JP2006064907A - Transmissive color liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To further improve contrast by reducing reflection on a color filter. <P>SOLUTION: This transmissive color liquid crystal display device comprises: a liquid crystal layer 17 which adjusts light transmittance by changing a plane of polarization of linearly polarized light passed through a lower side polarizing plate 18 and which is disposed between the lower side polarizing plate 18 of the light source side and an upper side polarizing plate 19 of the display side; and a color filter 20 disposed between the liquid crystal layer 17 and the upper side polarizing plate 19. A quarter wave plate 21 which rotates a plane of polarization of external light having passed through the upper side polarizing plate 19 and reflected on the color filter 20 by 90° so as to be absorbed with the upper side polarizing plate 19 is disposed between the upper side polarizing plate 19 and the color filter 20, and further another quarter wave plate 22 is disposed on the lower side of the color filter 20. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a transmissive color liquid crystal display device in which the contrast of a display image is increased by reducing reflection of external light.

  Currently, transmissive color liquid crystal display devices are widely used as displays for various devices. Among these, a TN (Twisted Nematic) mode liquid crystal display device that is widely used generally constitutes a liquid crystal layer by encapsulating liquid crystal molecules in a state twisted 90 degrees between two glass substrates having transparent electrodes, The liquid crystal layer has a structure in which two polarizing plates are arranged orthogonally outside the glass substrate. Image display in this type of liquid crystal display device is performed by changing the output of light passing through the liquid crystal by changing the orientation of the liquid crystal molecules by adjusting the voltage applied between the two substrates.

  That is, when the applied voltage is lower than the threshold value, the polarization direction of the linearly polarized light that has passed through the incident side polarizing plate is rotated (rotated) by 90 degrees along the twist of the liquid crystal molecules. A bright state is displayed by passing through the exit-side polarizing plate arranged orthogonal to the plate (white display). On the other hand, when the applied voltage becomes higher than the threshold value, the major axis of the liquid crystal molecule starts to rise from the center between the electrodes, and the optical rotation begins to decrease. When the applied voltage is further increased, the optical rotatory power finally disappears, and the linearly polarized light that has passed through the incident side polarizing plate reaches the outgoing side polarizing plate without rotating, so that it cannot be emitted and a dark state is displayed. (Black display).

  By the way, in such a liquid crystal display device, there is a problem that, for example, external light from a room or a window is reflected on the image display surface, so that, for example, black tightening at the time of black display is deteriorated and contrast is lowered. It was. The main causes are reflection at the air interface on the outermost surface of the display (surface reflection) and reflection at the color filter portion (internal reflection). So far, the former has generally had a higher reflectivity than the latter, and as a countermeasure, for example, a low reflectivity film incorporating a multilayer interference effect has been applied to the display surface to reduce the reflectivity.

  However, when the surface reflectance at the air interface is kept below a certain level by applying a low reflectance film, the internal reflection on the latter color filter surface, especially the reflection on the black matrix surface, improves the contrast. It has become a new problem in planning.

  Conventionally, as a black matrix of a color filter, (1) a relief formed by photoetching a chromium thin film formed by vapor deposition, sputtering, etc., (2) an electrodeposition of black electrodeposition paint, (3) Although there was what was formed by printing, etc. (for example, refer to patent documents 1), until now, there was no thing which reduced reflectance enough, ensuring the required dimensional accuracy, and the countermeasure is needed. It has become.

JP 2002-371204 A

  As described above, in the past, of the reflection of external light on the image display surface, a sufficient countermeasure has been taken for the surface reflection at the air interface, but for internal reflection on the color filter, Sufficient measures were not taken. Therefore, in order to further improve contrast, it has become important to solve the problem of internal reflection.

  In view of the above circumstances, an object of the present invention is to provide a transmissive color liquid crystal display device capable of reducing reflection on a color filter and further improving contrast.

  The transmissive color liquid crystal display device according to claim 1 is provided between a light source side lower polarizing plate, a display side upper polarizing plate, and between the lower polarizing plate and the upper polarizing plate, and transmits the lower polarizing plate. A transmissive color liquid crystal display device comprising: a liquid crystal panel for adjusting light transmittance by changing a polarization plane of the linearly polarized light; and a color filter provided between the liquid crystal panel and the upper polarizing plate, wherein the upper polarizing plate And a phase plate that rotates a polarization plane of external light that is transmitted through the upper polarizing plate and reflected by the color filter to an angle that can be absorbed by the upper polarizing plate.

  The transmissive color liquid crystal display device according to claim 2 is configured such that the phase plate is formed of a single quarter-wave plate and the polarization plane of the external light is changed to 90 by the reciprocating passage of the external light transmitted through the upper polarizing plate. Rotate.

  The transmissive color liquid crystal display device according to claim 3 further includes another quarter-wave plate between the color filter and the liquid crystal panel.

  According to a fourth aspect of the present invention, the liquid crystal panel is of a TN type, an IPS type, or a VA type.

  According to the first aspect of the invention, the phase plate is disposed between the upper polarizing plate and the color filter, and the polarization plane of the external light that is transmitted through the upper polarizing plate and reflected by the color filter is rotated by a predetermined angle. Therefore, the reflected light on the color filter can be absorbed by the upper polarizing plate. In this case, the external light (linearly polarized light) that has passed through the upper polarizing plate passes through the phase plate twice, going forward and backward, so that the angle at which the light cannot pass through the upper polarizing plate through the two passes. It is necessary to.

  Therefore, in the second aspect of the present invention, one quarter wavelength plate (λ / 4 plate) is used as the phase plate. When passing through the quarter-wave plate twice, the polarization plane of the linearly polarized light (external light) transmitted through the upper polarizing plate is rotated by 90 °. Therefore, since this light becomes polarized light having a polarization plane orthogonal to the upper polarizing plate, it cannot pass through the upper polarizing plate, and the reflected light does not return to the viewer side. That is, reflection can be reliably prevented.

  By the way, when the quarter wavelength plate is provided on the color filter in this way, light that has passed through the liquid crystal panel reaches the upper polarizing plate after passing through one quarter wavelength plate. The display may not be performed properly. Therefore, in the invention of claim 3, another polarization plate is placed under the color filter and the two polarization plates are passed through, thereby rotating the polarization plane of the transmitted light by 90 °. Let By doing so, light that has passed through the liquid crystal panel can be transmitted through the upper polarizing plate, and an appropriate display can be achieved.

Hereinafter, embodiments of the present invention will be described.
FIG. 1 is a schematic cross-sectional view of a transmissive color liquid crystal display device for explaining an embodiment of the present invention.
The transmissive color liquid crystal display device 100 transmits light by changing the polarization plane of linearly polarized light transmitted through the lower polarizing plate 18 between the lower polarizing plate 18 on the light source 11 side and the upper polarizing plate 19 on the display side. A liquid crystal panel 17 that adjusts the rate (switching control in the case of ON / OFF) is arranged, and a λ / 4 plate (a quarter wavelength plate) as a phase plate up and down between the liquid crystal panel 17 and the upper polarizing plate 19 The color filter 20 provided with 21 and 22 is arranged, and the backlight unit 10 is arranged below the lower polarizing plate 18. The liquid crystal panel 17 includes a liquid crystal layer and glass or driving electrodes sandwiching the liquid crystal layer.

  The backlight unit 10 is of an edge light type, and includes a light source 11 including a cold cathode tube for illuminating the liquid crystal panel 17 from the back side, a reflection plate 12 that reflects light from the light source 11, and the light source 11. And a light guide plate 14 for directing the light reflected by the reflection plate 12 toward the liquid crystal panel 17, and a diffusion plate for returning the light emitted from the light guide plate 14 to the light guide plate 14 or the liquid crystal panel 17 side. (Or reflector) 13. Further, between the light guide plate 14 and the lower polarizing plate 18, a diffusion film 15 that diffuses the light emitted from the light guide plate 14, and the light diffused by the diffusion film 15 is collected and incident on the liquid crystal panel 17. A condensing filter 16 is provided. A diffusion plate (or reflection plate) 13, a light guide plate 14, a diffusion film 15, a condensing filter 16, a lower polarizing plate 18, a liquid crystal panel 17, a λ / 4 plate 22, a color filter 20, a λ / 4 plate 21, The upper polarizing plate 19 is laminated in this order.

  As the liquid crystal panel 17, a twisted nematic (TN) type, a super twisted nematic (STN) type, an IPS type, or a VA type is used. The color filter 20 includes red (R), green (G), and blue (B) filters and a black matrix disposed in the gap.

  The lower polarizing plate 18 has a function of transmitting a specific polarization component in the omnidirectional light from the condensing filter 16, and the upper polarizing plate 19 has a function of transmitting a specific polarization component. Here, the polarization directions of the lower polarizing plate 18 and the upper polarizing plate 19 are set to be equal (indicated by the left and right arrows in the drawing) and are not orthogonal.

Next, the operation will be described.
The reflected light from the color filter 20 when outside light is incident through the upper polarizing plate 19 passes through the λ / 4 plate 21 twice in a reciprocating manner, so that the polarization plane is linearly polarized with respect to the linearly polarized light of the incident light. Converted. Therefore, the reflected light is absorbed by the upper polarizing plate 19 and cannot reach the eyes of the observer. From the above, liquid crystal display with high contrast is possible by suppressing reflection of external light. Here, the polarization plane of the external light is rotated by 90 ° using the λ / 4 plate 21, but the rotation angle is not necessarily 90 °, and the upper polarizing plate 19 reflects the light from the color filter 20. Any angle that can absorb light to an inconspicuous extent is acceptable. That is, the effect can be obtained by using a phase plate capable of rotating the polarization plane by the angle instead of the λ / 4 plate.

  On the other hand, the light from the backlight unit 10 is polarized by the lower polarizing plate 18, and only the light in the horizontal direction in FIG. 1 enters the liquid crystal panel 17. The light transmittance of the liquid crystal panel 17 is adjusted by a voltage applied between transparent electrodes (not shown). That is, the molecular arrangement of the liquid crystal is changed by the applied voltage, and the light transmittance is adjusted. In the case of white display, the polarization plane rotates by 90 ° when passing through the liquid crystal panel 17, and in the case of black display, the polarization plane does not change. When the light that has passed through the liquid crystal panel 17 passes through the λ / 4 plates 21 and 22 twice before and after passing through the color filter 20, the polarization plane rotates by 90 °. In the case of black display, the light passes through the polarizing plate 19 and is absorbed by the upper polarizing plate 19 so that black and white display is accurately performed.

  In order to suppress reflection of outside light, the λ / 4 plate 22 is not essential. However, without the λ / 4 plate 22, the light that has passed through the liquid crystal panel 17 passes only once through the λ / 4 plate 21. The polarizing plate 19 cannot pass through and accurate display cannot be performed. In view of this, in the liquid crystal display device 100, the λ / 4 plate 22 is provided between the liquid crystal panel 17 and the color filter 20, so that the display can be performed accurately as described above. Here, the polarization directions of the lower polarizing plate 18 and the upper polarizing plate 19 are set to be equal, but they may be in an orthogonal relationship. For example, when the polarization direction of the upper polarizing plate 19 is a direction perpendicular to the paper surface, the black display becomes white display and the white display becomes black display. It is possible to display an image accurately.

  Further, since the liquid crystal display device 100 uses a TN type, STN type, IPS type, VA type or the like as the liquid crystal panel 17, it can display an image even without the λ / 4 plates 21 and 22. Is possible. The purpose of providing the λ / 4 plate 21 is to improve contrast, and the purpose of providing the λ / 4 plate 22 is to prevent images from being displayed due to the provision of the λ / 4 plate 21. .

  In the above-described embodiment, the case where the λ / 4 plates 21 and 22 are used as the phase plates is shown, but other phase plates may be used.

Hereinafter, the effect of the present invention will be proved by examples.
As a transmissive color liquid crystal display device (referred to as device A) for the example, a λ / 4 plate 21 is attached to a glass substrate with a color filter, and λ / 4 plates 21 are provided above and below the color filter. As a transmissive color liquid crystal display device (referred to as device B) for a comparative example, a device without a λ / 4 plate above and below the color filter was prepared. For the device A, the distance between the upper polarizing plate 19 and the λ / 4 plate 21 was increased in order to eliminate reflection on the surface with respect to reflectance measurement. In this case, the degree of separation is such that the regular reflection light on the upper polarizing plate 19 and the regular reflection light on the color filter 20 are separated when the light source is at a position of 45 degrees with respect to the normal of the display surface. The position that can be measured in this way was determined experimentally, so that the determined position was reached.
Then, a fluorescent lamp (10000 cd / m ² ) is installed at a position where the incident angle is 45 ° from the vertical to the display surfaces of the devices A and B, and the specularly reflected light on the color filter 20 is Each parallel light reflectance (%) was measured. In addition, when the devices A and B were turned off, a white plate with a brightness of 100 cd / m ² was projected from a position of 45 ° from the vertical with respect to the display surfaces of both devices, and the visual reflection was evaluated. .

  As a result of the measurement, the parallel light reflectance is 0.42% for the device A and 2.7% for the device B, and the configuration in which two λ / 4 plates are provided with the color filter sandwiched as in the present invention. It was found that the reflectance can be suppressed. Further, as a result of the evaluation, a result of ○ for the device A (not worried about the reflection) and a result of × (not worried about the reflection) for the device B is obtained. It was also found that the configuration with two λ / 4 plates can suppress the reflection.

1 is a schematic cross-sectional view of a transmissive color liquid crystal display device for explaining an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Transmission type color liquid crystal display device 11 Light source 17 Liquid crystal layer 18 Lower polarizing plate 19 Upper polarizing plate 20 Color filter 21,22 λ / 4 plate (1/4 wavelength plate, phase plate)

Claims (4)

A light source side lower polarizing plate, a display side upper polarizing plate, and provided between the lower polarizing plate and the upper polarizing plate, change the polarization plane of linearly polarized light that has passed through the lower polarizing plate, and transmit light. A transmissive color liquid crystal display device including a liquid crystal panel for adjusting a rate, and a color filter provided between the liquid crystal panel and the upper polarizing plate,
A transmission type comprising a phase plate between the upper polarizing plate and the color filter that rotates a polarization plane of external light that is transmitted through the upper polarizing plate and reflected by the color filter to an angle that can be absorbed by the upper polarizing plate. Color liquid crystal display device. The transmissive color liquid crystal display device according to claim 1,
A transmissive color liquid crystal display device in which the phase plate is composed of one quarter-wave plate, and the polarization plane of the external light is rotated by 90 ° by the reciprocating passage of the external light transmitted through the upper polarizing plate. The transmissive color liquid crystal display device according to claim 2,
A transmissive color liquid crystal display device further comprising another quarter wavelength plate between the color filter and the liquid crystal panel. The transmissive color liquid crystal display device according to claim 1,
A transmissive color liquid crystal display device, wherein the liquid crystal panel is of a TN type, an IPS type, or a VA type.

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