Wide-viewing angle in-plane switching liquid crystal displays for television applications using optical compensation technology

Transcription

1 Journal of Electronic Imaging 17(1), (Jan Mar 2008) Wide-viewing angle in-plane switching liquid crystal displays for television applications using optical compensation technology Daisuke Kajita Ikuo Hiyama Yuka Utsumi Hitachi Limited Materials Research Laboratory 7-1-1, Omika-cho, Hitachi Ibaraki, , Japan Masahiro Ishii Kikuo Ono Hitachi Displays Limited 3300, Hayano, Mobara Chiba, , Japan Abstract. Viewing angle performance in in-plane switching liquid crystal displays (IPS-LCDs) has been greatly improved, using two technologies. One is optical compensation technology using a biaxial film. The other is a newly developed IPS-Pro cell structure with higher transmission efficiency. These technologies are successfully introduced into the fabrication of 32-in. IPS-LCDs with a minimum contrast ratio over three times that of conventional IPS-LCDs and with color saturation (area ratio to NTSC at CIE1931 xy chromaticity coordinates) of more than 70% at almost all viewing angles SPIE and IS&T. DOI: / Introduction To improve image performance, various liquid crystal display LCD modes have been investigated; these include the in-plane- switching IPS mode, 1 fringe-field-switching FFS mode, 2 vertical-aligned VA mode, 3 and opticallycompensated-bend OCB mode. 4 We developed AS-IPS LCDs 5 for TV applications. Their intrinsically good viewing angle performance is well known. In the case of bright images, such as that shown in Fig. 1, the image at oblique angles in the IPS-LCD maintains its image quality at normal viewing angles. IPS-LCDs are suitable for ordinary uses. However, in the case of dark images, such as that shown in Fig. 2, the viewing angle performance in IPS-LCDs is not satisfactory. Actually, it has been pointed out that the viewing angle performance in IPS-LCDs is not satisfactory for several applications, such as movies, in which dark image quality is very important. 6 Therefore dark image quality at oblique angles is important in large TVs. In this work, improvements of the viewing angle performance of IPS- LCDs using an optical compensation technology are described in detail. 2 Analysis 2.1 Evaluation Method For accurate analysis of viewing angle performance, we evaluated four viewing angle characteristics. Luminance and contrast ratio CR are well known characteristics. Here we introduce two new characteristics for evaluating the viewing angle. The first, color, 7 represents the dependence of chromaticity measured in an image on a viewing angle. The chromaticity obtained at oblique angles, where u v, 0.02, and at the normal angle azimuth angle polar angle =0 deg is similar. u v, is the chromaticity difference at 1976 CIE u v chromaticity coordinates over viewing angles, given by the following expression. u v, = u, u 0,0 2 + v, v 0,0 2 1/2. 1 A noticeable difference of 0.02 was determined based on Paper 07004RR received Jan. 12, 2007; revised manuscript received Sep. 5, 2007; accepted for publication Sep. 7, 2007; published online Apr. 2, /2008/17 1 /013019/7/$ SPIE and IS&T. Fig. 1 Photographs of a bright image at a normal angle in our conventional IPS-LCD AS-IPS, which is similar to that in a fourdomain VA-LCD; b an oblique angle in our conventional IPS-LCD; and c an oblique angle in a four-domain VA-LCD. Journal of Electronic Imaging

2 Fig. 2 Photographs of a dark image at a normal angle in our conventional IPS-LCD, which is similar to that in a four-domain VA-LCD; b an oblique angle in our conventional IPS-LCD; and c an oblique angle in a four-domain VA-LCD. Fig. 3 Contrast ratio viewing angle characteristics in a our conventional IPS-LCD and b a four-domain VA-LCD. the MacAdam ellipsis. The second viewing angle characteristic, color saturation, represents the dependence of color saturation area ratio to NTSC at CIE1931 xy chromaticity coordinates on viewing angles. For all viewing angle characteristics, the absorption axis of the analyzer is parallel to the direction =0 deg. All viewing angle characteristics are based on measurements made using the EZContrast160R produced by Eldim. 2.2 Viewing Angle Characteristics in Conventional In-Plane Switching Liquid Crystal Displays It has been pointed out that in conventional IPS-LCDs, the contrast ratio is insufficient in diagonal viewing directions, as shown in Fig. 3. Usually, the viewing angle performance of LCDs is evaluated using CR. However, this characteristic does not represent all aspects of the viewing angle performance of LCDs. To more fully analyze viewing angle performance and clarify its issues with regard to IPS-LCDs, we have evaluated it from various viewpoints. Figure 4 shows a comparison of the color viewing angle characteristics measured for a bright compound color R124/255, G83/255, B53/255 between our conventional IPS-LCD AS-IPS and a four-domain VA-LCD with protrusions. The IPS-LCD shows superior viewing angle performance in most grayscale representations compared with the VA-LCD. However, in low gray-level images, especially those consisting of primary colors with low grayscales, the viewing angle of the conventional IPS-LCD is not yet sufficient. Figure 5 shows a comparison of the color viewing angle characteristics measured for a primary color in a low grayscale R0/255, G0/255, B127/255. Figures 4 and 5 are correlated with Figs. 1 and 2, respectively. Relatively, in conventional IPS-LCDs, primary colors red, green, blue appear whitish from oblique viewing directions. Figure 6 shows a comparison of the color shifts at all viewing angles measured for the three primary colors R255/255, G0/255, B0/255, R0/255, G255/255, B0/255, and R0/255, G0/255, B255/255 at CIE1931 xy chromaticity coordinates. In conventional IPS-LCDs, there are large color shifts in the representation of primary colors at oblique angles. Additionally, at oblique angles the chromaticity shifts toward an achromatic. Therefore, in conventional IPS-LCDs, the color saturation is insufficient at oblique angles, which is easy to evaluate with the color saturation viewing angle characteristic, as shown in Fig Color and Color Saturation Viewing Angle Characteristics The factors affecting the color and color saturation viewing angle characteristics should be discussed. These are considered in terms of the principle of a display with three primary colors. In typical LCDs, chromaticity is determined by the ratio of the optical intensity from three kinds of subpixels for the respective three primary color representations. To achieve good color viewing angle characteristics, that ratio should be equal at all viewing angles. In the case of cathode ray tube CRT displays and plasma displays PDP, the ratio at all viewing angles is maintained. However, in the case of LCDs, the ratio depends on the viewing angle. This is clear from the dependence of luminance viewing angle characteristics in grayscale. Figure 8 shows the normalized luminance measured at =45 deg. In a conventional IPS-LCD, the luminance at viewing angles within 20 deg of is similar in all grayscales. In grayscales over 64, it is very similar at all viewing angles. However, in grayscale levels under 31, the luminance increases at large s. On the other hand, in a multidomain VA-LCD, there is slightly more luminance increase in lower grayscales at large than in a conventional IPS- LCD. However, the luminance viewing angle characteristic strongly depends on the grayscale. For example, when a grayscale is displayed by three primary colors R124/255, G83/255, B53/255, the three luminance viewing angle characteristics are relatively similar in the conventional IPS-LCD, whereas in the multidomain VA-LCD, they are not similar. Therefore, it is thought that the ratio of the optical intensity from the three kinds of subpixels is nearer. As a result, the color viewing angle characteristics in conventional IPS-LCDs are better than in Fig. 4 Color viewing angle characteristics measured for a bright compound color in a our conventional IPS-LCD and b a fourdomain VA-LCD. Color online only. Journal of Electronic Imaging

3 Fig. 5 Color viewing angle characteristics measured for a primary color in low grayscale in a our conventional IPS-LCD and b a four-domain VA-LCD. multidomain VA-LCDs with such bright compound colors. On the other hand, when a grayscale is displayed by three primary colors R0/255, G0/255, B127/255, the three kinds of luminance viewing angle characteristics are not similar in conventional IPS-LCDs, especially in the grayscales of 0. Therefore, the color viewing angle characteristic is not satisfactory with such primary colors or dark colors. Furthermore, it can deteriorate the color saturation viewing angle characteristic. However, in a multidomain VA-LCD, the luminance viewing angle characteristic in the grayscales of 0 is more similar to that in the grayscale of 127 than it is in a conventional IPS-LCD. To make drastic improvements of the viewing angle performance of IPS- LCDs, the three kinds of luminance viewing angle characteristics in grayscales of 0 should be similar to the luminance viewing angle characteristics in other grayscales. Namely, the following is needed: 1. decreasing luminance at oblique angles in the black representation increasing CR at oblique angles, and 2. decreasing color shift at oblique angles in the black representation. 2.4 Viewing Angle Characteristics in the Black Representation in Conventional In-Plane Switching Liquid Crystal Displays Figure 9 a shows the luminance viewing angle characteristic. Figure 9 b shows the color shift depending on the viewing angles in the black representation in a conventional IPS-LCD, where the chromaticity in the black representation is plotted within 80 deg of the polar angle at all azimuth angles. The luminance increases at a viewing direction of =45 deg, which causes the unsatisfactory CR viewing angle characteristic shown in Fig. 3. Furthermore, there is a large color shift in the black representation. The causes of these effects should be discussed. It has been pointed out that the viewing angle characteristics in the black representation in conventional IPS-LCDs are affected by the intrinsic properties of the polarizers. 8 In Fig. 10 a, two polarizers are stacked with their absorption Fig. 6 Color shifts of primary colors at all viewing angles in a our conventional IPS-LCD and b a four-domain VA-LCD. Color online only. Journal of Electronic Imaging

4 Fig. 7 Color saturation viewing angle characteristics in a our conventional IPS-LCD and b a fourdomain VA-LCD. Color online only. Fig. 8 Luminance viewing angle characteristics dependent on grayscales K1=grayscale/255 in a our conventional IPS-LCD and b a four-domain VA-LCD. Fig. 9 a Luminance and b color shift viewing angle characteristics in the black representation in our conventional IPS-LCD. Color online only. Journal of Electronic Imaging

5 Fig. 10 The relation of stacked polarizers axes at a a normal angle and b an oblique angle. c The viewing angle dependence of the two crossed absorption axes. axes crossed at an angle of 90 deg. A ax1, T ax1, A ax2, and T ax2 are the absorption axes and transparent axes of the first and second polarizers, respectively. The angles of the two crossed absorption axes is larger than 90 deg at oblique angles, as shown in Fig. 10 b. Figure 10 c shows the calculated dependence of the angle of the two crossed absorption axes on the viewing angle, where A ax1 is parallel to the direction of =0 deg. Several excellent optical compensation methods have been proposed from this viewpoint. 9,10 However, there is another factor affecting viewing angle characteristics in the black representation in conventional IPS-LCDs. This is unnecessary retardation due to the mutual relationship between the retardations of the polarizers and the LC layer. Figure 11 shows the configuration of a common polarizer. Usually, the polarizing layer is primarily made of polyvinyl alcohol PVA, which needs substrates with some rigidity. A triacetylcellulose TAC film is used as the substrate. It has low retardation at oblique angles, which affects the viewing angle characteristics in conventional IPS-LCDs. Considering this, the black representation at oblique angles in conventional IPS-LCDs should be discussed. The Poincaré sphere is useful for this discussion. Usually, it is used for a normal angle. However, it can be expanded to oblique angles with the relationship between the slow axes of birefringent media and the absorption axes of polarizers at oblique angles. 8 The former are obtained by solving the problem concerning the cross section of the index ellipsoid. The latter are obtained from Fig. 10 c. Figure 12 shows the configuration of a conventional IPS-LCD and the transformation of the polarization at oblique angles =45 deg in the black representation using the Poincaré sphere projecting onto the S 1 to S 3 plane. T in is the polarization of the incident light through the polarizer and A out is the polarization absorbed by the analyzer at an oblique angle. T in is transformed to P 1 by the low retardation of the previously mentioned TAC film of the polarizer. P 1 is transformed to P 2 by the retardation of the LC layer. P 2 is transformed to P out by the small retardation of the TAC film contained by the analyzer. The light leakage corresponds to the distance between the polarization P out and A out. Additionally, there is a long path for the polarization transformation through the LC layer in particular, which causes the large wavelength dependence of the polarization due to the positive wavelength dispersion of each birefringent medium. Therefore, large color shifts are caused at oblique angles in the black representation. 2.5 Improvement of Viewing Angle Performance of In-Plane Switching Liquid Crystal Displays by Optical Compensation To decrease the light leakage at oblique angles, a polarizer with a biaxial film has been used, as shown in Fig. 13 a. We decided on the concept and the configuration using the Poincaré sphere expression, as shown in Fig. 13 b. From there, it is thought that the appropriate Nz value of the biaxial film is from 0.2 to 0.4, because an Nz value decides the axis of the transformation of the polarization in the Poincaré sphere when the incident light passes through a birefringent medium at oblique angles. The Nz value is given by the following expression. Nz = n x n z n x n y, 2 n x n y where n x, n y, and n z are the refractive indexes and the subscripts x, y, and z express principal axes. The relationship between the optical constants of a biaxial film and the viewing angle characteristics in the black representation was calculated for the optimization of the film constants, using a 4 4 matrix simulation. 11 Two values were introduced in evaluating the viewing angle characteristics in the black representation. The first is the maximum luminance measured for all viewing angle directions; the other is the maximum color shift expressed by u v measured for all off-axis directions. The maximum luminance corresponds to the light leakage in the black representation, i.e., CR, at oblique angles. The maximum color shift u v eventually corresponds to the color shift at oblique angles in the black representation. Both values should be reduced. Figure 14 shows that when the Nz value of the biaxial film is set to 0.3, contrast ratio is greatly improved in the diagonal directions. However, the color shift still remains. Unfortunately, as shown in Fig. 14, it is difficult to get a high CR and a low color shift at an oblique angle at the same time. Fig. 11 Configuration of a normal polarizer. Journal of Electronic Imaging

6 Fig. 12 a Configuration of our conventional IPS-LCD and b transformation of the polarization at an oblique angle in the black representation in the configuration. To simultaneously achieve both a higher CR and a lower color shift at an oblique angle, the following approach was adopted. First, the transmittance at a normal angle in the white state was increased dramatically using the newly developed cell structure IPS-Pro, 12 as shown in Fig. 15 a. The transmittance in the white state at oblique angles was also increased. Decreasing the luminance of the backlight allowed the luminance in the black representation to be decreased while achieving the same value of white luminance as in a conventional IPS-LCD. Therefore, the CR at normal and oblique angles could be increased, as shown in Fig. 15 b. Next, the optical constants of the biaxial film were optimized, increasing the maximum luminance and reducing the color shift at all viewing angles. In IPS-Pro, the maximum luminance dependence on the constants of the biaxial film was calculated. Drastic improvements have been obtained. There is little difference in the color shift between a conventional IPS and IPS-Pro. Figure 16 shows the result when the Nz value of the biaxial film is set to 0.3. It is clear that for maximum luminance and maximum color shift u v at all viewing angles, IPS-Pro with a biaxial film provides better performance than the conventional IPS with a biaxial film, setting the retardation of the biaxial film from 150 to 210 nm. 3 Viewing Angle characteristics of Developed 32-Inch Diagonal IPS-Pro Panel A 32-in. diagonal IPS-Pro panel with the optimized biaxial film has been fabricated to demonstrate the effect of the results calculated in the previous chapter. The effect is clarified by comparing each of its characteristics with those Fig. 14 Calculation of the relationship between biaxial film retardation and viewing angle characteristics in the black representation in our conventional IPS-LCD with a biaxial film. of conventional LCDs, as shown in Figs. 3, 5, 7, and 9, which illustrate the CR, color, and color saturation viewing angle characteristics, and color shift in the black representation, respectively. From Figs. 17 and 18, we see that the viewing angle dependences of the CR and color shift have been greatly improved. Therefore, it is expected that the viewing angle performance will be improved in whole images, including the issues for conventional IPS-LCDs. For example, Fig. 19 shows two such characteristics. Figure 19 a shows the color viewing angle characteristic measured for the same color as Fig. 5. Figure 19 b shows the color saturation viewing angle characteristic. As expected, drastic improvements have been achieved. Actually, the image quality has been enhanced for all kinds of images. For dark images, the effect is especially remarkable, as shown in Fig Conclusion We develop a high-performance 32-in. diagonal LCD with a significantly enhanced viewing angle. It is found that the degradation of the viewing angle performance in our conventional IPS-LCD is caused by degradation of the viewing angle characteristics in the black representation. Optical compensation for IPS-LCDs is investigated, using the Poincaré sphere expression and an optical simulation. We obtain the optimum optical configuration with one biaxial Fig. 13 a Configuration of the developed IPS-LCD with a biaxial film and b transformation of the polarization at an oblique angle in the black representation in the configuration. Fig. 15 Comparisons of measured results of a transmittance at a normal angle and b CR at horizontal viewing angles between our conventional IPS AS-IPS and IPS-Pro. Journal of Electronic Imaging

7 Fig. 20 Photographs for a dark image at a normal angle and b an oblique angle in IPS-Pro with the optimized biaxial film. Fig. 16 Calculation of the relationship between biaxial film retardation and the viewing angle characteristics of an IPS-Pro with a biaxial film in the black representation. film with an Nz value of 0.2 to 0.4. The improvement is obtained by a technological combination of that configuration and a panel with a high transmission over 5%. As a result, both the minimum contrast ratio and the maximum color shift u v at all viewing angles are improved by over three times and 2/3, respectively, compared with conventional panels. Additionally, the color saturation area ratio to NTSC at CIE1931 xy chromaticity coordinates is maintained at more than 70% at almost all viewing angles. Fig. 17 Contrast ratio viewing angle characteristic in IPS-Pro with the optimized biaxial film. Fig. 18 Color shift in the black representation in IPS-Pro with the optimized biaxial film. Fig. 19 a Color viewing angle characteristic measured for the primary color in low grayscale and b color saturation viewing angle characteristic in IPS-Pro with the optimized biaxial film. Color online only. Acknowledgments The authors would like to thank the contributing engineers in the panel design, product engineering, and module development teams at Hitachi Displays Limited for the development of the TFT-LCD, and also sincerely thank the researchers in Hitachi Limited for useful discussions about the performance of the device. References 1. M. Ohta, M. Oh-e, and K. Kondo, Development of super-tft-lcds with in-plane switching display mode, Asia Display 95 Digest, pp S. H. Lee, S. L. Lee, and H. Y. Kim, Electro-optic characteristics and switching principle of a nematic liquid crystal cell controlled by fringe-field switching, Appl. Phys. Lett. 73, A. Takeda, S. Kataoka, T. Sasaki, H. Chida, H. Tsuda, K. Ohmuro, Y. Koike, T. Sasabayashi, and K. Okamoto, A super-high-imagequality multi-domain vertical alignment LCD by new rubbing-less technology, SID 98 Digest, pp T. Miyashita, C. L. Kuo, M. Suzuki, and T. Uchida, Properties of the OCB mode for active-matrix LCDs with wide viewing angle, SID 95, Digest, pp Y. Nakayoshi, N. Kurahashi, J. Tanno, E. Nishimura, K. Ogawa, and M. Suzuki, High transmittance pixel design of in-plane switching TFT-LCDs for TVs, SID 03 Digest, pp A. Badano, Viewing angle comparison of IPS and VA medical AM- LCDs, SID 05 Digest, pp D. Kajita, H. Hiyama, Y. Utsumi, M. Ishii, and K. Ono, Optically compensated IPS-LCD for TV applications, SID 05 Digest, pp J. Chen, K. H. Kim, J. J. Jyu, J. H. Souk, J. R. Kelly, and P. J. Bos, Optimum film compensation modes for TN and VA LCDs, SID 98 Digest, pp Y. Saitoh, S. Kimura, K. Kusafuka, and H. Shimizu, Optimum film compensation of viewing angle of contrast in in-plane-switchingmode liquid crystal display, Jpn. J. Appl. Phys., Part 1 37, T. Ishinabe, T. Miyashita, and T. Uchida, Novel wide viewing angle polarizer with high achromaticity, SID 00 Digest, pp D. W. Berreman, Optics in stratified and anisotropic media: 4 4-matrix formulation, J. Opt. Soc. Am. 62 4, K. Ono, I. Mori, R. Oke, Y. Tomioka, and Y. Satou, High performance IPS technology for LCD-TVs: AS-IPS2, IDW 04 Proc., pp Biographies and photographs of the authors not available. Journal of Electronic Imaging