JP2003029303A - Liquid crystal display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that viewing angle characteristics of white display and contrast are insufficient when a driving method in which a high voltage is periodically applied so as to keep bend alignment even in the case where the applied voltage is low in a liquid crystal display using an OCB liquid crystal. SOLUTION: An applied voltage for black display, a periodically applied high voltage and an applied voltage for white display are set for every display color.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a liquid crystal display device.

[0002]

2. Description of the Related Art Currently, TN is a liquid crystal display device for displaying images.
Those using (Twisted Nematic) mode are widely used. In recent years, OCB (Op (Op) has been developed to overcome the drawbacks of TN mode, such as narrow viewing angle and poor response performance.
A liquid crystal display device using a tically self-compensated birefringence mode has been reported (JP-A-7-84254, JP-A-9-96790, etc.).

The OCB liquid crystal, as disclosed in Japanese Patent Laid-Open No. 9-96790, must be initialized from the splay alignment to the bend alignment when used (hereinafter referred to as transition).

However, thereafter, the applied voltage is a constant voltage value Va.
When it is less than, it will return to the splay array (hereinafter referred to as reverse transfer). Therefore, most OCB liquid crystals are used by controlling the modulation degree within the voltage application range for maintaining the bend alignment ((a) in FIG. 3).

When the voltage applied to the liquid crystal is limited for the purpose of preventing the reverse transition as described above, the panel transmittance, and thus the brightness of the liquid crystal display device during white display (bright display) cannot be increased, It cannot fully meet the demand for display characteristics. In this specification, the liquid crystal panel is normally white.

The inventors have examined the occurrence of reverse metastasis. Even if there is a time during which the liquid crystal applied voltage is less than or equal to Va, there will be no reverse transition if a high voltage (hereinafter, reverse transition prevention voltage) is applied periodically at a different time. Found.

When displaying an image of one frame, 1
By applying the liquid crystal display device by dividing the frame period into a period for displaying an image and a period for applying a high voltage (FIG. 1), the voltage application range in the period for displaying an image was expanded to Vb which is less than Va. Hereinafter, such a driving method will be referred to as reverse transition prevention driving. When the reverse transition prevention drive is performed, the panel transmittance can be increased, and thus the maximum brightness of the liquid crystal display device can be increased ((b) in FIG. 3).

[0008]

However, when the above-mentioned reverse transition prevention drive is performed, a problem peculiar to the drive method arises.

First of all, the brightness of black display does not become sufficiently small, and thus the contrast (maximum brightness / minimum brightness)
There was a problem that With respect to this first problem, the inventors investigated the relationship between the liquid crystal applied voltage and the brightness (panel transmittance). As a result, as shown in FIG. 7, when OCB liquid crystal was used, it was found that the voltage at which the transmittance was minimum was different for each display color.

The cause of the result of FIG. 7 will be briefly described. OC
The B liquid crystal mode liquid crystal panel has a structure in which a film phase plate is arranged in a liquid crystal cell. Then, when the refractive index anisotropies of the liquid crystal layer and the film phase plate cancel each other out, black display is performed.
Since the liquid crystal panel used in this verification is normally white, the black brightness decreases with the increase of the applied voltage up to the voltage for displaying black. However, when a voltage higher than the voltage for displaying black is applied, the refractive index anisotropy in the liquid crystal layer decreases,
The balance of the offsetting is lost and the black luminance is increased again. For this reason, there is a liquid crystal applied voltage that minimizes the brightness. Further, the liquid crystal layer and the film phase plate have wavelength dependence, and the tendency of the wavelength dependence of both is different, so that the characteristics are different for each display color as shown in FIG.

From the results of FIG. 7, the liquid crystal applied voltage at the time of black display is smaller than that of the same voltage for all display colors when the liquid crystal applied voltage is set separately in view of the characteristics, and thus the black luminance of the liquid crystal display device is reduced. It is thought that the contrast can be lowered and the contrast can be improved.

Secondly, there is a case where the solution to the first problem is not always optimal. This is because the occurrence of the reverse transition is affected by a plurality of parameters such as the voltage value of the reverse transition prevention voltage, the time width, and the applied voltage for image display. The relationship between the occurrence of reverse transition, the voltage value of the reverse transition prevention voltage, the time width, and the applied voltage for image display is shown in FIGS. 8 (a) to 8 (c).

The above-mentioned parameters can be arbitrarily set based on the luminance characteristics required for the device, driver performance, user selection, and the like. However, if the voltage value of the reverse transition prevention voltage is to be changed significantly, it is necessary to change the driver itself. Further, if the applied voltage for image display is largely changed, the effect of the reverse transition prevention drive is diminished. Therefore, it is considered most efficient to set the voltage depending on the time width of the reverse transition prevention voltage application.

Thirdly, there is a problem that the chromaticity point in white display changes between the front view and the field of view other than the front view, and the amount of change becomes larger than that before the introduction of the reverse transition prevention drive. In the liquid crystal display device driven by reverse transition prevention, the chromaticity points of white display in the front view and in the deep field of view are significantly different ((a) of FIG. 6).
Here, ΔCu'v 'in the figure represents the amount of change in color when changing the front view and the visual field on the CIE1976UCS chromaticity diagram, expressed as a distance. The inventors have examined this problem. As a result, the viewing angle characteristics of the brightness of each display color are as shown in FIG. The difference in the characteristics for each display color seen in FIG. 11 is due to the wavelength dependence of the liquid crystal layer and the film phase plate respectively, and the long distance that the light transmitted in the oblique direction passes through the liquid crystal layer. It was clarified that the above difference in characteristics causes a change in the chromaticity point of white display.

Fourthly, there is a problem of lowering the luminance which is brought about to solve the third problem.

As shown in FIG. 5, the brightness (FIG. 5 (b)) of the liquid crystal display device which has solved the third problem when viewed from the front is higher than that of the reverse transition prevention drive (FIG. 5 (a)). It is falling. This is caused by limiting the applied voltage.

[0017]

In order to solve the first problem, the inventors investigated the relationship between the liquid crystal applied voltage and the brightness (panel transmittance), and the voltage at which the transmittance becomes the minimum is different for each display color. Revealed the facts. From this result, the liquid crystal applied voltage at the time of black display is set to be the same for all the display colors, but it is better to set the voltage separately in view of the characteristics so that the transmittance is smaller, which in turn reduces the black luminance of the liquid crystal display device and reduces the contrast. It can be improved. Further, it was confirmed that the improvement effect is maximized by applying the voltage optimized for the black display not only when the black display is performed but also when the reverse transition prevention voltage is applied. From the above, the first and second liquid crystal display devices of the present invention are characterized in that the reverse transition prevention voltage minimizes the luminance in black display.

With respect to the second problem, the inventors investigated the relationship between the occurrence of reverse transition, the voltage value of the reverse transition prevention voltage, the time width, and the applied voltage for image display.

The above-mentioned parameters can be arbitrarily set based on the luminance characteristics required for the device, driver performance, user selection and the like. However, if the voltage value of the reverse transition prevention voltage is to be changed significantly, it is necessary to change the driver itself. Further, if the applied voltage for image display is largely changed, the effect of the reverse transition prevention drive is diminished. Therefore, it is considered most efficient to set the voltage value according to the time width of the reverse transition prevention voltage application.

When the voltage that minimizes the black luminance set in the first problem is also applied to the reverse transfer prevention voltage, the critical time at which reverse transfer starts to occur as the time width of the reverse transfer prevention voltage is reduced. There is a width ta.

When the time width of the reverse transition prevention voltage application is equal to or longer than ta, the period during which the reverse transition prevention voltage is applied is long and the display is greatly affected. Therefore, in this case, by setting the voltage value of the reverse transition prevention voltage to the voltage that minimizes the black brightness as described in the first problem, it is possible to reduce the black brightness and improve the contrast.

On the other hand, when the time width of the reverse transition prevention voltage application is less than ta, it is necessary to prevent the reverse transition from occurring in a short time. Further, the period during which the reverse transition prevention voltage is applied is short and the influence on the display is small. Therefore, the reverse transition prevention voltage should be higher than the above-mentioned voltage that minimizes the black luminance.

FIG. 10 shows the relationship between the time width of the reverse transition prevention voltage and the black luminance.

As described above, the third liquid crystal display device of the present invention is characterized in that the voltage value of the reverse transition prevention voltage is set according to the time width of the reverse transition prevention voltage.

The setting of the voltage value depending on the time width of the reverse transition prevention voltage is fixed in consideration of the characteristics of the liquid crystal display device, is switched depending on other characteristics of the displayed image, or is switched as a result of user selection. Can be applied.

With respect to the third problem, the inventors investigated the influence of the liquid crystal applied voltage and the viewing angle on the chromaticity point. As a result, it was clarified that the third problem is that the viewing angle characteristics differ depending on the display color as shown in FIG.

Since the above-mentioned viewing angle characteristics change depending on the liquid crystal applied voltage as shown in FIG. 12, the liquid crystal applied voltage of each display color is set so that the viewing angle characteristics of each display color are similar, and the voltage is set. By displaying white with the combination of, the problem can be significantly reduced.

In practice, not only the luminance but also the chromaticity point changes for each display color, which causes an error, but there may be a combination of liquid crystal applied voltages that causes almost no color change even when the viewing angle is different. I was confirmed. Specifically, ΔCu'v 'could be minimized by setting the liquid crystal applied voltage to 0.1 V for red, 0.8 V for green, and 1.7 V for blue ((b) of FIG. 6).

From the above, the fourth liquid crystal display device of the present invention is characterized in that, in view of the viewing angle characteristics of each display color, at least one applied voltage range is different for each display color.

The above-mentioned fourth problem is that the luminance in a front view is reduced because the range of applied voltage is different in the fourth liquid crystal display device of the present invention.

However, according to the investigation by the inventors, as shown in FIG. 5, the luminance reduction rate in the front view is the maximum (about 16%), and the luminance reduction rate other than the front view is smaller than that in the front view. It became clear that he was slacking off. From this, when using an optical sheet that collects light emitted in a direction other than the front view in the front direction, it is considered that it is possible to suppress the brightness reduction rate in the front view due to the limitation of the applied voltage. It was From this, the fifth liquid crystal display device of the present invention is characterized in that the backlight device of the fourth liquid crystal display device of the present invention has means for condensing the light of the backlight in the front direction.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below based on its embodiments.

(Embodiment 1) FIG. 4 is a block diagram showing a configuration of a second liquid crystal display device of the present invention.

While the input video signal is digitized as necessary and then written into the line memory 42, the video data stored in the line memory 42 is read out at a speed twice that of the writing, and is then sent to the driver control circuit 43. input. From the driver control circuit 43, the gradation control circuit 48
The image data for one line read out from the line memory and the black display data for the reverse transition prevention voltage for one line are alternately transmitted to. The gradation control circuit 48 is a table conversion circuit including an SRAM and the like, and for the purpose of the second liquid crystal display device of the present invention, performs the table conversion at least during black display and during application of the reverse transition prevention voltage. On the other hand, from the driver control circuit 43 to the gate driver 44, a gate line whose display position is appropriate is selected when the image data is being sent to the source driver, and the image data is displayed when the reverse transition prevention voltage is applied. A control signal is sent to select a gate line different from the line. The gate driver 44 used here can select any line. By changing the phase relationship between the gate line selected when displaying the image data and the gate line selected when applying the reverse transition prevention voltage, the time width of applying the reverse transition prevention voltage in one frame can be changed.

For the sake of simplicity, the relationship between the above timings is 5
An example of using a panel having two gate lines is shown in FIG.

The rationale for such a configuration will be described below. When the OCB liquid crystal is used as shown in FIG. 7, the voltage at which the transmittance is minimum differs for each display color. This is caused by the wavelength dependence of the liquid crystal layer and the film phase plate, and is considered to be an unavoidable characteristic. Therefore, the liquid crystal applied voltage at the time of black display is smaller than the same for all display colors, the transmittance is smaller when the applied voltage is set to the minimum luminance characteristic of each color, and the black luminance of the liquid crystal display device is lowered. The contrast can be improved. Further, it was confirmed that the improvement effect is maximized by applying the voltage optimized for the black display not only when the black display is performed but also when the reverse transition prevention voltage is applied. Specifically, the liquid crystal applied voltage during black display was 6.3 V for all display colors, the red and green voltages were 6.3 V, and the blue voltage was 5.
By changing to 5V, the black brightness could be reduced by about 15%.

(Second Embodiment) FIG. 9 shows the configuration of a third liquid crystal display device of the present invention. The same as the first embodiment except that a user selection interface 91 is added and the operations of the driver control circuit 43 and the gradation control circuit 48 are different.

The user inputs the adjustment value of the time width of the reverse transition prevention voltage or the selection result from the user interface 91 as necessary.

Here, the visual effect exerted by the time width of the reverse transition prevention voltage will be described. Application of the reverse transition prevention voltage corresponds to black (dark) display. Therefore, when the time width is expanded, the display of images becomes intermittent and the visibility of moving images can be improved, but the display brightness decreases. On the other hand, when the time width is reduced, the visibility of the moving image is significantly reduced due to the occurrence of edge blurring, but the display brightness is increased. Therefore, the user can select a large time width when he / she wants to see an image with a lot of movement with reduced edge blur, and a small time width when he / she wants to display a still image or the like brightly.

The driver control circuit 43 changes the phase relationship between the gate line selected when the image data is displayed and the gate line selected when the reverse transition prevention voltage is applied, based on the input adjustment value of the time width, so that one frame The time width for applying the reverse transition prevention voltage is changed.

From the driver control circuit 43 to the gradation control circuit 4
The image data for one line and the black display data for the reverse transition prevention voltage for one line are alternately transmitted to 8. Further, the determination result as to whether the time width is equal to or larger than ta or smaller than ta is also output to the gradation control circuit 48.

The gradation control circuit 48 receives the judgment result,
When the time width of the reverse transition prevention voltage is equal to or more than ta, table conversion is performed during black display and when the reverse transition prevention voltage is applied so that the black luminance is minimized. On the other hand, when the time width of the reverse transition prevention voltage is less than ta, table conversion for black display is performed so that the black luminance is minimized, and table conversion when the reverse transition prevention voltage is applied is performed. The voltage value is set to be higher than the applied voltage value based on the table conversion result for black display.

Here, ta is the minimum time width of the reverse transition prevention voltage at which reverse transition does not occur when the voltage value of the reverse transition prevention voltage is set to a voltage at which the black luminance is minimized.

As a result, the third liquid crystal display device of the present invention can suppress the black luminance and improve the contrast according to the time width of the reverse transition prevention voltage.

(Embodiment 3) FIG. 4 shows the configuration of a fourth liquid crystal display device of the present invention. The operation is the same as that of the first embodiment except the operation of the gradation control circuit 48.

The gradation control circuit 48 is a table conversion circuit including an SRAM or the like, and performs the table conversion at least during white display for the purpose of the fourth liquid crystal display device of the present invention.

The reason for having such a configuration will be described below. As shown in FIG. 6, in the liquid crystal display device driven by the reverse transition prevention, the chromaticity points of white display in the front view and in the deep field of view are significantly different ((a) of FIG. 6). Here, ΔCu'v 'in the figure is CIE197
6UCS This is the distance change representing the amount of color change when the front view and the visual field are changed on the chromaticity diagram. It has been found that the change in the chromaticity point of white display is caused by the difference in viewing angle characteristics depending on the display color as shown in FIG.

Since the above-mentioned viewing angle characteristics change depending on the liquid crystal applied voltage as shown in FIG. 12, the liquid crystal applied voltage for each display color is set so that the viewing angle-relative luminance characteristics of each display color are approximated. By displaying white with the combination of the voltages, the problem could be significantly reduced.

In practice, not only the luminance but also the chromaticity point changes for each display color, which causes an error, but there may be a combination of liquid crystal applied voltages that causes almost no color change even when the viewing angle is different. I was confirmed. Specifically, ΔCu'v 'could be minimized by setting the liquid crystal applied voltage to 0.1 V for red, 0.8 V for green, and 1.7 V for blue ((b) of FIG. 6). When displaying a gradation other than the white display, the gradation control circuit 48 may perform table conversion based on the voltage so as to obtain desired input / output characteristics. Although the improvement effect in the horizontal direction is shown here, the horizontal direction is a direction orthogonal to the rubbing direction (vertical direction) of the liquid crystal panel used by the inventors.

(Embodiment 4) FIG. 4 shows the configuration of a fifth liquid crystal display device of the present invention. It is the same as the fourth invention except that the configuration of the backlight device 47 is changed. The backlight device 47 is provided with an optical sheet (called a lens sheet, a prism sheet, a BEF, etc.) having a light collecting property.

The reason for having such a configuration will be described below. As shown in FIG. 5, the luminance of the fourth liquid crystal display device of the present invention in front view ((b) of FIG. 5) is lower than the luminance of reverse transition prevention drive ((a) of FIG. 5). . This is because the applied voltage range is limited, but according to FIG. 5, the brightness reduction rate in the front view is the maximum (about 16%), and the brightness reduction rate in the areas other than the front view is smaller than that in the front view. is doing. Therefore, when the optical sheet that collects the light emitted in the direction other than the front view in the front is used, the luminance reduction rate in the front view due to the limitation of the applied voltage can be suppressed.

According to the experiments conducted by the inventors, the brightness reduction rate was improved by about 4% by adopting this embodiment. Here, the verification was performed using one type of optical sheet, but by changing the characteristics of the optical sheet used, various characteristics such as the viewing angle characteristic of the luminance are further reduced, as well as further reduction of the front luminance reduction. can do.

The viewing angle characteristic of chromaticity point change in white display of the present liquid crystal display device is as shown in FIG. 6 (c),
The use of the optical sheet did not deteriorate the viewing angle characteristic of chromaticity change. From the above, the present liquid crystal display device was able to suppress a decrease in maximum brightness and significantly reduce the change in chromaticity depending on the viewing angle.

As the optical sheet used for the verification, a so-called vertical grain having a groove in the longitudinal direction was used. The characteristics and the inserting direction of the optical sheet depend on the rubbing direction of the liquid crystal panel used by the inventors being vertical.

[0055]

The first liquid crystal display device of the present invention is an OCB.
In a liquid crystal display device including a liquid crystal panel using a liquid crystal mode, when a driving method is performed in which a period for displaying an image of one frame is divided into a period for displaying an image and a period for applying a high voltage, the high voltage Is characterized in that the brightness when displaying black is the minimum, and the minimum brightness is reduced and the contrast is expanded.

The second liquid crystal display device of the present invention is a liquid crystal display device provided with a liquid crystal panel using the OCB liquid crystal mode, in which a period for displaying one frame image, a period for displaying an image and a high voltage are applied. In the case of performing a driving method that is divided into a period for which the black display is performed, the applied voltage for black display in the period for displaying the high voltage and the image is such that the brightness when displaying black is minimum, The brightness was reduced and the contrast was expanded.

A third liquid crystal display device according to the present invention is a liquid crystal display device provided with a liquid crystal panel using an OCB liquid crystal mode, in which a period for displaying one frame image, a period for displaying an image and a high voltage application. In the case of performing a driving method in which the high voltage is applied for a period equal to or higher than a certain value, the applied voltage for black display during the period for displaying the high voltage and the image is changed to black. It is assumed that the brightness at the time of displaying is minimum, and when the ratio of the period for applying the high voltage is less than a certain value, the high voltage is higher than the applied voltage for black display in the period for displaying the image. According to the ratio of the period of applying the high voltage, the minimum luminance is lowered and the contrast is enlarged.

A fourth liquid crystal display device of the present invention is a liquid crystal display device comprising an OCB liquid crystal mode liquid crystal panel and a backlight device, wherein a period for displaying one frame image is defined as a period for displaying an image. It is divided into a high voltage application period and a voltage application range in the image display period is expanded, and at least one of the voltage application range in the image display period is different depending on the display color. The change in chromaticity point due to the viewing angle in white display is reduced.

A fifth liquid crystal display device of the present invention is characterized in that, in the backlight device of the liquid crystal display device of the fourth invention, it has means for condensing the light of the backlight to the front.
The improvement of the maximum brightness in white display and the reduction of the change of the chromaticity point depending on the viewing angle are both achieved.

[Brief description of drawings]

FIG. 1 is a timing diagram of the present invention.

FIG. 2 is a detailed timing diagram of the present invention.

FIG. 3 is a graph showing the relationship between liquid crystal applied voltage and transmittance.

FIG. 4 is a configuration diagram of first, third and fourth liquid crystal display devices of the present invention.

FIG. 5 is a graph showing the measurement results of the relationship between luminance and viewing angle.

FIG. 6 is a graph showing measurement results of the relationship between chromaticity change and viewing angle.

FIG. 7 is a graph showing measurement results of liquid crystal applied voltage and brightness.

FIG. 8 is a diagram showing conditions under which reverse transition occurs.

FIG. 9 is a configuration diagram of a second liquid crystal display device of the present invention.

FIG. 10 is a graph showing the relationship between the time width of the reverse transition prevention voltage and the black luminance in the second liquid crystal display device of the present invention.

FIG. 11 is a graph showing viewing angle characteristics of each display color in reverse transition prevention drive.

FIG. 12 is a graph showing the measurement results of the relationship between the luminance and the viewing angle in each display color.

[Explanation of symbols]

41 Liquid Crystal Display 42 line memory 43 Driver control circuit 44 gate driver 45 source driver 46 LCD panel 47 Backlight device 48 gradation control circuit 91 User selection interface

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G09G 3/36 G09G 3/36 (72) Inventor Katsuyuki Arimoto 1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial In-house (72) Inventor Kenji Nakao 1006, Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd. F-term (reference) 2H088 HA23 HA24 HA28 JA05 JA28 MA02 2H091 FA21X FA26X FA41Z HA07 LA17 2H093 NA51 NC11 ND04 5C006 AA22 AC11 BA15 BB16 FA54 5C080 AA10 BB05 CC03 DD09 DD30 FF11 JJ02 JJ05

Claims (5)

[Claims] 1. A driving method for dividing a period for displaying an image of one frame into a period for displaying an image and a period for applying a high voltage in a liquid crystal display device including a liquid crystal panel using an OCB liquid crystal mode. When the liquid crystal display device is applied, the applied voltage of the high voltage minimizes the brightness when displaying black. 2. A driving method in which a period for displaying an image of one frame is divided into a period for displaying an image and a period for applying a high voltage in a liquid crystal display device including a liquid crystal panel using an OCB liquid crystal mode. A liquid crystal display device characterized in that, when the black voltage is applied, the applied voltage for black display during the period of displaying the high voltage and the image is such that the brightness when displaying black is minimum. 3. A driving method for dividing a period for displaying an image of one frame into a period for displaying an image and a period for applying a high voltage in a liquid crystal display device provided with a liquid crystal panel using an OCB liquid crystal mode. When performing, when the ratio of the period for applying the high voltage is a certain value or more, the applied voltage for black display during the period for displaying the high voltage and the image has the minimum luminance when displaying black. If the ratio of the period of applying the high voltage is less than a certain value,
A liquid crystal display device, wherein the high voltage is higher than an applied voltage for black display during a period in which the image is displayed. 4. A liquid crystal display device comprising a liquid crystal panel using an OCB liquid crystal mode and a backlight device,
A period for displaying an image of one frame is divided into a period for displaying an image and a period for applying a high voltage, the voltage application range in the period for displaying an image is expanded, and the voltage application range in the period for displaying the image. The liquid crystal display device is characterized in that at least one is different depending on the display color. 5. The liquid crystal display device according to claim 4, further comprising means for condensing the light of the backlight in the front direction.