JP2605003B2 - Screw-nematic liquid crystal display - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a screw-nematic liquid crystal display device used for displaying an automobile or a watch, and more particularly to a negative display type liquid crystal display device in which a bright display pattern appears on a dark background. Things.

[Conventional technology]

The screw-nematic liquid crystal display device is roughly classified into a negative display type and a positive display type according to its appearance. With negative display type,
This is a format for displaying bright characters and figures on a dark display surface, and is already widely used for instruments and watches of automobiles.

FIG. 2 shows an example of a conventional negative-type screw-nematic liquid crystal display device. In general, back-side illumination means (1), polarizing plates (3), (5), TN cell (TN cell)
(4), and a color filter (2) may be used in combination depending on the application.

The TN cell is provided with electrodes of a desired shape opposed to two transparent supports such as glass and plastic. Between the transparent supports, nematic liquid crystal molecules are substantially parallel to the surface of the transparent support and 90 °. The orientation is controlled so as to form a helical structure. The polarizing axes of the polarizing plates (3) and (5) are
The liquid crystal molecules are set so as to be parallel or perpendicular to the alignment direction of the liquid crystal molecules on the adjacent substrate surfaces, and the polarization axes are parallel to each other. With such a configuration, in a portion where no voltage is applied, the light of the light source (1) becomes linearly polarized light by the polarizing plate (3) and enters the TN cell (4). In the TN cell (4), the plane of polarization of the linearly polarized light is rotated by about 90 °, and is shielded by the polarizing plate (5). On the other hand, in the portion of the TN cell where a voltage higher than the threshold voltage is applied, the polarization plane does not rotate, so that the polarization plane passing through the TN cell (4) matches the polarization axis of the polarizing plate (5), Produces a bright pattern. Further, if the color filter (2) is provided, the transmitted light can be colored, and a display device in which the display color is partially changed can be configured.

[Problems to be solved by the invention]

This conventional method has been theoretically analyzed. (OH Gooch et al., J. Phys., D, 8 , 1975, p. 157
5) The transmittance (Toff) of the background part is when an ideal polarizing plate is used u = 2dΔn / λ (2) d: gap of TN cell Δn: refractive index anisotropy of nematic liquid crystal λ: wavelength of light Further, the transmittance (Ton) of the display portion (voltage application portion) can be approximated by the transmittance when two polarizing plates are installed so that their polarization axes are parallel to each other.
≒ 0.5. Therefore, the contrast ratio (CR) can be approximated by CR = Ton / Toff ≒ 0.5 / Toff (3), and has a cell gap (d), a refractive index anisotropy (Δn) of the liquid crystal, and a wavelength dependence. I have.

On the other hand, it is difficult to control the liquid crystal cell gap (d) to be strictly constant, and the liquid crystal cell has a variation with respect to a set value, and Δn has both temperature dependence and wavelength dependence. Is displayed, in other words, there is a restriction condition that light of different wavelengths may be used.

As a result, the conventional device has the following problems.

(1) The contrast ratio is small.

(2) The background is not sufficiently dark, and the background looks bright to some extent. In other words, Toff is large.

(3) The temperature dependence of Toff is large, and the darkness of the background changes depending on the temperature.

Both problems are caused by the fact that Toff is not an ideal optical shutter because it follows the equation (1).

Furthermore, in the conventional method, in order to provide a fixed display portion that always displays transmitted light such as scales, numbers or characters, an electrode corresponding to the fixed display portion is formed, and an excitation voltage is always applied, or It is necessary to make the alignment of the liquid crystal molecules different only in the portion corresponding to the fixed display portion, for example, to vertically align, or to provide a cutout in at least one polarizing plate only in the portion corresponding to the fixed display portion,
There are problems that the number of electrodes increases, the driving circuit becomes complicated, the alignment process becomes complicated, and the transmitted light intensity differs between the normal display pattern and the fixed display portion, and the view becomes poor.

[Means for solving the problem]

The present invention has been made in order to solve such a problem of the conventional device, and has a large contrast negative light configured so that Toff has the same light amount as when the polarization axes of two polarizing plates are orthogonal to each other. An object of the present invention is to provide a negative-type screw-nematic liquid crystal display device having a fixed display portion by a simple method.

The present invention provides an illumination means, a nematic liquid crystal layer having a helix structure of approximately 90 °, a plurality of transparent supports sandwiching the nematic liquid crystal layer and having electrodes formed thereon corresponding to an operation display pattern, and a transparent support. At least a pair of polarizing plates provided outside, a light-shielding mask having a light-transmitting portion formed at a position corresponding to the operation display pattern, and means for applying a voltage to the electrodes. Arranged substantially orthogonally or parallel to the alignment direction of adjacent liquid crystal molecules, and the polarization axes of the respective polarizing plates are orthogonal to each other, and between electrodes other than the electrodes corresponding to a desired display portion of the operation display pattern. A screw-nematic liquid crystal display device (hereinafter referred to as an R-TN cell) characterized in that a desired display portion is made translucent by applying an excitation voltage. light Always be displayed transmitted light Yotsute, graduations or numerals or letters or the like to be a Nejirenemachitsuku liquid crystal display device according to claim.

FIG. 1 is a perspective view of a basic configuration example of a screw-nematic liquid crystal cell used in the present invention. The portion different from the conventional example is provided with a light-shielding mask (20), the shape of the operation display pattern is the same as the shape of the light-shielding mask, and the polarization axes of the polarizing plates (3) and (5) are substantially orthogonal to each other. And the means (6) for applying a voltage to the TN cell (4) applies the TN cell operating voltage only to the non-display segment. The reverse drive is performed. (Hereinafter, this is referred to as an R-TN cell.) The configuration of the present invention is not limited to the example shown in FIG.
In addition to using a color polarizing plate and various light guide plates, a dichroic dye may be added to a liquid crystal material, and an active element may be provided on a substrate.

Although the light-shielding mask of the present invention is arranged on the back side of the cell in the example of FIG. 1, it can be arranged on the front side of the cell and on the inner surface of the cell.

Further, in the present invention, since the background portion is in a dark state with a light-shielding mask, no light is necessary for that portion, and since the light of the light source is concentrated on the operation display pattern portion, a brighter display can be performed. It is possible to provide a reflection plate that reflects in the back direction on the back side of the cell or on the back side of the back side polarizing plate except for the operation display pattern portion, or to use a light guide plate that concentrates and emits light only to the operation display pattern portion. it can.

In the present invention, in a use requiring a wide viewing angle or a use requiring a fine display pattern, a display defect may occur if there is a parallax between the shape of the light shielding mask and the display segment. Therefore, in such a case, it is preferable to provide a light-shielding mask on the inner surface of the TN cell.

Further, Δn of the nematic liquid crystal used has a preferable region in consideration of a practical cell gap of 6 to 10 μm.
03 ≦ Δn ≦ 0.17, and more preferably 0.03 ≦ Δn ≦ 0.
It is 13. As a liquid crystal material having a small Δn, A compound having a trans-cyclohexane ring is used.

[Action]

In the present invention, by adopting such a configuration, that is, by performing a negative display with a positive cell configuration,
The background portion is shielded from light by a light-shielding mask, and the Toff in the operation display pattern portion is the same as the amount of light when the polarization axes of the two polarizing plates are orthogonal because the liquid crystal molecules are no longer in a twisted orientation by voltage application. , And the same dark state can be obtained irrespective of temperature and color. As a result, even when a color filter or a different color light source is partially provided, it is not recognized, and the contrast can be increased. Further, for practical use as a display device, the viewing angle is also important.

As a result of various studies, the present inventors have found that the seed Δnd between the refractive index anisotropy (Δn) of the liquid crystal and the cell gap (d) is 1.0 to 0.3 μm,
It has been found that a practical viewing angle can be obtained if the thickness is more preferably set to 0.8 to 0.3 μm.

In an R-TN cell, the nematic liquid crystal layer takes a twisted structure in a state where no excitation voltage is applied, so that the nematic liquid crystal layer is in a bright state.
A light-shielding mask is required for portions other than the operation display pattern. By providing a part of this light-shielding mask with a light-shielding cutout in the form of scales, numbers, letters, or the like according to the design, there is no need to apply a voltage or change the orientation partially. A fixed display can be provided. The fixed display unit thus obtained has the same structure as the display unit in which no voltage is applied and the display unit is turned on, so that the same bright state is obtained.

〔Example〕

FIG. 3 shows an example of a light shielding mask used by applying the present invention. In this example, a bar graph display section (22) for turning on and off by applying an excitation voltage and a fixed display section (21) for indicating a corresponding value are provided. In the first place, the light-shielding mask only needs to be light-opaque, so it is possible to print the light-shielding mask on the surface of the R-TN cell by partially changing the color, for example. It is also possible to obtain a display colored by using the colored paint of the above.

Further, the notch (22) of the light-shielding film provided in the display section which is turned on and off by applying an excitation voltage is enlarged as shown in FIG. Light display is possible. Fifth
The figure shows a case where the notch (22) of the light shielding film is set smaller than the electrode pattern (23). OFF in this case
Occasionally, even when viewed from an oblique direction, there is no transmitted light from the contour portion, and a good display is obtained.

FIG. 6 shows another example of a light-shielding mask used by applying the present invention. This example is an example of a vehicle speedometer used for an instrument panel of a car, for example. The light-shielding film (20) has a light-transmitting notch in the display (22), which turns on and off by applying an excitation voltage according to the vehicle speed, and in the portion (21), which performs fixed display regardless of the vehicle speed. Is provided.

Therefore, the contrast of the R-TN cell of the present invention is analyzed in the same manner as in the conventional example described above. When an ideal polarizing plate is used and a sufficient applied voltage is applied, Toff ≒ 0 (4) And the contrast ratio (Ton / Toff) is close to infinity. In practice, only a finite contrast ratio can be obtained due to the characteristics of the polarizing plate or the limit of the applied voltage, but a dramatic improvement can be achieved as compared with the conventional method. Table 1 shows the measurement results of the contrast ratio. The liquid crystal used is Merck ZL
I-1565, the polarizing plate was Q-12 manufactured by Nitto Denko Corporation, and the cell gap was 9.3 μm. The applied voltage was 10 V, the transmitted light intensity was measured in a direction perpendicular to the cell surface, and the values of Ton and Toff were represented by photometric output voltages.

Further, the viewing angle is important for practical use as a display device. As a result of various studies, the present inventors have found that the product Δnd of the refractive index anisotropy (Δn) of the liquid crystal and the cell gap (d) is 1.0 to 1.0.
It has been found that a practical viewing angle can be obtained by setting the thickness to 0.3 μm, more preferably 0.8 to 0.3 μm.

FIG. 7 shows the angle dependence of the contrast ratio together with a comparative example. FIG. 7A shows the relationship between the contrast ratio and the visual field. The cell gap was set to 5 μm in the R-TN cell according to the embodiment of the present invention, and 9.3 μm and 5 μm in the conventional example. The conditions of the liquid crystal, the polarizing plate and the like are the same as in Table 1. FIG. 7 (b) shows the measurement angle conditions, and the angle dependence was measured in the main viewing direction (θ> 0 °) and the anti-viewing direction (θ <0 °) of the TN cell. As shown in FIG. 7 (a), in the conventional cell, the viewing angle is widened by simply reducing Δnd, but the contrast ratio is small, while the vertical direction (θ =
In contrast, when the contrast (0 °) is increased, the viewing angle becomes narrower, there is a contradiction. However, in the present invention, both excellent features can be utilized.

Further, in the cell of the present invention, since each segment of the bar graph and the fixed display are shown with the same brightness, the visibility is excellent and the scale is fixedly displayed even when the cell is not driven. It is excellent.

〔The invention's effect〕

As described above, according to the present invention, the contrast and the viewing angle of the conventional negative TN cell are improved, and the fixed display unit can be provided by a simple method. That is, R-TN
By providing a cutout in the light-shielding film required for the cell, a fixed display can be obtained. Since this light-shielding film can be formed by printing or the like, the present invention is also effective in obtaining a fixed display of a fine pattern.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a configuration example of a negative display type TN liquid crystal display device of the present invention. (1) ... light source, (2) ... color filter, (3)
... Polarizing plate, (4) TN cell, (5) Polarizing plate (20) Shielding mask, (6) Voltage applying means FIG. 2 is a perspective view showing a conventional configuration example. . 3 to 6 show an embodiment of a light-shielding mask having a cutout in a fixed display portion used in the present invention. FIG. 7A is a graph showing the angle dependence of the contrast ratio, and FIG. 7B is a perspective view showing the measurement angle conditions. (20): a light-shielding mask; (21): a notch provided in a fixed display portion; (22): a notch provided in a display segment portion; (23): a display segment electrode pattern.

Claims (2)

(57) [Claims] An illumination means, a nematic liquid crystal layer having a helix structure of approximately 90 °, a plurality of transparent supports sandwiching the nematic liquid crystal layer and having electrodes formed thereon corresponding to an operation display pattern; At least a pair of polarizing plates provided outside the body, a light-shielding mask having a light-transmitting portion formed at a position corresponding to the operation display pattern, and means for applying a voltage to the electrodes; Are arranged substantially orthogonally or parallel to the orientation direction of the liquid crystal molecules adjacent to each other, and the polarization axes of the respective polarizing plates are orthogonal to each other, and between electrodes other than the electrodes corresponding to a desired display portion of the operation display pattern. A screw-nematic liquid crystal display device characterized in that a desired display portion is made translucent by applying an excitation voltage thereto, and further by making a part of the light-shielding mask translucent, A screw-nematic liquid crystal display device characterized by always displaying transmitted light such as numerals or characters. 2. The torsional nematic liquid crystal according to claim 1, wherein the product Δnd of the refractive index anisotropy (Δn) of the liquid crystal and the thickness (d) of the liquid crystal layer is 0.3 to 1.0 μm. Display device.