Display panel color reproduction and improved optical efficiency technology

In the current technology industry, who will become the protagonist? Some people say that the next technology will be the "Display Central" era, I believe that will be recognized by many people; the most important reason is that the display under the technical support, develop People have more and more demand for vision, 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 “unclear”, 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 to 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 Illumination (CIE) has defined a series of color-related definition criteria and expression spaces based on the response of the human visual system 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, a regression model, and a 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 is it compatible with the color gamut of each display device?

There are several common methods, such as keeping the Keep hue, reducing the Minimize lightness shift, and adjusting the color chroma.

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 uses 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 to make large size secondary waves by nano-imprint technology.

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