JP2009288525A - Liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make contrast reduction in a specified azimuthal angle ψ not conspicuous in an IPS type or a VA type liquid crystal display. <P>SOLUTION: An upper polarizing plate 11 and a lower polarizing plate 12 are stuck on an upper side and a lower side of the IPS type or VA type liquid crystal cell 10 having excellent visual field angle characteristics, respectively. A reflection type polarizing plate 14 and a retardation plate 13 are disposed in this order from a backlight 20 side between the lower polarizing plate 12 and the backlight 20. Increase of black display luminance when viewed from an oblique direction at 45° angle to an absorption axis direction of the lower polarizing plate 12 is reduced by cooperation of the reflection type polarizing plate 14 and the retardation plate 13. A real contrast reducing effect is not obtained since white display luminance is also reduced and, however, thereby an apparent contrast enhancing effect as the whole screen can be obtained. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a display device, and more particularly to a liquid crystal display device having a wide viewing angle and a thin display device.

  Since the liquid crystal display device is flat and lightweight, its application is expanding in various fields such as a large display device such as a TV, a mobile phone, and a DSC (Digital Still Camera). In a liquid crystal display device, there are a TFT substrate in which pixel electrodes and thin film transistors (TFTs) are formed in a matrix, and a counter substrate in which color filters are formed at locations corresponding to the pixel electrodes of the TFT substrate, facing the TFT substrate. The liquid crystal is sandwiched between the TFT substrate and the counter substrate. An image is formed by controlling the light transmittance of the liquid crystal molecules for each pixel. Since the liquid crystal can be controlled only for polarized light, an upper polarizing plate is attached to the counter substrate, and a lower polarizing plate is attached to the TFT substrate.

  In a liquid crystal display device, the viewing angle is a problem. In “Patent Document 1”, in order to improve the viewing angle of a liquid crystal display device using a TN (Twisted Nematic) liquid crystal conventionally used in a liquid crystal display device, between a liquid crystal cell and an upper polarizing plate, or A configuration in which an optical compensation member is disposed between a liquid crystal cell and a lower polarizing plate is described. The optical compensation member in “Patent Document 1” is one or a plurality of retardation plates and a third polarizing plate.

Japanese Patent Laid-Open No. 06-222357

  An IPS (In Plane Switching) type liquid crystal display device controls light transmission by rotating liquid crystal molecules in an in-plane direction of a glass substrate by an electric field in a direction parallel to the glass substrate. Even when viewed, it has excellent viewing angle characteristics with little contrast degradation.

  The IPS liquid crystal display device (hereinafter referred to as IPS) has excellent viewing angle characteristics. However, in the conventional IPS, even with the same viewing angle (hereinafter referred to as polar angle), the contrast depends on the in-plane direction (hereinafter referred to as azimuth angle). The phenomenon that there is a difference is observed. In this way, the image quality is adversely affected due to the black brightness increase (black floating) in a direction where the contrast is poor.

  IPS is also used in small liquid crystal display devices by taking advantage of excellent viewing angle characteristics. A small liquid crystal display device is required to have a small outer shape, and the liquid crystal display device is required to be thin.

  An object of the present invention is to obtain means for suppressing an increase in black luminance (black floating) in an azimuth direction having poor contrast and maintaining high image quality as a whole screen. Another object of the present invention is to obtain a means that does not require an extremely large thickness of the liquid crystal display device while maintaining high image quality.

  The present invention overcomes the above problems, and specific means are as follows.

  (1) A liquid crystal display device in which an upper polarizing plate is attached to the upper side of an IPS liquid crystal cell, a lower polarizing plate is attached to the lower side, and a backlight is disposed on the back surface of the lower polarizing plate. And, between the lower polarizing plate and the backlight, a reflective polarizing plate and a retardation plate are arranged in this order from the backlight side, and the retardation in the retardation plate is 150 nm to 400 nm. A liquid crystal display device characterized by the above.

  (2) The liquid crystal display device according to (1), wherein the phase difference is 200 nm to 300 nm.

  (3) The liquid crystal display device according to (1) or (2), wherein the retardation plate is a negative C-plate.

  (4) The liquid crystal display device according to (1) or (2), wherein the retardation plate is a Positive C-Plate.

  (5) The retardation plate is two positive A-plates having one slow axis direction coinciding with the lower polarizing plate absorption axis direction and orthogonal to each other, according to (1) or (2) Liquid crystal display device.

  (6) A liquid crystal display device in which an upper polarizing plate is attached to the upper side of a VA liquid crystal cell, a lower polarizing plate is attached to the lower side, and a backlight is disposed on the back surface of the lower polarizing plate. And, between the lower polarizing plate and the backlight, a reflective polarizing plate and a retardation plate are arranged in this order from the backlight side, and the retardation in the retardation plate is 150 nm to 400 nm. A liquid crystal display device characterized by the above.

  (7) The liquid crystal display device according to (6), wherein the phase difference is 200 nm to 300 nm.

  (8) The liquid crystal display device according to (6) or (7), wherein the retardation plate is a negative C-plate.

  (9) The liquid crystal display device according to (6) or (7), wherein the retardation plate is a Positive C-Plate.

  (10) (6) or (7), wherein the retardation plate is two positive A-plates having one slow axis direction coincident with a lower polarizing plate absorption axis direction and orthogonal to each other. Liquid crystal display device.

  (11) An upper polarizing plate is attached to the upper side of the IPS or VA liquid crystal cell, a lower polarizing plate is attached to the lower side, and a backlight is disposed on the back surface of the lower polarizing plate. In the liquid crystal display device, a reflective polarizing plate and a retardation plate are disposed in this order from the backlight side between the lower polarizing plate and the backlight, and the retardation in the retardation plate is A liquid crystal display device having a thickness of 150 nm to 400 nm, wherein the retardation plate is formed by applying to the lower polarizing plate.

  (12) An upper polarizing plate is attached to the upper side of the IPS or VA liquid crystal cell, a lower polarizing plate is attached to the lower side, and a backlight is disposed on the back surface of the lower polarizing plate. In the liquid crystal display device, a reflective polarizing plate is disposed between the lower polarizing plate and the backlight, the lower polarizing plate side substrate of the reflective polarizing plate, and the lower polarizing plate side Any of the base materials on the reflective polarizing plate side has a function of a retardation plate, the base material on the lower polarizing plate side of the reflective polarizing plate, and the reflective type on the lower polarizing plate side. The total retardation of the base material on the polarizing plate side is in the range of 150 nm to 400 nm.

  (13) An upper polarizing plate is attached to the upper side of the IPS or VA liquid crystal cell, a lower polarizing plate is attached to the lower side, and a backlight is disposed on the back surface of the lower polarizing plate. In the liquid crystal display device, a reflective polarizing plate is integrally formed with the lower polarizing plate below the lower polarizing plate, and the lower polarizing plate includes a polarizer sandwiched between an upper substrate and an intermediate substrate. The reflective polarizing plate is sandwiched between the middle base material and the lower base material, the middle base material has a function of a retardation plate, and the retardation of the middle substrate Is a liquid crystal display device having a thickness of 150 nm to 400 nm.

  According to the present invention, in the IPS liquid crystal display method or the VA liquid crystal display method, the brightness is lowered at a specific azimuth angle where the contrast is low. Can be provided.

  According to another embodiment of the present invention, in the IPS mode or VA mode liquid crystal display mode, the retardation plate disposed under the lower polarizing plate is formed by coating, so that the liquid crystal can be improved while improving the visual image quality. As a whole display device, an increase in thickness can be suppressed.

  According to still another embodiment of the present invention, a material having the effect of a retardation plate is used as the base material of the lower polarizing plate or the reflective polarizing plate, so that the retardation plate as a separate component is omitted. I can do it.

  Hereinafter, the contents of the present invention will be described in detail according to examples.

  In this embodiment, an example in which the present invention is applied to an IPS liquid crystal display device will be described. The IPS system controls the amount of light transmitted through the liquid crystal layer by rotating the liquid crystal with an electric field component parallel to the TFT substrate or the counter substrate, and has excellent viewing angle characteristics.

  In IPS, as will be described later, the rubbing direction for aligning the liquid crystal and the contrast in the direction of 90 degrees with respect to the rubbing direction are good, but the direction of ± 45 degrees with respect to the rubbing direction is black luminance BR. The contrast is lowered due to the increase of. The present invention maintains image quality by making the contrast decrease in a direction of ± 45 degrees with respect to the rubbing direction inconspicuous.

  FIG. 1 is a perspective view showing the configuration of the present invention. In FIG. 1, an upper polarizing plate 11 is attached to the upper side of the liquid crystal cell 10, and a lower polarizing plate 12 is attached to the lower side. The liquid crystal cell 10 has a liquid crystal layer sandwiched between a TFT substrate on which pixel electrodes and TFTs are formed and a counter substrate on which color filters and the like are formed.

  A backlight 20 is disposed on the back surface of the liquid crystal cell 10. A feature of the present invention is that a retardation plate 13 and a reflective polarizing plate 14 are disposed between the backlight 20 and the lower polarizing plate 12. The retardation plate 13 and the reflective polarizing plate 14 may be attached to the lower polarizing plate 12 as shown in FIG. 1, or may be attached to the backlight 20 as shown in FIG. Further, as shown in FIG. 3, the retardation plate 13 may be attached to the lower polarizing plate 12 and the reflective polarizing plate 14 may be attached to the backlight 20. Hereinafter, description will be made with reference to FIG.

  The phase difference plate 13 includes Negative C-Plate, Positive C-Plate, Positive A-Plate, and the like. The phase difference plate 13 has a refractive index anisotropy, the refractive index anisotropy in the x direction in the film plane is nx, the refractive index anisotropy in the y direction orthogonal to the x direction in the film plane is ny, and the film surface When the refractive index anisotropy in the z direction in the direction orthogonal to nz is nz, the case of nx = ny> nz is negative C-Plate, the case of nx = ny <nz is Positive C-Plate, nx> ny = nz This case is referred to as Positive A-Plate.

  FIG. 4 shows an example in which a negative C-plate is used as the phase difference plate 13. In FIG. 4, when the horizontal axis direction of the liquid crystal cell 10 is 0 degree, the rubbing axis RU of the liquid crystal cell 10 is 75 degrees. The transmission axis TR of the upper polarizer 11 is also 75 degrees, while the transmission axis TR of the lower polarizer 12 is 165 degrees. A negative C-plate is disposed under the lower polarizing plate 12 as the retardation plate 13.

  In FIG. 4, a reflective polarizing plate 14 is disposed under the retardation plate 13. A normal polarizing plate such as the upper polarizing plate 11 or the lower polarizing plate 12 has a degree of polarization of 99% or more, but the reflection type polarizing plate 14 in this embodiment may have a degree of polarization of about 90% or more. The degree of polarization is the transmittance (parallel transmittance Tp) measured by making the transmission axes of two films parallel, and the transmittance (orthogonal transmittance Tc) measured orthogonally ((Tp−Tc) / It is defined by the square root of (Tp + Tc)). In the reflective polarizing plate 14, light in a direction perpendicular to the transmission axis TR is not absorbed but reflected.

  The transmission axis TR of the reflective polarizing plate 14 is 165 degrees which is the same as the transmission axis TR of the lower polarizing plate 12. The reflective polarizing plate 14 and the retardation plate 13 have the effect of reducing the transmittance TRR in the direction of 45 degrees with respect to the rubbing axis RU and improving the visual image quality as a whole screen. A backlight 20 is disposed below the reflective polarizing plate 14.

  FIG. 5 shows an example in which Positive C-Plate is used as the phase difference plate 13. In FIG. 5, the configuration other than the phase difference plate 13 is the same as that of FIG. In FIG. 5, Positive C-Plate which is the retardation plate 13 cooperates with the reflective polarizing plate 14 to reduce the transmittance TRR in the direction of 45 degrees with respect to the rubbing axis RU, so that the entire screen looks It has the effect of improving the image quality.

  FIG. 6 shows an example in which Positive A-Plate is used as the phase difference plate 13. In FIG. 6, the configuration other than the phase difference plate 13 is the same as that of FIG. The phase difference plate 13 is a laminate of two positive A-plates having one slow axis RE coincident with the lower polarizing plate absorption axis direction and orthogonal to each other. In FIG. 6, the slow axis RE of the upper phase difference plate 13 in Positive A-Plate is 75 degrees, and the slow axis RE of the lower phase difference plate 13 is 165 degrees. In FIG. 6, Positive A-Plate which is the phase difference plate 13 cooperates with the reflective polarizing plate 14 to reduce the transmittance TRR in the direction of 45 degrees with respect to the rubbing axis RU, so that the entire screen looks It has the effect of improving the image quality.

  FIG. 7 is a definition of an angle for evaluating the visual characteristics of the liquid crystal display device. In FIG. 7, the polar angle θ is used as an angle indicating how much the screen is viewed at an angle with respect to the normal direction of the display region 100 of the liquid crystal display device. In addition, the horizontal axis of the display region 100 is set to zero degrees, and the angle counterclockwise from the zero degrees is set to the azimuth angle φ.

  FIG. 8 shows how the transmittance TRR of the liquid crystal display device, that is, the luminance BR changes depending on the polar angle θ and the azimuth angle φ in a conventional IPS liquid crystal display device in which the retardation plate 13 and the reflective polarizing plate 14 are not present. FIG. FIG. 8A shows the luminance BR for black display, and FIG. 8B shows the luminance BR for white display. 8A and 8B, A-A ′ indicates the rubbing direction.

  In FIG. 8A, the luminance BR is suppressed small even when the polar angle is large in A-A ′ that is the rubbing direction and C-C ′ that is perpendicular to the rubbing direction. Therefore, this direction shows excellent black display characteristics. On the other hand, in the B-B ′ direction and the D-D ′ direction, which are 45 degrees from the rubbing direction, the brightness BR increases as the polar angle θ increases. Therefore, in this direction, the polar angle θ increases, and the image quality deterioration due to black floating becomes remarkable.

  FIG. 8B shows the dependence of the luminance BR on the polar angle θ and the azimuth angle φ in the case of white display. In FIG. 8B, when the transmission is 0.3, D has a wide viewing angle characteristic in the BB ′ direction 45 degrees larger than the rubbing axis RU, and is perpendicular to BB ′. The −D ′ direction has a relatively narrow viewing angle characteristic. However, when the transmission is 0.2, the luminance BR has almost no dependency on the azimuth angle φ. As shown in FIG. 8B, the dependence of the luminance BR on the polar angle θ and the azimuth angle φ is smaller in the case of white display than in the case of black display. Therefore, the contrast is determined by the polar angle θ and the azimuth angle φ dependency of black display, and the B-B ′ direction and the D-D ′ direction are directions of contrast deterioration.

  The dependence of the luminance BR of the liquid crystal display device on the polar angle θ and the azimuth angle φ varies depending on the presence or absence of the retardation plate 13 and the reflective polarizing plate 14. FIG. 9 is a graph showing the azimuth angle φ dependency when the polar angle θ of the luminance BR in black display and white display is fixed to 70 degrees and the azimuth angle φ is changed.

  FIG. 9A shows the azimuth angle φ dependency of the luminance BR in the case of black display. In FIG. 9A, the horizontal axis is the azimuth angle φ, and the vertical axis is the brightness BR. The unit of the vertical axis a. u. Is an arbitrary unit, which means a relative value. A. u. Is the same. AA is a characteristic when the retardation plate 13 and the reflective polarizing plate 14 are not provided (conventional). BB is the case where the retardation plate 13 and the reflective polarizing plate 14 are present (the present invention), and the retardation Rth of the retardation plate 13 (= ((nx + ny) / 2−nz) d, d: film thickness This is a characteristic when the thickness is 300 nm. In both AA and BB, the luminance BR varies greatly depending on the azimuth angle φ. In FIG. 9A, the direction of AA ′ and CC ′ that showed excellent black display characteristics in FIG. 8A (75 degrees in the rubbing direction, 165 degrees in the direction perpendicular to the rubbing direction). Etc.), the luminance BR is small in BB as in AA. Therefore, excellent contrast characteristics are maintained at this azimuth angle even when the retardation plate 13 and the reflective polarizing plate 14 are present, while the BB ′ and DD ′ whose contrast deteriorates in FIG. 8A. BB (direction of 45 degrees relative to the rubbing direction, such as 30 degrees or 120 degrees), the brightness BR is almost half of the decrease AA compared to AA, and the retardation plate 13 and the reflection type polarizing plate 14, it can be seen that the black luminance is selectively suppressed in the contrast deterioration direction.

    FIG. 9B shows the azimuth angle φ dependency of the luminance BR in the case of white display. In FIG. 9B, the horizontal axis represents the azimuth angle φ, and the vertical axis represents the brightness BR. AA is a characteristic when the retardation plate 13 and the reflective polarizing plate 14 are not provided. BB is a characteristic when the retardation plate 13 and the reflective polarizing plate 14 are present, and the phase difference Rth of the negative C-plate which is the retardation plate is 300 nm. FIG. 9B shows the azimuth angle φ dependency when the polar angle θ is fixed at 70 degrees.

  In FIG. 9B, when the phase difference plate 13 and the reflective polarizing plate 14 are present in the case of white display, as shown by BB, A− showing excellent black display characteristics in FIG. In the direction of A ′ and CC ′, the brightness BR is the same as that of AA. On the other hand, in the direction of BB ′ and DD ′ where the contrast is deteriorated in FIG. The brightness BR is also reduced.

Thus, as shown in FIG. 9A and FIG. 9B, the black luminance BR is also reduced in the direction 45 degrees away from the rubbing direction by inserting the phase difference plate 13 and the reflective polarizing plate 14. Since the white luminance BR is also reduced and the rate of reduction is almost the same, the contrast improvement effect is not recognized. By inserting the retardation plate 13 and the reflective polarizing plate 14 as described above, the contrast is deteriorated. In the direction 45 degrees away from the rubbing direction, the contrast does not change, but the brightness BR of the black display is reduced, so that the human eye can suppress the black floating in all directions and the viewing angle contrast of the image display device can be reduced. Can give the impression that it has risen.

  FIG. 10 shows the change in the brightness BR when the azimuth angle φ is fixed at 30 degrees (DD ′ direction), which is the contrast deterioration direction, and the polar angle θ is changed. The case where the phase difference plate 13 is inserted and the case where neither the reflection type polarizing plate 14 nor the phase difference plate 13 is inserted are shown for black luminance BR and white luminance BR.

  FIG. 10A shows the polar angle dependency of the brightness BR in the case of black display. In FIG. 10A, the horizontal axis is the polar angle θ, and the vertical axis is the brightness BR. AA has no retardation plate 13 and reflective polarizing plate 14, BB has negative C-Plate (phase difference Rth = 300 nm) and reflective polarizing plate 14, CC has positive C-Plate (phase difference Rth = 300 nm) and the reflective polarizing plate 14, DD is the case where there is a Positive A-Plate (two phase differences Δnd = 260 nm are used) and the reflective polarizing plate 14. As can be seen from FIG. 10A, as the polar angle θ increases, the characteristics AA in the case where the three retardation films 13BB, CC, and DD are used do not use the retardation film 13. However, the black luminance BR is small, and it can be seen that the black luminance reduction effect is obtained in the contrast deterioration direction.

  FIG. 10B shows the polar angle dependency of the brightness BR in the case of white display. In FIG. 10B, the horizontal axis is the polar angle θ, and the vertical axis is the brightness BR. AA, BB, CC, and DD in FIG. 10B are the same as those described in FIG. As can be seen from FIG. 10B, as the polar angle θ increases, all of the BB, CC, and DD, which are the cases where the three types of phase difference plates 13 are used, are the characteristics when the phase difference plate 13 is not used. The white luminance BR is smaller than AA.

  Comparing FIG. 10A and FIG. 10B, when the reflective polarizing plate 14 and various retardation plates 13 are used, as shown by BB, CC, DD, the reflective polarizing plate 14 and Both the black luminance BR and the white luminance BR are reduced to the same extent as compared with AA in the case where the phase difference plate 13 is not used. Even if the reflection type polarizing plate 14 and various phase difference plates 13 are inserted, the contrast is reduced. Although there is no increase, when the azimuth angle φ is 30 degrees, when the reflective polarizing plate 14 and various retardation plates 13 are inserted, the black luminance is lowered in the contrast deterioration direction. This is the same when the azimuth angle φ is 120 degrees.

  On the other hand, as can be seen from FIG. 9 and the like, the rubbing direction and the rubbing direction have a high contrast in the direction of 90 degrees, and the white luminance BR does not decrease. Therefore, by inserting the phase difference plate 13 and the reflective polarizing plate 14, it is possible to give the human eye the impression that the black angle is suppressed in all directions and the viewing angle contrast of the image display device is increased. .

  FIG. 11 shows the operating principle of the present invention. As shown in FIG. 11A, the change in transmittance TRR when the retardation plate 13 and the reflective polarizing plate 14 are arranged below the polarizing plate. This is evaluated by changing the azimuth angle φ and the phase difference Rth of the negative C-Plate used as the phase difference plate 13. In FIG. 11A, the transmission axis TR of the polarizing plate and the reflective polarizing plate 14 is the same and is 165 degrees. The absorption axis AB is 75 degrees. In FIG. 11, the polar angle is fixed at 70 degrees for evaluation.

  FIG. 11B shows the dependency of the transmittance TRR on the azimuth angle φ in the configuration of FIG. 11A. The horizontal axis represents the azimuth angle φ and the vertical axis represents the transmittance TRR. The polar angle is fixed at 70 degrees. In FIG. 11B, when the phase difference plate 13 is not present, the evaluation is performed for the case where the phase difference Rth of the negative C-plate of the phase difference plate 13 is 200 nm, 400 nm, and 600 nm.

  As can be seen from FIG. 11B, in the direction of the absorption axis AB of the polarizing plate, the transmittance TRR is constant regardless of the presence of the phase difference plate 13 or the phase difference Rth of the phase difference plate 13. However, in the direction of 45 degrees with respect to the absorption axis AB of the polarizing plate, that is, in the direction of 30 degrees, 120 degrees, etc., the transmittance TRR is large depending on the presence of the retardation plate 13 and the value of the retardation Rth of the retardation plate 13. Change. That is, this effect is the same when the liquid crystal display panel displays black or white.

  FIG. 11C shows the dependence of the transmittance TRR on the phase difference Rth of the negative C-Plate when the azimuth angle φ is fixed at 30 degrees and the polar angle is fixed at 70 degrees in the configuration of FIG. Is. In FIG. 11C, the transmittance TRR varies greatly depending on the value of the phase difference Rth. In FIG. 11C, there is a minimum of the transmittance TRR when the phase difference Rth is around 600 nm.

  For the purpose of the present invention to reduce the influence of the poor contrast range on the entire screen by reducing the transmittance TRR in the direction of 45 ° and the direction of the transmission axis TR of the polarizing plate, a phase difference Rth of around 600 nm is suitable. looks like. However, when the phase difference Rth is around 600 nm, the white luminance BR becomes too small, and an unnatural image is formed. Therefore, in the present invention, the phase difference Rth of Negative C-Plate is set to 150 nm to 400 nm, more preferably 200 nm to 300 nm.

  As a result, the luminance at the time of black display in the direction of 45 degrees with respect to the transmission axis TR of the polarizing plate, which is the direction of contrast deterioration, is suppressed, and an unnatural portion is not generated in the brightness in the entire viewing angle direction. I can do it. In FIG. 11, a negative C-plate is used as the phase difference plate 13, but positive C-plate, one of the positive A-, the slow axis direction of which coincides with the lower polarizing plate absorption axis direction and is orthogonal to each other. The same effect can be obtained when using Plate. That is, also when Positive C-Plate is used, a suitable phase difference Rth is 150 nm to 400 nm, more preferably 200 nm to 300 nm. In addition, a preferable value of the phase difference Δnd of one phase difference plate when using two positive A-plates in which one slow axis direction coincides with the lower polarizing plate absorption axis direction and is orthogonal to each other is It is 150 nm to 400 nm, more preferably 200 nm to 300 nm.

  FIG. 12 is a diagram for explaining a mechanism for generating the effect of FIG. FIG. 12A is a diagram in which the phase difference plate 13 and the reflective polarizing plate 14 are arranged to overlap each other. As the phase difference plate 13, a Negative C-Plate is used. In FIG. 12A, the transmission axis TR of the reflective polarizing plate 14 is 165 degrees. FIG. 12 shows an evaluation of how the polarization state of linearly polarized light transmitted through the reflective polarizing plate 14 changes.

  FIG. 12B shows the dependence of the degree of polarization on the phase difference Rth in the direction of −45 degrees with respect to the reflection axis direction of the reflective polarizing plate 14, that is, when the azimuth angle φ is 30 degrees. The vertical axis in FIG. 12B is the Stokes parameter S3 indicating the polarization state of light. When the polarization state of transmitted light is linearly polarized when it is 0, it is clockwise and counterclockwise circularly polarized when +1 and −1, respectively. It shows that. The horizontal axis represents the phase difference Rth of the phase difference plate 13. As shown in FIG. 12B, the polarization state of the reflective polarizing plate 14 changes greatly depending on the value of the phase difference Rth in the direction of 45 degrees with respect to the polarization axis direction, that is, in the direction of the azimuth angle φ30 degrees. .

  FIG. 12C shows the dependence of the degree of polarization on the phase difference Rth when the reflective polarizing plate 14 has a reflection axis direction, that is, when the azimuth angle φ is 75 degrees. In the reflection axis direction of the reflective polarizing plate 14, the degree of polarization hardly changes compared to the case of FIG. That is, by inserting the reflective polarizing plate 14 and changing the phase difference Rth of the retardation plate 13, the polarization state is changed with respect to a specific azimuth, and the polarization state with respect to a specific azimuth. Can be prevented from changing. For this reason, it was possible to obtain the luminance reduction effect in a specific direction as described above.

  As a method for improving the viewing angle characteristics of a liquid crystal display device, there is a VA (Vertical Alignment) method (hereinafter referred to as VA). The VA controls the transmittance TRR of the liquid crystal display device by setting the initial alignment of the liquid crystal in a direction perpendicular to the TFT substrate or the counter substrate and applying a voltage between the TFT substrate and the counter substrate.

  Also in the VA liquid crystal display device, the phenomenon that the contrast is lowered at a specific azimuth occurs similarly to the case of IPS. Therefore, the present invention as described in Embodiment 1 can be applied to a VA liquid crystal display device. Also in VA, the upper polarizing plate 11 is attached to the upper side of the liquid crystal cell 10, the lower polarizing plate 12 is attached to the lower side, and the reflective polarizing plate 14 and the retardation plate are provided between the backlight 20 and the lower polarizing plate 12. 1 is arranged in the same manner as in the case of IPS. In addition, the fact that FIG. 2, FIG. 3 and the like exist as variations of the arrangement of FIG.

  FIG. 13 is a diagram showing how the transmittance TRR of the liquid crystal display device, that is, the luminance BR changes depending on the polar angle θ and the azimuth angle φ in the VA system. The dependency of the luminance BR on the polar angle θ and the azimuth angle φ varies depending on the presence or absence of the phase difference plate 13 and the reflection type polarizing plate 14 shown in FIG. It is a graph when it does not exist. FIG. 13A shows the luminance BR for black display, and FIG. 13B shows the luminance BR for white display. In VA, rubbing is not necessary.

  In FIG. 13A, the transmittance TRR is very small in the A-A ′ direction, which is the absorption axis AB direction of the polarizing plate, and in the C-C ′ direction, which is a direction perpendicular to the absorption axis AB direction of the polarizing plate. Therefore, this direction shows excellent contrast. On the other hand, as the polar angle θ increases in the BB ′ direction and the DD ′ direction, which are 45 degrees with respect to the AA ′ direction that is the absorption axis AB direction of the polarizing plate, the luminance BR increases. growing.

  FIG. 13B shows the dependence of the luminance BR on the polar angle θ and the azimuth angle φ in the case of white display. In FIG. 13B, there is almost no dependency of the luminance BR on the azimuth angle φ, regardless of whether the transmission is 0.3 or 0.2. That is, as shown in FIG. 13B, the dependency of the luminance BR on the polar angle θ and the azimuth angle φ is smaller in white display than in black display. Therefore, even in the case of VA, the contrast is determined by the dependency of black display on the polar angle θ and the azimuth angle φ, and the B-B ′ direction and the D-D ′ direction in which the black luminance increases are the direction of contrast deterioration.

  The dependence of the brightness BR on the polar angle θ and the azimuth angle φ differs depending on the presence or absence of the retardation plate 13 and the reflective polarizing plate 14. FIG. 14 is a graph showing the dependency of the azimuth angle φ when the polar angle θ of the brightness BR in black display and white display is fixed to 70 degrees and the azimuth angle φ is changed.

FIG. 14A shows the dependency of the luminance BR on the azimuth angle φ in the case of black display. In FIG. 14A, the horizontal axis is the azimuth angle φ, and the vertical axis is the brightness BR. EE is a characteristic when the retardation plate 13 and the reflective polarizing plate 14 are not provided (conventional). Further, FF is a characteristic when the retardation plate 13 and the reflective polarizing plate 14 are present (the present invention), and the phase difference Rth of the negative C-Plate which is the retardation plate is 400 nm. In both EE and FF, the brightness BR varies greatly depending on the azimuth angle φ. In FIG. 14A, the directions of A, C, A ′, and C ′ (the absorption axis AB direction of the polarizing plate and the absorption axis AB of the polarizing plate) that exhibited excellent black display characteristics in FIG. 13A. In the direction perpendicular to the direction), the luminance BR is small in FF as well as in EE. Accordingly, excellent contrast characteristics are maintained at this azimuth even when the retardation plate 13 and the reflective polarizing plate 14 are present, while B, D, B ′, D ′ whose contrast has deteriorated in FIG. In the direction of (a direction of 45 degrees with respect to the absorption axis AB direction of the polarizing plate, a direction of 0 degrees or 90 degrees, etc.)
In FF, the brightness BR is lower than that of EE, and is less than half of EE. When the phase difference plate 13 and the reflective polarizing plate 14 are provided, the black brightness is selectively suppressed in the contrast deterioration direction. I understand that.

  FIG. 14B shows the azimuth angle φ dependency of the luminance BR in the case of white display. In FIG. 14B, the horizontal axis is the azimuth angle φ, and the vertical axis is the brightness BR. EE is a characteristic when the retardation plate 13 and the reflective polarizing plate 14 are not provided. FF is a characteristic when the retardation plate 13 and the reflective polarizing plate 14 are present, and the phase difference Rth of the negative C-Plate which is a retardation plate is 300 nm. FIG. 14B shows the dependence of the azimuth angle φ when the polar angle θ is fixed at 70 degrees.

  In FIG. 14B, when the phase difference plate 13 and the reflective polarizing plate 14 are present in the case of white display, as shown by FF, A, which has excellent black display characteristics in FIG. In the directions of C, A ′, and C ′, the brightness BR is the same as that of EE. On the other hand, in the directions of B, D, B ′, and D ′ in which the contrast is deteriorated in FIG. The brightness BR is also reduced. That is, as shown in FIG. 14A and FIG. 14B, by inserting the phase difference plate 13 and the reflective polarizing plate 14, black is removed in the direction 45 degrees away from the absorption axis AB direction of the polarizing plate. Since the brightness BR and the white brightness BR are both reduced, and the rate of reduction is almost the same as the rate of reduction of the black brightness BR, no effect of improving the contrast is recognized.

  By inserting the phase difference plate 13 and the reflective polarizing plate 14 as described above, the contrast does not change in the direction 45 degrees away from the absorption axis AB direction of the polarizing plate, which is the direction of contrast deterioration, but the black display Since the brightness BR is lowered, it is possible to give the human eyes the impression that the black floating is suppressed in all directions and the viewing angle contrast of the image display device is increased. This phenomenon is the same as in the case of IPS.

  FIG. 15 shows the change in luminance BR when the azimuth angle φ is fixed to 0 degree (DD ′ direction), which is the direction of contrast deterioration, and the polar angle θ is changed. The case where the phase difference plate 13 is inserted and the case where neither the reflection type polarizing plate 14 nor the phase difference plate 13 is inserted are shown for black luminance BR and white luminance BR.

  FIG. 15A shows the polar angle dependency of the luminance BR in the case of black display. In FIG. 15A, the horizontal axis is the polar angle θ, and the vertical axis is the brightness BR. EE has no retardation plate 13 and reflective polarizing plate 14, FF has negative C-Plate (phase difference Rth = 400 nm) and reflective polarizing plate 14, GG has positive C-Plate (phase difference Rth = 400 nm) and the reflective polarizing plate 14, HH is the case where there is a Positive A-Plate (two phase differences Δnd = 260 nm are used) and the reflective polarizing plate 14. As can be seen from FIG. 15 (a), as the polar angle θ increases, the three types of phase difference plates 13 are used, and any of FF, GG, and HH does not use the phase difference plate 13. The black luminance BR is smaller than the characteristic EE, and it can be seen that the black luminance reduction effect is obtained in the contrast deterioration direction.

  FIG. 15B shows the polar angle dependence of the brightness BR in the case of white display. In FIG. 15B, the horizontal axis is the polar angle θ, and the vertical axis is the brightness BR. EE, FF, GG, and HH in FIG. 15B are the same as those described in FIG. As can be seen from FIG. 15 (b), as the polar angle θ increases, all of the three types of phase difference plates 13 are used, and any of FF, GG, and HH when the phase difference plate 13 is not used. The white luminance BR is smaller than the characteristic EE.

  Comparing FIG. 15A and FIG. 15B, in FF, GG, and HH, which is the case where the reflective polarizing plate 14 and various retardation plates 13 are used, the reflective polarizing plate 14 and the position are compared. Both the black luminance BR and the white luminance BR are reduced to the same extent as compared with EE in the case where the phase difference plate 13 is not used, and the contrast increases even if the reflective polarizing plate 14 and various phase difference plates 13 are inserted. However, when the azimuth angle φ is 0 degree, the luminance of black luminance decreases in the contrast deterioration direction in which the reflective polarizing plate 14 and various retardation plates 13 are inserted.

  On the other hand, as can be seen from FIG. 13 and the like, there is a high contrast in the 90 ° direction with respect to the absorption axis AB direction of the polarizing plate and the absorption axis AB direction of the polarizing plate, and there is no decrease in white luminance BR. Therefore, by inserting the phase difference plate 13 and the reflective polarizing plate 14, black floating is suppressed in all directions, and an impression that the viewing angle contrast of the image display device is increased can be given. That is, the same effect as described in IPS can be obtained in VA.

  The reason why the effects as shown in FIGS. 14 and 15 occur is the same as that described in FIGS. 11 and 12 in the first embodiment. Therefore, in the range where the overall contrast of the screen is prevented from lowering the apparent contrast and the luminance BR of the screen is not unnatural, the phase difference Rth of the Negative C-Plate is set to 150 nm to 400 nm, more preferably 200 nm to 300 nm. Is good.

  In FIG. 14 and FIG. 15, Negative C-Plate is used as the phase difference plate 13. However, as the phase difference plate 13, Positive C-Plate, one slow axis direction coincides with the lower polarizing plate absorption axis direction. The same effect can be obtained when two positive A-plates orthogonal to each other are used. That is, also when Positive C-Plate is used, a suitable phase difference Rth is 150 nm to 400 nm, more preferably 200 nm to 300 nm. In addition, when two positive A-plates whose one slow axis direction coincides with the lower polarizing plate absorption axis direction and are orthogonal to each other, a preferable value of the phase difference Δnd of the single-layer retardation plate is 150 nm. ˜400 nm, more preferably 200 nm to 300 nm.

  A feature of the present invention is that a reflective polarizing plate 14 and a retardation plate 13 are disposed between the lower polarizing plate 12 and the backlight 20 attached to the liquid crystal cell 10. In this case, two components, a reflective polarizing plate 14 and a phase difference plate 13, are added as compared with the conventional example. Therefore, there arises a problem that the number of parts increases and a problem that the thickness of the entire liquid crystal display device increases.

  In this embodiment, the retardation plate 13 is applied to the lower polarizing plate 12 to omit the retardation plate 13 as a separate component and to reduce the thickness of the liquid crystal display device. FIG. 16 is a cross-sectional view of a normal lower polarizing plate 12. The polarizing plate has a shape in which a polarizer 30 for polarizing light is sandwiched between base materials 31. As the polarizer 30, a uniaxially stretched PVA (polyvinyl alcohol) impregnated with iodine I is used. Since such a polarizer 30 is mechanically weak, the mechanical strength is ensured by sandwiching with a base material 31 formed of TAC (triacetyl cellulose) or the like.

  FIG. 17A shows a case where the retardation film 13 is formed by coating the lower polarizing plate 12 as shown in FIG. FIG. 18 shows a specific process flow for forming the retardation film 13 on the lower polarizing plate 12 by coating. The retardation plate 13 by application is formed by applying a liquid crystal material (thermosetting liquid crystal material) that solidifies when heat is applied and then solidifies by heating.

  In FIG. 18, process A is a process of forming an alignment film for aligning the thermosetting liquid crystal material on the lower polarizing plate 12. Process B is a step of baking and solidifying the alignment film. In the next process C, a retardation film material having a retardation effect is applied. Since the retardation plate 13 is basically a thermosetting liquid crystal material, the liquid crystal molecules are aligned in the direction of the alignment film, so that the slow axis RE is formed. Thereafter, in process D of FIG. 18, baking is performed to solidify the applied retardation plate material. Depending on the phase difference plate material, an alignment film may be unnecessary, and in this case, processes A and B can be omitted.

  After applying the retardation plate 13 to the lower polarizing plate 12 as shown in FIG. 17A in the process of FIG. 18, the reflective polarizing plate 14 is overlaid as shown in FIG. 17B. FIG. 19 is a cross-sectional view showing a state in which the retardation plate 13 is applied to the lower polarizing plate 12 and the reflective polarizing plate 14 is overlaid. Similarly to the lower polarizing plate 12, the reflective polarizing plate 14 has a configuration in which a polarizer 30 is sandwiched between base materials 31.

  Thereafter, as shown in FIG. 17C, the backlight 20 is installed behind the reflective polarizing plate 14 to complete the liquid crystal display device. The above process is the same when the negative C-Plate is used as the phase difference plate 13 and when the positive C-Plate is used.

  In this embodiment, two positive A-plates having one slow axis direction coincident with the lower polarizing plate absorption axis direction and orthogonal to each other are used as the phase difference plate 13, and the positive A-plate is used as the lower polarizing plate 12 and the reflection plate. The process in the case of apply | coating to the type | mold polarizing plate 14 is shown. Positive A-Plate has a configuration in which two phase difference plates 13 having different slow axes RE are stacked.

Therefore, as shown in FIG. 20A, the retardation film 13 is applied to both the lower polarizing plate 12 and the reflective polarizing plate 14. FIG. 21 shows a process of forming the retardation plate 13 by coating for each of the lower polarizing plate 12 and the reflective polarizing plate 14.
In FIG. 21, process E is a step of forming an alignment film for aligning the thermosetting liquid crystal material on the lower polarizing plate 12. Process F is a step of baking and solidifying the alignment film. The next process G is a rubbing step for orienting the phase difference plate 13 material in a specific direction. The rubbing direction in the process G is different between the alignment film applied to the lower polarizing plate 12 and the alignment film applied to the reflective polarizing plate 14.

  Thereafter, in process H, a retardation film material having a retardation effect is applied. Since the retardation plate 13 is basically a thermosetting liquid crystal material, the liquid crystal molecules are aligned in the direction of the alignment film, thereby having a retardation effect. Thereafter, in process I of FIG. 21, baking is performed to solidify the applied retardation plate material.

  After applying the retardation plate 13 to the lower polarizing plate 12 and the reflective polarizing plate 14 as shown in FIG. 20 (a), the reflective type with the retardation plate 13 applied as shown in FIG. 20 (b). The polarizing plate 14 and the reflective polarizing plate 14 coated with the retardation plate 13 are overlapped. Thereafter, as shown in FIG. 20C, the backlight 20 is disposed on the back surface of the reflective polarizing plate 14 to complete the liquid crystal display device.

  In the first to fourth embodiments, the retardation plate 13 is arranged as a separate component in addition to the reflective polarizing plate 14. Incidentally, the base material 31 of the polarizing plate has a retardation effect, that is, a retardation plate 13 effect. TAC is an example. Therefore, the retardation plate 13 as a separate part can be omitted by using a material having a retardation effect for the base material 31 of the polarizing plate.

  FIG. 22 shows an example. In FIG. 22, the lower polarizing plate 12 and the reflective polarizing plate 14 are overlaid. The base material 31 on the lower polarizing plate 12 side in the reflective polarizing plate 14 and the base material 31 on the reflective polarizing plate 14 side in the lower polarizing plate 12 are materials having a retardation effect such as TAC. In FIG. 22, the materials and plate thicknesses of the base material 31 on the lower polarizing plate 12 side in the reflective polarizing plate 14 and the base material 31 on the reflective polarizing plate 14 side in the lower polarizing plate 12 have a total phase difference Rth of 150 nm to 400 nm. More preferably, it is set to be 200 nm to 300 nm.

  FIG. 23 shows an example in which the lower polarizing plate 12 and the reflective polarizing plate 14 are formed simultaneously. In FIG. 23, the polarizer 30 of the lower polarizing plate 12 is sandwiched between an upper substrate 311 and an intermediate substrate 312, and the polarizer 30 of the reflective polarizing plate 14 is sandwiched between an intermediate substrate 312 and a lower substrate 313. . In FIG. 23, the middle base material 312 has the role of the retardation plate 13. A material such as TAC may be used as the material of the middle substrate 312, but a thermosetting liquid crystal material may be used.

  In the configuration of FIG. 23, the mechanical strength of the polarizer 30 of the lower polarizing plate 12 and the polarizer 30 of the reflective polarizing plate 14 is maintained by the upper base material 311 and the lower base material 313. May not necessarily have high mechanical strength. The phase difference Rth of the intermediate substrate 312 having a role as a phase difference plate is 150 nm to 400 nm, more preferably 200 nm to 300 nm.

It is a block diagram of the liquid crystal display device of this invention. It is another block diagram of the liquid crystal display device of this invention. FIG. 6 is still another configuration diagram of the liquid crystal display device of the present invention. It is a disassembled perspective view of the liquid crystal display device of this invention. It is another disassembled perspective view of the liquid crystal display device of this invention. FIG. 10 is still another exploded perspective view of the liquid crystal display device of the present invention. It is a definition diagram of polar angle θ and azimuth angle φ. It is a figure which shows the polar angle (theta) and the azimuth angle (phi) dependence of the transmittance | permeability. It is a figure which shows the azimuth angle (phi) dependence of a brightness | luminance. It is a figure which shows the polar angle (theta) dependence of a brightness | luminance. It is a figure which shows the azimuth angle (phi) and Rth dependence of the transmittance | permeability. It is a figure which shows the azimuth angle (phi) and Rth dependence of a polarization degree. It is a figure which shows the polar angle (theta) and azimuth | direction angle (phi) dependence of the brightness | luminance in VA. It is a figure which shows the azimuth angle (phi) dependence of the brightness | luminance in VA. It is a figure which shows the polar angle (theta) dependence of the brightness | luminance in VA. It is sectional drawing of a polarizing plate. It is a process figure at the time of forming a phase difference plate by application | coating. It is a flowchart which forms a phase difference plate by application | coating. It is sectional drawing of the state which piled up the lower polarizing plate and reflection type polarizing plate at the time of forming a phase difference plate by application | coating. It is another process figure at the time of forming a phase difference plate by application | coating. It is another flowchart which forms a phase difference plate by application | coating. It is sectional drawing at the time of forming a phase difference plate with a base material. FIG. 5 is a cross-sectional view of an example in which the base material is used both as a lower polarizing plate and a reflective polarizing plate, and the base material has a retardation plate function.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Liquid crystal cell, 11 ... Upper polarizing plate, 12 ... Lower polarizing plate, 13 ... Phase difference plate, 14 ... Reflective polarizing plate, 20 ... Back light, 30 ... Polarizer, 31 ... Base material, 100 ... Display area, 311 ... Upper base material, 312 ... Middle base material, 313 ... Lower base material, TR ... Transmission axis, RU ... Rubbing axis, RE ... Slow axis, AB ... Absorption axis, TRR ... Transmission rate, φ ... Azimuth angle, θ ... polar angle.

Claims (13)

An upper polarizing plate is attached to the upper side of an IPS liquid crystal cell, a lower polarizing plate is attached to the lower side, and a backlight is disposed on the back of the lower polarizing plate. ,
Between the lower polarizing plate and the backlight, a reflective polarizing plate and a retardation plate are arranged in this order from the backlight side, and the retardation in the retardation plate is 150 nm to 400 nm. A characteristic liquid crystal display device.   The liquid crystal display device according to claim 1, wherein the phase difference is 200 nm to 300 nm.   The liquid crystal display device according to claim 1, wherein the retardation plate is a negative C-plate.   The liquid crystal display device according to claim 1, wherein the retardation plate is a positive C-plate.   3. The liquid crystal display according to claim 1, wherein the retardation plate is two positive A-plates having one slow axis direction coincident with a lower polarizing plate absorption axis direction and orthogonal to each other. 4. apparatus. An upper polarizing plate is attached to the upper side of a VA liquid crystal cell, a lower polarizing plate is attached to the lower side, and a backlight is disposed on the back of the lower polarizing plate. ,
Between the lower polarizing plate and the backlight, a reflective polarizing plate and a retardation plate are arranged in this order from the backlight side, and the retardation in the retardation plate is 150 nm to 400 nm. A characteristic liquid crystal display device.   The liquid crystal display device according to claim 6, wherein the phase difference is 200 nm to 300 nm.   The liquid crystal display device according to claim 6, wherein the retardation plate is a negative C-plate.   The liquid crystal display device according to claim 6, wherein the retardation plate is a positive C-plate.   8. The liquid crystal display according to claim 6, wherein the retardation plate is two positive A-plates having one slow axis direction coincident with a lower polarizing plate absorption axis direction and orthogonal to each other. apparatus. A liquid crystal display device in which an upper polarizing plate is attached to the upper side of an IPS or VA liquid crystal cell, a lower polarizing plate is attached to the lower side, and a backlight is disposed on the back surface of the lower polarizing plate. Because
Between the lower polarizing plate and the backlight, a reflective polarizing plate and a retardation plate are arranged in this order from the backlight side, and the retardation in the retardation plate is 150 nm to 400 nm,
The liquid crystal display device, wherein the retardation film is formed by applying to the lower polarizing plate. A liquid crystal display device in which an upper polarizing plate is attached to the upper side of an IPS or VA liquid crystal cell, a lower polarizing plate is attached to the lower side, and a backlight is disposed on the back surface of the lower polarizing plate. Because
A reflective polarizing plate is disposed between the lower polarizing plate and the backlight, the lower polarizing plate side base material of the reflective polarizing plate, and the lower polarizing plate side of the reflective polarizing plate side. Any of the base materials has the action of a retardation plate,
The total phase difference of the base material on the lower polarizing plate side of the reflective polarizing plate and the base material on the reflective polarizing plate side on the lower polarizing plate side is in the range of 150 nm to 400 nm. Display device. A liquid crystal display device in which an upper polarizing plate is attached to the upper side of an IPS or VA liquid crystal cell, a lower polarizing plate is attached to the lower side, and a backlight is disposed on the back surface of the lower polarizing plate. Because
A reflective polarizing plate is formed integrally with the lower polarizing plate below the lower polarizing plate,
The lower polarizing plate has a configuration in which a polarizer is sandwiched between an upper substrate and an intermediate substrate, and the reflective polarizing plate has a configuration sandwiched between the intermediate substrate and the lower substrate.
The intermediate substrate has a function of a retardation plate, and the retardation of the intermediate substrate is 150 nm to 400 nm.

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