Who will be the protagonist in many technology industries? Some people say that the next technology will be the era of "Display (Display Central)", and I believe it will be recognized by many people; the main reason is that under the technical support of the display, people have developed more and more strong visual needs, and almost All electronic products have display applications, making flat displays an indispensable protagonist in the technology industry. This article takes the liquid crystal display technology as the main axis, and talks about the technical development of the flat panel display and the research direction in the laboratory.
Liquid crystal displays are increasingly demanding colors
In the past, most of the research that valued color performance was mainly in the art and design industries; in contrast, engineers with a background in science and technology such as electronics and motors were simply "knowledgeless," but the display panel technology has developed for a long time The following, the display industry has a further demand for "color". Because the image signal is converted at the same time as the display device, the picture or image will easily have the disadvantage of color distortion, plus the level of engineering technology covered by the color performance of the display device is quite high. Therefore, "color expression" will become one of the key technologies for the future development of the display industry.
The main purpose of the color reproduction technology used in the display device is to convert the signal of the input terminal into a signal of the output device through a series of color calculation and processing under a visual system and environmental conditions, so that the output picture The color of the image is consistent with the color of the input image. This is the color reproduction technology of the display device. However, in terms of color formation and expression, since colors are formed by the combination of light sources, objects, and human visual perception, under the same object and different light sources (reflection), the colors produced by the display device and the actual colors will have The difference. Therefore, to achieve color reproduction technology in display devices, parameters such as light sources, objects, and human visual perception must be taken into consideration.
In the past, the International Lighting Association (The InternaTIonal Commission on IlluminaTIon; CIE) based on the human visual system's response to visible light, set a series of 列 color-related definition standards and expression spaces, each with its own characteristics and applicability. For example, the CIE 1931 XYZ color space, where X, Y, and Z are tristimulus values, describe 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 the incident light of the same environment and objects, that is, the same X, Y, and Z values, it is regarded as having the same color. Therefore, if the relevant input signal in the display device is converted to a color space similar to the XYZ color space, which is independent of the characteristics of the instrument, it will contribute to the calculation of color reproduction.
Display color reproduction steps and unresearched scope
Color profile: The most basic method of constructing a set of color profile is to look up the table method, and directly correspond each group of 數 bit signals on the instrument to the three stimulus values ​​it belongs to. The color profile described by this method The file is the most correct and complete, but the memory space occupied is very considerable. For example, with a set of colors (8, 8, 8), then the color characteristic description file has as many as 256 x 256 x 256 groups, The project is vast, and generally will try to adopt other methods to approach. The methods of constructing the color characteristic narrative file include analytical model (analyTIcal model), regression model (Regression) and interpolated model of reference table (LUT + interpolaTIon). For example, the interpolation model of the reference table is to look up the table and cooperate with the interpolation method, first find out the remaining sets of data and use the interpolation method to approximate, so as to save a lot of time.
Color Appearance Model: Changes in the surrounding environment will affect the human visual system's perception of color. In order to extract the environmental impact 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 environment (light source) on color, and is used to calculate the influence of different environments (light source) on color perception. For example, if the same observer observes the same object illuminated by different light sources, different color perception may occur if possible. In addition, if we consider the influence of psychological level on color recognition, human beings have the ability to adapt to the surrounding environment, so the same object under different light sources, due to the adaptability of the visual system, may also be recognized as very similar colors. Therefore, the effect of environmental (light source) factors on color perception is quite complicated. Based on this complexity, if the color appearance mode is used to divide the tristimulus value contributed by the ambient light source in the visual system, then the tristimulus that is not related to the environment Value for processing operations.
As mentioned earlier, CIE formulates many color spaces, and each has its own characteristics and applicability. Among them, CIE 1971 LUV, CIE 1971 LAB and other spaces, because the parameters of the ambient light source have been independent of the defined parameters, so these The space is not only the color space, but also the available color appearance model, and the LCH space also belongs to this type of color space, namely clear (L), hue (C), and saturation (H). Therefore, if you want to consider the role of the environmental light source, if you can convert the XYZ tristimulus value to the LCH model for processing.
Gamut mapping: The range of colors that can be displayed in a display device is called the "color gamut", and it is also regarded as the color rendering energy of the display device 力. However, each display device has a different color gamut. If some colors at the input end cannot be displayed at the output end, severe distortion will inevitably occur; therefore, the color gamut corresponds to a very important step in the color reproduction process. In addition, why the color gamut of each display device is the same, and then there are several common methods for matching the color gamut, such as: keeping the hue (Keep hue), reducing the brightness shift (Minimize lightness shift), and adjusting the chromaticity (Move chroma) and other methods.
Improve the development bottleneck of traditional liquid crystal display technology and improve the color gamut and saturation of the system
At present, the market demand for flat panel displays is expanding, and various types of display technologies are also developing vigorously. They are distinguished by the size of mass production. Although LCD monitors are still the largest application in the display market; Organic light-emitting diode panels, or field-emission displays ... etc., each have their own characteristics that are stronger than liquid crystal displays. For example: self-luminescence, fast response, contrast, color saturation, flexibility ... and many other advantages Display technology has suffered a considerable degree 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 advantages, it is necessary to continue to invest more manpower and research and development costs for the performance, display quality and price of the liquid crystal display technology.
For example, taking Field-Sequential Full-Color (FS-FC) technology as an example, it can not only improve the bottleneck of the development of traditional liquid crystal display technology, but also improve the color gamut and saturation of the system and reduce the cost of materials. It can even significantly increase the electro-optical conversion efficiency of the display panel by about 40% to meet the current global requirements for green and environmentally friendly products. Therefore, the display research technology of Jiaotong University is also funded by the research on key components and technologies of high-display image quality system panels of the Ministry of Economics and the panel industry, and the FS-FC LCD research technology, including high-efficiency integrated panels The architecture of the light source uses LED as the light source, combined with a high-reflectivity optical cavity to form a high-efficiency direct-lit backlight module.
On the other hand, since most display panels generally use a color filter with lower luminous efficiency, the optical use efficiency of the display panel is less than 8%, or the polarization conversion efficiency of the polarizer is low. The introduction of a control circuit that uses R, G, and B as the main light source and color sequential technology to further achieve the light mixing effect, which can reduce the operating voltage and improve the optical efficiency without the use of color filters. , So that the display panel has better color performance characteristics.
Using RGB-LED as the main backlight source can remove the color filter to improve the brightness
The polarization conversion efficiency and high-efficiency backlight module development technology in the display panel can be improved by combining the development concept of sub-wavelength grating. Analysis of the traditional liquid crystal display with CCFL as the backlight source shows that when the backlight source provides a brightness of 8500 nits, the actual light output brightness is about 800 nits after the loss of each component. The same is the brightness of 800nits. For this high-efficiency backlight module, only 1650nits is required. In other words, for a display constructed with this high-efficiency backlight module, the backlight source only needs to provide 1650 nits to be equivalent to the light output brightness when the conventional backlight source provides 8500 nits. In addition, because the biggest advantage of this design is to remove the color filter to increase the light efficiency by three times, even if the traditional polarizer is still used, it can achieve a light efficiency of about 40%; in addition, if RGB-LED is used as the main backlight source, only Need to provide 2650nits, you can achieve the equivalent brightness of 8500nits provided by traditional backlight.
The sub-wavelength grating can define the nano-grating pattern by the technique of direct writing of electron beam, and the sample is produced by the semiconductor manufacturing process. The experimental results show that the polarization conversion efficiency is 1.7 times that of the traditional LCD polarizer. This method adopts the technology of electron beam direct writing to define the grating, which will be time-consuming and high-cost when manufacturing a large size, so we propose a method of manufacturing a large-size sub-wavelength grating polarizer using Nano-imprint technology . The key to its technology lies in "precision fabrication of mold core structure". In addition, the improvement of laser stereotype technology is also expected to become a new mold production method. The master mold is defined by the laser stereotype technology, and then the mold core is made by the injection molding method. For large-sized mold cores, laser stereotypes are fast and inexpensive, so this method is expected to make nano grating masters quickly.
At present, the nano laser read / write head technology has reached a line width of <100nm. If it can be applied to laser engraving technology, not only can the nano grating master be quickly manufactured to meet the grating conditions with high spectral efficiency (grating period = 200nm ). After the grating mold kernel is fabricated, it must first be printed into a silicone gel (PDMS) mold kernel, followed by nano-imprinting and etching steps to complete the production of sub-wavelength grating polarizers.
Color separation effect left and right display quality
During the display time of the primary color light source of the display device, it can be positioned in the performance time of the color field of the image; the light of three consecutive color field times stimulates the incidence to the human eye, and the effect of the visual system is sufficient to form a color image. The ideal image forming condition is that each pixel in the three-image color field contained in a color image is projected to the same position corresponding to each pixel on the retina, and the color information of each pixel will be Completely reproduced visually.
If it is a three-image color field contained in a color image, and its corresponding pixels are projected on different positions on the retina and is detected by the visual system, the observer will see the image with the color field separated and misaligned. This is called color separation (CBU). And because CBU usually forms a ribbon arrangement on the edge of the object in the image, like a rainbow stripe, so CBU is also called the Rainbow Effect. Basically, the color separation phenomenon will not only reduce the quality of perception, but also some research reports indicate that after watching the FS-FC type display for a long time, it is easy to cause uncomfortable feelings such as dizziness. Therefore, for the potential color separation effect of liquid crystal displays, it will be the primary goal that FS-FC LCD must improve.
Laboratory research and actual verification process
In the laboratory stage, the 5.6-inch light guide plate was used for the preliminary verification and development. The power and brightness conversion capability of the LED is 378nits / W. Comparing the measurement results with the analysis chart provided on 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, its conversion capacity is about 40 nits / W. In other words, if the backlight module designed at 37 inches needs to provide a brightness of 2800 nits (greater than the aforementioned 2650 nits to ensure better performance than the conventional CCFL backlight module when providing 8500 nits), the required LED power is 70W. Coupled with the power consumption of the circuit, the total power is about 88W.
From the light distribution measurement results and photos of R, G, B alone and mixed white light, it can be seen that the uniformity is good and the brightness is stable. The result can be seen that the brightness has been very stable after 20 minutes of lighting, although due to heat The brightness is reduced, but the decrease is only 4%, which is within the range that the human eye cannot perceive. The measured viewing angle is 35 degrees, which is wider than the 30 degrees required by the specifications; 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 improve the competitiveness of display products
The high-efficiency RGB LED array backlight module architecture of “color sequence method to replace color filters†and “sub-wavelength grating polarizer to replace traditional polarizers†can achieve 37-inch 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 also greatly improved by more than 3 times, and at the same time, extremely high contrast (> 10,000) images can also be passed through the backlight source And the input image is processed and presented, and the verification results of other performances are also better than the existing specifications. In addition, in the sub-wavelength grating polarizer, it can replace the traditional polarizer experiment, in addition to the verification can improve the polarization conversion efficiency of 1.7 times.
In terms of overall application advantages, the light efficiency of the backlight module can be increased by more than 5 times. The high-efficiency backlight module is a display component with both high light efficiency and low electrical power consumption, which can further improve the existing characteristics of the flat panel display The advantages of the display panel. At the same time, environmental protection can save power consumption, reduce heat, improve the competitiveness and added value of display products.
As a display that provides visual stimulation, the viewing quality of FS-FC LCD is no longer a result independent of viewing conditions. The use of FS-FC technology, in addition to the traditional SCF LCD must consider the display component characteristics, light source and backlight module characteristics, must be considered in the design of the user's viewing conditions, including the objective physical conditions and visual aforesaid viewing Physiological reactions, even psychophysical perceptions. Because the color separation phenomenon is extremely difficult to eliminate in the FS-FC type display under the condition that the display response rate is limited, we can only try to make the color separation effect as invisible to the observer as possible. Therefore, the application of FS-FC technology to LCD is a simple system, but the overall design is more complex. A large amount of research investment is still required in order to have a new type of low power consumption, high color saturation, and high contrast in the next 3 to 5 years. The TFT-LCD panel appears.
Liquid crystal displays are increasingly demanding colors
In the past, most of the research that valued color performance was mainly in the art and design industries; in contrast, engineers with a background in science and technology such as electronics and motors were simply "knowledgeless," but the display panel technology has developed for a long time The following, the display industry has a further demand for "color". Because the image signal is converted at the same time as the display device, the picture or image will easily have the disadvantage of color distortion, plus the level of engineering technology covered by the color performance of the display device is quite high. Therefore, "color expression" will become one of the key technologies for the future development of the display industry.
The main purpose of the color reproduction technology used in the display device is to convert the signal of the input terminal into a signal of the output device through a series of color calculation and processing under a visual system and environmental conditions, so that the output picture The color of the image is consistent with the color of the input image. This is the color reproduction technology of the display device. However, in terms of color formation and expression, since colors are formed by the combination of light sources, objects, and human visual perception, under the same object and different light sources (reflection), the colors produced by the display device and the actual colors will have The difference. Therefore, to achieve color reproduction technology in display devices, parameters such as light sources, objects, and human visual perception must be taken into consideration.
In the past, the International Lighting Association (The InternaTIonal Commission on IlluminaTIon; CIE) based on the human visual system's response to visible light, set a series of 列 color-related definition standards and expression spaces, each with its own characteristics and applicability. For example, the CIE 1931 XYZ color space, where X, Y, and Z are tristimulus values, describe 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 the incident light of the same environment and objects, that is, the same X, Y, and Z values, it is regarded as having the same color. Therefore, if the relevant input signal in the display device is converted to a color space similar to the XYZ color space, which is independent of the characteristics of the instrument, it will contribute to the calculation of color reproduction.
Display color reproduction steps and unresearched scope
Color profile: The most basic method of constructing a set of color profile is to look up the table method, and directly correspond each group of 數 bit signals on the instrument to the three stimulus values ​​it belongs to. The color profile described by this method The file is the most correct and complete, but the memory space occupied is very considerable. For example, with a set of colors (8, 8, 8), then the color characteristic description file has as many as 256 x 256 x 256 groups, The project is vast, and generally will try to adopt other methods to approach. The methods of constructing the color characteristic narrative file include analytical model (analyTIcal model), regression model (Regression) and interpolated model of reference table (LUT + interpolaTIon). For example, the interpolation model of the reference table is to look up the table and cooperate with the interpolation method, first find out the remaining sets of data and use the interpolation method to approximate, so as to save a lot of time.
Color Appearance Model: Changes in the surrounding environment will affect the human visual system's perception of color. In order to extract the environmental impact 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 environment (light source) on color, and is used to calculate the influence of different environments (light source) on color perception. For example, if the same observer observes the same object illuminated by different light sources, different color perception may occur if possible. In addition, if we consider the influence of psychological level on color recognition, human beings have the ability to adapt to the surrounding environment, so the same object under different light sources, due to the adaptability of the visual system, may also be recognized as very similar colors. Therefore, the effect of environmental (light source) factors on color perception is quite complicated. Based on this complexity, if the color appearance mode is used to divide the tristimulus value contributed by the ambient light source in the visual system, then the tristimulus that is not related to the environment Value for processing operations.
As mentioned earlier, CIE formulates many color spaces, and each has its own characteristics and applicability. Among them, CIE 1971 LUV, CIE 1971 LAB and other spaces, because the parameters of the ambient light source have been independent of the defined parameters, so these The space is not only the color space, but also the available color appearance model, and the LCH space also belongs to this type of color space, namely clear (L), hue (C), and saturation (H). Therefore, if you want to consider the role of the environmental light source, if you can convert the XYZ tristimulus value to the LCH model for processing.
Gamut mapping: The range of colors that can be displayed in a display device is called the "color gamut", and it is also regarded as the color rendering energy of the display device 力. However, each display device has a different color gamut. If some colors at the input end cannot be displayed at the output end, severe distortion will inevitably occur; therefore, the color gamut corresponds to a very important step in the color reproduction process. In addition, why the color gamut of each display device is the same, and then there are several common methods for matching the color gamut, such as: keeping the hue (Keep hue), reducing the brightness shift (Minimize lightness shift), and adjusting the chromaticity (Move chroma) and other methods.
Improve the development bottleneck of traditional liquid crystal display technology and improve the color gamut and saturation of the system
At present, the market demand for flat panel displays is expanding, and various types of display technologies are also developing vigorously. They are distinguished by the size of mass production. Although LCD monitors are still the largest application in the display market; Organic light-emitting diode panels, or field-emission displays ... etc., each have their own characteristics that are stronger than liquid crystal displays. For example: self-luminescence, fast response, contrast, color saturation, flexibility ... and many other advantages Display technology has suffered a considerable degree 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 advantages, it is necessary to continue to invest more manpower and research and development costs for the performance, display quality and price of the liquid crystal display technology.
For example, taking Field-Sequential Full-Color (FS-FC) technology as an example, it can not only improve the bottleneck of the development of traditional liquid crystal display technology, but also improve the color gamut and saturation of the system and reduce the cost of materials. It can even significantly increase the electro-optical conversion efficiency of the display panel by about 40% to meet the current global requirements for green and environmentally friendly products. Therefore, the display research technology of Jiaotong University is also funded by the research on key components and technologies of high-display image quality system panels of the Ministry of Economics and the panel industry, and the FS-FC LCD research technology, including high-efficiency integrated panels The architecture of the light source uses LED as the light source, combined with a high-reflectivity optical cavity to form a high-efficiency direct-lit backlight module.
On the other hand, since most display panels generally use a color filter with lower luminous efficiency, the optical use efficiency of the display panel is less than 8%, or the polarization conversion efficiency of the polarizer is low. The introduction of a control circuit that uses R, G, and B as the main light source and color sequential technology to further achieve the light mixing effect, which can reduce the operating voltage and improve the optical efficiency without the use of color filters. , So that the display panel has better color performance characteristics.
Using RGB-LED as the main backlight source can remove the color filter to improve the brightness
The polarization conversion efficiency and high-efficiency backlight module development technology in the display panel can be improved by combining the development concept of sub-wavelength grating. Analysis of the traditional liquid crystal display with CCFL as the backlight source shows that when the backlight source provides a brightness of 8500 nits, the actual light output brightness is about 800 nits after the loss of each component. The same is the brightness of 800nits. For this high-efficiency backlight module, only 1650nits is required. In other words, for a display constructed with this high-efficiency backlight module, the backlight source only needs to provide 1650 nits to be equivalent to the light output brightness when the conventional backlight source provides 8500 nits. In addition, because the biggest advantage of this design is to remove the color filter to increase the light efficiency by three times, even if the traditional polarizer is still used, it can achieve a light efficiency of about 40%; in addition, if RGB-LED is used as the main backlight source, only Need to provide 2650nits, you can achieve the equivalent brightness of 8500nits provided by traditional backlight.
The sub-wavelength grating can define the nano-grating pattern by the technique of direct writing of electron beam, and the sample is produced by the semiconductor manufacturing process. The experimental results show that the polarization conversion efficiency is 1.7 times that of the traditional LCD polarizer. This method adopts the technology of electron beam direct writing to define the grating, which will be time-consuming and high-cost when manufacturing a large size, so we propose a method of manufacturing a large-size sub-wavelength grating polarizer using Nano-imprint technology . The key to its technology lies in "precision fabrication of mold core structure". In addition, the improvement of laser stereotype technology is also expected to become a new mold production method. The master mold is defined by the laser stereotype technology, and then the mold core is made by the injection molding method. For large-sized mold cores, laser stereotypes are fast and inexpensive, so this method is expected to make nano grating masters quickly.
At present, the nano laser read / write head technology has reached a line width of <100nm. If it can be applied to laser engraving technology, not only can the nano grating master be quickly manufactured to meet the grating conditions with high spectral efficiency (grating period = 200nm ). After the grating mold kernel is fabricated, it must first be printed into a silicone gel (PDMS) mold kernel, followed by nano-imprinting and etching steps to complete the production of sub-wavelength grating polarizers.
Color separation effect left and right display quality
During the display time of the primary color light source of the display device, it can be positioned in the performance time of the color field of the image; the light of three consecutive color field times stimulates the incidence to the human eye, and the effect of the visual system is sufficient to form a color image. The ideal image forming condition is that each pixel in the three-image color field contained in a color image is projected to the same position corresponding to each pixel on the retina, and the color information of each pixel will be Completely reproduced visually.
If it is a three-image color field contained in a color image, and its corresponding pixels are projected on different positions on the retina and is detected by the visual system, the observer will see the image with the color field separated and misaligned. This is called color separation (CBU). And because CBU usually forms a ribbon arrangement on the edge of the object in the image, like a rainbow stripe, so CBU is also called the Rainbow Effect. Basically, the color separation phenomenon will not only reduce the quality of perception, but also some research reports indicate that after watching the FS-FC type display for a long time, it is easy to cause uncomfortable feelings such as dizziness. Therefore, for the potential color separation effect of liquid crystal displays, it will be the primary goal that FS-FC LCD must improve.
Laboratory research and actual verification process
In the laboratory stage, the 5.6-inch light guide plate was used for the preliminary verification and development. The power and brightness conversion capability of the LED is 378nits / W. Comparing the measurement results with the analysis chart provided on 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, its conversion capacity is about 40 nits / W. In other words, if the backlight module designed at 37 inches needs to provide a brightness of 2800 nits (greater than the aforementioned 2650 nits to ensure better performance than the conventional CCFL backlight module when providing 8500 nits), the required LED power is 70W. Coupled with the power consumption of the circuit, the total power is about 88W.
From the light distribution measurement results and photos of R, G, B alone and mixed white light, it can be seen that the uniformity is good and the brightness is stable. The result can be seen that the brightness has been very stable after 20 minutes of lighting, although due to heat The brightness is reduced, but the decrease is only 4%, which is within the range that the human eye cannot perceive. The measured viewing angle is 35 degrees, which is wider than the 30 degrees required by the specifications; 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 improve the competitiveness of display products
The high-efficiency RGB LED array backlight module architecture of “color sequence method to replace color filters†and “sub-wavelength grating polarizer to replace traditional polarizers†can achieve 37-inch 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 also greatly improved by more than 3 times, and at the same time, extremely high contrast (> 10,000) images can also be passed through the backlight source And the input image is processed and presented, and the verification results of other performances are also better than the existing specifications. In addition, in the sub-wavelength grating polarizer, it can replace the traditional polarizer experiment, in addition to the verification can improve the polarization conversion efficiency of 1.7 times.
In terms of overall application advantages, the light efficiency of the backlight module can be increased by more than 5 times. The high-efficiency backlight module is a display component with both high light efficiency and low electrical power consumption, which can further improve the existing characteristics of the flat panel display The advantages of the display panel. At the same time, environmental protection can save power consumption, reduce heat, improve the competitiveness and added value of display products.
As a display that provides visual stimulation, the viewing quality of FS-FC LCD is no longer a result independent of viewing conditions. The use of FS-FC technology, in addition to the traditional SCF LCD must consider the display component characteristics, light source and backlight module characteristics, must be considered in the design of the user's viewing conditions, including the objective physical conditions and visual aforesaid viewing Physiological reactions, even psychophysical perceptions. Because the color separation phenomenon is extremely difficult to eliminate in the FS-FC type display under the condition that the display response rate is limited, we can only try to make the color separation effect as invisible to the observer as possible. Therefore, the application of FS-FC technology to LCD is a simple system, but the overall design is more complex. A large amount of research investment is still required in order to have a new type of low power consumption, high color saturation, and high contrast in the next 3 to 5 years. The TFT-LCD panel appears.
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