JPH04153622A - Tn type liquid crystal display element - Google Patents

Abstract

PURPOSE:To obtain a good visual angle characteristic which fewer phase plates by interposing one sheet of the phase plate having the optical axis in the direction nearly equal to the transmission axis direction of a 1st polarizing plate between the 1st polarizing plate and a TN type liquid crystal cell. CONSTITUTION:One sheet of the phase plate 22 having the optical axis 24 nearly equal to the transmission axis direction of the 1st polarizing plate 12 is interposed between the 1st polarizing plate 12 and the TN type liquid crystal cell 11. A retardation arises between the vibration (1st light) of the light in the optical axis 24 direction of the phase plate 22 and the vibration (2nd light) of the light in the direction orthogonal therewith and the polarization state of the light changes if incident light passes the phase plate 22. Only the 1st light vibrating in the optical axis 24 direction of the phase plate 22 is, however, transmitted, if the transmission axis directions 16, 17 of the polarizing plates 12, 13 are parallel with the optical axis 24 direction of the phase plate 22 and, therefore, the transmission quantity of the light is constant regardless of the presence or absence of the retardation. The reduction of the phase plate 22 to one sheet is possible in this way and the stage for sticking films is simplified. The production is facilitated and this initial cost is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a TN (twisted nematic) type liquid crystal display element, and particularly relates to improvement of viewing angle characteristics thereof.

[Prior Art] TN type LCDs (also referred to as liquid crystal display elements or liquid crystal displays) are widely used as front panel displays such as liquid crystal televisions and lamp-top personal computers. The panel structure is shown in FIG. 9 and FIG. 1O. That is, polarizing plates 12 and 13 are attached to the upper and lower sides of the TN type liquid crystal cell 11, respectively. Arrows 14 and 15 indicate the alignment directions of the liquid crystals on the upper and lower sides of the cell, and numerals 16 and 17 indicate the transmission axis directions of the polarizing plates 12 and 13, respectively. arrow 14
．． 15 are substantially perpendicular to each other, and arrows 15, 16, and 17 are substantially parallel to each other. Arrows 18 and 19 define the horizontal and vertical directions of the display screen, respectively, and when the viewing angle is tilted horizontally or vertically with the viewing angle being 0° perpendicular to the display screen of the LCD. The viewing angles in each direction are called the horizontal viewing angle and the vertical viewing angle. Note that in FIG. 1O, light enters from either the lower surface or the upper surface of the TN type LCD 10 and exits from the other surface.

However, a major problem with this LCD is that its viewing angle characteristics are inferior to other displays. As the viewing angle increases, contrast decreases and display quality deteriorates.

Regarding this problem, the IEICE Technical Research Report EI states that the viewing angle characteristics can be improved by using two phase plates.
D89-94 Wide viewing angle LCD using r phase film
It is reported in J. The structure is shown in FIGS. 11 and 12, and the same reference numerals are given to the parts corresponding to those in FIGS. 9 and 10.
Omit duplicate explanations. A TN type liquid crystal cell 1 is mounted on a keyaki in which the optical axes 23 and 24 of the two phase plates 21 and 22 are almost orthogonal to each other.
1 and a polarizing plate 12. The phase plates 21, 22 have the same retardation Δnd.

(n means refractive index, d means thickness.) Considering the polarization state of light passing through the liquid crystal panel 10 at a viewing angle of 0°, the light passing through only one phase plate 22 is
2 slow axis direction (in most cases, the phase plate has positive crystalline characteristics, so if the 0 phase plate coincides with the optical axis direction 24 and has negative crystalline characteristics, it will be perpendicular to the optical axis direction 24).
receive retardation. Yet another phase plate 2
1 will further receive the same retardation in the direction perpendicular to it.

Therefore, the retardation 5 is canceled and the polarization state of the light becomes the same before and after passing through the two phase plates 21 and 22. This means that at a viewing angle of 0°, the transmission spectrum distribution of the liquid crystal panel 10 is the same regardless of the presence or absence of the phase plates 21 and 22, and there is no change in the brightness and color tone of the emitted light.

When the viewing angle is shifted from 0°, the retardation of the phase plate 2122 is no longer canceled, and the polarization state of the light changes before and after passing through the two phase plates. That is, the addition of the two phase plates changes the viewing angle-transmittance characteristics of the liquid crystal baffle 10, thereby improving the viewing angle characteristics.

(Problem to be solved by the invention) In the conventional TN type LCD described above, the number of films attached to the liquid crystal cell 11 has been increased from two to four (from only two polarizing plates to two additional phase plates). Normally, films are pasted with adhesive, but when pasting, it is necessary to prevent air bubbles from forming and the adhesive layer to be disturbed. For example, boundary reflections occur when air bubbles or disturbances in the adhesive layer occur.

Boundary reflections increase with increasing viewing angle. This is because, as a result, the contrast and viewing angle characteristics of the liquid crystal panel 10 will deteriorate. Care must also be taken when setting the angle of the film when pasting. (In particular, the phase plate requires accuracy within about 1 degree.) Furthermore, if excessive force is applied to the phase plate during attachment, the retardation of the phase plate may change partially or entirely. (The retardation changes as the phase plate expands and contracts.) If the angle or retardation of the phase plate deviates from the settings, it will cause the contrast and viewing angle characteristics of the panel to deteriorate.

The above manufacturing difficulties increase as the size of the panel increases and therefore the area of the film attached to the panel increases.

As described above, the conventional method using two phase plates increases the number of films, causing manufacturing difficulties.

An object of the present invention is to eliminate the conventional drawbacks and provide a TN type LCD with fewer phase plates and good viewing angle characteristics.

[yAl! ! [Means for Solving] (First and second polarizing plates having transmission axes in substantially the same direction as the alignment direction of liquid crystal molecules on the top side or bottom side of the liquid crystal cell are provided on the top and bottom surfaces of the 1jTN type liquid crystal cell. In the TN type liquid crystal display elements arranged respectively, the above-mentioned first
A phase plate having an optical axis in substantially the same direction as the transmission axis direction of the first polarizing plate is interposed between the polarizing plate and the TN type liquid crystal cell.

(2) In the above item (1), the transmission axis directions of the first and second polarizing plates and the optical axis direction of the phase plate are ± with respect to the alignment direction of liquid crystal molecules on the upper surface side of the TN type liquid crystal cell. 10
It is desirable that each setting be within the same range.

(3) In the above item (1) or (2), it is desirable that the retardation of the phase plate is set in a range of 150 to 350 nm.

[Example] In a conventional panel, by using two phase plates whose optical axes are perpendicular to each other, a
The polarization state of the light before and after passing through the two phase plates was made the same, so that the transmission spectrum of the LCD did not change depending on whether or not the phase plate was added.

However, as a result of the inventor's research, even if there is only one phase plate, by making the transmission axis direction of the polarizing plate parallel to the optical axis direction of the phase plate, even if the polarization state changes depending on the phase plate, after passing through the polarizing plate. It was found that the transmission spectrum was the same as without the phase plate. In other words, when the incident light passes through the phase plate, a retardation noise (Δnd) is generated between the vibration of the light in the optical axis direction of the phase plate (first light) and the vibration of light in the direction perpendicular to it (second light). However, the polarization state of the light changes.

However, when the transmission axis direction of the polarizing plate is parallel to the optical axis direction of the phase plate, only the first light vibrating in the optical axis direction of the phase plate is transmitted. The amount of permeation becomes constant.

Furthermore, the inventor's research has revealed that viewing angle characteristics can be improved with a single phase plate.

Research on conventional panels revealed that of the two phase plates, phase plate 22, which is installed so that the transmission axis of the polarizing plate and the optical axis of the phase plate are parallel, contributes to improved viewing angle characteristics. However, it was found that the other phase plate 21 made a much smaller contribution to the improvement of viewing angle characteristics. The situation during this time is shown in Figures 13 and 14. Figure 13 shows a conventional two-plate phase plate system in which the retardation of the phase plate parallel to the transmission axis of the polarizing plate is varied between 0 and 400 nm, and the retardation of the other phase plate 21 is changed to 300 nm.
The horizontal viewing angle-transmittance characteristics in the dark state are shown in comparison with the case where no phase plate is added. The addition of a phase plate improves viewing angle characteristics in dark conditions. On the other hand, the viewing angle characteristics in the bright state do not change much due to the addition of the phase plate. Therefore, the viewing angle dependence of contrast is improved by improving it in the dark state. In this case, it can be seen that the viewing angle characteristics change significantly depending on the retardation value. In addition, FIG. 14 shows that the retardation noon of the phase plate whose optical axis is perpendicular to the transmission axis direction of the polarizing plate is varied between 0 and 400 nm, and the retardation noon of the other phase plate 22 is changed to 300 nm.
It can be seen that the change in viewing angle characteristics is extremely small compared to FIG. 13, which shows the case where In the vertical viewing angle, almost the same results as in FIGS. 13 and 14 are obtained.

Liquid crystal panel 1 is mounted on the keyaki with a phase plate parallel to the polarizing plate.
It is thought that the reason why the viewing angle-transmittance characteristic of 0 can be improved is that the optical axis direction of the phase plate changes depending on the viewing angle.

Previously, it was thought that this was due to the retardation of the phase plate changing depending on the viewing angle, but considering that the transmittance at a viewing angle of 0° does not depend on the retardation of the phase plate, it seems that this is due to the change in the viewing angle of the retardation 7 run. It is reasonable to think that this is because the direction of the optical axis changes depending on the viewing angle.

In order to consider the viewing angle characteristics of the phase plate, the viewing angle characteristics in the case shown in Figure 15 in which two polarizing plates and a phase plate are arranged are shown in Figures 16 and 17 in comparison with the case in which only two polarizing plates are used. . 16th
The figure shows the vertical viewing angle vs. transmittance 18 ratio characteristic, and FIG. 17 shows the case where the viewing angle is taken in a direction perpendicular to the X3 hyperaxial direction of the polarizing plate (this will be referred to as the oblique viewing angle).

From Figures 16 and 17, if the optical axis direction of the phase plate and the transmission axis direction of the polarizing plate are perpendicular, the viewing angle characteristics do not change regardless of the addition of the phase plate, and the optical characteristics of the phase plate do not change. It can be seen that when the axial direction and the transmission axis direction of the polarizing plate are parallel or perpendicular, the oblique viewing angle characteristics do not change due to the addition of the phase plate.

Regarding the viewing angle characteristics of the phase plate, it is known that the retardation changes depending on the viewing angle, but this phenomenon cannot be explained only by the retardation.
It is thought that in any combination of the phase plates shown in FIGS. 6 and 17, the retardenotan changes depending on the viewing angle. Nevertheless, results similar to those obtained when no phase plate is added may be obtained in some cases.

As a result of considering this problem, it was concluded that there is an effect in which the direction of the optical axis of the phase plate apparently changes depending on the viewing angle, and that this effect is large. This is shown in FIG.

In FIG. 18, the optical axis direction of the phase plate is shown in relation to the optical axis direction of the polarizing plate. Here, it should be noted that in many cases, the optical axis direction of the polarizing plate is not the transmission axis direction of the polarizing plate but the absorption axis direction.

As is well known, a polarizing plate has a property of absorbing polarized light in the absorption axis direction of the dye due to the dichroic dye (a dye having optical anisotropy). In many cases, the absorption axis of the dye is the optical axis, so the absorption axis of the polarizing plate is the optical axis, which is orthogonal to the transmission axis. When the optical axes of the polarizing plate and the phase plate are parallel, that is, in the case of FIG. 15B, they remain substantially parallel even if the viewing angle is changed. However, when the optical axes are orthogonal to each other, that is, in the case of FIG. 15A, the intersection angle changes depending on the viewing angle direction, as shown in FIG. 18B and C. This change in the intersection angle corresponds to the change in transmittance shown in FIGS. 16 and 17.

The keyaki described above has the effect that the optical axis direction of the phase plate appears to change depending on the viewing angle, and this effect is thought to play an important role when a phase plate is added to a liquid crystal panel.

Now, the embodiment of the present invention is shown in FIG. 1 and FIG. 2, and the same reference numerals are given to the parts corresponding to the conventional FIGS. 9 to 12, and redundant explanation will be omitted. In this embodiment, the retardation of the phase plate 22 is 250 nm, and the transmission axis direction 16.17 of the polarizing plate 12.13 and the optical axis direction 24 of the phase plate 22 are parallel to the alignment direction 14 of the TN type liquid crystal cell 11. The retardation of the TN type liquid crystal cell 11 was 470 nm, the liquid crystal was left-handed, and the twist angle was 90°.

FIG. 3 shows data on the viewing angle characteristics of this embodiment in comparison with the conventional system shown in FIG. 9 in which no phase plate is added. The viewing angle characteristics in the dark state are improved, and the viewing angle characteristics of the contrast ratio (the ratio between the bright state and the dark state) are improved. (Here, in the bright state,
The rate of change in transmittance from a viewing angle of 0° is not so large since the state is originally bright, and its contribution to the contrast ratio is small. ) Next, a case will be described in which parameters are changed in this embodiment.

4 and 5 show viewing angle characteristics in a dark state when the retardation of the phase plate 22 is changed. FIG. 4 shows the case in the vertical viewing direction, and FIG. 5 shows the case in the horizontal viewing direction. Although there is some deviation between Figures 4 and 5 due to the different viewing angle directions, it can be seen that in both cases, the optimum value for retardane 5 is around 250 nm, and that the viewing angle characteristics deteriorate as the value deviates from this value. . It is desirable to set the retardation to 150 to 350 nm, taking into consideration changes in transmittance (and thus contrast ratio) in a dark state and manufacturing variations of display panels.

As is clear from a comparison between FIG. 5 and FIG. 13, which uses two conventional phase plates, it can be seen that almost the same characteristics can be obtained with just one phase plate.

FIG. 6 shows the case where the angle φ formed by the optical axis direction 24 of the phase plate and the orientation direction 4 of the liquid crystal cell 11 is changed. It can be seen that as φ deviates from 0, the transmittance at the side viewing angle increases, making it impossible to obtain a sufficient dark state.

FIGS. 7 and 8 show cases in which the angle α or B of the polarizing plate 12 or 13 with respect to the alignment direction 14 of the liquid crystal cell 11 is changed, respectively. In both of these cases, the polarizing plate 12.13
transmission axis directions 16 and 17 and orientation direction 14 of the liquid crystal cell 11
It can be seen that as the angle deviates from parallel (0°), the transmittance in the dark state at a horizontal viewing angle increases and the contrast ratio changes.

The cases in which the parameters were changed in the above embodiments were shown using FIGS. 4 to 8, but the viewing angle was approximately Oo
When viewing the screen at an angle of 20 degrees, the transmittance in the dark state becomes minimum and the best contrast ratio is obtained. However, by changing the parameters, the viewing angle direction in which the best contrast ratio is obtained changes.

Therefore, if the viewing angle direction to be emphasized differs depending on how the LCD is used, it may be possible to slightly change the parameters accordingly.

The phase plate 22, the polarizing plate 12.
13 Angle to each liquid crystal cell 11, φ, α, β
is desirably set in the range of -10° to +10°.

In this embodiment, the liquid crystal is twisted to the left by 90 degrees, but the same structure may be used even if it is twisted to the right or the twist angle is shifted by 90 degrees.

The present invention applies regardless of whether the driving method of the liquid crystal panel 11 is simple matrix driving or active matrix driving, and whether there is a color filter or a multi-gap (a method of changing the cell thickness of each dot depending on the color of the color filter). It is valid regardless.

〔Effect of the invention〕

As explained above, according to the present invention, the number of phase plates added to improve viewing angle characteristics can be reduced from the conventional two to one, which simplifies the film bonding process and facilitates manufacturing. At the same time, the effect of reducing material costs can be obtained.

[Brief explanation of the drawing]

FIG. 1 is an exploded perspective view showing an embodiment of the present invention, FIG. 2 is a sectional view of the embodiment of FIG. 1, and FIG. 3 is a side view angle-transmission of the embodiment of FIG. Figures 4 and 5 are diagrams showing the vertical viewing angle and horizontal viewing angle versus transmittance characteristics in the dark state when the retardation of the phase plate is changed in the embodiment shown in Figure 1, respectively. 6 is a diagram showing the horizontal viewing angle-transmittance characteristics in a dark state when the angle φ between the optical axis direction 24 of the phase plate in FIG. 1 and the alignment direction 14 of the liquid crystal cell is changed from Oo, and FIG. and FIG. 8 show the horizontal viewing angle in the dark state when the angle α or β between the transmission axis of the polarizing plate 12 or 13 in FIG. 1 and the alignment direction 14 of the liquid crystal cell 11 is changed from 0°. Figures 9 and 10, which show transmittance characteristics, are an exploded perspective view and a cross-sectional view of a conventional TN-type LCD that does not use a phase plate, respectively.
Figures 1 and 12 each show a conventional T using two phase plates.
Exploded perspective view and cross-sectional view of N-type LCD, Figures 13 and 1
Figure 4 shows the horizontal viewing angle-transmittance characteristics in the dark state when the retardation of the phase plate 22 or 21 in Figure 11 is changed from zero, and Figure 15 shows the horizontal viewing angle-transmittance characteristics in the dark state when the retardation of the phase plate 22 or 21 in Figure 11 is changed from zero. FIG. 16 is a perspective view of the model with plates arranged, and FIG.
Figure 7 is a diagram showing the diagonal viewing angle-transmittance characteristics of the model in Figure 15, and Part 18 is a diagram to explain that the optical axis direction of the phase plate changes depending on the viewing angle in the model in Figure 15A. It is.

Claims (3)

[Claims] (1) A TN type liquid crystal cell in which first and second polarizing plates having transmission axes in substantially the same direction as the alignment direction of liquid crystal molecules on the top side or bottom side of the liquid crystal cell are arranged on the top and bottom surfaces of the TN type liquid crystal cell, respectively. type liquid crystal display element, the first polarizing plate and the TN type liquid crystal cell are provided with the first
1 with an optical axis in almost the same direction as the transmission axis direction of the polarizing plate
A TN type liquid crystal display element characterized in that a phase plate is interposed therebetween. (2) In claim (1), the transmission axis directions of the first and second polarizing plates and the optical axis direction of the phase plate are ± with respect to the orientation direction of liquid crystal molecules on the upper surface side of the TN type liquid crystal cell. 10
TN characterized in that each is set within a degree.
type liquid crystal display element. (3) A TN type liquid crystal display element according to claim (1) or (2), wherein the retardation of the phase plate is set in a range of 150 to 350 nm.