JPH05273550A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To provide a liquid crystal display device which maintains an always good display grade without making display images dark and hardly visible in diversified illumination environments from a bright place to a dark place and features a small thickness, lightness in weight, lower electric power consumption, etc. CONSTITUTION:The transmittance of a translucent reflection plate 12 changes with the voltage to be impressed to a high polymer dispersion type liquid crystal layer 25 existing between transparent electrodes 22a and 22b. Then, the characteristic of the translucent reflection plate 13 is adjusted to mainly the transmission type or mainly the reflection type by changing the transmittance or reflectivity thereof, by which the brightness of the display screen is adjusted to be most suitable to the external illumination environment of this time. The display screen is also adjusted to the optimum brightness by adjusting the transmittance of the translucent reflection plate 13 in correspondence to the various external illumination environments.

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 having a semi-transmissive reflection means for effectively adjusting the brightness of a screen.

[0002]

2. Description of the Related Art Liquid crystal display devices have advantages of thinness, light weight, and low power consumption, and are used as image display devices such as OA devices such as word processors and personal computers and portable TVs, and for instruments such as automobiles and aircrafts. In recent years, it has been used in a wide range of fields as a display device for displaying information such as watches and measuring instruments.

Unlike a self-luminous element such as a CRT (cathode ray tube) or an ELP (electroluminescent panel), this liquid crystal display device does not emit light by itself, and therefore is used as a display device. Must use some kind of illumination means or the like for the display element.

Illuminating means of such a liquid crystal display device is roughly classified into a transmission type, a reflection type and a semi-transmission reflection type.
There are two types.

The transmissive type is provided with illumination means such as a small fluorescent lamp or an EL light emitting element on the back surface of the liquid crystal display element, and the liquid crystal display element is supplied from this illumination means by changing the transmittance and transmitted to the front surface of the display panel. The image is expressed by adjusting the intended light and used as a transmitted illumination for display. Therefore, the transmissive type can be used in a dark place like the self-luminous element, and the brightness of the display screen can be adjusted by controlling the intensity of the illumination.

On the other hand, on the other hand, the power consumption of the illumination means is large, so that the advantage of the low power consumption of the liquid crystal display element cannot be utilized, and a power source or the like for supplying power to the illumination means is required. The entire display device is relatively large and heavy. For this reason, the transmissive type is mainly used as a display device of a relatively large size that is rarely carried around, such as an OA device such as a desktop word processor.

On the other hand, the reflective type displays in the environment in which the liquid crystal display device is used, using external light such as sunlight or indoor lighting applied to the liquid crystal display device as the reflective illumination. Therefore, unlike the transmissive type, the reflective type can omit the power consumption of the illuminating means, so that the advantage of the low power consumption of the liquid crystal display element can be utilized. Taking advantage of this feature, the reflection type is mainly used for wristwatches and calculators intended for carrying, and for portable OA devices such as notebook type personal computers and portable word processors.

However, on the other hand, the brightness of the display greatly depends on the lighting condition of the environment in which it is used, and the display is very difficult to see, especially in a dark place, so that it is not suitable for use in a dark place.

In order to solve this problem, a small display area such as a wristwatch is provided with auxiliary lighting means on the side surface or the front surface, which is used in a dark place where the illuminance of external lighting is insufficient. In some cases, the auxiliary lighting means is turned on. However, a portable OA such as a notebook computer having a larger display area
In devices and the like, in order to realize a sufficient auxiliary lighting effect, the auxiliary lighting means must be large enough to fit the display area. For this reason, the portable OA device and the like as described above are usually not equipped with auxiliary lighting.

As described above, the reflection type is not suitable for use in a dark place.

The semi-transmissive reflective type is a semi-transmissive plastic sheet or a semi-transmissive plastic sheet or a magic mirror such as a magic mirror whose main part is composed of a mesh metal reflective film disposed on the diffuser plate. A transflective plate is arranged between a liquid crystal display element of a transmissive liquid crystal display device and an illuminating means on the back surface thereof. In a bright lighting environment, the back lighting means is not turned on and the transflective plate is used as a reflective plate in a reflective manner to display, and in a dark lighting environment the back lighting means is turned on. Then, the semi-transmissive reflector is used like a transmissive type for display.

As described above, the semi-transmissive reflective type can be used as both a transmissive type and a reflective type, thereby compensating for the shortcomings of both types, thus enabling low power consumption and use in a dark place. Is.

However, the characteristics of the transflective liquid crystal display device as described above are fixedly determined by setting the transmissivity and the reflectivity of the material such as the mesh metal reflective film used for the transflective plate. It That is, when a semi-transmissive reflector having a high transmittance and a low reflectance is used, the illumination characteristics of the transmissive main body are obtained, and conversely, when a semi-transmissive reflector having a low transmittance and a high reflectance is used, It has lighting characteristics.

Such a characteristic has a trade-off relationship between the transmittance and the reflectance, if a loss such as light scattering in the semi-transmissive reflector is subtracted, which is inherent in the semi-transmissive reflector. Takes the value of.

Therefore, in such a semi-transmissive reflection type liquid crystal display device, since the transmittance is lower than that of the transmission type liquid crystal display device, the utilization efficiency of the light of the back lighting is low and the displayed image in a dark place looks dark. There is a problem of becoming painful. Also, since a part of the incident light is transmitted, the reflectance is low,
There is a problem that a display image in a bright place is dark and hard to see as compared with a reflective liquid crystal display device.

Further, since the relationship between the transmittance and the reflectance is in a trade-off relationship, if the transmission type is set, the display image becomes dark and hard to see when used as the reflection type, and the reflection type is used. When set to, there is a problem that the display image is dark and difficult to see when used as a transmissive type.
If it is set so as to be compatible with both the transmissive type and the reflective type, there is a problem that the range of correspondence to the external lighting environment is narrowed.

[0017]

As described above, in the liquid crystal display device according to the prior art, the transmissive type can be used even in a dark place, and the brightness of the display screen can be controlled by controlling the intensity of illumination. Although it can be adjusted, there is a problem that the power consumption of the illuminating means is large and a power source for supplying power to the illuminating means is required, so that the entire liquid crystal display device becomes relatively large and heavy.

Further, since the reflective type can omit the power consumption of the illuminating means, it can take advantage of the low power consumption of the liquid crystal display element. However, on the other hand, the brightness of the display greatly depends on the lighting condition of the environment in which it is used, and the display is very difficult to see especially in a dark place, which is not suitable for use in a dark place.

Further, the semi-transmissive reflection type is intended to have low power consumption and can be used in a dark place. However, since the relationship between the transmittance and the reflectance is almost in a trade-off relationship, if the transmissive type is set, the display image becomes dark and hard to see when used as the reflective type, and the reflective type is set. If you set it so that the display image is dark and difficult to see when used as a transmissive type, setting it so that it is compatible with both the transmissive type and the reflective type narrows the range of correspondence to the external lighting environment, There is a problem that sufficient brightness of a displayed image cannot be obtained as a mold or a reflection type. In addition, in portable OA devices such as notebook computers, which have been receiving attention in recent years, since they are thin and lightweight, have low power consumption, and can be carried around, they can be used in various lighting environments. Characteristics are becoming more and more demanding.

The present invention has been made to solve such a problem, and an object thereof is to prevent a displayed image from being dark and difficult to see in a variety of lighting environments from a bright place to a dark place. An object of the present invention is to provide a liquid crystal display device which is always suitable for a portable OA device such as a notebook type personal computer, which has features such as thin and light weight and low power consumption while maintaining good display quality.

[0021]

In order to achieve the above-mentioned object, a liquid crystal display device of the present invention has two substrates having electrodes such that the electrodes face each other and a liquid crystal composition is sandwiched between the electrodes. A liquid crystal display device having a liquid crystal display element disposed on the liquid crystal display device, and a back lighting device that is provided outside the liquid crystal display device and supplies back transmitted light to the liquid crystal display device, wherein the liquid crystal display device and the back lighting device are provided. And a semi-transmissive reflection unit in which at least one of the transmittance and the reflectance is variable.

As a specific embodiment, a light scatterer whose transmittance and reflectance are electrically variable is used as the semi-transmissive / reflecting means. Specifically, as this light scatterer, for example, a polymer dispersed liquid crystal element or a fine particle dispersed liquid crystal element using fine particles such as alumina is suitable.

It is also possible to adjust the reflection characteristics of the semi-transmissive reflection means by adhering a light-transmitting reflector adjacent to at least one main surface of the semi-transmissive reflection means.

Further, a metal thin film having a structure like a mesh may be used as the above-mentioned light-transmitting reflector, and may also be used as an electrode for applying an electric field to the above-mentioned light-scattering body.

[0025]

In the liquid crystal display device of the present invention, at least one of the transmissivity and the reflectivity of the semi-transmissive / reflecting means is made variable, and when used in a dark illumination environment, the optimum transmissive liquid crystal display device is used. The transmissivity of the semi-transmissive reflection means is adjusted so that it becomes the optimum value, and the reflection of the semi-transmissive reflection means is adjusted to the optimum reflectivity as a reflective liquid crystal display device when used in a bright illumination environment. Since the rate can be adjusted, good display with optimum brightness can always be realized in any lighting environment.

In addition, the liquid crystal display device of the present invention is characterized by the semi-transmissive reflection type, and when the back lighting is unnecessary such as when used in a bright place, the display is performed by the reflection lighting of the external light. It can be turned off, and since it uses a polymer dispersion type liquid crystal element or a fine particle dispersion type liquid crystal element using fine particles such as alumina as a semi-transmissive reflection means, its power consumption is small, and the whole device can be used. As a result, it is possible to reduce the size of a power source or the like that supplies power to the lighting means.

Therefore, it is possible to realize characteristics such as thinness and lightness and power saving as the entire apparatus. From this characteristic, the liquid crystal display device of the present invention is suitable as a portable OA device such as a notebook type personal computer and a portable word processor.

[0028]

【Example】

(Embodiment 1) Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a sectional view showing the structure of the liquid crystal display device of the present invention, and FIG. 2 is a sectional view showing the structure of the transflective plate.

The liquid crystal display device of the present invention comprises a liquid crystal display element 9
The rear illumination device 11 and the semi-transmissive reflection plate 13 as the semi-transmissive reflection means constitute a main part thereof.

The liquid crystal display element 9 comprises a liquid crystal cell 6 and polarizing plates 7 and 8. The liquid crystal cell 6 has a transparent electrode 2
a, 2b and substrates 1a, 1b having polyimide alignment films 3a, 3b arranged so as to cover them
Are arranged so that the transparent electrodes 2a and 2b face each other, the liquid crystal composition 4 is sandwiched in the substrate gap (cell gap), and the peripheries of the substrates 1a and 1b are fixed and sealed with a sealant 5 made of an epoxy adhesive. It must be stopped. As the liquid crystal composition 4, STN (super twist nematic) having a positive dielectric anisotropy and twisting the major axis direction of liquid crystal molecules in the liquid crystal cell counterclockwise by 240 degrees is used. )
Type liquid crystal is used. Polarizing plates 7 and 8 are attached to the display surface side and the back surface side of the liquid crystal cell 6, respectively. Display capacity of such a liquid crystal display element 9 (substantially the number of display pixels)
Is 640 × 400 pixels.

A back lighting device 11 is arranged on the back side of such a liquid crystal display element 9. And the liquid crystal display element 9
A semi-transmissive reflection plate 13 is disposed between the back lighting device 11 and the back lighting device 11.

The back lighting device 11 is connected to a back lighting power source and a back lighting switch (both not shown). When the back lighting switch is turned on, the back lighting device 11 receives power from the back lighting power source and operates as described above. Light is supplied to the liquid crystal display element 9 from the back surface.

The semi-transmissive reflection plate 13 uses a polymer dispersion type liquid crystal element, and a minute liquid crystal composed of a liquid crystal composition having a positive dielectric anisotropy in a polymer substrate 23 made of an ultraviolet curable resin. The polymer-dispersed liquid crystal layer 25 in which the particles 24 are dispersed is disposed on the transparent electrodes 22a, 22 from the upper and lower surfaces thereof.
Two substrates 21a and 2 each having b disposed on the inward side
It is sandwiched by 1b. The transparent electrodes 22a and 22b are connected to an AC variable power source (not shown), and the voltage applied to the polymer dispersed liquid crystal layer 25 between the transparent electrodes 22a and 22b is adjusted by adjusting the voltage of the AC variable power source. It is possible to change.

In the transflective plate 13 having such a structure, as shown in the applied voltage-transmittance characteristic curve of FIG. 3, the transmittance is applied to the polymer dispersed liquid crystal layer 25 between the transparent electrodes 22a and 22b. It changes depending on the voltage. That is, the polymer dispersed liquid crystal layer 25 has a threshold voltage Vth of about 10 [V] and a saturation voltage Vsat of about 20 [V], and has a transmittance of 1 when the applied voltage is equal to or lower than the threshold voltage Vth. It is as low as [%] or less, and becomes cloudy, and the reflectance is high, while the saturation voltage Vsa
When the value is t or more, the transmittance is as high as 95 [%], and it becomes almost transparent and the reflectance becomes low. When the applied voltage is a voltage between the threshold voltage Vth and the saturation voltage Vsat, the transmittance changes according to the value of this voltage, and the higher the voltage, the higher the transmittance.

Therefore, the transmittance and the reflectance can be changed by appropriately adjusting the applied voltage to the voltage value during this period, whereby the characteristics of the semi-transmissive reflection plate 13 are changed to those of the transmission type.
Alternatively, the display screen can be adjusted to a brightness that is most suitable for the external lighting environment at that time by adjusting to a reflective type. In a place where the external illumination environment is bright, the voltage applied to the polymer-dispersed liquid crystal layer 25 is lowered to lower the transmittance of the semi-transmissive reflection plate 13 to adjust it mainly to the reflection type, and the back lighting device 11 is turned off to prevent external light. The display is made by the effective use of the reflective illumination, and in a dark place, the voltage applied to the polymer dispersion type liquid crystal layer 25 is increased to increase the transmissivity of the semi-transmissive reflection plate 13 so that the transmissive type is mainly adjusted and the back lighting device is used. 11 is turned on, and the display is made by back lighting that effectively uses the light. Then, the applied voltage is adjusted in accordance with various external lighting environments to change the transmissivity of the semi-transmissive reflection plate 13 to an appropriate value, so that the display screen has an optimum brightness.

The liquid crystal display device of the present invention equipped with such a semi-transmissive reflection plate 13 was multiplex driven under various illumination environments to display a test pattern, and the display quality was visually verified.

As a result, in a very bright place such as a sunny daytime outdoors, the back lighting device 11 is turned off, and the voltage applied to the polymer dispersed liquid crystal layer 25 is also turned off, so that the semi-transmissive reflection plate 13 is turned on. It was confirmed that the display screen had good brightness when it was used as a reflective type by lowering the transmittance of the device to a threshold voltage value Vth or less. Further, in a very dark place such as a room where the light is turned off, the back lighting device 11 is turned on and the voltage applied to the polymer dispersed liquid crystal layer 25 is set to a saturation voltage value Vsat or more to increase the transmittance of the semi-transmissive reflection plate 13. It was confirmed that the display screen had good brightness when it was used as a reflective type after being lowered to the minimum value. In addition, the same display as above is performed under various lighting environments such as indoor lighting such as a fluorescent lamp or an incandescent lamp, and the voltage applied to the polymer dispersed liquid crystal layer 25 is adjusted appropriately. When the brightness of the display screen is changed according to the lighting environment, there is no problem that the display screen is difficult to see due to insufficient lighting unlike the liquid crystal display device according to the related art, and it is always displayed in any lighting environment. I was able to adjust the brightness of the screen so that it was easy to see.

The voltage applied to the transparent electrodes 22a and 22b of the transflective plate 13 may be adjusted by manually adjusting the voltage supplied from the AC variable power source using a volume switch. Alternatively, an optical sensor for detecting the brightness of the external lighting environment is provided near the display panel of the liquid crystal display device, and means for automatically adjusting the supply voltage based on the information of the external lighting environment detected by the optical sensor is provided. It may be used and automatically adjusted.

Further, as the liquid crystal display element, in addition to the STN type liquid crystal display element as described above, a TN type liquid crystal display element or the like may be used.

(Embodiment 2) A semi-transmissive reflection plate 1 composed of a polymer dispersed liquid crystal element in the liquid crystal display device of the first embodiment.
Instead of 3, a transflective plate 213 composed of a fine particle dispersion type liquid crystal element was used. The structure of this fine particle dispersion type liquid crystal element is shown in the sectional view of FIG. That is, the semi-transmissive reflection plate 213 of the liquid crystal display device of the second embodiment has a diameter of about 1 in the liquid crystal layer 223 made of a liquid crystal composition having a positive dielectric anisotropy.
The fine particle-dispersed liquid crystal layer 225 in which the alumina particles 224 of μm are dispersed is disposed on the transparent electrode 22 from the upper and lower surfaces thereof.
2a and 222b are sandwiched between two substrates 221a and 221b, which are respectively arranged on the inward side. Then, as in the first embodiment, the transparent electrodes 222a, 22a
Reference numeral 2b is connected to an AC variable power source (not shown). By adjusting the voltage of the AC variable power source, the transparent electrode 222a,
The voltage applied to the fine particle dispersed liquid crystal layer 225 between 222b can be changed.

The transflective plate 213 having such a structure is
The transmittance changes depending on the voltage applied to the fine particle dispersed liquid crystal layer 225 between the transparent electrodes 222a and 222b. That is, the fine particle dispersed liquid crystal layer 225 has a threshold voltage V
th is about 5 [V] and saturation voltage Vsat is about 10 [V],
The transmittance and reflectance can be changed by appropriately adjusting the applied voltage to a voltage value in the meantime, whereby the characteristics of the semi-transmissive reflector 213 are adjusted to a transmissive main body or a reflective main body. The brightness of the display screen most suitable for the external lighting environment can be adjusted. In a bright place as an external lighting environment, the fine particle dispersed liquid crystal layer 2
The transmissivity of the semi-transmissive reflection plate 213 is lowered by adjusting the voltage applied to 25 to adjust it to a reflective type, the back lighting device 211 is turned off, and the display is performed by the reflection illumination that effectively uses the external light. Then, the voltage applied to the fine particle dispersion type liquid crystal layer 225 is increased to increase the transmissivity of the semi-transmissive reflection plate 213 so that the transmissive type is mainly adjusted, and the back lighting unit 211 is turned on to perform the back lighting effectively using the light. Displayed by The semi-transmissive reflection plate 213 corresponds to various external lighting environments.
Adjust the transmittance of to an appropriate value to obtain the optimum brightness on the display screen.

The liquid crystal display device of the present invention having such a semi-transmissive reflection plate 213 was multiplex driven under various illumination environments to display a test pattern, and the display quality was visually verified.

As a result, there is no problem that the display screen is difficult to see due to lack of illumination, and the brightness of the display screen can be adjusted to be easy to see in any lighting environment as in the first embodiment. It was

The semi-transmissive reflection plate 213 of the liquid crystal display device according to the second embodiment consumes only about half the power as compared with the semi-transmission reflection plate 13 of the liquid crystal display device of the first embodiment. It is unnecessary, and the power consumption is smaller than that of the liquid crystal display device of the first embodiment.

(Embodiment 3) In the liquid crystal display device of the first embodiment, a reflector 17 formed by patterning an aluminum vapor-deposited thin film on the surface of the semi-transmissive reflection plate 13 facing the polarizing plate 8 in a fine mesh structure. Was attached, and the reflectance of the semi-transmissive reflection plate 13 was supplemented to obtain a liquid crystal display device having improved reflection characteristics.
In the liquid crystal display device of the third embodiment, the reflective display screen in a bright place is brighter and easier to see than the liquid crystal display device of the first embodiment. Further, the number of occasions in which the back lighting device 11 is required is smaller than that in the first embodiment, so that the power consumption can be further reduced. On the other hand, the aperture ratio of the fine mesh structure of the aluminum vapor-deposited thin film is set to an appropriate value so that even if the reflectance is improved as described above, the transmittance does not significantly decrease, and it can be used in a dark place. Even when used as a transmissive display device, the display screen is bright and easy to see as in the liquid crystal display device according to the first embodiment.

As a result, the brightness of the display screen could be adjusted so that it was easy to see in any lighting environment. In particular, the display screen of the reflective type is brighter and easier to see than the first embodiment.

(Embodiment 4) In the liquid crystal display device of the first embodiment, the reflector 17 used in the third embodiment is formed on the surface of the substrate 1a facing the substrate 1b instead of the transparent electrode 2a, This was used as an electrode for applying a voltage to the polymer dispersed liquid crystal layer 25. The reflector 17 has a fine mesh structure of an aluminum vapor deposition thin film as described above,
Compared with the transparent electrode 2a made of a material such as ITO, it has a low electric resistance and excellent conductivity, and therefore is suitable as an electrode, and also serves as a reflector and an electrode. It is possible to omit the step of disposing the transparent electrode 2a and the material cost as in the embodiment to avoid the complexity of the manufacturing process and reduce the cost.

(Comparative Example 1) In the liquid crystal display device of the first embodiment, a conventional semi-transmissive reflective polarizing plate was used in place of the polarizing plate 9 and the semi-transmissive reflective plate 13, and
In the same manner as in Example 1, multiplex driving was performed under various illumination environments to display a test pattern, and the display quality was visually verified.

As a result, in the semi-transmissive reflective polarizing plate according to the prior art as described above, the light transmittance and the reflectance cannot be changed because they are fixed, so that the reflectance is low in a bright place. As a result, the apparent contrast of the entire screen was significantly reduced, and the brightness of the entire screen was insufficient due to the low transmittance in a dark place, resulting in a dark display, which made the display screen very unsightly.

[0051]

As is clear from the detailed description above, the liquid crystal display device of the present invention is always good under a variety of lighting environments from bright places to dark places without causing a display image to be dark and difficult to see. It is a liquid crystal display device suitable for a portable OA device such as a notebook computer, which has features such as excellent display quality, thinness, light weight, and low power consumption.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the structure of a liquid crystal display device according to a first embodiment.

FIG. 2 is a sectional view showing the structure of a semi-transmissive reflector according to the first embodiment.

FIG. 3 is a diagram showing a voltage applied to the semi-transmissive reflector according to the first embodiment.
The figure which shows a transmittance characteristic curve.

FIG. 4 is a sectional view showing the structure of a semi-transmissive reflector according to a second embodiment.

[Explanation of symbols]

 1a, 1b ... Substrate 2a, 2b ... Transparent electrode 3a, 3b ... Alignment film 4 ... Liquid crystal composition 5 ... Sealing agent 6 ... Liquid crystal cell 7, 8 ... Polarizing plate 9 ... Liquid crystal display element 11 ... Back lighting device 13 ... Semi-transmission Reflectors 21a, 21b ... Substrate of semi-transmissive reflector 22a, 22b ... Transparent electrode of semi-transmissive reflector 23 ... Polymer substrate 24 ... Liquid crystal particles 25 ... Polymer dispersed liquid crystal layer

Claims (1)

[Claims] 1. A liquid crystal display device comprising two substrates having electrodes arranged so that the electrodes face each other and sandwich a liquid crystal composition between the electrodes, and the liquid crystal display device disposed outside the liquid crystal display device. In a liquid crystal display device having a back lighting device that supplies back transmitted light to a display element, at least one of transmissivity and reflectance is variable semi-transmission between the liquid crystal display device and the back lighting device. A liquid crystal display device comprising a reflection means.