantages of the display panel. At the same time, it can save power consumption, reduce heat and improve the competitiveness and added value of display products.
The FS-FC LCD is a display that provides visual stimuli, and its viewing quality is no longer independent of viewing conditions. Using FS-FC technology, in addition to the traditional SCF LCD must consider the characteristics of the display components, light source and backlight module characteristics, must be considered in the design of the user's viewing conditions, including the objective physical conditions, visual Physiological reactions, even psychophysical perceptions. Because the color separation phenomenon is extremely difficult to eliminate in the case of a display with a limited response rate in the FS-FC type display, it is only possible to try to make the color separation effect as unobtrusive as possible by the observer. Therefore, the application of FS-FC technology to LCD is a simple system, but the overall design is more complex. It still requires a lot of research investment, so as to have a new type of low power consumption, high color saturation and high contrast in the next 3 to 5 years. A TFT-LCD panel appeared.
LCD monitors are increasingly demanding color
In the past, most of the research focusing on color performance was mainly in the art and design industries. In contrast, engineers with backgrounds in electronics, electrical engineering, etc., are simply “unclearâ€, but the long-term development of display panel technology Underneath, the display industry has a further need for "color." Because the image signal is converted by the display device, the picture or image is easy to produce color distortion, and the engineering technology covered by the color performance of the display device is quite high. Therefore, “color performance†will become one of the technologies that show the industry and focus on future development.
In the color reproduction technology used in the display device, the main purpose is to convert the device signal at the input end into a signal of the output device through a series of color calculations and processing processes under the visual system and environmental conditions, so that the output image is output. The color of the image is the same as the color of the input image, which is the color reproduction technology of the display device. However, in terms of color formation and performance, since color is formed by the combination of light source, object, and human visual experience, and the same object and different light sources are illuminated (reflected), the color and actual color produced by the display device will be The difference. Therefore, in order to achieve color reproduction technology in display devices, parameters such as light source, object, and human visual perception must be considered.
In the past, The InternaTIonal Commission on Illumina TIon (CIE) has defined a series of color-related definition criteria and expression spaces based on the human vision system's response to twilight, each with its own characteristics and applicability. For example, the CIE 1931 XYZ color space, where X, Y, and Z are tristimulus values, describes the response of the human visual system to incident light. Because this space is the true response of the visual system, if the visual system has the same response to different environments and incident light of the object, that is, the same X, Y, and Z values, it is considered to have the same color. Therefore, if the relevant input signal in the display device is converted to a color space that is independent of the instrument characteristics like the XYZ color space, it will facilitate the calculation of color reproduction.
Display color reproduction steps and future research areas
Profile Description: The most basic method for constructing a set of color characteristics is to look up the table and directly map each group of digits on the instrument to the associated tristimulus value. This method establishes the color characteristics of the file. The file is the most correct and complete, but the memory space is very impressive. For example, with a set of colors (8, 8, 8), the color feature narrative file has 256 x 256 x 256 groups. The project is vast, and generally try to adopt other methods to approach. The methods for constructing the color characteristic narrative file include an analytical model (analyTIcal model), a regression model (Regression), and a comparison table interpolation model (LUT + interpolaTIon). For example, the interpolated model of the comparison table is the look-up table method combined with the interpolation method. First, find out the remaining data of several sets of data to use the interpolation method to approach, so that a lot of time can be saved.
Color Appearance Model: Changes in the surrounding environment can affect the perception of color in the human visual system. In order to converge environmental factors, there are many modules, such as CIELAB, CIELUV, Hunt, Nayatani, etc. model. The color appearance model is a model that considers the influence of the environment (light source) on color and is used to calculate the influence of different environments (light sources) on color perception. For example, the same observer, if observing the same object illuminated by different light sources, may produce different color perceptions if possible. In addition, if we consider the influence of the psychological layer on color perception, because the human body has the ability to adapt to the surrounding environment, the same object under different light sources may also be distinguished as a very similar color due to the adaptability of the visual system. Therefore, the effect of the environment (light source) on color perception is quite complicated. Based on this complexity, if the color appearance mode is used, the tristimulus value contributed by the ambient light source in the visual system is divided, and then the environment-independent tristimulus remains. Value, for processing operations.
As mentioned earlier, CIE has a lot of color space, and each has its own characteristics and applicability. Among them, space such as CIE 1971 LUV, CIE 1971 LAB, etc., because the parameters of the ambient light source have been separated from the defined parameters, these Space is not only a color space, but also a usable color appearance model, and LCH space is also a color space of this type, which is clear (L), hue (C), and chroma (H). Therefore, if the role of the ambient light source is to be considered, the XYZ tristimulus value can be converted to the LCH model for processing.
Gamut mapping: The range of colors that can be exhibited in a display device is called the "gamut" and is also considered the color rendering capability of the display device. However, the color gamut of each display device is not the same. If the color of the input end cannot be displayed at the output end, serious distortion will inevitably occur; therefore, the gamut correspondence becomes a very important step in the color reproduction process. In addition, why, in conjunction with the color gamut of each display device, there are several common methods for processing the gamut corresponding, such as: Keep hue, Minimize lightness shift, Adjust chromaticity (Move) Chroma) and other methods.
Improve the development bottleneck of traditional liquid crystal display technology, improve system color gamut and saturation
At present, the market demand for flat panel displays is expanding, and various types of display technologies are also booming, which are differentiated by the size of mass production, although LCD display applications are the largest in the display market; however, along with other display technologies, such as plasma TV, Organic light-emitting diode panels, or field-type displays, etc., each have advantages over liquid crystal displays, such as: self-illumination, fast response, contrast, color saturation, flexibility, etc., making LCD Display technology has suffered a considerable amount of impact in the flat panel display industry. Therefore, in order to ensure that the liquid crystal display technology can continue to have the current competitive advantage, it is necessary to continuously invest more manpower and research and development costs for the performance, display quality and price of the liquid crystal display technology.
For example, Field-Sequential Full-Color (FS-FC) technology, for example, can not only improve the development bottleneck of traditional liquid crystal display technology, but also improve system color gamut and saturation, and reduce material cost. Even more can greatly improve the electro-optical conversion performance of the display panel by about 40%, in response to the current global requirements for green products. Therefore, the University of Transportation's display research technology is also funded by the FS-FC LCD research technology, including high-performance integrated panels, under the auspices of the Ministry of Economics, the "High-definition image quality system panel key components and technology research" and the panel industry. The architecture of the light source uses LED as the light source and combines a high-reflectivity optical cavity to form a high-efficiency direct-type backlight module.
On the other hand, since the general display panel mostly uses a color filter with a lower luminous efficiency, the optical efficiency of the display panel is less than 8%, or the polarizing conversion efficiency of the polarizer is low. In the introduction of the control circuit with R, G, B as the main light source and the color sequential technology, the light mixing effect is further achieved, and the operating voltage can be lowered and the optical efficiency can be improved without using the color filter. To make the display panel have better color performance characteristics.
Color filter can be removed with RGB-LED as the main backlight to enhance brightness
In the display panel, the polarization conversion efficiency and high-efficiency backlight module development technology can be combined with the development concept of sub-wavelength grating. Analysis of the traditional liquid crystal display using CCFL as the backlight, it can be known that when the backlight provides 8500 nits of brightness, the actual light output brightness is about 800 nits after the loss of each component. The same is the light output of 800nits. For this high-efficiency backlight module, you only need to provide 1650nits. In other words, the display of this high-efficiency backlight module frame can only provide 1650 nits of backlight, which is equivalent to the brightness of 8500 nits provided by the conventional backlight. In addition, since the biggest advantage of this design is that the color filter is removed to increase the light efficiency by three times, even with the conventional polarizer, the light efficiency can be achieved by about 40%; in addition, if the RGB-LED is used as the main backlight, only 2650nits is required to achieve the brightness of 8500nits provided by an equivalent conventional backlight.
The sub-wavelength grating can define the nano-grating pattern by the technique of electron beam direct writing, and the sample is fabricated by the semiconductor process. The experimental results show that the polarization conversion efficiency is 1.7 times that of the conventional LCD polarizer. This method adopts the technology of electron beam direct writing to define the grating, which will be time-consuming and costly in the production of large size. Therefore, we propose a method for fabricating large-sized sub-wavelength grating polarizer by nano-imprint technology. . The key to its technology lies in the "precision fabrication of the structure of the mold." In addition, the promotion of laser stereotypes is also expected to become a new way of making molds. The master mold is defined by the laser-engraving technique, and the mold is made by the injection molding method. For large-sized mold cores, the laser engraving speed is fast and inexpensive, so this method is expected to quickly produce a nano-scale master mold.
At present, the nano laser read/write head technology can reach the line width of <100nm. If it can be applied to the laser engraving technology, it can not only quickly produce the grating condition that the nano grating mother satisfies the high spectral efficiency (grating period = 200nm) ). After the grating mold is made, it must be reprinted into a silica gel (PDMS) mold, and then subjected to nano-imprinting and etching to complete the fabrication of the sub-wavelength grating polarizer.
Color separation effect, left and right display, delicate image quality
At the time when the primary color light source of the display device is displayed, it can be positioned at the performance time of the color field; the light of three consecutive color field times is incident on the human eye, and after the visual system is effective, it is sufficient to form a color. image. The ideal image forming condition is that under the respective pixel light stimuli in the three image color fields included in the color image, the image is projected to the same position corresponding to each pixel on the retina, and the color information of each pixel will be Reproduced visually and completely.
If the three image color fields contained in a color image are projected on different positions on the retina and are visually recognized by the visual system, the observer will separate the color field from the misaligned image, which is called the color separation (CBU) phenomenon. And because the CBU usually forms a ribbon arrangement at the edge of the object in the image, like a rainbow stripe, the CBU is also called the Rainbow Effect. Basically, the color separation phenomenon not only reduces the perceived quality, but also reports that it is easy to cause an uncomfortable feeling such as dizziness after watching the FS-FC type display for a long time. Therefore, the potential color separation effect of liquid crystal displays will be the primary goal of FS-FC LCD improvement.
Laboratory research and actual verification process
In the laboratory stage, 5.6 å‹ light guide plate was used for early verification and development, and the LED power and brightness conversion capability was 378 nits/W. Comparing the measurement results with the analysis charts provided by the LuxeonTM website, the conversion capabilities of the two are similar. Therefore, it can be inferred from this that if the design is extended to a 37-inch backlight module, the conversion capability is about 40 nits/W. In other words, when the backlight module is designed to provide 2800 nits of light at 37 ( (greater than the 2650 nits mentioned above, to ensure better performance than the conventional CCFL backlight module provides 8500 nits), the required LED power is 70 W. In addition to the power consumption of the circuit, the power after summing is about 88W.
The light distribution measurement results and photos of R, G, and B alone and mixed white light show that the uniformity is good and the brightness is stable. The results show that the brightness is very stable after 20 minutes of lighting, although it is hot. The brightness is reduced, but the decline is only 4%, which is not perceived by the human eye. The measured viewing angle is 35 degrees, which is wider than the specification requirement of 30 degrees; the color gamut distribution is also close to NTSC, far exceeding the current CCFL's 72% NTSC color gamut.
Save power consumption and reduce heat to increase the competitiveness of display products
The high-efficiency RGB LED array backlight module architecture of “color-sequence method instead of color filter†and “sub-wavelength grating polarizer replaces traditional polarizer†can achieve 37å‹ backlight module power consumption less than 90W. The experimental verification and analysis of the RGB-LED color-sequence method show that in addition to low power consumption, the efficiency of the backlight module is greatly improved by more than three times, and the image with extremely high contrast (>10,000) can also be passed through the backlight. And the processing of the input image is presented, and the verification results of other performances are also superior to the existing specifications. In addition, in the case of sub-wavelength grating polarizers, it can replace the experiment of the conventional polarizing plate, except that it has been verified that the polarization conversion efficiency can be improved by 1.7 times.
As far as the overall application advantages are concerned, the light efficiency of the backlight module can be improved by more than 5 times. The high-efficiency backlight module is a display component having both high light efficiency and low electric power consumption, which can further improve the existing characteristics of the flat panel display and play a flat surface. The advantages of the display panel. At the same time, it can save power consumption, reduce heat and improve the competitiveness and added value of display products.
The FS-FC LCD is a display that provides visual stimuli, and its viewing quality is no longer independent of viewing conditions. Using FS-FC technology, in addition to the traditional SCF LCD must consider the characteristics of the display components, light source and backlight module characteristics, must be considered in the design of the user's viewing conditions, including the objective physical conditions, visual Physiological reactions, even psychophysical perceptions. Because the color separation phenomenon is extremely difficult to eliminate in the case of a display with a limited response rate in the FS-FC type display, it is only possible to try to make the color separation effect as unobtrusive as possible by the observer. Therefore, the application of FS-FC technology to LCD is a simple system, but the overall design is more complex. It still requires a lot of research investment, so as to have a new type of low power consumption, high color saturation and high contrast in the next 3 to 5 years. A TFT-LCD panel appeared.
YESWITCH ELECTRONICS CO., LTD. , https://www.yeswitches.com