1. Field
The present invention relates to a liquid crystal display element that drives liquid crystal to display an image, a method of driving the same, and an electronic paper including the same.
2. Description of the Related Art
In recent years, many companies and universities actively advance development of electronic papers. Promising fields of application of the electronic paper include the field of an electronic books first of all and include the field of portable apparatus such as sub-displays of mobile terminals and IC card display units or the like. As an example of a display element used for the electronic paper, there is a liquid crystal display element that uses a liquid crystal composition having a cholesteric phase formed therein (which is referred to as cholesteric liquid crystal or chiral nematic liquid crystal and hereinafter, referred to as cholesteric liquid crystal). The cholesteric liquid crystal has, for example, a semipermanent display retention characteristic (memory characteristics), a vivid color display characteristic, a high-contrast characteristic, and a high-resolution characteristic.
The B display unit 46b includes, a blue (B) liquid crystal 43b interposed between a pair of upper and lower substrates 47b and 49b, and a pulse voltage source 41b that applies a predetermined pulse voltage to the B liquid crystal layer 43b. The G display unit 46g includes, a green (G) liquid crystal 43g interposed between a pair of upper and lower substrates 47g and 49g, and a pulse voltage source 41g that applies a predetermined pulse voltage to the G liquid crystal layer 43g. The R display unit 46r includes, a red (R) liquid crystal 43r interposed between a pair of upper and lower substrates 47r and 49r, and a pulse voltage source 41r that applies a predetermined pulse voltage to the R liquid crystal layer 43r. A light absorbing layer 45 is provided on the rear surface of the lower substrate 49r of the R display unit 46r.
The cholesteric liquid crystal used for each of the B, G, and R liquid crystal layers 43b, 43g, and 43r is a liquid crystal mixture of nematic liquid crystal and a relatively large amount of chiral additive, for example, several tens of percent by weight of additive (which is also called a chiral material). When a relatively large amount of chiral material is added to the nematic liquid crystal, it is possible to form a cholesteric phase having a nematic liquid crystal molecules strongly twisted into a helical shape.
The cholesteric liquid crystal has bistability (memory characteristics) and is possible to be in either of a planar state, a focal conic state, or an intermediate state between the planar state and the focal conic state by adjusting the strength of an electric field applied to the liquid crystal. When the cholesteric liquid crystal is in either of the planar state, the focal conic state, or the intermediate state therebetween once, the cholesteric liquid crystal stably maintains its state even when no electric field is applied.
The planar state is obtained by applying a predetermined high voltage between the upper and lower substrates 47 and 49 to apply a strong electric field to the liquid crystal layer 43 and then rapidly reducing the electric field to zero. The focal conic state is obtained by applying, for example, a predetermined voltage that is lower than the high voltage between the upper and lower substrates 47 and 49 to apply an electric field to the liquid crystal layer 43 and then rapidly reducing the electric field to zero.
The intermediate state between the planar state and the focal conic state is obtained by applying, for example, a voltage that is lower than that used to obtain the focal conic state between the upper and lower substrates 47 and 49 to apply an electric field to the liquid crystal layer 43 and then rapidly reducing the electric field to zero.
Next, the display principle of the liquid crystal display element 51 using the cholesteric liquid crystal will be described using the B display unit 46b as an example.
In the planar state, light having a predetermined wavelength corresponding to the helical pitch of the liquid crystal molecules 33 is selectively reflected from the liquid crystal layer. When the average refractive index of the liquid crystal layer is n and the helical pitch is p, a wavelength λ where the highest reflectance is obtained is represented by λ=n·p.
Therefore, in order to selectively reflect blue light from the B liquid crystal layer 43b of the B display unit 46b in the planar state, the average refractive index n and the helical pitch p are determined such that, for example, the wavelength λ is 480 nm. The average refractive index n can be adjusted by selecting a liquid crystal material and a chiral material, and the helical pitch p can be adjusted by adjusting the content of the chiral material.
In the intermediate state between the planar state and the focal conic state, the ratio of the reflected light and the transmitted light is adjusted by the ratio of the planar state and the focal conic state, and the intensity of the reflected light varies. Therefore, it is possible to perform halftone display corresponding to the intensity of the reflected light.
As described above, it is possible to control the amount of light reflected by the alignment state of the cholesteric liquid crystal molecules 33 twisted in the helical shape. Similar to the B liquid crystal layer 43b described above, the cholesteric liquid crystal that selectively reflects green and red light in the planar state is injected into the G liquid crystal layer 43g and the R liquid crystal layer 43r to manufacture the liquid crystal display element 51 capable of performing full color display. The liquid crystal display element 51 has memory characteristics and can perform full color display without consuming power except screen rewriting.
Patent Document 1: JP-A-2002-14324
Patent Document 2: JP-A-2004-117404
However, the time required for the liquid crystal display element using the cholesteric liquid crystal to perform data write scanning for screen rewriting is 10 to 100 times longer than that in a liquid crystal display element according to the related art using twisted nematic (TN) liquid crystal or super twisted nematic (STN) liquid crystal. Therefore, about 0.5 to 10 seconds are required to perform screen rewriting, and it takes a long time to perform screen rewriting. In particular, the response characteristics of liquid crystal are lowered at a low temperature, and it takes further a long time to perform screen rewriting.
According to an aspect of the invention, there is a liquid crystal display element including, a display unit that includes liquid crystal; a driving control unit that can determine a driving method on the basis of an external environment; and a driving unit that drives the liquid crystal using the determined driving method.
A liquid crystal display element, a method of driving the same, and an electronic paper including the same according to a first embodiment will be described with reference to
As shown in
The B display unit 6b includes a pair of upper and lower substrates 7b and 9b opposite to each other and the B liquid crystal layer 3b that is sealed between the two substrates 7b and 9b. The B liquid crystal layer 3b includes B cholesteric liquid crystal having an average refractive index n and a helical pitch p that are adjusted so as to selectively reflect blue light.
The G display unit 6g includes a pair of upper and lower substrates 7g and 9g opposite to each other and the G liquid crystal layer 3g that is sealed between the two substrates 7g and 9g. The G liquid crystal layer 3g includes G cholesteric liquid crystal having an average refractive index n and a helical pitch p that are adjusted so as to selectively reflect green light.
The R display unit 6r includes a pair of upper and lower substrates 7r and 9r opposite to each other and the R liquid crystal layer 3r that is sealed between the two substrates 7r and 9r. The R liquid crystal layer 3r includes R cholesteric liquid crystal having an average refractive index n and a helical pitch p that are adjusted so as to selectively reflect red light.
A liquid crystal composition forming the B, G, and R liquid crystal layers 3b, 3g, and 3r is cholesteric liquid crystal obtained by adding 10 to 40 wt % of chiral material to a nematic liquid crystal mixture. The content of the chiral material added is represented by a value when the sum of the amount of nematic liquid crystal component and the amount of chiral material is 100 wt %. Various kinds of known liquid crystal materials may be used as the nematic liquid crystal.
However, it is preferable to use nematic liquid crystal having dielectric anisotropy Δε in a range of 20≦Δε≦50 in order to relatively reduce a driving voltage for the liquid crystal layers 3b, 3g, and 3r. In addition, the refractive index anisotropy Δn of the cholesteric liquid crystal is preferably in a range of 0.18≦Δn≦0.24. When the refractive index anisotropy Δn is smaller than the above-mentioned range, the reflectances of the liquid crystal layers 3b, 3g, and 3r in the planar state are lowered. On the other hand, in the case in which the refractive index anisotropy Δn is larger than the above-mentioned range, as the scatter reflections of the liquid crystal layers 3b, 3g, and 3r in a focal conic state increase, the viscosities of the liquid crystal layers 3b, 3g, and 3r increase, which results in a low response speed.
The chiral material added to the B and R cholesteric liquid crystals and the chiral material added to the G cholesteric liquid crystal are optical isomers having different optical rotatory powers. Therefore, the B and R cholesteric liquid crystals have the same optical rotatory power, but the optical rotatory powers of the B and R cholesteric liquid crystals are different from that of the G cholesteric liquid crystal.
As shown in
The upper substrates 7b, 7g, and 7r and the lower substrates 9b, 9g, and 9r need to be transmissive. In this embodiment, two polycarbonate (PC) film substrates each having a size of 10 (cm)×8 (cm) are used. Instead of the PC substrates, glass substrates or polyethylene terephthalate (PET) film substrates may be used. These film substrates have sufficient flexibility. In this embodiment, all of the upper substrates 7b, 7g, and 7r and the lower substrates 9b, 9g, and 9r can transmit light. However, the lower substrate 9r of the R display unit 6r, which is arranged at the lowermost layer, may not transmit light.
As shown in
Similar to the B display unit 6b, the G display unit 6g is provided with 240 scanning electrodes 17g, 320 data electrodes 19g, and G pixels 12g (not shown) that are arranged in a matrix of 240 rows by 320 columns. Similarly, the R display unit 6r is provided with scanning electrodes 17r, data electrodes 19r, and R pixels 12r (not shown). A set of the B, G, and R pixels 12b, 12g, and 12r forms one pixel 12 of the liquid crystal display element 1. The pixels 12 are arranged in a matrix to form a display screen.
The scanning electrodes 17b, 17g, and 17r and the data electrodes 19b, 19g, and 19r are typically formed of, for example, an indium tin oxide (ITO). These electrodes may be formed of a film of a transparent conductive material, such as an indium zinc oxide (IZO), amorphous silicon, or bismuth silicon oxide (BSO), or a metallic material, such as aluminum or silicon.
A scanning electrode driving circuit 25 having a scanning electrode driver IC for driving a plurality of scanning electrodes 17b, 17g, and 17r is connected to the upper substrates 7b, 7g, and 7r. In addition, a data electrode driving circuit 27 having a data electrode driver IC for driving a plurality of data electrodes 19b, 19g, and 19r is connected to the lower substrates 9b, 9g, and 9r. The scanning electrode driving circuit 25 and the data electrode driving circuit 27 form a driving unit 24.
The scanning electrode driving circuit 25 selects predetermined three scanning electrodes 17b, 17g, and 17r on the basis of predetermined signals output from the control circuit 23, and simultaneously outputs scanning signals to the selected three scanning electrodes 17b, 17g, and 17r. Meanwhile, the data electrode driving circuit 27 outputs image data signals corresponding to the B, G, and R pixels 12b, 12g, and 12r on the selected scanning electrode 17b, 17g, and 17R to the data electrodes 19b, 19g, and 19r, on the basis of predetermined signals output from the control circuit 23. For example, general-purpose STN driver ICs having a TCP (tape carrier package) structure are used as the scanning electrode driver IC and the data electrode driver IC.
In this embodiment, driving voltages for the B, G, and R liquid crystal layers 3b, 3g, and 3r can be substantially equal to each other. Therefore, each input terminals of the scanning electrodes 17b, 17g, and 17r are commonly connected to a predetermined output terminal of the scanning electrode driving circuit 25. In this way, it is not necessary to provide the scanning electrode driving circuit 25 for each of the B, G, and R display units 6b, 6g, and 6r, and thus it is possible to simplify the structure of a driving circuit of the liquid crystal display element 1. In addition, it is possible to reduce the number of scanning electrode driver ICs and thus reduce the manufacturing costs of the liquid crystal display element 1. The output terminals of the B, G, and R scanning electrode driving circuit 25 may be made in common, if necessary.
It is preferable that a functional film, such as an insulating film (not shown) or an alignment film (not shown) that controls the alignment of liquid crystal molecules, be coated on each of the two electrodes 17b and 19b. The insulating film prevents a short circuit between the electrodes 17b and 19b, and serves as a gas barrier layer to improve the reliability of the liquid crystal display element 1. In addition, the alignment film may be formed of an organic material, such as polyimide resin, polyamid-imide resin, polyetherimide resin, polyvinylbutiral resin, or acrylic resin, or an inorganic material, such as silicon oxide or aluminum oxide. In this embodiment, for example, alignment films are formed (coated) on the entire surfaces of the substrates on the electrodes 17b and 19b. The alignment films may also serve as the insulating films.
As shown in
Since the G display unit 6g and the R display unit 6r have the same structure as the B display unit 6b, a description thereof will be omitted. A visible light absorbing layer 15 is provided on the outer surface (rear surface) of the lower substrate 9r of the R display unit 6r. The visible light absorbing layer 15 can effectively absorb light not reflected from the B, G, and R liquid crystal layers 3b, 3g, and 3r. Therefore, the liquid crystal display element 1 can display an image with a high contrast ratio. The visible light absorbing layer 15 may be optionally provided.
Next, a method of driving the liquid crystal display element 1 will be described with reference to
Here, an example in which a predetermined voltage is applied to a blue (B) pixel 12b(1, 1) arranged at an intersection of the first column data electrode 19b and the first row scanning electrode 17b of the B display unit 6b shown in
During a non-selection period T1′ after the selection period T1, a voltage of, for example, +28 V or +4 V is applied to the first row scanning electrode 17b in a period corresponding to half the selection period T1. Meanwhile, a predetermined data signal voltage Vd is applied to the first column data electrode 19b. In
When the voltage applied to the liquid crystal molecules in the homeotropic state is changed from VP100 (±32 V) to VF0 (±4 V) and the electric field is sharply reduced to approximately zero, the liquid crystal molecules are helically twisted such that their helical axes are aligned with a direction that is substantially vertical to the two electrodes 17b and 19b, and turn to the helical state, which is the planar state that selectively reflects light corresponding to a helical pitch. Therefore, the B liquid crystal layer 3b of the B pixel 12b(1, 1) turns to the planar state to reflect light. As a result, the B pixel 12b(1, 1) displays blue.
Meanwhile, as shown in
In the state in which the liquid crystal molecules having the helical structure are not completely untwisted, when the voltage applied to the cholesteric liquid crystal is changed from VF100b (±24 V) to VF0 (±4 V) and the electric field is rapidly reduced to approximately zero, the liquid crystal molecules are helically twisted such that their helical axes are aligned with a direction that is substantially parallel to the two electrodes 17b and 19b, and turn to the focal conic state that transmits incident light. Therefore, the B liquid crystal layer 3b of the B pixel 12b(1, 1) becomes the focal conic state and transmits light. As shown in
Further, in this embodiment, multi-tone display is performed by using cumulative response characteristics of the cholesteric liquid crystal. When a pulse voltage is applied to the cholesteric liquid crystal plural times, it is possible to change the cholesteric liquid crystal from the planar state to the focal conic state or from the focal conic state to the planar state using the cumulative response characteristics.
As represented by the curved line A in
As shown in
Next, a detailed method of multi-tone display according to this embodiment will be described with reference to
In
As shown in the drawings, in this embodiment, cumulative response processing is performed in four steps from Step S1 to Step S4. In step S1, a pulse voltage Vlc corresponding to the level 7 or the level 0 is applied for an application time T1 (=2.0 ms). As shown in
Further, as shown in
Then, in Steps S2 to S4, a predetermined pulse voltage Vlc is applied for predetermined application times T2 to T4. As shown in
In Steps S2 to S4, the application times T2 to T4 of the pulse voltage are different from each other. It is possible to change the state of the cholesteric liquid crystal by changing the pulse width of a pulse voltage applied as well as by changing the level of a pulse voltage applied. In the halftone region A shown in
It is possible to control the pulse voltage application times T1 to T4 by lowering the frequency of clocks for driving the scanning electrode driving circuit 25 and the data electrode driving circuit 27 to lengthen an output period. In order to stably switch the pulse width, it is more preferable to logically change the division ratio of a clock generating unit that generates a clock input to a driver than to change the clock frequency in an analog manner.
In this way, 23 (=8) driving patterns are obtained by a combination of two kinds of pulse voltages (±24 V and ±12 V) and three kinds of pulse widths (2.0 ms, 1.5 ms, and 1.0 ms) that are arranged in time series. Table 1 shows the above-mentioned driving patterns. Specifically, Table 1 shows the pulse width (the period for which the pulse voltage is applied) (ms) of the pulse voltage applied to the B pixel 12b(1, 1) in Steps S1 to S4 and the level (V) of the pulse voltage applied in Steps S1 to S4 for each of the grayscale levels 7 (blue) to 0 (black).
In order to make the grayscale of the B pixel 12b(1, 1) at level 7 (blue), as shown in Table 1 and
In order to make the grayscale of the B pixel 12b(1, 1) at level 6, as shown in Table 1 and
In order to make the grayscale of the B pixel 12b(1, 1) at level 5, as shown in Table 1 and
In order to make the grayscale of the B pixel 12b(1, 1) at level 4, as shown in Table 1 and
In order to make the grayscale of the B pixel 12b(1, 1) at level 3, as shown in Table 1 and
In order to make the grayscale of the B pixel 12b(1, 1) at level 2, as shown in Table 1 and
In order to make the grayscale of the B pixel 12b(1, 1) at level 1, as shown in Table 1 and
In order to make the grayscale of the B pixel 12b(1, 1) at level 0 (black), as shown in Table 1 and
During a non-driving period between steps, as described with reference to
In the multi-tone display method according to this embodiment, a pulse voltage Vlc is also repeatedly applied plural times to make the pixel in a pure black state (level 0). When a pulse voltage is applied only one time, light black is likely to be obtained due to weak scatter reflection. However, according to this embodiment, it is possible to perform high-contrast display with deep black. In addition, since a low pulse voltage is used, it is possible to stably prevent crosstalk in a non-selection region.
In this embodiment, display is performed at 8 grayscale levels. However, it is possible to perform display at 16 or more grayscale levels by increasing the number of driving times (the number of steps). Whenever the number of driving times is increased one by one, the number of grayscale levels can be increased two times. For example, when the number of driving times is 5, it is possible to display 16 grayscale levels. When the number of driving times is 7, it is possible to display 64 grayscale levels. When the number of driving times is 1, it is possible to display 2 grayscale levels. As such, in the multi-tone display method according to this embodiment, the number of driving times depends on the number of grayscale levels.
It is possible to display 512 colors (in the case of 8 grayscale levels) or more (multi-tone display) on the pixel 12(1, 1), which is a laminate of three B, G, and R pixels 12b(1, 1), 12g(1, 1), and 12r(1, 1), by driving the green (G) pixel 12g(1, 1) and the red (R) pixel 12r(1, 1) by the same method as that driving the B pixel 12b(1, 1). In addition, it is possible to output display data to all of the pixels 12(1, 1) to 12(240, 320) by performing so-called line sequential driving (line sequential scanning) on the first to two hundred fortieth row scanning electrodes 17b, 17g, and 17r and rewriting a data voltages of each of the data electrodes 19b, 19g, and 19r one by one of rows a predetermined number of driving times, thereby displaying one frame of color image (display screen).
In the above-described multi-tone display method, it is possible to perform multi-tone display using binary inexpensive general-purpose drivers, without using a specific driver IC capable of generating a multi-level driving waveform. Therefore, it is possible to perform multi-tone (multi-color) display at a low cost.
As shown in
The response characteristics of the cholesteric liquid crystal are lowered when the temperature is reduced. Therefore, as the temperature is reduced, the width of a driving voltage pulse (the time for which a pulse voltage is applied. In the case of 8-grayscale display, the application times T1 to T4 shown in
In the above-mentioned multi-tone display method, when the number of driving times is large, the operation of the liquid crystal display element 1 may cause problems at a low temperature. For example, when the temperature is 10° C., the liquid crystal display element 1 completes screen rewriting within 20 seconds, regardless the number of driving times (the number of grayscale levels), and there is no great difference in the screen rewriting time. However, at the low temperature, there is a great difference in the screen rewriting time according to the number of driving times. For example, at a temperature of −20° C., the screen rewriting time is about 30 seconds when the number of driving times is 1 (2-grayscale display). The screen rewriting time is about 80 seconds when the number of driving times is 4 (8-grayscale display). The screen rewriting time is about 110 seconds when the number of driving times is 5 (16-grayscale display). The screen rewriting time is about 160 seconds when the number of driving times is 7 (64-grayscale display). When the number of driving times is large, a very long time is required for screen rewriting at the low temperature.
Therefore, as the number of driving times is increased, the quality of a displayed image can be improved. However, at the low temperature, the screen rewriting time is increased, which is impractical. When the number of driving times is 7 (64-grayscale display), the screen rewriting time of the liquid crystal display element 1 is about 10 seconds at a temperature of 20° C., about 20 seconds at a temperature of 10° C., about 30 seconds at a temperature of 5° C., about 40 seconds at a temperature of 0° C., about 60 seconds at a temperature of −5° C., about 85 seconds at a temperature of −10° C., about 120 seconds at a temperature of −15° C., and about 160 seconds at a temperature of −20° C. At a temperature of 5° C. or less, the screen rewriting is not completed after 30 seconds have elapsed after the screen rewriting started, and thus it is difficult to display a high-quality image. Therefore, when the number of times is set to 7 and the screen rewriting is set to be performed within 30 seconds, the liquid crystal display element 1 can be operated only in a temperature range of 5 to 70° C.
Meanwhile, when the number of times is small, for example, 1 (2-grayscale display), the liquid crystal display element 1 can rewrite a screen in a short time. Therefore, the number of grayscale levels is small, which makes it difficult to display a high-quality image.
In order to solve the above problems, in the method of driving the liquid crystal display element 1 according to this embodiment, as the temperature is reduced, the number of driving times (the number of grayscale levels) is gradually decreased. For example, when the screen rewriting time is set within 30 seconds, the number of driving times is set to 7 (64-grayscale display) at a temperature of 5° C. to 70° C. The number of driving times is set to 5 (16-grayscale display) at a temperature of 0° C. to 5° C., and the number of driving times is set to 4 (8-grayscale display) at a temperature of −5° C. to 0° C. The number of driving times is set to 1 (2-grayscale display) at a temperature of −20° C. to −5° C. In this way, the liquid crystal display element 1 can operate at a temperature of −20° C. to 70° C. even when the screen rewriting time is set within 30 seconds.
For example, when the screen rewriting time is set within 60 seconds, the number of driving times is set to 7 (64-grayscale display) at a temperature of −5° C. to 70° C. The number of driving times is set to 5 (16-grayscale display) at a temperature of −10° C. to −5° C., and the number of driving times is set to 4 (8-grayscale display) at a temperature of −15° C. to −10° C. The number of driving times is set to 1 (2-grayscale display) at a temperature of −20° C. to −15° C. In this way, the liquid crystal display element 1 can operate at a temperature of −20° C. to 70° C. even when the screen rewriting time is set within 60 seconds.
As described above, as the temperature is reduced, the number of driving times (the number of grayscale levels) is gradually decreased. Therefore, it is possible to reduce the screen rewriting time at the low temperature, and thus it is possible to obtain a wide operation temperature range even when the screen rewriting time is limited in a predetermined range. In addition, when the temperature is not low, it is possible to display a high grayscale level image, for example, a 64-grayscale image, thus displaying a high quality image.
Table 2 shows the above-mentioned driving patterns. Table 2 shows the temperature (° C.) range in which the number of driving times (1, 4, 5, and 7) and the number of grayscale levels (2, 8, 16, and 64 grayscale levels) corresponding thereto are used when the screen rewriting time is set within 30 seconds (screen rewriting time: 30 seconds) and within 60 second (screen rewriting time: 60 seconds).
Next, an image processing method and a driving method of the liquid crystal display element 1 when the number of driving times (the number of grayscale levels) varies on the basis of a variation in temperature will be described with reference to
The grayscale conversion control unit 61 is connected to a temperature sensor (temperature detecting unit) 65 that detects the outside air temperature (external environment) around the liquid crystal display element 1. The temperature sensor 65 outputs the measured outside air temperature to the grayscale conversion control unit 61. The grayscale conversion control unit 61 determines the number of grayscale levels and the number of driving times corresponding to the number of grayscale levels, on the basis of the outside air temperature. The temperature range in which the number of grayscale levels and the number of driving times are used is set as shown in Table 2 on the basis of a desired screen rewriting time.
Display data for each pixel is input from an external system (not shown) to the grayscale conversion control unit 61. In this embodiment, the display data is 6 bits for each pixel (the number of grayscale levels: 64). For example, 6-bit display data of the B pixel 12b(i, j), 6-bit display data of the G pixel 12g(i, j), and 6-bit display data of the R pixel 12r(i, j) of the pixel 12(i, j) (where i is an integer satisfying 1≦i≦240 and j is an integer satisfying 1≦j≦320) are sequentially input from the external system to the grayscale conversion control unit 61 in synchronization with a predetermined clock signal.
A data converting unit 63 is connected to the grayscale conversion control unit 61. The data converting unit 63 converts the 64-grayscale display data (grayscale value) that is sequentially input from the external system into driving voltage data corresponding to the number of driving times determined by the grayscale conversion control unit 61, on the basis of the measured result by the temperature sensor 65 and the determined number of driving times. The data converting unit 63 includes a 2-grayscale data converting unit 63a, an 8-grayscale data converting unit 63b, a 16-grayscale data converting unit 63c, and a 64-grayscale data converting unit 63d. The 2-grayscale data converting unit 63a is used when the number of driving times determined by the grayscale conversion control unit 61 is 1 (2-grayscale display). Similarly, the 8-grayscale, 16-grayscale, and 64-grayscale data converting units 63b, 63c, and 63d are used when the number of driving times is 4 (8-grayscale display), 5 (16-grayscale display), and 7 (64-grayscale display), respectively.
The grayscale conversion control circuit 61 selects any one of the data converting units 63a to 63d of the data converting unit 63 corresponding to the determined number of grayscale levels and the determined number of driving times, and outputs display data to the selected one of the data converting units 63a to 63d.
A scan data memory 71 is connected to the data converting unit 63. The scan data memory 71 includes first to seventh scan data memories 71a to 71g. The scan data memory 71 temporarily stores the driving voltage data generated by the data converting unit 63. In this embodiment, the first to seventh scan data memories 71a to 71g can store 240×320×3 driving voltage data corresponding to 240 rows×320 columns B pixels 12b(1, 1) to 12b(240, 320), 240 rows×320 columns G pixels 12g(1, 1) to 12g(240, 320), and 240 rows×320 columns R pixels 12r(1, 1) to 12r(240, 320), respectively. The scan data memory 71 is connected to the control circuit 23.
Next, a description will be made of an image processing method and a driving method for displaying an image on the B display unit 6b assuming that the grayscale conversion control unit 61 determines to perform driving times four times on the basis of external temperature information and only display data for the B pixel 12b(i, j) is input from the external system for clarity of description. The grayscale conversion control circuit 61 outputs 6-bit display data of the B pixel 12(i, j) to the 8-grayscale data converting unit 63b. The 8-grayscale data converting unit 63b converts the display data into four driving voltage data for the B pixel 12b(i, j), that is, first driving voltage data Dbs1(i, j), second driving voltage data Dbs2(i, j), third driving voltage data Dbs3(i, j), and fourth driving voltage data Dbs4(i, j). The first to fourth driving voltage data Dbs1(i, j) to Dbs4(i, j) are binary data that designates the level of the pulse voltage Vlc applied in Steps S1 to S4 shown in
As such, the 8-grayscale data converting unit 63b converts 64-grayscale display data into 8-grayscale display data. When display data is converted into lower grayscale display data, image quality is likely to deteriorate. For example, an ordered dither method, an error diffusion method, or a blue noise mask method is used as an algorithm for image processing in the 8-grayscale data converting unit 63b. Any one of these algorithms can be used to prevent the deterioration of the quality of a displayed image even when the number of grayscale levels is small. In addition, a threshold method may be used as an algorithm for grayscale conversion. These algorithms are used for the image processing of the 2-grayscale and 16-grayscale data converting units 63a and 63c, which will be described below.
The generated first driving voltage data Dbs1(i, j) is stored at an address B1(i, j) of the first scan data memory 71a. Similarly, the generated second to fourth driving voltage data Dbs2(i, j) to Dbs4(i, j) are stored at addresses B2(i, j) to B4(i, j) of the second to fourth scan data memories 71b to 71d, respectively.
The above operation is repeatedly performed on the B pixels 12b(1, 1) to 12b(240, 320) to store the first driving voltage data Dbs1(1, 1) to Dbs1(240, 320) at the addresses B1(1, 1) to B1(240, 320) of the first scan data memory 71a.
Similarly, the second driving voltage data Dbs2(1, 1) to Dbs2(240, 320) are stored at the addresses B2(1, 1) to B2(240, 320) of the second scan data memory 71b. The third driving voltage data Dbs3(1, 1) to Dbs3(240, 320) are stored at the addresses B3(1, 1) to B3(240, 320) of the third scan data memory 71c. The fourth driving voltage data Dbs4(1, 1) to Dbs4(240, 320) are stored at the addresses B4(1, 1) to B4(240, 320) of the fourth scan data memory 71d.
Grayscale number (driving times number) information indicating that the number of grayscale levels is 8 (the number of driving times is 4) is input from the grayscale conversion control unit 61 to the control circuit 23. The control circuit 23 sequentially receives the first driving voltage data Dbs1(i, 1) to Dbs1(i, 320) from the first scan data memory 71a on the basis of the grayscale number (driving times number) information, and sequentially outputs the data to the data electrode driving circuit 27. The data electrode driving circuit 27 receives the first driving voltage data corresponding to one scanning electrode, latches the data, and simultaneously outputs it to 320 data electrodes 19b(1) to 19b(320). In synchronization with this operation, the scanning electrode driving circuit 25 selects an i-th row scanning electrode 17b(i) and outputs a predetermined scanning signal voltage. In this way, Step S1 shown in
Then, the control circuit 23 sequentially receives the second driving voltage data Dbs2(i, 1) to Dbs2(i, 320) from the second scan data memory 71b, and sequentially outputs the data to the data electrode driving circuit 27. The data electrode driving circuit 27 receives the second driving voltage data corresponding to one scanning electrode, latches the data, and simultaneously outputs it to 320 data electrodes 19b(i, 1) to 19b(i, 320). In synchronization with this operation, the scanning electrode driving circuit 25 selects an i-th row scanning electrode 17b(i) and outputs a predetermined scanning signal voltage. In this way, Step S2 shown in
Similarly, the third driving voltage data Dbs3 is written to 320 B pixels 17b in the i-th row, and Step S3 is performed. Then, the fourth driving voltage data Dbs4 is written to 320 B pixels 17b in the i-th row, and Step S4 is performed.
As described above, the control circuit 23 controls the driving unit 24 (the scanning electrode driving circuit 25 and the data electrode driving circuit 27) on the basis of the grayscale number (driving times number) information and the acquired first to fourth driving voltage data. The driving unit 24 performs Steps S1 to S4 shown in
The same process as described above is performed on the G and R display units 6g and 6r to output the first to fourth driving voltage data to all of the pixels 12(1, 1) to 12(240, 320), thereby displaying one frame of image (display screen).
When the number of driving times is 1, the grayscale conversion control circuit 61 outputs display data to the 2-grayscale data converting unit 63a. The 2-grayscale data converting unit 63a converts the display data to generate a piece of driving voltage data (the first driving voltage data) for one pixel 12b. The first driving voltage data is binary data that designates whether the pulse voltage Vlc applied Step S1 shown in
When the number of driving times is 5, the grayscale conversion control circuit 61 outputs display data to the 16-grayscale data converting unit 63c. The 16-grayscale data converting unit 63c converts the display data to generate five driving voltage data (the first to fifth driving voltage data). The first to fifth driving voltage data are binary data that designate the pulse voltage Vlc applied in five Steps S1 to S5 when the number of driving times is 5. The generated first to fifth driving voltage data are stored in the first to fifth scan data memories 71a to 71e, respectively.
When the number of driving times is 7, the grayscale conversion control circuit 61 outputs display data to the 64-grayscale data converting unit 63d. The 64-grayscale data converting unit 63d converts the display data to generate seven driving voltage data (the first to seventh driving voltage data). The first to seventh driving voltage data are binary data that designate the pulse voltage Vlc applied in seven Steps S1 to S7 when the number of driving times is 7. The generated first to seventh driving voltage data are stored in the first to seventh scan data memories 71a to 71g, respectively.
Therefore, display data input to the 64-grayscale data converting unit 63d is converted into seven driving voltage data (the first to seventh driving voltage data) for one B pixel 12b. The number of driving times is fixed to 7 without depending on the temperature. In the image processing method according to the related art, when screen rewriting is set within 30 seconds, some of the pulse voltages Vlc corresponding to the first to seventh driving voltage data are not applied to the B, G, and R pixels 12b, 12g, and 12r at a temperature of 5° C. or less. As a result, a whiten image with some halftone pixels deleted is displayed, resulting in the deterioration of image quality.
Next, an example of a method of manufacturing the liquid crystal display element 1 will be simply described.
ITO transparent electrodes are formed on two polycarbonate (PC) film substrates each having a size of 10 (cm)×8 (cm) and then patterned by etching to form strip-shaped electrodes (the scanning electrodes 17 and the data electrodes 19) at a pitch of 0.24 mm. Then, strip-shaped electrodes are formed on two PC film substrates so as to support a 320×240 QVGA resolution. Then, a polyimide-based alignment film material is applied with a thickness of about 700 Å on the strip-shaped transparent electrodes 17 and 19 of the two PC film substrates 7 and 9 by a spin coating method. Then, the two PC film substrates 7 and 9 having the alignment film material applied thereon are baked in an oven at a temperature of 90° C. for one hour, thereby forming alignment films. Subsequently, an epoxy-based sealing material 21 is applied at the edge of one of the PC film substrates 7 and 9 by a dispenser to form a wall having a predetermined height.
Then, spacers (produced by Sekisui Fine Chemicals Co., Ltd.) having a diameter of 4 μm are dispersed in the other substrate of the two PC film substrates 9 and 7. Then, the two PC film substrates 7 and 9 are bonded to each other and then heated at a temperature of 160° C. for one hour to harden the sealing material 21. Subsequently, B cholesteric liquid crystal LCb is injected by a vacuum injection method and an inlet for liquid crystal injection is sealed by an epoxy-based sealing material, thereby manufacturing the B display unit 6b. The G and R display units 6g and 6r are manufactured by the same method as described above.
Then, as shown in
As described above, according to this embodiment, as the temperature is reduced, the number of driving times (the number of grayscale levels) is gradually decreased, and thus it is possible to reduce the screen rewriting time at a low temperature. Therefore, it is possible to display an image in a short time during screen rewriting even at a low temperature. In addition, it is possible to obtain a wide operation temperature even when the screen rewriting time is limited to a predetermined range.
A liquid crystal display element, a method of driving the same, and an electronic paper including the same according to a second embodiment will be described with reference to
A method of driving the liquid crystal display element 101 according to this embodiment is characterized in that it determines whether an image is a still picture or a moving picture to decide the number of driving times, unlike the first embodiment in which the driving method of the liquid crystal display element 1 determines the number of driving times on the basis of the outside air temperature around the liquid crystal display element 1. In the following description, components having the same functions and operations as those in the first embodiment are denoted by the same reference numerals, and a description thereof will be omitted.
As shown in
The still picture/moving picture determining unit 67 is connected to the grayscale conversion control unit 61. Display data is input to the grayscale conversion control circuit 61 and the still picture/moving picture determining unit 67. The still picture/moving picture determining unit 67 performs subtraction or division on the input time-series grayscale data for each of the pixels 12b, 12g, and 12r to determine whether the display data is a still picture or a moving picture, and outputs information (still picture/moving picture information) indicating whether the display data is a still picture or a moving picture to the grayscale conversion control unit 61.
The grayscale conversion control unit 61 determines the number of grayscale levels and the number of driving times determined by the number of grayscales, on the basis of the still picture/moving picture information output from the still picture/moving picture determining unit 67. For example, when the display data is a still picture, the number of driving times is set to 7 (64-grayscale display). When the display data is a moving picture, the number of driving times is set to 4 (8-grayscale display). The number of driving times and the number of grayscale levels are not limited thereto.
The grayscale conversion control unit 61 selects the 8-grayscale display data converting unit 63b or the 64-grayscale display data converting unit 63d corresponding to the determined number of grayscale levels and the determined number of driving times from the data converting unit 63, and outputs display data to the data converting unit 63b or 63d. The operations of the data converting unit 63, the scan data memory 71, the control circuit 23, and the driving unit 24 are the same as those in the image processing method and the driving method of the liquid crystal display 1 shown in
When the display data is a moving picture, 64-grayscale display data is converted into 8-grayscale display data having less grayscale. In order to display the moving picture, the data converting unit 63b uses, for example, an ordered dither method, an error diffusion method, or a blue noise mask method as an algorithm for image processing. These algorithms can prevent the deterioration of the quality of a displayed image even when the number of grayscale levels is small. In addition, a threshold method may be used as an algorithm for grayscale conversion.
According to this embodiment, it is determined whether an image is a still picture or a moving picture, and the number of driving times when the image is a moving picture is set to be smaller than that when the image is a still picture. Therefore, it is possible to reduce the screen rewriting time when a moving picture is displayed.
The invention is not limited to the above-described embodiments, but various modifications and changes of the invention can be made.
In the above-described embodiments, a line sequential driving (line sequential scanning) method is used as an example of the driving method, but the invention is not limited thereto. For example, a dot sequential driving method may be used as the driving method.
In the above-described embodiments, a three-layer liquid crystal display element including the B, G, and R display units 6b, 6g, and 6r is used as an example, but the invention is not limited thereto. For example, the invention may be applied to a two-layer liquid crystal display element or a four-or-more-layer liquid crystal display element.
In the above-described embodiments, the liquid crystal display element including the display units 6b, 6g, and 6r respectively provided with the liquid crystal layers 3b, 3g, and 3r that reflect blue, green, and red light in the planar state is used as an example, but the invention is not limited thereto. For example, the invention may be applied to a liquid crystal display element that includes three display units having liquid crystal layers that reflect cyan, magenta, and yellow light in the planar state.
In the above-described embodiments, a passive matrix liquid crystal display element is used as an example, but the invention is not limited thereto. For example, the invention may be applied to an active matrix liquid crystal display element in which a switching element, such as a thin film transistor (TFT) or a diode, is provided in each pixel.
In the above-described embodiments, a plurality of frames (for example, four frames in the case of 8-grayscale display) form one image in order to perform grayscale display, but the invention is not limited thereto. For example, in the case of 8-grayscale display, during one frame period, the same scanning electrode 17 may be driven four times to perform Steps S1 to S4 on the pixels 12 on the scanning electrodes 17.
In the above-described embodiments, four driving times are performed to display 8 grayscale levels, but the invention is not limited thereto. The invention can be applied to a liquid crystal display element that displays a predetermined grayscale image by a predetermined number of driving times. For example, the invention can be applied to a driving method of a liquid crystal display element capable of displaying 8 grayscale levels by three driving times.
In the above-described embodiments, the number of driving times is 1, 4, 5, and 7, but the invention is not limited thereto. For example, two or three of the numbers of driving times may be used. In addition, the number of driving times may be, for example, 2, 3, or 6 (32-grayscale display).
In the first embodiment, the temperature sensor 65 measures the outside air temperature around the liquid crystal display element 1, but the invention is not limited thereto. The temperature sensor 65 may directly measure the temperature of the liquid crystal display element 1.
In the multi-tone display method described with reference to
Number | Date | Country | |
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Parent | PCT/JP2006/306639 | Mar 2006 | US |
Child | 12239993 | US |