JP2000321565A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the same display states for both of transmission and reflection modes and to avoid positive-negative inversion by disposing a separation element for P/S polarized light consisting of a selective reflection layer interposed between a pair of 1/4 wavelength plates on the back side of a liquid crystal panel, and further disposing a second polarizing element on the back side. SOLUTION: A separation element 14 for P/S polarized light consisting of a pair of 1/4 wavelength plates 11, 13 and a selective reflection layer 12 interposed between the wavelength plates 11, 13 is disposed on the back side of a liquid crystal panel, and a second polarizing element 15 is further disposed on the back side. The transmittance of the second polarizing element 15 for polarized light with a perpendicular polarization plane is controlled to be lower than that of the separation element 14 for P/S polarized light. By aligning the transmission axis for the polarized light of the separation element 14 for polarized light to transmit S-polarized light, the P- polarized light is cut by the second polarizing element 15 and only the S-polarized light can be transmitted. In order to cut the P-polarized light by the second polarizing element 15, the angle between the polarization axis of the second polarizing element 15 and the direction of the axis of the polarized light transmitted by the separation element 14 for P/S polarized light is specified to the range of 90 deg.±30 deg..

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transflective liquid crystal display device which uses a P / S polarization splitting element having selective reflection and is bright and does not cause positive / negative inversion.

[0002]

2. Description of the Related Art As a flat display, a liquid crystal display device is widely used in various fields. Liquid crystal display devices have been commercialized from small ones such as calculators and watches to large ones as displays for word processors and personal computers because of their thinness, light weight, low power consumption, and ease of colorization. .

In particular, recently, a so-called paper white liquid crystal display device having a white background like paper has been demanded. One candidate for this bright liquid crystal display device is P /
A device using an S-polarized light separating element has been proposed. P /
A liquid crystal display device using an S-polarized light separating element uses a P / S polarized light separating element instead of a polarizing plate on the back side of a liquid crystal panel to reuse light not used in a conventional polarizing plate, and is bright. The display is enabled.

[0004]

However, in the case of a so-called semi-transmissive type, which is a reflective type when the external light is bright, and a transmissive type using a backlight when the external light is dark, as shown in FIG. Since the polarization state of the light used differs between the two, the positive / negative inversion occurs in the display state. That is, in the case of the monochrome display, the background color is white and the display represents a black positive display at the time of reflection, but the background color is black and the display is a white negative display at the time of transmission. In the case of multi-color display, the display color is inverted and changes to a complementary color.
Therefore, this method cannot be applied to a transflective liquid crystal display device that performs a reflective display and a transmissive display with one liquid crystal display device.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has a dielectric anisotropy between two substrates each having a transparent electrode and an alignment film and provided substantially in parallel. A liquid crystal display device comprising: a liquid crystal panel in which nematic liquid crystal having a positive property is sandwiched; a first polarizing element disposed on the front side of the liquid crystal panel; and a P / S polarization separating element and a backlight on the back side of the liquid crystal panel. In the above P / S
A polarization splitting element including a pair of quarter-wave plates and a selective reflection layer sandwiched therebetween, a second polarizing element disposed between the liquid crystal panel and the backlight; The orthogonal transmittance is lower than the orthogonal transmittance of the P / S polarization separation element, and the angle between the polarization axis of the second polarization element and the transmission polarization axis of the P / S polarization separation element is 90 ° ± 30 °.
It is characterized by being. With such a configuration, the same display state is obtained in both transmission and reflection,
No positive / negative reversal occurs.

Further, the second polarization element is disposed between the P / S polarization separation element and a backlight. With such a configuration, a brighter display can be obtained.

Further, the orthogonal transmittance of the second polarizing element with respect to the main wavelength component of the backlight is lower than the orthogonal transmittance of the P / S polarization splitting element. With such a configuration, options for the second polarizing element are increased, and the application range of the liquid crystal display device is widened.

The liquid crystal panel is 180-360.
It is a super twisted type liquid crystal panel having a twist angle of °. With such a configuration, the display capacity is increased, and the use of the liquid crystal display device can be expanded.

Furthermore, the product Δnd of the refractive index anisotropy Δn of the liquid crystal of the liquid crystal layer of the liquid crystal panel and the thickness d of the liquid crystal layer is 1.
0 to 1.5 μm, and by selectively applying three or more different voltage values to the transparent electrode,
It is characterized in that colors are generated in the same display unit.
With such a configuration, multicolor display can be performed without using a color filter, and the display color is not inverted to a complementary color in any state.

In addition, it is characterized in that the P / S polarization separation element is a polarization separation element having a diffusion layer on the surface on the liquid crystal panel side. With such a structure, the viewing angle of the liquid crystal display device can be increased, and a display with improved visibility with a hue of whitish can be obtained.

[0011]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. FIG. 1 is a conceptual diagram illustrating the principle of the positive / negative reversal prevention of the present invention, and FIG. 2 is a conceptual diagram illustrating the conventional positive / negative reversal (a polarizing element and a liquid crystal panel on the front side are not shown). .

First, referring to FIG. 2, a P /
The S polarization splitting element will be briefly described. The P / S polarization splitting element 24 includes a selective reflection layer 22 and two quarter-wave plates 21 and 23, and has a structure in which the selective reflection layer is sandwiched between quarter-wave plates.

[0013] The selective reflection is a phenomenon that, in a spirally wound element such as a cholesteric liquid crystal, when the pitch of the spiral is on the order of the wavelength of light, one circularly polarized light is transmitted and the other circularly polarized light is reflected. Also, a quarter-wave plate is
Although it is a kind of retardation plate, it has a retardation value for converting linearly polarized light to circularly polarized light. However, depending on the vibration direction of the linearly polarized light incident on the quarter-wave plate, the light may be right circularly polarized light or left circularly polarized light.

For this reason, for example, as shown in FIG. 2A, P-polarized light is incident on the P / S polarized light separating element 24, and the quarter-wave plate 21 is installed in a direction for converting the incident P-polarized light into right circularly polarized light. When the selective reflection layer 22 is configured to transmit right circularly polarized light and reflect left circularly polarized light, P polarized light is transmitted through the element 24 and S polarized light is reflected to form a P / S polarized light separating element. Become. On the other hand, as shown in FIG.
The same phenomenon as in (a) occurs, P-polarized light passes through this element, and S-polarized light is reflected.

For this reason, when this P / S polarization splitting element 24 is used in place of the back side polarizing plate, S
In the case of polarized light and transmission, P-polarized light is the main use light.
Negative reversal had occurred. Therefore, in order to prevent the positive / negative inversion, it is only necessary to use one of the polarized light at the time of reflection and at the time of transmission. Specifically, S-polarized light may be used, and when reflected, S-polarized light is used as it is.
At the time of transmission, only S-polarized light may be transmitted.

That is, as shown in FIG.
A second polarizing element 15 is arranged on the back side of a P / S polarization separating element 14 composed of wavelength plates 11 and 13 and a selective reflection layer 12 sandwiched between the quarter-wave plates. If the orthogonal transmittance is lower than the orthogonal transmittance of the P / S polarization separation element 14 and the transmission polarization axis of the polarization separation element 14 is adjusted so that S-polarized light is transmitted, the P-polarized light can be converted into the second polarization element. The light is cut at 15, and only S-polarized light can be transmitted. In the case of the P / S polarization splitting element having selective reflection shown in FIG. 5, about 10% of S-polarized light is transmitted on average.

Whether the light transmitted is mainly S-polarized light or mainly P-polarized light depends on the degree of polarization separation of the P / S polarization separation element, the degree of polarization of the second polarization element, and the polarization axis angle of both. You can decide on a balance. In order to increase the S-polarized component as much as possible during transmission, a second polarizing element having a degree of polarization as high as possible is used. In particular, it is preferable that the orthogonal transmittance of the second polarizing element is lower than the orthogonal transmittance of the P / S polarization splitting element.

Further, when transmitted, it is preferable that S-polarized light is transmitted at all wavelengths of visible light, but it is sufficient that S-polarized light is transmitted at a specific wavelength or a specific wavelength region of visible light. The same effect can be obtained by using a light source having these wavelengths or wavelength regions as main wavelength components for the backlight used in the present invention.

In order to cut the P-polarized light by the second polarizing element 15, the angle θ1 between the polarization axis 46 of the second polarizing element and the transmitted polarization axis direction 45 of the P / S polarized light separating element is 9
It is important that the angle is in the range of 0 ° ± 30 °. Outside of this range, positive / negative reversal occurs during transmission and reflection, or a display with low contrast occurs even without positive / negative reversal. More preferably 9
0 ° ± 10 °.

It is important that the P / S polarization splitting element used in the present invention is a P / S polarization splitting element having selective reflectivity. This P / S polarization splitting element has a pair of 1
It consists of a selective reflection layer sandwiched between wavelength plates. Here, the pair of quarter-wave plates are quarter-wave plates arranged on the front and back sides of the selective reflection layer, and are arranged such that their slow axes are orthogonal to each other.

The P / S polarization splitting element 14 shown in FIG.
Is taken as an example, the transmission polarization axis of the P / S polarization splitting element and 1/4
The relationship with the slow axis of the wave plate will be briefly described. As shown in FIG. 1, it can be seen that the P-polarized light has a vibration direction parallel to the paper surface, and the transmission polarization axis of the P / S polarized light separation element is also in a direction parallel to the paper surface. FIG. 3 shows an axial direction when FIG. 1A is viewed from above, and reference numeral 45 denotes a transmission polarization axis of the P / S polarization separation element. In order to convert the P-polarized light of FIG. 1 to right-handed circularly polarized light with the front side (upper part of the drawing) quarter-wave plate, the slow axis 31 is arranged at an angle of 45 ° with the transmission polarization axis 45. Have been.

The selective reflection layer may have any property of selectively reflecting circularly polarized light, and examples thereof include a cholesteric liquid crystal layer, a discotic liquid crystal layer, and a polymer liquid crystal layer in which the liquid crystal layer has a helical structure. A cholesteric liquid crystal layer in which the helical pitch tends to be on the order of the wavelength of light is preferable in terms of selective reflection performance. Further, a selective reflection layer having a uniform helical structure may be used, but a mixture of a plurality of helical pitches,
It is preferable to adopt a structure in which layers having different helical pitches are superposed on any number of layers, because wavelength dependence and angle dependence of selective reflectivity can be reduced.

Further, in order to diffuse S-polarized light and enlarge the viewing angle of the liquid crystal display device, it is preferable that the P / S polarized light separating element is a polarized light separating element having a diffusion layer on the surface on the liquid crystal panel side.

The polarizing element in the present invention may have a polarizing function. For example, a crystal polarizer using calcite or the like may be used, but a film-shaped polarizing plate is preferably used in practical use.

The arrangement relationship between the P / S polarization separation element and the second polarization element is such that the liquid crystal panel, the P / S polarization separation element, the second polarization element, and the backlight are arranged in this order from above, so that the reflection Although a brighter liquid crystal display device can be obtained, it is preferable that the liquid crystal panel, the second polarizing plate, the P / S polarized light separating element, and the backlight have sufficient effects in this order.

The liquid crystal panel to which the present invention can be applied is not particularly limited, and may be an ordinary twisted nematic (TN) type,
A super twisted nematic (STN) type using a liquid crystal to which a chiral substance is added so that a twist angle becomes 180 to 360 ° and a TFT type are exemplified. In particular, portable communication devices as applications for transflective liquid crystal display devices,
For example, they are often used for mobile phones, pagers, etc., and are advantageous in terms of large display capacity and ease of manufacture.
The effect is remarkable when applied to the TN type.

The present invention can be applied to a multi-color display system, and can provide a multi-color display device which does not reverse the display color when switching from reflection to transmission, ie, does not cause a change to a complementary color. The method of multi-color display is not limited. In addition to the method using a color filter, a method in which three or more colors are formed in the same display unit using a change in retardation of a liquid crystal layer due to an applied voltage without using a color filter. The present invention is also applicable to a color liquid crystal display device. Here, the same display unit is one segment formed by opposing electrodes in the segment display unit, and is one dot formed by opposing electrodes in the dot matrix display unit.

Known materials and techniques used in ordinary liquid crystal display devices can be applied as long as the effects of the present invention are not impaired.

[0029]

EXAMPLE 1 (Example) In the configuration of FIG. 4, an STN element having a twist angle of 240 ° and a Δnd of a liquid crystal layer of 0.86 μm is used as the liquid crystal panel 16, and a retardation value of 585 n is used as the retardation plate 17.
m of polycarbonate was used.

As the P / S polarization splitting element 14, the front side of the cholesteric liquid crystal layer (selective reflection layer) (above the drawing)
A transmax (trade name) manufactured by Merck and having a 1/4 wavelength plate adhered thereto, and a 1/4 wavelength plate attached to the back side thereof, and the slow axis of the 1/4 wavelength plate. Are arranged orthogonally.

The polarizing plate NPF-F1225DU manufactured by Nitto Denko is used as the front polarizing element (first polarizing element) 18, and the high polarizing degree (99.95%) is used as the back polarizing element (second polarizing element) 15. Nitto Denko polarizing plate NPF-EG14
25DU was used. A white LED was used as the backlight 19 used at the time of transmission. And this P / S
Transmission polarization axis 45 of polarization separation element and polarization axis 4 of back-side polarizing plate
6 was 90 °.

The polarization axes of the front and back polarizing plates, the optical axes of the retardation plates,
FIG. 5 shows the relative relationship between the transmission polarization axis of the P / S polarization separation element and the rubbing direction of the liquid crystal panel. With such a configuration, display in the positive mode can be performed. Here, θ2 is the twist angle of the liquid crystal panel,
3, θ4 are the angles of the phase difference plate and the front-side polarizing plate measured from the rubbing direction on the front side, and are 90 ° and 140 °, respectively.
Θ5 and θ6 are P / P measured from the rubbing direction on the back side.
The angles of the S-polarized light separating element and the back-side polarizing plate were set at 45 ° and 135 °, respectively.

FIGS. 6 and 7 show the P / S polarization separating element (Transmax + 1 manufactured by Merck) used in this embodiment.
/ 4 wavelength plate), polarizing plate on the back side (Nitto Denko polarizing plate NPF)
-EG1425DU) in a parallel arrangement and an orthogonal arrangement. As is clear from the transmittances of the orthogonal arrangement in FIGS. 6 and 7, the P / S polarization splitter has a higher orthogonal transmittance than the polarizing plate on the back side in almost all wavelength regions of visible light. Therefore, the S-polarized light has a P / S of this configuration.
The light can pass through the polarization separation element.

This liquid crystal display device is 1/65 duty,
When driven with a 1/6 bias, the display at the time of reflection was a positive display, bright, and had an excellent appearance with a contrast ratio of 8. In addition, the display at the time of transmission using a backlight was also a positive display, and a display without positive / negative inversion was obtained.

Example 2 (Example) The same configuration as in Example 1 was adopted except that the angle of attachment of the polarizing plate on the front side was rotated by 90 degrees with respect to Example 1 and the angle θ4 in FIG. This time the display was in negative mode,
A display without positive / negative reversal was obtained in both reflection and transmission.

Example 3 (Example) The structure was exactly the same as that of Example 1 except that the rear polarizing plate was disposed between the liquid crystal panel and the P / S polarization splitting element. In this case, the brightness at the time of reflection is slightly reduced as compared with Example 1, but the transmittance at the time of transmission is almost the same as that of Example 1.
A display with no inversion of positive / negative was obtained.

Example 4 (Example) The same configuration as in Example 1 was used, and a blue LED having a main wavelength component near 450 nm was used as a backlight. FIG.
As shown in the above, since the orthogonal transmittance of the P / S polarization separation element near 450 nm is about 20%, a brighter display than that of the white LED used in Example 1 is obtained, and the positive / negative light is obtained. There was no inversion.

Example 5 (Embodiment) As another embodiment of the present invention, the configuration shown in FIG. 8 is used, and the twist angle of the liquid crystal panel 16 is 90 ° and the liquid crystal layer Δnd is 1.
A TN type liquid crystal panel of 2 μm was used. The other members were the same as in Example 1, and the polarizing plate 18 and the P / S polarization splitting element 14 on the front side were arranged such that their transmission axes were parallel to the rubbing direction so as to be in a normally white mode.

This liquid crystal display device is 1/8 duty, 1
When driven with a / 4 bias, the same effect as the STN panel was obtained in the TN panel. The display at the time of reflection was a positive display, and was bright and had an excellent appearance with a contrast ratio of 20. In addition, the display at the time of transmission is also a positive display, and display without positive / negative inversion is possible.

Example 6 (Embodiment) As another embodiment of the present invention, as shown in FIG. 9, the structure is the same as that of Example 1 except that the diffusion plate 10 is arranged on the front side (upper side) of the P / S polarization splitting element 14. did. Even in this case, a display without positive / negative reversal was obtained by reflection and transmission, but at the time of reflection, the specular reflection due to selective reflection was diffused by the diffusion plate, the hue became whitish, and the display with better visibility Became possible.

Example 7 (Example) In the configuration of FIG. 4, an STN type liquid crystal panel having a twist angle of 240 ° and a liquid crystal layer Δnd of 1.27 μm is used as the liquid crystal panel 16, and a retardation value of 1490 nm is used as the retardation plate 17. Polycarbonate was used respectively. Also, θ3 and θ4 in FIG.
The configuration was the same as that of Example 1 except that 0 ° and θ5 and θ6 were set to 145 ° and 55 °, respectively.

This liquid crystal display device is 1/65 duty,
When driving with a ５ bias and applying a quaternary driving voltage, four colors of red, blue and green are displayed on a white background during reflection,
The display was bright and of high display quality. In addition, almost the same color was displayed during transmission, and a bright display without positive / negative inversion was obtained.

Example 8 (Comparative Example) The configuration of Example 1 was the same except that the rear polarizing plate 15 was omitted. The obtained liquid crystal display device was driven under the same driving conditions as in Example 1, and when the display state was confirmed, a positive display with a contrast ratio of 5 was obtained at the time of reflection, but a negative display was obtained at the time of transmission.

Example 9 (Comparative Example) The configuration of Example 4 was the same except that the backlight 19 was a white backlight. When the display state of this liquid crystal display device was confirmed, a positive display was obtained during reflection and a negative display was obtained during transmission, and positive / negative inversion occurred.

Example 10 (Comparative Example) The structure of Example 5 was the same as that of Example 5, except that the rear polarizing plate 15 was omitted. When the display state of the obtained liquid crystal display device was confirmed, the display state at the time of reflection was a positive display and the contrast ratio was 8, but the display state at the time of transmission was a negative display.

[0046]

According to the structure of the present invention, it is possible to eliminate the phenomenon that the positive / negative display is inverted between reflection and transmission when only the conventional P / S polarization separation element is used. As a result, the application range can be expanded.

Further, according to the structure of the present invention, the effect of greatly increasing the contrast at the time of reflection was obtained. Conventionally, in the configuration using only the P / S polarization separation element using selective reflection in the reflection mode, the transmitted P-polarized light is reflected by a backlight or the like and returns to the observer side again, thereby lowering the contrast. It is because of that. On the other hand, according to the configuration of the present invention, since the P-polarized light is almost completely blocked by the rear polarizer at the time of reflection, almost no P-polarized light returns to the observer side, and the contrast ratio is greatly improved. There is also an effect that can be done. further,
When the second polarizing element is arranged behind the P / S polarization splitting element, brighter display is possible.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram illustrating the principle of preventing positive / negative reversal according to the present invention.

FIG. 2 is a conceptual diagram illustrating a conventional positive / negative inversion.

FIG. 3 is a diagram illustrating a relationship between a transmission polarization axis of a P / S polarization separation element and a delay axis of a quarter-wave plate according to an embodiment of the present invention.

FIG. 4 is a schematic sectional view illustrating an embodiment of the present invention.

FIG. 5 is a diagram showing the relationship between the optical axes of various optical elements according to an embodiment of the present invention.

FIG. 6 is a diagram of a spectral transmittance of a P / S polarization splitting element used in one embodiment of the present invention.

FIG. 7 is a diagram showing the spectral transmittance of a back-side polarizing plate used in one example of the present invention.

FIG. 8 is a schematic cross-sectional view illustrating one embodiment (Example 5) of the present invention.

FIG. 9 is a schematic cross-sectional view illustrating one embodiment (Example 6) of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 Diffusion plate 11 1/4 wavelength plate 12 Selective reflection layer 13 1/4 wavelength plate 14 P / S polarization splitting element 15 2nd polarizing element (back side polarizing plate) 16 Liquid crystal panel 17 Phase difference plate 18 1st polarizing element (Front-side polarizing plate) 19 Backlight 31 Delay axis of front quarter-wave plate 32 Delay axis of rear quarter-wave plate 41 Rubbing direction of front substrate 42 Rubbing direction of rear substrate 43 Optical axis direction of retardation plate 44 Polarization axis direction of front side polarizing plate 45 Transmission polarization axis direction of P / S polarization separation element 46 Polarization axis direction of back side polarization plate θ1 Between polarization axis of second polarization element and transmission polarization axis of P / S polarization separation element Angle θ2 Twist angle of liquid crystal panel θ3 Angle of optical axis of retardation plate measured counterclockwise from front rubbing direction θ4 Angle of polarization axis of front side polarizing plate measured counterclockwise from front rubbing direction θ5 P / measured counterclockwise from the back side rubbing direction
Angle of transmission polarization axis of S-polarized light separating element θ6 Angle of polarization axis of back-side polarizing plate measured counterclockwise from rubbing direction of back side

Claims (6)

[Claims] 1. A liquid crystal panel in which a nematic liquid crystal having a positive dielectric anisotropy is sandwiched between two substrates each having a transparent electrode and an alignment film and provided substantially in parallel, and disposed on a front side of the liquid crystal panel. In a liquid crystal display device having a first polarizing element to be formed and a P / S polarization separating element and a backlight on the back side of the liquid crystal panel, the P / S polarizing separating element is sandwiched between a pair of quarter-wave plates. A second polarizing element is disposed between the liquid crystal panel and the backlight, and the orthogonal transmittance of the second polarizing element is equal to the orthogonal transmittance of the P / S polarization splitting element. A liquid crystal display device wherein the angle between the polarization axis of the second polarization element and the transmission polarization axis of the P / S polarization separation element is 90 ° ± 30 °. 2. The apparatus according to claim 1, wherein said second polarizing element is disposed between said P / S polarization separating element and a backlight.
3. The liquid crystal display device according to 1. 3. The liquid crystal display device according to claim 1, wherein the orthogonal transmittance of the second polarizing element with respect to the main wavelength component of the backlight is lower than the orthogonal transmittance of the P / S polarization splitting element. 4. The liquid crystal display device according to claim 1, wherein the liquid crystal panel is a super twisted type liquid crystal panel having a twist angle of 180 to 360 °. 5. The product Δnd of the refractive index anisotropy Δn of the liquid crystal of the liquid crystal layer of the liquid crystal panel and the thickness d of the liquid crystal layer is 1.0 to 1.0.
The liquid crystal display device according to claim 4, wherein at least three colors are generated in the same display unit by selectively applying three or more different voltage values to the transparent electrode. 6. The P / S polarization separation element is a polarization separation element having a diffusion layer on a surface on a liquid crystal panel side.
6. The liquid crystal display device according to 2, 3, 4, or 5.