JP3342940B2 - Liquid crystal display - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device,
In particular, the present invention relates to a super twisted nematic liquid crystal display device (hereinafter, referred to as STN-LCD) in which a contrast ratio depending on a viewing angle is improved by using a retardation film having a birefringence retardation that changes constantly in a vertical direction.

[0002]

2. Description of the Related Art In general, display devices are classified into a projection type such as a cathode ray tube (CRT) and a direct view type such as a liquid crystal display (LCD). In an LCD, liquid crystal is injected between two glass substrates on which electrodes are formed. When the state of alignment of the liquid crystal is changed according to the presence or absence of a voltage applied to the liquid crystal cell, the optical properties of the liquid crystal cell are changed. The image is shown by using the change.

Such an LCD has a simple matrix driving type using an averaging method of a driving voltage and a data signal voltage using time division driving, and an independent active element is attached to each pixel electrode. And an active matrix drive type that independently drives each pixel.

[0004] Here, the simple matrix drive type largely includes a TN-LCD (Twisted Nematic-LCD) and an STN-L.
It is classified as CD (Super Twisted Nematic-LCD).
The STN-LCD shows that when the number of pixels is increased by driving the TN-LCD by simple matrix addressing, the average voltage ratio between the selected pixel and the non-selected pixel becomes low, and the contrast ratio is essentially unsuitable for the display. It is to overcome. In this case, the twist angle is 180
By increasing the angle to between 〜 ° and 270 °, the inclination of the transmission curve due to the voltage is increased near the dislocation voltage in order to give a suitable contrast ratio to the display even in the LCD having the increased number of pixels. At this time, a certain amount of cholesteric (or Chiral) dopant is added to the liquid crystal used to give helical twisting power, and pretilt (pretilt) is added to give structural stability.
) Angles are given in the range of 3 ° to 10 °.

[0005] In such a type of STN-LCD, first, as a standard STN-LCD, two polarizing plates are polarized at 60 ° and 30 ° with respect to the rubbing direction, respectively.
There is a yellow mode STN-LCD that is arranged so as to form an angle and shows black on a yellow background, and a blue mode that shows one white polarizer by rotating one polarizing plate by 90 ° in the yellow mode.

[0006] Standard S which is difficult to colorize
As a device for latching a TN-LCD, there is one that adjusts an optical path difference to indicate black / white.

[0007] This is accomplished by adjusting the optical path difference to a single layer,
Alternatively, a single or double black / white STN- which is manufactured as a double layer for generating an optical path difference and compensating the optical path difference so as to reduce the overall optical path difference to zero so that high contrast and black / white can be displayed. There is an LCD.

The conventional double black / white STN-LCD will now be described with reference to the accompanying drawings.

FIG. 1 shows a conventional double black / white S
FIG. 2 is a component layout diagram of a TN-LCD, and FIG. 2 is a vertical birefringence phase difference graph of a phase delay compensation film in a conventional liquid crystal display device. A conventional double black / white STN-LCD has a liquid crystal cell 1 in which electrodes are formed on two transparent glass substrates and liquid crystal is injected between the two substrates.
And two phase delay compensation films 2 for delaying the phase based on the wavelength of the polarized light, and two polarizing films 3 disposed on both sides of the liquid crystal cell 1 for polarizing the light. As shown in FIG. 1A, a form in which two phase delay compensation films 2 are all arranged on one side of a liquid crystal cell 1,
As shown in FIG. 1B, there is a form in which two phase delay compensation films 2 are arranged one by one in both liquid crystal cells 1.

The above-mentioned phase delay compensation film 2 delays the phase based on the wavelength of the elliptically polarized light passing through the liquid crystal cell 1 and transmits the light immediately before passing through the polarizing film 3 and finally exiting before polarization. It plays the role of restoring to the state.

The black-and-white display is realized by the compensation function of such a system by actually recognizing the birefringence phase difference and twist angle of the liquid crystal cell 1, the phase delay compensation film 2 and the birefringence phase difference (R),
The angle of the optical axis, the angle of the passing axis of the polarizing film 3, etc.
It can be realized by an appropriate and optimized arrangement of various design parameters.

The conventional STN-LC configured as described above
In D, as shown in the graph of FIG. 2, the phase delay compensation film 2 having a constant phase difference in the vertical and horizontal directions is used.

Here, a method of manufacturing the conventional phase delay compensation film 2 will be described. When a polymer layer such as polycarbonate is increased with a uniform strength in a specific direction, the birefringence in the increased direction is increased. The birefringence in a direction perpendicular to the direction becomes different, and the birefringence becomes anisotropic. The birefringence phase difference of this substance is determined by the product of the thickness (d) of the substance and the refractive index anisotropy (Δn).
= About 200 to 500 nm.

FIG. 3 is an explanatory diagram of a general liquid crystal panel and a viewing angle, and FIG. 4 is a graph showing a contrast ratio depending on the viewing angle of a conventional phase delay phase compensation film. The STN-L thus formed with the phase delay compensation film 2
When a liquid crystal panel is actually viewed on a CD, as shown in FIG. 3, a viewing angle difference of about 10 ° appears in the vertical direction, and a birefringent phase difference that produces an optimum contrast ratio is different for each viewing angle position. different.

[0015]

However, in the conventional STN-LCD, a birefringent retardation film having a constant up and down birefringence so as to have a maximum contrast ratio at a viewing angle of 0 ° is used. As shown in FIG. 4, at 0 °, the contrast ratio is high at the viewing angle, but there is a problem that the contrast ratio decreases at the remaining viewing angles.

An object of the present invention is to solve such a problem. In a liquid crystal display device using a phase delay compensation film, the birefringence phase difference of the phase delay compensation film is determined for each part of the compensation film. Another object of the present invention is to improve the contrast ratio based on the viewing angle.

To achieve the above object, the present invention provides
Two polarizing films positioned at the top and bottom of the nematic liquid crystal cell to polarize light, and installed between the nematic liquid crystal cell and each polarizing film, and the birefringence phase difference Ro when the viewing angle is 0 °. And two phase delay compensation films formed such that the birefringence phase difference is constantly increased from the lower side having a lower viewing angle to the upper side having a large viewing angle. 300 nm at 0 °
It has a birefringence phase difference of 450 nm.

The present invention will be described below in more detail with reference to the accompanying drawings.

FIG. 5 is a graph showing the birefringence phase difference of the phase delay compensation film according to the present invention, and FIG. 6 is a graph showing the contrast ratio based on the viewing angle of the phase delay compensation film of the present invention. . As shown in FIG. 1, the STN-LCD structure of the present invention includes a liquid crystal cell in which liquid crystal is injected between two transparent substrates on which electrodes are formed, and a liquid crystal cell that delays a phase based on the wavelength of polarized light. And a phase delay compensating film 2 disposed on both sides of the liquid crystal cell 1 for polarizing light.
And three polarizing films 3. The birefringence retardation of the phase delay compensation film 2 is set to have the characteristics shown in FIG.

More specifically, as shown in FIG. 5, the birefringence phase difference that can maximize the contrast ratio at the horizontal center line of the phase delay compensation film 2, that is, at a viewing angle of 0 °. And the birefringence phase difference Ro is linearly reduced as the viewing angle decreases, and the birefringence phase difference at the lowermost stage of the phase delay compensation film 2 becomes Ro- △ R. Two films are provided on one side or both sides of a liquid crystal cell.

At this time, the birefringence phase difference (Ro) at a viewing angle of 0 ° is set to about 300 nm to 450 nm,
When the effective viewing angle range of the present invention is ± 10 °, the birefringence phase difference ΔR is set to about ５ to 1/12 of the above Ro.

The STN-LCD of the present invention using the phase retardation compensation film having such a birefringence phase difference has a structure shown in FIG.
As can be seen from FIG. 7, the contrast ratio becomes maximum when the birefringence phase difference is Ro−ΔR (FIG. 7A), and thereafter, the contrast ratio decreases as the birefringence phase difference increases.
Then, when approaching Ro after passing the point of Ro-1 / 2 △ R, the contrast ratio sharply increases, and has the maximum contrast ratio again at Ro (FIG. 7B), and the birefringence phase difference is larger than that of Ro. When it gets bigger it starts to fall again.

However, when the contrast ratio rises again from the point of Ro + 1/2 △ R and reaches the point of Ro + △ R, it has the maximum contrast ratio again (FIG. 7C).

Therefore, as can be seen from FIG. 7, according to the present invention, the contrast ratio is maximized at the point where the viewing angle is 0 °. Up to the angle limit (± 10 °), a substantially necessary contrast ratio can be maintained.

[0025]

As described above, as described above, the STN-
In the LCD, the phase retardation compensation film is formed so that the birefringence phase difference linearly decreases as the viewing angle decreases, and the birefringence phase difference increases linearly as the viewing angle increases. Thus, an effect of having a necessary contrast ratio in a wide viewing angle can be obtained.

[Brief description of the drawings]

FIG. 1 is a component layout diagram of a conventional liquid crystal display device.

FIG. 2 is a graph showing a birefringence phase difference in a vertical direction of a phase delay compensation film in a conventional liquid crystal display device.

FIG. 3 is a graph showing a viewing angle of a conventional liquid crystal display device.

FIG. 4 is a graph showing a contrast ratio depending on a viewing angle of a phase delay compensation film in a conventional liquid crystal display device.

FIG. 5 is a graph showing a birefringence phase difference when the phase delay compensation film according to the present invention is used in a liquid crystal display device.

FIG. 6 is a graph showing a birefringence phase difference when a phase delay compensation film according to the present invention is employed in a liquid crystal display device.

FIG. 7 is a graph showing a contrast ratio depending on a birefringence phase difference of the phase delay compensation film according to the present invention.

[Explanation of symbols]

 Reference Signs List 1 liquid crystal cell 2 phase delay compensation film 3 polarizing film

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-323311 (JP, A) JP-A-5-196931 (JP, A) JP-A-5-196930 (JP, A) JP-A-5-196930 225322 (JP, A) JP-A-4-258920 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/13363

Claims (4)

(57) [Claims] 1. Two polarizing films for polarizing light, located at the upper and lower parts of a nematic liquid crystal cell, and disposed between the nematic liquid crystal cell and each polarizing film, when a viewing angle is 0 °. Two phase delay compensation films formed so that the birefringence phase difference is constantly increased from the lower side having a low viewing angle to the upper side having a large viewing angle around the birefringence phase difference Ro; A liquid crystal display device, wherein the film has a birefringence retardation of 300 nm to 450 nm at a viewing angle of 0 °. 2. The nematic liquid crystal cell according to claim 1, wherein
The liquid crystal display device according to claim 1, wherein the liquid crystal display device has a twist angle of 0 °. 3. The viewing angle range of the phase delay compensation film is ± 1.
Assuming 0 °, the birefringence phase difference of the lower surface (−10 °) is Ro−ΔR and the birefringence phase difference of the upper surface (+ 10 °) is Ro + ΔR. ~ Ro /
The liquid crystal display device according to claim 1, wherein the liquid crystal display device has a value of 12. 4. The liquid crystal display device according to claim 1, wherein one phase delay compensation film is formed between the polarizing film on both sides of the liquid crystal cell and the liquid crystal cell.