1. Field of the Invention
The present invention relates to image display devices and display drive methods and, more particularly, to an image display device which is rewritable repeatedly and display drive method thereof.
2. Description of the Related Art
Conventionally, there have been proposed, as repeatedly rewritable display mediums, twisting ball displays (display with rotation of two-colored particles), electrophoretic display mediums, magnetophoretic display mediums, thermal-rewritable display mediums, storable liquid crystal display mediums.
Among these display mediums, the thermal-rewritable and storable liquid crystal display mediums are excellent in storing images but cannot display sufficient paper-like whiteness in the background. Therefore, in displaying images, the imaging and non-imaging regions have insufficient contrast, thus making it difficult to display a clear image.
Moreover, in the display medium utilizing electrophoresis or magnetophresis, for example, coloring particles which can move under electric or magnetic field are dispersed in a white liquid. In the imaging region, the color of coloring particles is displayed by putting the coloring particles on the display surface. In the non-imaging region, the coloring particles are removed from the display surface to display white with the white liquid. Thus, an image is formed. The coloring particles do not move unless an electric or magnetic field is applied thereto, thus making it possible to store the image displayed.
In these display mediums, white which is displayed with the white liquid is clear. However, when displaying the color of coloring particles, white liquid intrudes in the gap between coloring particles, thereby lowering the density of image. This, accordingly, lowers the contrast between the imaging region and the non-imaging region, making it difficult to display a clear image. Also, because the display medium is filled with the white liquid, when the display medium is removed from the image display apparatus and is roughly handled, the white liquid may leak out of the display medium.
Moreover, in the twisting ball display, spherical particles painted white on a half surface and black on the remaining half are rotated under the action of an electric field. Display is conducted such that, for example, in the imaging region the black surface is directed toward the display surface while in the non-imaging region the white surface is directed toward the display surface. Since the particles are not rotated if an electric-field is not applied to the particles, an image can be stored. Oil exists only in the cavity around the particles. However, because the interior of the display medium is mainly solid, the display medium is comparatively easily made in a sheet form.
In this display medium, however, it is difficult to perfectly rotate the particles. The contrast is lowered by the particles not perfectly rotated, making it difficult to form a vivid display image. Further, even if the white-painted hemisphere be perfectly directed toward the display side, it is still difficult to display paper-like white because of light absorption and scatter in the cavity region and it is difficult to display a vivid image. Furthermore, because the particles are required to have a size smaller than a size of the pixel, fine spherical particles must be produced for displaying images with high resolution, which requires high-level manufacturing technology.
Moreover, there are recent proposals, as completely-solid display medium, of the display mediums in which coloring particles, such as a powder toner, are enclosed in a space between a pair of substrates. For example, these are the display mediums described in Japan Hardcopy, ""99 theses, pp. 249-252. Japan Hardcopy, ""99 Fall Proceedings, pp. 10-13, JP A No. 2000-347483, and the display mediums or the like described in JP-A No. 2001-33833.
These display mediums have a structure having a conductive coloring toner (e.g., black toner) and an insulating coloring toner (e.g., white particle) in a space between a transparent front substrate and a rear substrate. Electrodes are formed on the front and rear substrates. The inner surfaces of the substrates are coated with a charge-transport material to transport only one charge polarity (e.g., holes).
If a voltage is applied to the substrates, holes are injected only to the conductive black toner. The black toner, electrified positive, moves between the substrates while pushing away the white particles according to an electric field formed between the substrates. Herein, black is displayed by moving the black toner toward the front substrate while white of the white particles is displayed by moving the black toner toward the rear substrate. Accordingly, a black and white image can be displayed by applying a voltage to the Substrate to desirably move the black toner according to image information.
The above display medium using coloring particles can store images because the particles do not move if an electric field is not applied thereto. Also, liquid spill does not occur because the display medium is solid. The use of two kinds of coloring particles (e.g., white and black particles) results in image display with high contrast.
Further, the display medium described in Japanese Patent Application No. 2000-165138 proposed by the present inventors has a structure in which two kinds of coloring particles different in color and electrifying characteristics are enclosed in a space between a transparent front substrate and a rear substrate. As the two kinds of coloring particles, particles having different polarities are selected. Consequently, if an electric field is formed between the substrates of the display medium, the two kinds of coloring particles respectively move toward the different substrates. If a voltage is applied to the substrates according to image information, a clear image with high contrast can be displayed.
However, in the display medium enclosing the coloring particles in a space between a pair of substrates, adhesion and coagulation gradually occur as images are displayed repeatedly. Thus, there has been a problem of raising defective display in a dot-like form.
Moreover, in the structure having a gap member to keep a gap between substrates and divide the space between the substrates into a plurality of cells, the particles gradually adhere onto the gap member. Thus, there have been problems in that display contrast is lowered due to a decrease in the number of particles which can actually move or defective display is caused by particles adhering to the gap member.
Also, when the display medium is disposed vertically and used as such and the coloring particles move toward the substrate according to an electric field formed between the substrates, the coloring particles move slightly downward from their previous height due to the action of gravity. Accordingly, change of display if repeated causes the coloring particles to gradually fall, ultimately causing a serious problem of impossible display.
Incidentally, in the structure having a gap member to keep a gap between the substrates and divide the space between the substrates into a plurality of cells, if the cell size is reduced, the movement of coloring particle in the gravity direction can be suppressed within a practical level. However, if the cell size is reduced, there is increase in the ratio of a gap member area to the actual display area on the display surface (area of the region enclosing coloring particles to effect actual display), resulting in lowered contrast of display.
The present invention has been made in view of the above fact, and it is a first object to provide an image display device having coloring particles in a space between a pair of substrates and display drive method thereof capable of preventing the coloring particles from coagulating even when display is repeatedly conducted, and, if the display medium comprises cells, preventing the coloring particles from adhering and coagulating onto a gap member defining the cells.
Also, a second object is to provide an image display device comprising cells formed between a pair of substrates and enclosing coloring particles and display drive method that, when the image display device is disposed vertically and used as such, coloring particles can be prevented from falling and, even if they falls, they can be restored to their initial height, thereby maintaining high display contrast, without having to reduce the size of cells compared with a conventional display medium.
In order to achieve the object, the present invention provides an image display device comprising: an image display medium having a pair of substrates, an electrode provided on each respective substrate, and a plurality of kinds of particles which are enclosed in a space between the substrates and which are movable due to an electric field formed between the electrodes and which have different colors and electrifying properties; and a voltage applying unit for applying to the electrodes an alternating voltage having a frequency to move the plurality of kinds of particles.
A plurality of kinds of particles that are different in color and electrifying characteristics are enclosed in the space between the pair of substrates of the image display medium. The pair of substrates is provided with electrodes. The electrodes may be provided on the inner surfaces of the pair of substrates, on the outer surfaces of the pair of substrates or at the inside of each substrate. By applying an electric field between the pair of electrodes, the particles different in color can be moved between the substrates depending upon the electrifying characteristic of the particles to display images. Incidentally, at least one of the pair of substrates can be made with dielectric substance such as insulating resin which is transparent, semi-transparent or colored transparent. Also, besides insulating particles, conductive, hole-transportable or electron-transportable particles can be used.
The particle involves variation in particle size or electrification amount. Due to this, variation occurs in an electrostatic drive force that the particles accept from an electric field formed between the substrates. Moreover, depending upon an adhesion state of the particles to the substrate or contact state between adjacent particles, the mobilities of particles are different under the same electric field. Accordingly, if an electric field is applied between the substrates, the easily movable particles move but the particles which can not easily move do not move and continue to adhere to the substrate or adjacent particle. Thus, the particles which can not easily move form coagulation with repeating change of display.
Consequently, the voltage applying unit applies to the electrodes an alternating voltage having a frequency to move the plurality of kinds of particles. The alternating voltage is applied as initialization (initializing drive), e.g. each time when image display is changed.
If an alternating electric field is formed between the substrates, the particles which can easily move are reciprocally moved between the substrates. By the collisions of such particles and the particles which can not easily move, the particles which can not easily move are dissociated from adhesion to the substrate or adjacent particles, and can be moved. As a result, particle coagulation is prevented. Moreover, even after already forming particle coagulation, the particles which do not coagulate are reciprocally moved and collide repeatedly against the coagulation, thereby dissociating the coagulation.
Herein, emphasis is placed on the frequency for switching the alternating electric field. The foregoing effect would not be obtained by simply applying an alternating voltage.
Consequently, it is preferred that the frequency is from 20 Hz to 20 kHz.
If the frequency of an alternating voltage is lower than 20 Hz, the particles move toward the opposite substrate under the electric field and adhere once to the substrate in a stable state, and then begin moving again toward the opposite substrate due to an electric field having the reverse direction. This is the same as the state in which display change is rapidly repeated and accelerates particle coagulation, thus rendering conspicuous the occurrence of defective display.
Moreover, if the frequency of alternating voltage is higher than 20 kHz, the movement of particles can not follow the switch rate of electric field, thus extremely lowering the particle moving amount. Thus, the foregoing particle coagulation preventing effect due to collisions cannot be obtained. Furthermore, the particles having less momentum tend to form coagulation.
Accordingly, the frequency of an alternating voltage applied to the substrates is to be set such that the particles favorably, reciprocally move continuously between the substrates. Thus, it is preferred that the frequency is from 20 Hz to 20 kHz.
An initializing drive voltage to form an alternating electric field between the substrates may be applied to the substrates before or after the application of a display drive voltage for image display. However, where display is not conducted for a long time, it is preferred to conduct initialization before applying a display drive voltage. This is because the electrifying amount of some particles slightly decline and the initialization before display also provides the effect that the particle electrifying amount is restored due to frictional electrification by collision between the particles or the particle and the substrate.
Moreover, the initializing drive voltage may be applied simultaneously to all electrodes or to respective electrodes. However, the initializing drive voltage is preferably applied simultaneously to all electrodes. This is because, if a voltage for forming an alternating electric field is applied to some electrodes of the image display medium, the particles between the voltage-applied electrodes which reciprocally move also moves in a direction other than voltage-applied direction and particles may be localize in the image display medium. The simultaneous initialization of all electrodes enables uniform initialization of the display surface.
It is preferred that the image display medium further comprises a gap member for holding the pair of substrates with a predetermined gap and dividing a space between the pair of substrates into a plurality of cells and that the voltage applying unit applies the alternating voltage per each of the cells.
In this manner, when the image display medium has a plurality of cells divided by the gap member, some particles may adhere to the gap member. However, by applying a predetermined alternating voltage to the electrodes as described above, particle coagulation can be prevented and further the particles which adhere to the gap member can be effectively detached therefrom by mechanical collision between the particles reciprocally moving at a high speed.
Incidentally, if the frequency for switching alternating electric field is lower than 20 Hz or exceeds 20 kHz, the particle coagulation preventing effect due to particle collision cannot be obtained and the particles remarkably adhere to the gap member defining the cells as described above.
Moreover, the initializing drive voltage may be applied simultaneously to all electrodes or sequentially to respective electrodes or respective cells. It is however preferred to carry out simultaneous initialization of at least one cell. This is because when, for example, a plurality of electrodes correspond to one cell and an alternating electric voltage is applied to some electrodes within the cell, the particles between the voltage-applied electrodes which reciprocally move also move in a direction other than a voltage-applied direction and particles may localize within the cell. Contrary to this, initialization of at least one cell ensures a uniform initialization within the cell and in turn uniform initialization of the entire display surface.
Furthermore, when an image display medium comprises the electrodes respectively formed on the pair of substrates each of which corresponds to each pixel and to each cell, if initialization is conducted based on the electrode corresponding to each cell, initializing drive voltage can be combined with display drive voltage into one drive voltage thus eliminating the necessity to especially provide a initialization sequence. Also, eliminated is flicker on the display surface as observed when applying initializing voltage simultaneously to the entire display surface. Thus, change of display can be carried out continuously.
When inclining the image display medium with respect to a horizontal direction, it is preferred that the frequency is from 50 Hz to 10 kHz.
The present inventors confirmed that when the image display medium is inclined (e.g., vertically disposed) to repeatedly display images and a high-frequency alternating electric field is applied to the image display medium in a state where the coloring particles have fallen due to gravity, the coloring particles that have fallen and are deposited at the bottom are diffused upward to a certain constant height to thereby restore a display state. It was also confirmed that, by applying a high-frequency alternating electric field at a proper interval during successive display on the vertically disposed image display medium, falling coloring particles can be halted at a certain constant height thereby maintaining the height of display.
Herein, emphasis is placed still on the frequency for switching alternating electric field. The above particle diffusion effect is obtainable at a frequency for alternating electric field of from 20 Hz to 20 kHz. However, the effective effect is obtainable at from 50 Hz to 10 kHz. Particularly, it is preferred to set it at from 100 Hz to 3 kHz. In this case, the display height when applying a high-frequency alternating electric field for initialization (height of the uppermost particle from the lower most of the image display medium) can be from several times to 10-20 times as high as the display height when no alternating electric field is acted upon. Accordingly, when using the display medium such that it is inclined relative to the horizontal direction, the application of an alternating electric field based on that frequency range can effectively prevent localization of the particle due to gravity,
Moreover, if the cells are set to a size that the particle is to be diffused by applying a high-frequency alternating electric field, the particles can be completely prevented from falling even if the image display medium is disposed vertically and used. In this case, by applying as initialization a high-frequency alternating electric field, the cell size can be from several times to tens times as large as that of the conventional scheme. Accordingly, it is possible to achieve high contrast free from lowering of the contrast due to scale-down of the cell, even if the image display medium is disposed vertically and used.
Moreover, the initialization is not necessarily conducted each time when image is changed. The voltage applying unit may apply the alternating voltage to the electrodes every several times that image display medium is switched.
Namely, particles gradually coagulate, adhere to the gap member, and fall due to gravity as images are changed. These phenomena would not be recognized as defective display, if the number of change of images is from several to tens times. Accordingly, alternating voltage is applied, e.g. once per several to tens of changes of images. By thus carrying out initialization prior to recognition of defective display, it is possible to prevent defective display from being recognized.
Moreover, because initialization utilizes a mechanical collision force due to particle reciprocating motion, deformation of the particles or wear of the substrate surface due to the collisions between the particles or between the particle and the substrate may occur. Also, mechanical or characteristic change in the particle or substrate due to the above or deterioration in the display characteristics resulting from them may occur.
Accordingly, initialization is preferably suppressed to the minimum degree. If initialization is once per a plurality of changes of images, the deterioration of the image display medium can be suppressed to the minimum degree.
Moreover, the voltage applying unit may apply to the electrodes an alternating voltage lower than a display drive voltage for displaying images on the image displaying medium.
This is because initialization utilizes mechanical collision force due to particle reciprocating motion as described above and this may result in the deterioration of the image display medium.
During initialization, because the particles readily moves by mechanical collision of the reciprocally moving particles due to an alternating electric field, initialization can be carried out favorably even if the voltage is lower than a display drive voltage for image display. In this manner, by applying to the pair of electrodes an alternating voltage lower than the display drive voltage for image display on the image display medium, reduced is the collision force between the particles or between the particle and the substrate during initialization, thereby making it possible to further reduce the deterioration of the image display medium due to initialization.
Moreover, when the image display medium is disposed vertically and used, the voltage applying unit may apply to the electrodes an alternating voltage higher than a display drive voltage for displaying images on the image displaying medium.
Namely, if the initializing drive voltage is higher than the display drive voltage for image display, a greater collision force is obtained. Accordingly, when the image display medium is inclined in relation to a horizontal direction, an alternating voltage higher than the display drive voltage is applied to the electrodes. Because this can diffuse the particles upward, the particles can be prevented more effectively from depositing in the lower region of the medium due to the action of gravity.
However, the deterioration in the image display medium may be accelerated because the particle collision force increase by raising the initializing drive force. Accordingly, initialization is preferably carried out once per a plurality of changes of images.
The voltage applying unit may apply to the electrodes, at a predetermined ratio, an alternating voltage equal to or lower than the display drive voltage and an alternating voltage higher than the display drive voltage.
For example, initialization is basically carried out with the alternating voltage lower than the display drive voltage, to conduct initialization with the alternating voltage higher than the display drive voltage every several times. This can effectively prevent the occurrence of particle coagulation and adhesion of the particles to the gap member and also suppress to some extent the deterioration in the image display medium due to initialization. Thus, further effectively secured is a larger cell even if the image display medium is disposed vertically and used.
Also, the voltage applying unit may apply to the pair of substrates an alternating voltage superposed with a predetermined direct current voltage on the alternating voltage.
Namely, electrifying amount of the particles and adhesive force of the particles to the substrate, etc, are different depending on the components and the constitution of the particles, and the mobility of particles is different depending on the kind thereof. Accordingly, by superposing a direct current voltage on the alternating voltage, the intensity of applying alternating voltage is matched to the mobility of particles to be used. This provides further stable initialization.
Moreover, the voltage applying unit may include a changing unit for changing a duty of the alternating voltage.
In this manner, by properly changing the duty in accordance with the type of the particles, obtained is the effect similar to the above.
The present invention also provides a display drive method for an image display medium having a pair of substrates, an electrode provided on each substrate respectively, and a plurality of kinds of particles which are enclosed in a space between the substrates and which are movable due to an electric field formed between the electrodes and which have different color and electrifying properties. The display drive method comprises applying to the electrodes an alternating voltage having a frequency to move the plurality of kinds of particles.
This can prevent particle coagulation and allow images with high contrast to be displayed.
Incidentally, the above process can be implemented with a program for executing a process to apply an alternating voltage for moving the plurality of kinds of particles to the electrodes of an image display medium having a pair of substrates, electrode provided on each respective substrate, and a plurality of kinds of particles which are enclosed in a space between the pair of substrates and which can move due to an electric field formed between the electrodes and which have different color and electrifying properties. Moreover, the program may be recorded in a recording medium to be read by the computer.
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