JPH08292429A - Liquid crystal display device - Google Patents

Abstract

PURPOSE: To provide a liquid crystal display device which causes neither burning nor a flowing even when the same display is held for a long time. CONSTITUTION: Polarizing plates 13 and 14 are arranged on the same surfaces of a liquid crystal panel 11 which changes in the polarized state of transmitted light according to whether or not an electric field is applied, and a phase shift plate 12 which shifts the phase of transmitted light or does not shift it according to whether or not the electric field can be applied is arranged between the polarizing plate 14 and liquid crystal panel 11. Then the polarized state of the liquid crystal panel 11 is inverted at a constant period and the phase shift state of the phase shift plate 12 is also switched accordingly to display the same image.

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,
More specifically, the present invention relates to a liquid crystal display device that does not have a screen burn-in or disturbance even though the same screen is displayed for a long time, and particularly to a liquid crystal display device using a ferroelectric liquid crystal.

[0002]

2. Description of the Related Art In a ferroelectric liquid crystal device, a chiral smectic liquid crystal is filled in a gap between two horizontally aligned substrates, which is sandwiched between two polarizing plates, and the molecular axis direction changes depending on the direction of an electric field. This is used to control the transmission and blocking of light. When the substrate gap is extremely thin, around 1 μm, the smectic screwing structure is unraveled and two or more stable states appear. At this time, there is a property (memory property) of maintaining the previous state even if the electric field is cut off.
The contrast does not decrease even when the number of divisions increases in so-called time division driving, and it can be said that the element is suitable for a large-capacity display device.

[0003]

However, some problems have been pointed out in the ferroelectric liquid crystal device having the memory property. The first is what is called burn-in, which is a phenomenon that when the same picture is continuously displayed for a long time, the liquid crystal alignment is fixed in that state and cannot be switched to another state. The cause is not always clear, but it is because the ionic impurities are accumulated and fixed near the interface due to the spontaneous polarization of the liquid crystal, and the interface molecules that were initially oriented in the layer normal direction gradually tilted. It is explained that it is for doing so.

Another problem is the phenomenon that the liquid crystal flows little by little and moves when the same picture is continuously displayed for a long time. It is considered that this is a result of the flow caused by the liquid crystal vibrating around the stable position by the non-selection voltage of the time division drive.

Both of these are phenomena caused by keeping the liquid crystal in the same stable state for a long time of several hours or more. This phenomenon has been a serious problem in practical use because it is common to use the display device for the same display for a long time.

It is an object of the present invention to provide a liquid crystal display device which does not cause burning or flow even if the same display is performed for a long time.

[0007]

The present invention provides a liquid crystal display device comprising a liquid crystal panel in which polarizing plates are arranged on both sides and the polarization state of transmitted light is switched depending on whether or not an electric field is applied. A phase shift plate that changes the phase shift state of transmitted light depending on whether or not an electric field is applied is disposed between the liquid crystal panels to invert the polarization state of the liquid crystal panel at a constant period, and the phase shift of the phase shift plate accordingly. It is characterized in that the state is switched and the same image is displayed.

The liquid crystal panel may be composed of a ferroelectric liquid crystal element, and the phase shift plate may be composed of a ferroelectric liquid crystal element having the same optical characteristics.

Further, it is preferable that the axes of the liquid crystal panel and the phase shift plate are orthogonal to each other.

The phase shift plate may be a TN liquid crystal element having a twist of 90 degrees.

Further, the phase shift plates are arranged on both sides of the liquid crystal panel, and the magnitude of the twist is equal to twice the tilt angle of the liquid crystal panel made of a ferroelectric element, and the direction is opposite. You may comprise.

[0012]

Since the present invention is constructed as described above,
Inverts the polarization state of the transmitted light of the liquid crystal panel at a constant cycle,
By switching the phase shift state of the phase shift plate periodically in the inversion so that the amount of light passing through the polarizing plate does not change,
Although the image displayed on the screen looks the same, the application state of the electric field to the liquid crystal of the liquid crystal panel changes, preventing the liquid crystal from being kept in the same state for a long time, preventing screen burn and image flow. It becomes possible to do.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 shows the components of this embodiment, which comprises a liquid crystal panel (display panel) 11, a phase shift plate 12, polarizing plates 13 and 14, and a lighting device 15. The liquid crystal panel is a ferroelectric liquid crystal element that takes two stable states, and matrix display can be performed by two groups of orthogonal stripe electrodes. The phase shift plate is made of the same ferroelectric liquid crystal as the display panel, and is different from the display panel except that there is no matrix electrode and a single electrode can switch between two states at once. It has the same element characteristics, and the first stable state of the display panel and the first phase shift plate.
And the second stable state of the display panel and the second stable state of the phase shift plate are arranged so as to be orthogonal to each other. A configuration similar to this is disclosed in JP-A-4-1862.
No. 30 is disclosed.

FIG. 2 shows the polarization state after the light passes through the respective constituent elements of FIG. 1. (a) shows the case where both the display panel and the phase shift plate have the first liquid crystal arrangement, (b). When the display panel has the second liquid crystal arrangement and the phase shift plate has the first liquid crystal arrangement, (c) shows when the display panel has the first liquid crystal arrangement and the phase shift plate has the second liquid crystal arrangement. , (D) shows the case where both the display panel and the phase shift plate have the second liquid crystal arrangement.

Light incident from a rear light source (not shown) is linearly polarized in the x direction by the polarizing plate 14. The display panel 11 is placed so that the molecular axis switches between the x direction and a direction forming an angle of 2θ (θ is the tilt angle of the ferroelectric liquid crystal) with the molecular axis facing the x direction. When it is, the light polarized in the x direction is transmitted as it is ((a) and (c)), and when it is in the 2θ direction, it is converted into linearly polarized light in the almost 4θ direction ((b) and (d)). . θ
Is usually selected to have an optimum value of 22.5 ° in order to maximize the transmittance, but in the present invention, it does not have to be that value.

Further, it is desirable that the retardation of the liquid crystal is set to approximately 1/2 of the wavelength in the wavelength region of the incident light from the viewpoint of the transmittance, but this condition is not essential to the present invention. Absent. FIG. 2 shows that θ = 22.
Although the drawing is made with 5 ° and retardation = 1/2 wavelength, the same effect can be obtained even if the condition is deviated from this condition.

The light transmitted through the liquid crystal enters the phase shift plate 12. The phase shift plate 12 is made of ferroelectric liquid crystal similarly to the display panel 11, but is arranged so that the first liquid crystal arrangement and the second liquid crystal arrangement are orthogonal to the display panel 11. .

Therefore, the light transmitted through the phase shift plate 12 is
(A) shows the incident light as it is, and (b) shows the polarized light in the 4θ direction is −.
It is converted to polarized light in the 4θ direction.
It is converted to polarized light in the θ direction, and in (d), polarized light in the 4θ direction is x.
Is converted into directional light.

In the cases of (b) and (c), depending on the wavelength of the incident light, elliptically polarized light is deviated from the ± 4θ directions, but in the case of (d), the retardation of the phase shift plate 12 is made the same as that of the display panel. Therefore, the phase shift plate transmittance returns to the original phase shift deviation, and becomes linearly polarized light in the x direction regardless of the wavelength.

The intensity of light that finally passes through the front polarizing plate
Therefore, assuming that the incident light intensity is 1, (a) is 0,
(B) is (sin4θ)² , (C) is (sin4θ)²,
(D) becomes 0. However, (b) and (c) have wavelength dependence
However, normally, the retardation is performed so that the transmitted light becomes almost white.
Is designed.

As described above, the function of the phase shift plate is reversed between when the phase shift plate 12 is placed in the first liquid crystal arrangement and when it is placed in the second liquid crystal arrangement, and the display panel 1 is also adjusted accordingly.
For 1 as well, if the two states are exchanged, light transmission /
The interruption remains.

FIG. 3 shows how a picture displayed in a matrix on the display panel 11 looks after passing through the polarizing plate 13. When the state of the phase shift plate 12 is reversed as shown in FIGS. 3A and 3B and the display panel is negative-positive reversed in synchronization with the state, the overall display state does not change. If this inversion is performed in a fixed cycle, the display state does not change and the arrangement of the liquid crystal can be switched in a fixed cycle, so that both the burn-in and the flow caused by leaving one state for a long time can be prevented.

In this embodiment, the display panel 11 and the phase shift plate 1 are used.
2 is made up of ferroelectric liquid crystal elements orthogonal to each other. It is known that the ferroelectric liquid crystal element has a viewing angle characteristic derived from the orientation processing direction. In this case, however, since the orientation processing directions of the display panel 11 and the phase shift plate 12 are orthogonal to each other, the viewing angle characteristics cancel each other out. There is also an effect that uniform contrast can be obtained over a wide angle.

In this embodiment, the display panel and the phase shift plate are made of the same ferroelectric liquid crystal, but it is clear that different ferroelectric liquid crystals may be used as long as the tilt angle and the retardation are the same. Is. For example, a polymer ferroelectric liquid crystal sandwiched between flexible resin films can be used as the phase shift plate.

Negative / positive inversion of display can be easily performed by switching image signals. The inversion cycle can be set arbitrarily, but if it is too frequent, it may cause flicker, so a long cycle such as 1
Hours to 2 hours are desirable.

FIG. 4 shows a control circuit of this embodiment. In addition to the drive circuits 41 and 42 for normal display and the control circuit 43, a drive circuit 44 for the phase shift plate 12, a timer circuit 45 for generating an inversion signal, and a signal from the timer circuit to receive an image signal and invert the image signal. It is composed of a circuit 46.

FIG. 5 shows the timing of the signals of the control circuit of FIG. A pulse 51 is generated from the timer circuit 45 every hour, and the flip-flop of the inverting circuit 46 is switched every time. The flip-flop output 52 is input to the XOR gate with the image signal. From the XOR gate,
When the flip-flop output is at the low level, the image signal 53 remains as it is, and the flip-flop output 52 is high.
At the level, an inverted image signal is output. The flip-flop output 52 is also transmitted to the phase shift plate drive circuit 44 to generate the phase shift plate drive voltage 54, and when the phase shift plate is at the low level, the phase shift plate is output as shown in FIG. 3 (a), and when it is at the high level, the phase shift plate is brought into the states shown in FIGS. 2 (c) (d) and 3 (b).

(Second Embodiment) FIG. 6 shows a second embodiment of the present invention, which is the same as the first embodiment except that a 90 ° TN (twisted nematic) liquid crystal element is used as the phase shift plate 61. Is. When an electric field is applied to the TN device, liquid crystal molecules become perpendicular to the substrate, so that light transmitted through the display panel passes through the phase shift plate as it is. When the electric field is cut off, a 90 ° twisted orientation occurs, and the polarization plane of the transmitted light of the display panel is rotated by 90 ° in any state. Therefore, in the final transmission state after passing through the second polarizing plate, transmission / blocking is reversed by application / non-application of an electric field to the TN element. As in the first embodiment, the phase shift plate is switched at a constant cycle and the display is negative-positive inverted, so that the liquid crystal arrangement can be switched at a constant cycle without changing the apparent display state.

(Embodiment 3) FIG. 7 shows a third embodiment of the present invention. The difference from the second embodiment is that the TN element phase shifter 71a,
The twist of 71b is set to 2θ, and two sheets are arranged in front of and behind the display panel 11 plate. However, the molecular twist of the front phase shift plate 71a is opposite to that of the latter phase shift plate 71b. In the second embodiment, of the light transmitted through the display panel, the one having the phase shift caused by the birefringence has a slightly different polarization state depending on the wavelength, and therefore, the phase shift plate causes
The ratio of being blocked by the second polarizing plate after being twisted by 0 ° also differs depending on the wavelength. That is, there is a problem that some light is transmitted even in the cutoff state, resulting in a reduction in contrast.

In this embodiment, the state in which the electric field is simultaneously applied to the two phase shift plates and the state in which the electric field is cut off are switched at a constant cycle, and the image on the display panel 11 is also inverted in synchronization with this.

The polarization of light in each state will be described below with reference to FIG. In the following description, it is assumed that the two TN elements forming the phase shift plate are both ideal in that the twist pitch is sufficiently larger than the wavelength and the polarization plane is rotated by 2θ regardless of the wavelength.

As shown in (a), when an electric field is applied to the phase shift plate and the liquid crystal of the display panel 11 is in the first arrangement, the light transmitted through the first polarizing plate changes its polarization state as it is. Without reaching the second polarizing plate, the light is blocked. Birefringence does not occur by any liquid crystal plate, so that it is blocked regardless of the wavelength.

As shown in (b), when the liquid crystal of the display panel 11 is in the second arrangement, the plane of polarization is 4θ due to the birefringence effect.
(Since θ is set to around 22.5 ° as described above, 4θ is about 90 °), and the light reaches the second polarizing plate, so that the light is transmitted.

As shown in (d), when the electric field of the phase shift plate is turned off, the light transmitted through the first polarizing plate 14 has a polarization plane of 2θ.
Only twisted to reach the display panel 11. Display panel 1
If the liquid crystal of 1 is in the second arrangement, the light passes through the display panel as it is, and the plane of polarization is returned by 2θ in the opposite direction by the second phase shift plate, so that it is blocked by the polarizing plate.

As shown in (c), when the liquid crystal of the display panel 11 is in the first arrangement, the light undergoes birefringence by the display panel and the plane of polarization rotates in the opposite direction by 4θ. After that, since the second phase shift plate further rotates by 2θ, when reaching the second polarizing plate, the polarization plane is rotated by −4θ as compared with immediately after passing through the first polarizing plate. Therefore, the light is in the transmissive state.

In this structure, the light that has been subjected to birefringence is always in the transmission state, so that there is no light leakage in the cutoff state due to the difference in birefringence depending on the wavelength.

In the above embodiments, it is assumed that all liquid crystals have two states. However, even when displaying an intermediate state by changing the area ratio within one pixel, the liquid crystal arrangement can be similarly switched at a constant cycle. It goes without saying that you can do it.

[0039]

As described above, according to the present invention,
An electro-optical element including a liquid crystal panel that switches the polarization states of two transmitted lights by an electric field and a phase shift plate that takes two phase shift states by the electric field, and a first polarizing plate on a light transmission path in this order. By arranging, the polarization state of the transmitted light of the liquid crystal panel is inverted at a constant cycle, and by switching the phase shift state of the phase shift plate in synchronization with the inversion so that the light amount of the light transmitted through the polarizing plate does not change, It was possible to prevent the liquid crystal from being kept in the same state for a long time, and as a result, it was possible to enhance the durability of the device. The viewing angle characteristics were also improved at the same time.

[Brief description of drawings]

FIG. 1 is a schematic view of a liquid crystal display device according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing transmitted light according to the first embodiment.

FIG. 3 is a diagram showing a usage pattern of the first embodiment.

FIG. 4 is a diagram illustrating a drive circuit according to a first embodiment.

FIG. 5 is a timing diagram of drive control signals according to the first embodiment.

FIG. 6 is an explanatory diagram showing transmitted light according to a second embodiment of the present invention.

FIG. 7 is an explanatory diagram showing transmitted light according to a third embodiment of the present invention.

[Explanation of symbols]

 11 Display Panel (Liquid Crystal Panel) 12 Phase Shift Plate 13 Polarizing Plate 14 Polarizing Plate 15 Illuminating Device 61 Phase Shifting Plate 71a Phase Shifting Plate 71b Phase Shifting Plate

Claims (6)

[Claims] 1. A liquid crystal display device comprising a liquid crystal panel in which polarizing plates are disposed on both sides and the polarization state of transmitted light is switched depending on whether or not an electric field is applied, wherein a liquid crystal display device is provided between the polarizing plate and the liquid crystal panel. By arranging a phase shift plate whose phase shift state of transmitted light changes depending on whether or not an electric field is applied, the polarization state of the liquid crystal panel is reversed at a constant cycle, and the phase shift state of the phase shift plate is also switched accordingly, thereby obtaining the same image. A liquid crystal display device characterized by being configured to display. 2. The liquid crystal display device according to claim 1, wherein the liquid crystal panel is composed of a ferroelectric liquid crystal element. 3. The liquid crystal display device according to claim 1, wherein the liquid crystal panel and the phase shift plate are ferroelectric liquid crystal elements having the same optical characteristics. 4. The liquid crystal display device according to claim 1, wherein axes of the liquid crystal panel and the phase shift plate are orthogonal to each other. 5. The TN in which the phase shift plate has a twist of 90 degrees.
The liquid crystal panel according to claim 1, which is a liquid crystal element. 6. The phase shift plate is disposed on both sides of a liquid crystal panel, and the TN liquid crystal element has a twist size equal to twice a tilt angle of a liquid crystal panel including a ferroelectric element and has a reverse direction. The liquid crystal display device according to claim 1, wherein