Patent Application
20070290975
Embodiments of the present invention will be described in detail based on the following figures, wherein:
As will be described later, an aspect of the present invention enables various reflectance control operations appropriate for the composition of liquid crystal, by means of operation (often merely called “adjustment voltage application operation”) which is immediately subsequent to voltage application for writing purpose and is performed in the adjustment voltage application step of applying a voltage (hereinafter sometimes called an “adjustment voltage”)—differing in frequency from the applied voltage for writing purpose—to the selective reflection layer.
As exemplary aspects of the present invention, aspects (A) and (B), which are provided below as specific, effective examples of utilization of the adjustment voltage-applying operation, are now mentioned. (A) denotes an aspect of the present invention in relation to adjustment voltage-applying operation being applied to the method for driving the stacked reflective liquid crystal display by means of the threshold shifting method; and (B) denotes an aspect of the present invention in relation to adjustment voltage-applying operation being applied for the purpose of enhancing brightness contrast of a single-layer optical writing reflective liquid crystal display.
According to the aspect (A), a reflective liquid crystal display including a plurality of selective reflection layers stacked without insertion of electrodes between the layers is taken as an object on which an image is recorded, the respective selective reflection layers selectively reflecting light of different wavelengths in the visible light region and differing from one another in terms of a lower threshold value serving as an operation threshold value for a voltage externally applied to induce a change from a planar texture to a focal conic texture and a higher threshold value serving as an operation threshold value for a voltage externally applied to induce a change from the focal conic texture to a homeotropic texture.
The operation threshold value employed in the writing operation (a first writing operation) is an upper threshold value for any of the selective reflection layers, and in the writing operation, two or more kinds of voltages including the voltage V1H and the voltage V1L is applied to the respective selective reflection layers to thus select, in each of the selective reflection layers, an area where the upper threshold value is exceeded or other area where the upper threshold value is not exceeded.
The driving method of the aspect further comprises a second writing operation, in succession to the adjustment voltage-applying operation, of applying two or more kinds of voltages (hereinafter sometimes called simply a “write voltage”), including a voltage V3H exceeding the lower threshold value of any selective reflection layer other than the selective reflection layer selected in the writing operation as having the upper threshold value and a voltage V3L not exceeding the lower threshold value, to each of the selective reflection layers, thereby selecting, in each of the selective reflection layers, an area where the lower threshold value is exceeded or other area where the lower threshold value is not exceeded.
As is evident from
When the F reset and the H reset in both graphs are viewed in more detail, no major changes are found in the reflectance at the F reset and the H reset in connection with the display layer B. However, the reflectance of the display layer A at the F reset is found to increase from the neighborhood of a point where the adjustment voltage has exceeded 80V, and the luminous reflectance Y approaches 10 in the neighborhood of a point where the adjustment voltage is 130 V, and the display layer is understood to be in a selective reflection state (i.e., the planer texture). However, in relation to the H reset, major changes are not found in the luminous reflectance Y when the adjustment voltage is 80 V or higher.
This aspect of the present invention is intended for ensuring an operation margin at the upper threshold value by means of utilizing a difference between characteristics of changes in the reflectance of the display layers A and B with respect to the adjustment voltage.
In the embodiment shown in
A voltage—at which normalization reflectance of each of the display layers assumes a value of 90% when each of the display layers shifts from the planar texture to the focal conic texture (at the lower threshold value)—is taken as Vpf 90. A voltage at which normalization reflection assumes a value of 50% is taken as Vpf 50. A voltage at which normalization reflection assumes a value of 10% is taken as Vpf 10. A prefix “b” affixed to numerals denotes the display layer “b” requiring high upper and lower threshold voltages. Similarly, a prefix “a” affixed to numerals denotes the display layer “a” requiring lower upper and lower threshold voltages. An operation margin Vm achieved at the lower threshold value can be expressed as Vm=2×(Vpfb90−Vpfa10)/(Vpfb50+Vpfa50).
The operation margin is desirably a positive value.
Likewise, a voltage—at which normalization reflectance of each of the display layers assumes a value of 90% when each of the display layers (selective reflection layers) shifts from the focal conic texture to the homeotropic texture (at the upper threshold value)—is taken as Vfh 90. A voltage at which normalization reflection assumes a value of 50% is taken as Vfh 50. A voltage at which normalization reflection assumes a value of 10% is taken as Vfh 10. In the same manner as mentioned previously, prefixes “a” and “b” are affixed to numerals. In that case, an operation margin Vm achieved at the upper threshold value can be expressed as Vm=2×(Vfhb10−Vfha90)/(Vfhb50+Vfha50).
When difficulty is encountered in sufficiently assuring the operation margin Vm at the upper threshold value, the adjustment voltage-applying operation of the present invention can be utilized.
First, in the writing operation, a voltage application section 17 performs selective application of either the voltage for the period Vc that is lower than the upper threshold value Vfhb of the display layer 7b but higher than the upper threshold value Vfha of the display layer 7a or the voltage for the period Vd that is higher than the upper threshold value Vfhb of the display layer 7b. When the operation margin Vm is sufficiently ensured at the upper threshold value, areas of the display layer 7b applied with the voltage for the period Vc enter a focal conic texture, and areas of the display layer 7a applied with the same enter a homeotropic texture. Meanwhile, areas of the display layer 7a applied with the voltage for the period Vd enter a homeotropic texture as in the case of the voltage for the period Vc. However, a voltage achieved in areas of the display layer 7b applied with the voltage for the period Vd exceeds the upper threshold value Vfhb, and hence the areas enter the homeotropic texture.
In short, by means of determining whether the voltage for the period Vc or the voltage for the period Vd is selected as the voltage applied by the voltage application section 17, either the focal conic texture or the homeotropic texture is selected as the texture of the display layer 7b. When the applied voltage is rapidly stopped in this state, the homeotropic texture changes to the planar texture, and the focal conic texture is maintained. Meanwhile, the display layer 7a is in the homeotropic texture before stoppage of application of a voltage, regardless of whether the applied voltage is the voltage for the period Vc or the voltage for the period Vd. All of the display layers change to the planar texture as a result of rapid stoppage of application of a voltage.
However, in a case where the operation margin Vm is not sufficiently ensured at the upper threshold value, it is assumed that, when a voltage is applied to the display layer 7b so as to definitely select either the focal conic texture or the homeotropic texture for the display layer 7b, the texture of portions of the display layer 7a remain in the focal conic texture and do not change. Under the threshold shifting method, if a layer whose texture is desired to be selected at the upper threshold value is the display layer 7b, all areas of the other display layer 7a are desired to be aligned to the homeotropic texture in this writing operation.
Accordingly, in the present applied invention, operation pertaining to the adjustment voltage-applying operation is performed continually subsequent to the writing operation.
In the adjustment voltage-applying operation, a voltage which differs in frequency from the voltage applied in the writing operation; namely, an adjustment voltage, is applied. When a divided voltage applied to the display layer 7a at this time is assumed to be, e.g., 130V, the areas (areas in the H-reset state) of homeotropic texture in the display layer 7a maintain the homeotropic state intact. The texture of the areas of focal conic texture (areas in the F-reset state) changes to the homeotropic texture. All of the areas finally enter the homeotropic texture. As indicated by the graph shown in
As can be seen from the graphs of
As has been described above, even when difficulty is encountered in ensuring operation margin at the upper threshold value for reasons of the composition of liquid crystal, ensuring of intended performance, or the like, application of an adjustment voltage-applying operation featuring the present invention can yield an advantage that is the same as that yielded when substantially wide operation margin is ensured. Therefore, the freedom degree of design and selection of a liquid crystal configuration is improved. For instance, even at the time of the operation margin being ensured at the lower threshold value to be described later, limitations are reduced. Thus, a high operation margin can be ensured as a whole. Stable threshold shift driving can be implemented while each of the display layers (the selective reflection layers) of the stacked reflective liquid crystal display is provided with sufficient reflectance.
As mentioned above, an essentially-wide operation margin can be ensured at the upper threshold value by utilization of the adjustment voltage-applying operation featuring the present invention. In order to sufficiently ensure operation margin for the lower threshold value, the frequency (FV3) of second write voltages (V3H and V3L) applied during the second writing operation may be made lower (FV3<FV1) than the frequency (FV1) of the write voltages (V1H and V1L) applied during the writing operation. The reason for this will now be described.
In order to increase the ratio of the electric field applied to the display layer 7a to that applied to the display layer 7b, a conceivable method is to ensure a wide ratio of the dielectric constant of the display layer 7a to the dielectric constant of the display layer 7b and to increase a capacitive division ratio. However, as mentioned previously, a positive correlation generally exists between the specific inductive capacity of cholesteric liquid crystal and refractive anisotropy Δn. Hence, under the above-mentioned method, a bright display is hardly achieved by the display layer of low dielectric constant (the display layer 7a in this case). When the dielectric constant of a liquid crystal material increases, the threshold electric field tends to become lower. Hence, the threshold electric field of the display layer of high dielectric constant (the display layer 7b in this case) having a small capacitive division ratio becomes smaller, which in turn poses difficulty in enlarging the operation margin.
A change in orientation arising from the planar texture to the focal conic texture at the lower threshold value is considered to be caused by accumulation of field energy exerted on the display layer including cholesteric liquid crystal. The present inventors have found that the drawbacks mentioned above are solved by making the specific resistance values of the respective liquid crystal materials different from each other and applying a voltage pulse of such a low frequency as to relax the potential division ratio between the display layers from a capacitive division ratio to a resistive division ratio (a ratio of dependence on the resistive division ratio is increased).
Specifically, potential division is performed by utilization of a resistive division ratio between the display layers, thereby obviating a necessity to ensure a large dielectric ratio between liquid crystal materials of the display layers. Accordingly, a liquid crystal material having high refractive anisotropy Δn can be used for each of the display layers, so that a bright display can be obtained. Reversal of threshold field becomes unlikely to arise as a result of a reduction in the dielectric ratio, and hence operation margin can be readily enlarged. The waveform of an applied pulse may be a square wave. However, an ascending triangular wave, a sinewave, or a d.c. pulse, which is more susceptible to a resistance component, is desirable.
In order to implement the present invention, resistance of each of the layers is also adjusted (namely, the liquid crystal material of each layer is selected such that the display layer 7a exhibits high resistance and such that the display layer 7b exhibits low resistance).
When a pulse wave having a frequency of 50 Hz is applied to the display layers, the voltage applied in the form of a pulse wave tends to increase or decrease with lapse of time but to transition linearly, as represented by the left-side graph of each of the upper and lower rows. In contrast, when the pulse wave having a frequency of 5 Hz is applied to the display layers, the voltage applied in the form of a pulse waveform continually exhibits the tendency to increase or decrease with lapse of time until the tendency becomes constant, as represented by the right-side graph of each of the upper and lower rows. Consequently, a potential division ratio between the display layer 7a and the display layer 7b is understood to have been widened.
Specifically, the time constant of relaxation of the capacitive division ratio of the voltage applied to the respective display layers to the resistive division ratio is reduced with regard to the operation margin of the lower threshold value, and a pulse having such a frequency and a waveform as to increase the influence of a resistance component is added. Limitations on the dielectric constant are diminished by means of utilization of the resistance ratio between the respective layers, thereby enhancing the freedom degree of design of the liquid crystal material.
However, upon application of the voltage pulse of a waveform (a low frequency) utilizing relaxation of a capacitive element into a resistive element, a disturbance arises in a waveform for reasons of a residual potential after application of a final pulse.
The frequency of a voltage to be applied is changed by means of the writing operation performed at the upper threshold value and the writing operation performed at the lower threshold value, thereby solving the drawbacks. In the above example, the frequency of an applied voltage is lowered in order to ensure the operation margin at the lower threshold value, thereby relaxing the capacitive division ratio to the resistive division ratio. Further, the frequency is increased with regard to the switching operation performed at the upper threshold value susceptible to the disturbance that arises in the waveform for reasons of the low-frequency voltage, thereby realizing stable operation and ensuring operation margin.
The related-art waveform utilizing capacitive division; namely, a high-frequency voltage, may be applied in connection with the upper threshold value. In connection with the lower threshold value, a waveform utilizing resistive division; namely, a low-frequency voltage, is applied. As mentioned above, voltages of different frequencies (or waveforms) are applied as a voltage (a reset voltage) applied at the upper threshold value and a voltage (a select voltage) applied at the lower threshold value. Thus, a reflective liquid crystal display can be driven while display layers of high reflectance are realized and an operation margin at the lower threshold value is ensured. Thus, the practical utility of driving of the reflective liquid crystal display based on the threshold shifting method is enhanced to a much greater extent.
In relation to the frequency of an applied voltage which is to be changed between the writing operation and the second writing operation, a frequency FV3 of the applied voltage V3 employed in the second writing operation (at the time of lower threshold switching) is preferably set so as to fall within a range of 0 to 100 Hz; more preferably a range of 0 to 30 Hz. When the frequency FV3 exceeds 100 Hz, relaxation of the capacitive division ratio to the resistive division ratio becomes insufficient, thereby leading to a case where difficulty is encountered in ensuring operation margin. Thus, a frequency of 100 Hz or more is not preferable.
Under a reflective liquid crystal display-driving method according to one aspect of the present invention, the selective reflection layer may be of a minimum of two-layer configuration or a three-layer configuration which enables formation of a full-color image.
The aspect (A) of the present invention can also be applied even when the reflective liquid crystal display to which an image is recorded is an optical-writing substance having a structure including a photoconductive layer. Specifically, the reflective liquid crystal display-driving method of the applied invention having a configuration including a photoconductive layer comprises the configuration of the aspect (A) of the present invention.
Further, an optical-writing reflective liquid crystal display is taken as an object on which an image is recorded, a photoconductive layer being stacked on one surface of the plurality of stacked selective reflection layers, and the selective reflection layers with the photoconductive layer being sandwiched between the pair of electrodes.
The writing operation (first writing operation) is performed by selectively exposing to an address light, as required, while applying to the pair of electrodes the bias voltage V1 in which the bias voltage V1 is divided into the voltage V1H applied to the selective reflection layer during exposure by the address light and the voltage V1L applied to the selective reflection layer during non-exposure by the address light.
The second writing operation is performed by selectively exposing to an address light, as required, while applying to the pair of electrodes the bias voltage V3 in which the bias voltage V3 is divided into the voltage V3H applied to the selective reflection layer during exposure by the address light and the voltage V3L applied to the selective reflection layer during non-exposure by the address light.
In the optical-writing reflective liquid crystal display, bias voltages V1 to V3, by means of which a voltage V1L or V3L is applied to the selective reflection layers in a divided manner during non-exposure, is applied to the pair of electrodes in the writing operation and the second writing operation. During exposure by the address light, the voltage V1H or V3H is applied to the selective reflection layers. As a result, there can be achieved operation and advantages which are essentially the same as those achieved by the configuration for writing an image by means of only the magnitude of a write voltage. Thus, the operation and advantage of the present invention and those of the applied invention are yielded.
According to the aspect (A), an optical-writing reflective liquid crystal display is taken as an object on which an image is recorded, a photoconductive layer being stacked along with the selective reflection layer formed from only one layer, and the selective reflection layer with the photoconductive layer being sandwiched between the pair of electrodes.
The writing operation is performed by selectively exposing to an address light, as required, while applying to the pair of electrodes the bias voltage V1 in which the bias voltage V1 is divided into the voltage V1H applied to the selective reflection layer during exposure by the address light and the voltage V1L applied to the selective reflection layer during non-exposure, thereby selecting, in the selective reflection layer, an area where the threshold value is exceeded or other area where the threshold value is not exceeded.
An application time TV2 during which the voltage V2 is applied in the adjustment voltage-applying operation is shorter than an application time TV1 during which the voltage V1 is applied in the writing operation (TV1>TV2). At this time, the voltage V2 employed in the adjustment voltage-applying operation may be a half period component (a d.c. component) of one pulse.
As mentioned previously in connection with the related-art section, there is a case where, depending on a liquid crystal material, the reflective liquid crystal display provides a display image of high reflectance by application of a high-frequency pulse as a write voltage, under the influence of moisture or ion content. For instance, in an optical-writing reflective liquid crystal display into which a liquid crystal layer (a selective reflection layer) and an OPC layer (an organic photoconductive layer) are stacked, when a high-frequency pulse is applied for writing operation, the potential division ratio is not relaxed to the resistive division ratio between the liquid crystal layer and the OPC layer. Hence, there is a case where difficulty is encountered in achieving a sufficient contrast ratio.
In this aspect of the present invention, the writing operation is performed by means of applying a voltage of the same frequency as that employed in the related art. Subsequently, a short pulse voltage having a different frequency is applied in the adjustment voltage-applying operation. A period of time during which the pulse voltage is applied is considerably short and can be grasped as a half wavelength of a high-frequency rectangular pulse. Consequently, an advantage yielded when a high-frequency pulse is used for writing operation is acquired, and a display image of high reflectance is obtained.
General descriptions about a reflective liquid crystal display-driving method of the present invention and the aspects (A), (B) utilizing the method have thus been provided above. The reflective liquid crystal display-driving method of the present invention is intended for proposing a new reflectance control method that is separate from and independent of the driving operation for writing an image on the reflective liquid crystal display. Various reflectance control operations are enabled by means of combining the technique with the related-art known techniques. Specifically, applied embodiments of a reflective liquid crystal display-driving method of the present invention are not limited to the aspects (A) and (B) of the present invention. By means of adjusting the adjustment voltage-applying operation or other conditions, as appropriate, various reflectance control operations become feasible.
Under the reflective liquid crystal display-driving methods (including the aspects (A) and (B)) of the present invention, when the reflective liquid crystal display serving as an object on which an image is recorded is an optical-writing substance having a configuration including a photoconductive layer, the photoconductive layer may be formed from an organic photoconductor.
Moreover, under these reflective liquid crystal display-driving methods (including the aspects (A) and (B)) of the present invention, the one selective reflection layer or the two or more selective reflection layers can be formed by means of dispersing the cholesteric liquid crystal in a polymer.
Exemplary embodiments of the present invention will be described in detail hereinbelow by reference to the drawings.
A first embodiment is an exemplary embodiment of an aspect (A) in which the configuration of the present invention is applied to a method for driving a voltage-writing, stacked, reflective liquid crystal display by means of a threshold shift method.
As has been described,
The display medium 1 of the present embodiment is a member which enables selective driving of a plurality of liquid crystal layers (selective reflection layers) by application of a bias signal; specifically, a reflective liquid crystal display.
In the present embodiment, the display medium 1 is a subject formed by means of stacking, in sequence from a display surface side, a transparent substrate 3; a transparent electrode 5; a display layer (a selective reflection layer) 7b; a display layer (a selective reflection layer) 7a; a transparent electrode (electrode) 6; and a transparent substrate 4.
The transparent substrates 3, 4 are members intended for retaining respective functional layers on interior surfaces of the substrates, to thus maintain the structure of a display medium. The transparent substrates 3, 4 are sheet-shaped subjects having strength withstanding external force, and have at least the function of enabling transmission of incident light. The transparent substrates 3, 4 may possess flexibility. An inorganic sheet (e.g., glass, silicon), a polymeric film (e.g., polyethylene terephthalate, polysulfone, polyethersulfone, polycarbonate, polyethylene naphthalate), and the like, can be mentioned as a specific material for the transparent substrates 3, 4. A known functional film, such as an antifouling film, an abrasion resistant film, an anti-reflection film, a gas barrier film, and the like, may be formed on the exterior surface of the transparent substrate.
Transparent electrodes 5, 6 are members intended for uniformly applying a bias voltage applied by the writing apparatus 2 over the surface of each of the functional layers in an optical address element. The transparent electrodes 5, 6 have uniform conductivity over the surfaces thereof, and permit transmission of at least incident light and address light. Specifically, a conductive thin film formed from metal (e.g., gold, aluminum), a metallic oxide (e.g., an indium oxide, a tin oxide, an indium-tin oxide (ITO)), a conductive organic polymer (e.g., polythiophene-based conductive organic polymers and polyaniline-based conductive organic polymers), and the like, can be mentioned as the materials for the transparent electrodes. Known functional films, such as an adhesion improvement film, an anti-reflection film, a gas barrier film, and the like, may also be formed over the surface of the transparent electrodes. In the present invention, an electrode provided opposite the electrode on the display side (i.e., the transparent electrode 6 of the present embodiment) may also be nontransparent.
The display layers of the present invention have the function of reflecting specific color light in incident light and modulating transmission state of the light by means of an electric field; and have a characteristic of enabling retaining a selected state in a field-free state. The display layer can have a structure which does not become deformed by external force, such as flexure or pressure.
In the present embodiment, a liquid crystal layer of a self-holding-type liquid crystal composite made from cholesteric liquid crystal and transparent resin is formed as the display layer. Specifically, the display layer is a liquid crystal layer which does not require a composite a spacer, or the like, for imparting a self-holding characteristic. Although not illustrated in the present embodiment, cholesteric liquid crystal remains dispersed in a polymeric matrix (transparent resin).
In the present invention, the display layer being a liquid crystal layer of a self-holding-type liquid crystal composite is not indispensable. The display layer may also be formed from only liquid crystal.
Cholesteric liquid crystal has the function of modulating the reflecting and transmitting state of specific color light of the incident light. Liquid crystal molecules are oriented in a helically-twisted manner. Of the light having entered in the helical axis, specific light dependent on a helical pitch is subjected to interference and reflection. By means of an electric field, the orientation and the reflecting state can be changed. When the display layer is formed into a self-retaining liquid crystal composite, a drop size may be uniform, and the composite may be arranged densely in a single layer.
Specific liquid crystal which can be used as cholesteric liquid crystal includes liquid crystals formed by means of adding an optically active material (e.g., steroid-based cholesterol derivatives, Schiff-base-based materials, azo-based materials, ester-based materials, biphenyl-based materials) to nematic liquid crystals, and smectic liquid crystals (e.g., Schiff-base-based liquid crystals, azo-based liquid crystals, azoxy-based liquid crystals, benzoic-ester-based liquid crystals, biphenyl-based liquid crystals, terphenyl-based liquid crystals, cyclohexyl-carboxylate-based liquid crystals, phenylcyclohexane-based liquid crystals, biphenylcyclohexane-based liquid crystals, pyrimidine-based liquid crystals, dioxane-based liquid crystals, cyclohexyl-cyclohexane-ester-based liquid crystals, cyclohexyl-ethane-based liquid crystals, cyclohexane-based liquid crystals, tolan-based liquid crystals, alkenyl-based liquid crystals, stilbene-based liquid crystals, multiply-fused-ring-based liquid crystals), or mixtures thereof; or the like.
The helical pitch of cholesteric liquid crystal is adjusted by means of the amount of chiral agent added to nematic liquid crystal. For instance, when blue, green, and red are taken as display colors, the center wavelengths of selective reflection fall within a range of 400 nm to 500 nm; a range of 500 nm to 600 nm; and a range of 600 nm to 700 nm, respectively. In order to compensate for temperature dependence of the helical pitch of cholesteric liquid crystal, there may also be used a known technique of addition of a plurality of chiral agents which twist in different directions or exhibit reverse temperature dependence characteristics.
In a mode where the display layers 7a, 7b form a self-holding liquid crystal composite made up of cholesteric liquid crystal and a macromolecular matrix (transparent resin), there can be used a PNLC (Polymer Network Liquid crystal) structure including mesh-shaped resin in a continuous texture of cholesteric liquid crystal or a PDLC (Polymer Dispersed Liquid crystal) structure where cholesteric liquid crystal is dispersed in the form of a droplet in a polymeric framework structure. By means of adoption of either the PNLC structure or the PDLC structure, an anchoring effect arises in an interface between cholesteric liquid crystal and polymer, and retention of the planar texture or the focal conic texture in a field-free state can be made more stable.
The PNLC structure and the PDLC structure can be formed according to a known method for subjecting polymer and liquid crystal to texture separation; for example, a PIPS (Polymerization Induced Texture Separation) method for mixing liquid crystal with polymeric precursors which effect polymerization by means of heat, light, electron beams, and the like; for example, acrylic precursors, thiol-based precursors, epoxy-based precursors and the like, and for polymerizing the mixture in a uniform texture and subjecting the thus-polymerized mixture to texture separation; an emulsion method for mixing liquid crystal with a polymer exhibiting low liquid crystal solubility, such as polyvinyl alcohol, agitating and suspending the mixture, and dispersing the liquid crystal in a polymer in the form of droplets; a TIPS (Thermally-Induced Texture Separation) method for mixing thermoplastic polymer with liquid crystal and cooling the mixture heated in a uniform texture to thus effect texture separation; and an SIPS (Solvent Induced Texture Separation) method for dissolving polymer and liquid crystal into a solvent such as chloroform and evaporating the solvent to thus subject the polymer to liquid crystal to texture separation. However, no limitations are imposed on the method for forming the PNLC structure and the PDLC structure.
The polymeric matrix has the function of retaining cholesteric liquid crystal and preventing the flow of liquid crystal (changes in an image), which would otherwise be caused by deformation of a display medium. A polymeric material—which is not dissolved in a liquid crystal material and uses, as a solvent, a liquid incompatible with liquid crystal—can be used. The polymeric matrix is desired to be a material which has strength withstanding external force and exhibits high transmissivity for at least reflected light and address light.
Materials which can be adopted as the polymeric matrix include a water-soluble polymeric material (e.g., gelatin, polyvinyl alcohol, a cellulose derivative, a polyacrylic polymer, ethyleneimine, a polyethylene oxide, polyacrylamide, polystyrenesulfonate, polyamidine, and an isoprene-based sulfonic polymer); a material which can be formed into an aqueous emulsion (e.g., a fluororesin, a silicone resin, an acrylic resin, a urethane resin, an epoxy resin); and the like.
In the display medium 1 serving as a reflective liquid crystal display used in the threshold shift method, the upper and lower threshold values are desired to be separated, as appropriate, from each other in each of the display layers 7a and 7b, to thereby ensure an operation margin for the threshold shift method. In the present invention, a capacitor ratio and a resistor ratio are suitably adjusted, by means of appropriately adjusting the thickness of a liquid crystal material and the thickness of each of the layers.
The specific resistance of the liquid crystal material can be controlled by means of, e.g., mixing a fluorine-based material of high resistance with a cyano-based material of low resistance or doping the liquid crystal material with ionic impurities. At this time, the respective display layers must be made slightly different from each other in terms of capacitance. The capacitive difference between the display layers can be controlled by means of making the liquid crystal materials different from each other in terms of a dielectric constant or making the display layers different from each other in terms of a thickness.
In addition, the switching behaviors of the display layers 7a, 7b can also be controlled by means of dielectric anisotropy, an elastic modulus, a helical pitch, a polymeric framework structure, and side chains of cholesteric liquid crystal forming the display layers 7a, 7b; a texture separation process; morphology of an interface between the polymeric matrix and the display layers 7a, 7b; the degree of anchoring effect achieved between the polymeric matrix and the display layers 7a, 7b, which is determined by all of these factors; and the like.
More specifically, the factors used for controlling the switching behaviors include the type and composition ratio of nematic liquid crystal; the type of a chiral agent; the type of a resin; the type and composition ratio of monomer and oligomer which are starting materials of a polymeric resin; the type and composition ratio of an initiator and a cross-linking agent; a polymerization temperature; the exposure light source, exposure intensity, an exposure time, and an ambient temperature for photopolymerization; electron intensity, an exposure time, and an ambient temperature for electronic polymerization; the types and composition ratio of solvents used for application; the concentration of a solution; the thickness of a wet film; a drying temperature; the starting temperature of a temperature drop; a temperature drop rate; and the like. However, the factors are not limited to these.
In order to control the influence of the liquid crystal composition on the adjustment voltage-applying operation unique to the present invention, appropriate adjustment of the above-described liquid crystal composition can be performed. However, behaviors in response to the adjustment voltage-applying operation vary according to a liquid crystal material. Therefore, the display layers are configured such that a desired effect is exhibited at the time of application of an adjustment voltage while a liquid crystal composition is being adjusted. For instance, specific objects of adjustment include selection of a liquid crystal material, the composition and viscosity of liquid crystal, adjustment of impedance, and the like.
As a matter of course, even when the composition of liquid crystal has been adjusted, the effects of adjustment of the composition are not necessarily exhibited in an optimal manner. Therefore, it is desirable to temporarily form, from a liquid crystal composition, a liquid crystal layer constituting a display layer; to verify characteristics of the liquid crystal layer; and to set conditions (operations) for respective steps in accordance with the characteristics.
Display layers of two types which differ from each other to a certain extent in terms of a liquid crystal composition (e.g., a display layer having large amounts of specific composition components, another display layer having small amounts of specific composition components, and the like) are formed, and characteristics of the display layers are determined in advance. As a result of this, a characteristic of a liquid crystal composition which is in the process of preparation can be estimated.
In relation to the display layer of the display layer composition A shown in
When the three graphs are compared with each other, the display layer of the display layer composition B is understood to exhibit intermediate characteristics between those of the display layer of the display layer composition A and those of the display layer of the display layer composition C.
Thus, the display layer exhibiting a desired reflection characteristic in response to the adjustment voltage can be realized.
In the present embodiment, the writing apparatus (an apparatus for driving the stacked optical modulation element) 2 is a device for writing an image on the display medium 1, and comprises, as a principal constituent element, a voltage application section (a power supply) 17 for applying a voltage to the display medium 1. Further, the writing apparatus is provided with a control circuit 16 for controlling the operation of the voltage application section.
The voltage application section (power supply) 17 has the function of applying a predetermined bias voltage to the display medium 1. The voltage application section may be of any circuit, so long as it can apply a desired voltage waveform to the display medium (between the respective electrodes) in accordance with an input signal from the control circuit 16. However, in this case, the circuit is required to produce an AC output and achieve a high through rate. In the present embodiment, there is a necessity for appropriately changing the frequency of a voltage to be applied (or applying a d.c. voltage) by means of writing operation performed at the lower threshold value, the adjustment voltage-applying operation, and writing operation performed at the higher threshold value. Accordingly, the frequency being variable (including 0 Hz) is indispensable. For instance, a bipolar high-voltage amplifier can be used for the voltage application section 17.
The power supply application section 17 applies a voltage to an area between the transparent electrodes 5, 6 of the display medium 1 via a contact terminal 19.
The contract terminal 19 is a member which contacts the voltage application section 17 and the display medium 1 (the transparent electrodes 5, 6) to thus bring them into mutual conduction, and has high conductivity. A contact terminal which exhibits small resistance upon contact with the transparent electrodes 5, 6 and the voltage application section 17 is selected. The contact terminal can have a structure for enabling separation of either the transparent electrodes 5, 6 and the voltage application section 17 or separation of the transparent electrodes 5, 6 and the voltage application section 17 from each other, so that the display medium 1 can be separated from the writing apparatus 2.
A terminal—which is formed from metal (e.g., gold, silver, copper, aluminum, or iron), carbon, a composite formed by dispersing metal and carbon in a polymer, a conductive polymer (e.g., a polythiophene-based conductive polymer or a polyaniline-based conductive polymer), and the like, and which assumes the shape of a clip or connector for pinching an electrode—is mentioned as the contact terminal 19.
The control circuit 16 is a member having the function of controlling operation of the voltage application section 17 in accordance with image data input from the outside (an image-capturing apparatus, an image receiver, an image processor, an image reproducing apparatus, an apparatus having a combination of these functions, or the like). Specific control operations of the control circuit 16 are embodied by processing pertaining to three operations (steps); namely, a “(first) writing operation (step),” an “adjustment voltage-applying operation (step),” and the “second writing operation (step),” all of which feature the present applied invention. Details of these operations will be described later.
The method for driving a reflective liquid crystal display and actuation (operation) of the apparatus for driving the reflective liquid crystal display, both of which pertain to the present embodiment, are described in detail hereunder by reference to simple verification experiments.
In a case where characteristics of the influence of operation margins of the respective display layers and the influence of adjustment voltage-applying operation are evaluated, difficulty is encountered in observing changes in reflectance of each of the display layers if the two display layers have been stacked directly.
Therefore, in the verification experiment provided below, each of the display layers is formed alone, and the thus-formed display layers are electrically connected in series. Thus, changes in reflectance of each of the display layers and characteristics of the influence of operation margin on the adjustment voltage-applying operation are measured, thereby analogously evaluating the operation margin.
In the following experiment, a display layer of low dielectric constant and high resistivity is described as the display layer 7a; and a display layer of high dielectric constant and low resistivity is described as the display layer 7b.
The display layers 7a, 7b are formed into such a structure (having a thickness of 15 μm) that a pair of polyethylene terephthalate (PET)substrates, each having an ITO electrode (and a thickness of 125 μm), are provided opposite each other with the ITO electrodes facing inwardly and cholesteric liquid crystal of polymer dispersed type is sealed between the ITO electrodes.
Liquid crystal materials having different specific resistance values are used for the respective display layers 7a, 7b. Specifically, a liquid crystal material whose resistance value has been optimized by mixing fluorine-based liquid crystal into cyano-based liquid crystal is used for the display layer 7a of low dielectric constant and high resistivity. Cyano-based liquid crystal is used for the display layer 7b of high dielectric constant and low resistivity.
Since a slight capacitance difference must exist between the display layers 7a, 7b, the capacitance of the display layer 7a of low dielectric constant and high resistivity is set to 0.4 nF (50 Hz); and the capacitance of the display layer 7b of high dielectric constant and low resistivity is set to 0.6 nF (50 Hz). In order to control the influence of an operation margin on the adjustment voltage-applying operation, the liquid crystal composition of the display layer 7a and that of the display layer 7b are controlled. Specifically, a liquid crystal material—exhibiting low viscosity and high threshold steepness—is added to the display layer 7a, thereby enhancing sensitivity to an adjustment voltage.
A rectangular-wave voltage (a reset pulse) (V1H=150V and V1L=60V) of 50 Hz (FV1=50 Hz) is applied to the display layers 7a, 7b for 200 mS, thereby bringing the texture of the liquid crystal into a focal conic texture (an F reset) or a homeotropic texture (an H reset texture). Soon after, a DC pulse (having a frequency FV2=0) of 10 mS is applied as an adjustment voltage V2 to the display layers 7a, 7b. At this time, verification is performed while the magnitude of the adjustment voltage V2 is switched within a range of 0V (i.e., without involvement of adjustment voltage-applying operation) to 150V in increments of 10V.
As shown in
As is evident from the graphs, the texture of the display layer 7a changes to a planar texture when the adjustment voltage V2 to be applied is 130V or thereabouts or more at the time of focal-conic resetting. In contrast, the display layer 7b retains the focal conic texture even when an adjustment voltage V2 of 130V or thereabouts is applied at the time of focal-conic resetting. Meanwhile, at the time of homeotropic resetting, the texture of the display layer 7a and that of the display layer 7b change to a planar texture by application of an adjustment voltage V2 of about 130V. The applied invention (A) is to make an attempt to essentially enlarge operation margin at the upper threshold value by utilization of a change in the characteristics of the display layers.
During operation performed at the upper threshold value, the texture of the display layer “b” having a high upper threshold voltage is selected to be a focal conic texture or a homeotropic texture. When application of a voltage is released, the homeotropic texture changes to the planar texture, and selective transmission realized by the focal conic texture and selective reflection realized by the planar texture are selected. At this time, a voltage having sufficiently exceeded the upper threshold voltage is applied to the display layer “a” having a low upper threshold voltage, and the texture of the display layer evenly changes to the homeotropic texture. When application of the voltage is released, the texture changes to a planar texture, and the entire surface of the display layer enters a reflecting state. If not, difficulty is encountered in driving the display layer under the threshold shifting method.
As can be seen from
Specifically, even when an attempt is made to drive the display layer “b” at the upper threshold value, driving conditions for a write voltage which enable the display layer “a” to be evenly brought into a reflecting state are not found. The relationship between the display layer “a” and the display layer “b” is said to be in a state where the operation margin can hardly be ensured.
In the present embodiment, even in the case of a combination of display layers where an operation margin can hardly be ensured at the upper threshold value as mentioned above, an operation margin can be ensured substantially.
The way of the adjustment voltage actually acting on the display medium 1 shown in
First, it is assumed that a write voltage (300V), by means of which both the display layers 7a, 7b enter a homeotropic texture, is applied to the display layers, thereby bringing the display layers 7a, 7b into a homeotropic reset state; and that, immediately after the application of the write voltage, a DC pulse of about 260V is applied for 10 mS. In this case, an adjustment voltage V2 of about 130V is applied to each of the display layers 7a, 7b, and application of the voltage is released while the display layers remain in a homeotropic reset state, whereby the texture of both layers enters the planer texture (see
The adjustment voltage applied to each of the display layers 7a, 7b is divided by means of a capacitor ratio between the display layers 7a and 7b. In this case, the capacitance of the display layer 7b is set so as to become higher than the capacitance of the display layer 7a (i.e., a higher voltage is applied to the display layer 7a).
Meanwhile, it is assumed that the write voltage (120V), by means of which both the display layers 7a, 7b enter a focal conic texture, is applied to the display layers, thereby bringing the display layers into a focal-conic reset state; and that, immediately after application of the write voltage, a DC pulse of about 260V is applied to the display layers for 10 mS. Even in this case, the adjustment voltage V2 of about 130V is applied to each of the display layers 7a, 7b, and the display layer 7b maintains a focal conic texture. After the texture of the display layer 7a has changed to the homeotropic texture, the texture of the display layer 7a changes to a planar texture along with release of application of a voltage.
As mentioned above, the display layers 7a, 7b can be independently controlled by means of a single drive signal, by utilization of a difference in behaviors between the reset states of the respective display layers 7a, 7b. Thus, an operation margin for the upper threshold value can be substantially ensured.
Even when mere appropriate selection of the magnitude of the write voltage (300V or 120V) in the writing operation cannot prevent changing of the texture of the display layer 7a to an inappropriate texture, a state where selective reflection is realized by the planar texture and selective transmission is realized by the focal conic texture in the display layer 7b is maintained during the writing operation as a result of operation pertaining to the adjustment voltage-applying operation having been performed. Eventually, the entirety of the display layer 7a can be brought into a desired planar texture.
Actuation (operation) pertaining to the writing operation and the adjustment voltage-applying operation is summarized below by reference to
:Writing Operation:
In the writing operation, a write voltage V1 of a single frequency FV1 is selectively applied, during a given period of time TV1, in the form of two magnitudes; namely, a voltage V1H exceeding the upper threshold value of the display layer 7b and a voltage V1L not exceeding the upper threshold value, thereby selecting areas of the display layer 7b where the voltage does not exceed the upper threshold value and areas of the same where the voltage exceeds the upper threshold value. As a result, the areas of the display layer 7b —where the voltage exceeds the upper threshold value—enter a homeotropic texture; and the areas of the display layer 7b—where the voltage does not exceed the upper threshold value—enter a focal conic texture.
At this time, the entirety of the display layer 7a is desired to have remained in the homeotropic texture under a common threshold shifting method. However, in the present embodiment, the operation margin achieved at the upper threshold value is not ensured, and the homeotropic texture and the focal conic texture exist mixedly.
:Adjustment Voltage-Applying Operation:
In the adjustment voltage-applying operation in succession to the writing operation, the adjustment voltage V2, which is a direct current, is applied to the display layers 7a, 7b. The influence of the adjustment voltage V2 on the display layers 7a, 7b is described by reference to
When the applied voltage is released, both the display layer “a” whose entirety remains in a homeotropic texture and the areas of the display layer “b” selected to remain in the homeotropic texture enter a planar texture. However, the areas of the display layer “b” selected to be in the focal conic texture remain intact. Namely, in connection with the display layer “b,” selection of reflection realized by the planar texture and transmission realized by the focal conic texture in the writing operation are retained in an unmodified manner.
Thus, the display layer 7b can be subjected to selective writing at the upper threshold value in the writing operation. The entirety of the display layer 7a can be brought into a planar texture in the adjustment voltage-applying operation. The display layer 7a is subjected to selective writing at the lower threshold value in a second writing operation that is subsequent to the writing operation.
The above descriptions relate to the steps up to adjustment voltage-applying operation unique to the present invention. In relation to the applied invention, no problems arise, so long as operation pertaining to the subsequent second writing operation is performed by means of a known technique; that is, the threshold shifting method. It is desired to effect stable threshold shift driving while a sufficient operation margin is ensured at the lower threshold value even at the time of writing operation.
Therefore, in the present embodiment, a frequency FV3 of voltages (hereinafter sometimes called simply “second write voltages”) V3H and V3L applied during the course of operation performed in the second writing operation is made lower than the frequency FV1 of the write voltages V1H and V1L employed in the writing operation (FV3<FV1).
Actuation (operation) performed in the second writing operation of the present embodiment will now be described.
The display layers 7a, 7b are electrically connected in series, and a voltage pulse is applied from an external power supply capable of outputting an arbitrary waveform. During the operation performed at the lower threshold value, a rectangular pulse of 5 Hz is applied for 200 mS.
In the case of a comparatively-low-frequency pulse of the order of 5 Hz, the potential division ratio applied to each of the display layers is relaxed from a momentary capacitive division ratio to a resistive division ratio determined by a leakage current, and the influence of a difference between the resistance value of the display layer 7a and that of the display layer 7b appears. Since a greater voltage is applied to the display layer 7a of low dielectric constant and high resistance, an operation margin at the lower threshold value can be sufficiently ensured by means of making the operation threshold values of the respective display layers 7a, 7b different from each other.
Meanwhile, in the case of a comparatively-high-frequency pulse of the order of 50 Hz, the potential division ratio applied to the display layers 7a, 7b is not relaxed to the resistive division ratio. The capacitor ratio between the display layers 7a, 7b is expressed in an unmodified form as a potential division ratio. As mentioned previously, in relation to the upper threshold value, an operation margin is substantially ensured by means of the adjustment voltage-applying operation unique to the present invention. Accordingly, the essential requirement pertaining to the capacitor ratio between the display layers 7a, 7b is taken into consideration a potential division ratio of the adjustment voltage. There is no necessity for imparting a considerable difference of capacitance between the display layers.
When the display medium 1 shown in
In the following descriptions, display layers 107a, 107b are described as if they were the display layers 7a, 7b of the present embodiment, by use of the graph of
A texture is selected for each of areas in the display layer 7b by means of the operation pertaining to the previously-described “writing operation” and the operation pertaining to the previously-described “adjustment voltage-applying operation.” After the entirety of the display layer 7a has been aligned to the planar texture, the voltage for the period Va, which is lower than the lower threshold value Vpfa of the display layer 7a, or the voltage for the period Vb, which is higher than the lower threshold value Vpfa of the display layer 7a and lower than the lower threshold value Vfpb of the display layer 7b, is selectively applied, in the form of a pulse waveform having a frequency of 5 Hz. In the areas of the display layer “a” applied with the voltage for the period Vb, the voltage exceeds the lower threshold value Vpfa, and the texture of the areas changes to a focal conic texture. In the areas of the display layer “a” applied with the voltage for the period Va, the voltage does not exceed the lower threshold value Vpfa, and the areas maintain the planar texture.
Meanwhile, in the display layer 7b, the voltage is lower than the lower threshold value Vfpb regardless of the magnitude of the applied voltage. Hence, the planer texture or the focal conic texture selected by the upper threshold value is maintained as mentioned previously.
In the second writing operation, a texture is selected for each of the areas of the display layer 7a.
Operation pertaining to the “writing operation,” operation pertaining to the 2“adjustment voltage-applying operation,” and operation pertaining to the “second writing operation” are sequentially performed. The magnitude of a voltage to be applied is selected for each of the voltage signals of two levels; namely, the upper and lower threshold values. In accordance with a combination of the voltages to be applied, an arbitrary one of the display layers 7a and 7b or both the display layers 7a and 7b can be brought into a reflection state; or both the display layers 7a and 7b can be brought into the transmission state. Thus, the texture is selected, and writing of an image to the reflective liquid crystal display (or driving of the reflective liquid crystal display) is performed.
As has been described above, in the present embodiment, in parallel with the liquid crystal composition of each of the display layers 7a, 7b being controlled, an appropriate adjustment voltage is applied to the display layers in the adjustment voltage-applying operation. Accordingly, the operation margin at the upper threshold value can be substantially enlarged, and stable threshold shift driving can be realized. Even at the lower threshold value, the frequency of a write voltage achieved in the second writing operation is reduced, to thus relax the capacitive division ratio to the resistive division ratio. Thus, the operation margin is enlarged, and stable threshold shift driving can be implemented.
Specifically, according to the present embodiment, operation margins at the upper and lower threshold values are enlarged while sufficient reflectance is being imparted to each of the display layers 7a, 7b, thereby enabling stable threshold shift driving.
A second embodiment is an exemplary embodiment of the aspect (A) of the present invention in which the configuration of the present invention is applied to a method for driving an optical-writing, stacked, reflective liquid crystal display by means of a threshold shift method.
The following descriptions chiefly provides differences between the first and second embodiments in terms of configuration, operation, and advantages. Those elements having the same functions as those of the first embodiment are assigned the same reference numerals, and their repeated explanations are omitted, as applicable.
A display medium of the present embodiment is an element which enables selective driving of a plurality of liquid crystal layers (selective reflection layers) by means of exposure by an address light or application of a bias signal. Specifically, the display medium is a reflective liquid crystal display.
In the present embodiment, the display medium 1′ is a substance formed by means of stacking, in sequence from a display surface side, a transparent substrate 3; a transparent electrode 5; a display layer (a selective reflection layer) 7b; a display layer (a selective reflection layer) 7a; a laminate layer 8; a coloring layer (a light-shielding layer) 9; an OPC layer (a photoconductive layer) 10; a transparent electrode 6; and a transparent substrate 4. Specifically, the display medium 1′ has a structure in which the laminate layer 8, the coloring layer (a light-shielding layer) 9, and the OPC layer (a photoconductive layer) 10 are sandwiched between the first display layer (a selective reflection layer) 7a of the display medium 1 and the transparent electrode 6, both of which pertain to the first embodiment. Only these layers featured exclusively in the present embodiment will be described in detail hereunder.
The OPC layer (a photoconductive layer) 10 is a layer which has an internal photoelectric effect and whose impedance characteristic changes according to the intensity of radiation of address light. The OPC layer 10 can perform AC operation and must be symmetrically activated in response to the address light. The OPC layer is formed into a three-layer structure, wherein charge generation layers (CGL) are formed on upper and lower sides of a charge transport layer (CTL). In the present embodiment, an upper charge generation layer 13, a charge transport layer 14, and a lower charge generation layer 15 are stacked as the OPC layer 10 in sequence from an upper layer in
The charge generation layers 13, 15 have the function of generating photo carriers by absorbing address light. The charge generation layer 13 controls the amount of photo carriers flowing from the transparent electrode 5 on the display surface side to the transparent electrode 6 on the write surface side. The charge generation layer 15 controls the amount of photo carriers flowing from the transparent electrode 6 on the write surface side toward the transparent electrode 5 on the display surface side. Used as the charge generation layers 13, 15 can be layers which generate excitons upon absorption of address light and efficiently separate the excitons into free carriers in the CGL or along an interface between the CGL and the CTL.
The charge generation layers 13, 15 can be generated by a dry process for directly forming a film of a charge generation material (e.g., metal or metal-free phthalocyanine; squalirium compounds; azulenium compounds; perylene pigments; indigo pigments; bis-azo pigments or tris-azo pigments; quinacridone pigments; pyrrolopyrrole dyes; polycyclic quinone pigments; fused aromatic pigments such as dibromo anthrone, and the like; cyanine dyes; xanthene pigments; charge-transfer complexes such as polyvinyl carbazole, nitrofluoren, and the like; and eutectic complexes formed from pyrylium-salt dye and polycarbonate resin); or a wet application method or the like for dispersing or dissolving the charge generation material into an appropriate solvent along with a polymeric binder (e.g., a polyvinyl butyral resin, a polyarylate resin, a polyester resin, a phenol resin, a vinylcarbazole resin, a vinyl formal resin, a partially-denatured vinyl acetal resin, a carbonate resin, an acrylic resin, a vinyl chloride resin, a stylene resin, a vinyl acetate resin, a silicone resin, and the like) to thus prepare a coating fluid, and applying and drying the coating fluid, to thus form a film.
The charge transport layer 14 has the function of being implanted with the photo carriers generated by the charge generation layers 13, 15 and drifting the photo carriers in the direction of the electric field applied by means of a bias signal. In general, the CTL has a thickness which is tens of times as large as that of the CGL. Hence, the contrast impedance of the entire OPC layer 10 is determined by the capacitance and a dark current of the charge transport layer 14 and the photo carrier current in the charge transport layer 14.
The charge transport layer 14 can be efficiently implanted with free carriers originating from the charge generation layers 13, 15 (the charge transport layer 14 can be close to the charge generation layers 13, 15 in terms of ionization potential), and the implanted free carriers can migrate in a hopping manner as fast as possible. In order to increase dark impedance, a lower dark current induced by hot carriers can be used.
The charge transport layer 14 is formed by means of dissolving or dispersing a low-molecular positive-hole transport material (e.g., trinitrofluorene-based compounds; polyvinyl-carbazole-based compounds; oxadiazole-based compounds; hydrazone-based compounds such as benzylamino-based hydrazone, quinoline-based hydrazone, or the like; stilbene-based compounds, triphenylamine-based compounds, triphenylmethane-based compounds, benzidine-based compounds) or a low-molecular electron transport material (e.g., quinone-based compounds, tetracyanoquinodimethane-based compounds, furfreon compounds, xanthone-based compounds, benzophenone-based compounds) into an appropriate solvent along with a polymeric binder (e.g., a polycarbonate resin, a polyalylate resin, a polyester resin, a polyimide resin, a polyamide resin, a polystyrene resin, a silicon-containing crosslinked resin, and the like), to thus prepare a coating fluid; and applying and drying the coating fluid.
The coloring layer (the light-shielding layer) 9 is provided for the purpose of optically separating address light from incident light to thus prevent occurrence of malfunction, which would otherwise be caused by mutual interference between the address light and the incident light. In the present invention, the coloring layer is not an indispensable constituent element. In order to enhance the performance of the display medium 1′, the coloring layer is desirably provided. To this end, the coloring layer 9 is required to have the function of absorbing at least light in an absorption wavelength range of the CGL.
Specifically, the coloring layer 9 can be formed by means of applying inorganic pigments (e.g., cadmium-based pigments, chromium-based pigments, cobalt-based pigments, manganese-based pigments, carbon-based pigments) or organic dyes or organic pigments (azo-based pigments, anthraquinone-based pigments, indigo-based pigments, triphenylmethane-based pigments, nitro-based pigments, phthalocyanine-based pigments, perylene-based pigments, pyrrolopyrrole-based pigments, quinacridone-based pigments, polycyclic-quinone-based pigments, squarium pigments, azulenium pigments, cyanine-based pigments, pyrylium-based pigments, anthrone-based pigments) directly on the surface of the OPC layer 10 facing the charge generation layer 13 or dissolving or dispersing them into an appropriate solvent along with a polymeric binder (e.g., a polyvinyl alcohol resin, a polyacryl resin, and the like), to thus prepare a coating fluid; and applying and drying the coating fluid.
The laminate layer 8 is formed from a polymeric material having a low glass transition point. A material which enables bonding and intimate contact between the display layers 7a, 7b and the coloring layer 9 by means of heat and pressure is selected for the laminate layer 8. Another requirement for the laminate layer 8 is to exhibit transmissivity to at least incident light and address light.
An adhesive polymeric material (e.g., an urethane resin, an epoxy resin, an acrylic resin, a silicone resin) can be mentioned as a material suitable for the laminate layer 8.
The laminate layer 8 is not an indispensable constituent element of the present invention.
The writing apparatus (the apparatus for driving a reflective liquid crystal display) 2′ of the present embodiment is an apparatus for writing an image on the display medium 1′, and comprises, as principal constituent elements, a light irradiation section (an exposure apparatus) 18 for radiating address light to the display medium 1′, and a voltage application section (a power supply) 17 for applying a bias voltage to the display medium 1′. Further, the writing apparatus is provided with a control circuit 16′ for controlling the operation of the voltage application section. Specifically, the writing apparatus 2 of the first embodiment is provided with the light irradiation section 18. The control circuit 16′ has the function of controlling the operation of the light irradiation section 18 along with operation of the voltage application section 17, as appropriate. Only the light irradiation section 18 and the control circuit 16′, both of which are features of the present embodiment, are described in detail hereunder.
The light irradiation section (the exposure apparatus) 18 may be embodied as any section and is not susceptible to limitations, so long as the section has the function of radiating a predetermined address light pattern, which forms an image, on the display medium 1′ and is able to radiate a desired optical image pattern (a spectrum, intensity, and a spatial frequency) on a display medium 1′ (more specifically an OPC layer) in accordance with an input signal delivered from the control circuit 16.
Light complying with the following conditions can be selected as the address light radiated by the light irradiation section 18.
Spectrum: Higher energy in the absorption wavelength of the OPC layer 10 is desirable.
Radiation Intensity: Intensity at which the voltage applied to the respective display layers 7a, 7b during a bright period is divided by the OPC layer 10 to become greater than the upper and lower threshold voltages, thereby changing the texture of liquid crystal in the display layers 7a, 7b, but becoming equal to the upper and lower threshold voltages or less during a dark period.
Light desired to be the address light radiated by the light irradiation section 18 has peak intensity in an absorption wavelength of the OPC layer 10 and the narrowest band.
The following elements are mentioned as the light irradiation section 18.
(1-1) A uniform light source in which light sources (e.g., a cold-cathode tube, a Xenon lamp, a halogen lamp, an LED, an EL, and the like) are arranged in the form of an array or in which a light source and a light guide plate are combined together, and the like.
(1-2) A combination of light-modulating elements which generate a light pattern (e.g., an LCD, a photomask, and the like).
(2) A surface-emitting display (e.g., a CRT, a PDP, an EL, an LED, an FED, and an SED).
The control circuit 16′ is a member having the function of controlling operation of the voltage application section 17 and operation of the light irradiation section 18, as required, in accordance with the image data supplied from the outside (an image-capturing apparatus, an image receiver, an image processor, an image reproducing apparatus, an apparatus having a combination of these functions, or the like). Specific control operations of the control circuit 16′ are embodied by processing pertaining to three steps (operations); namely, a “writing operation (operation),” an “adjustment voltage-applying operation (operation),” and a “second writing operation (operation)” performed subsequent to the writing operation, all of which feature the present invention. Details of these operations will be described later.
Even in the present embodiment, the verification experiment performed in the first embodiment, where the layered structure of the display layer is basically identical with that of the second embodiment, shows that threshold shifting driving operation can be sufficiently performed.
When a reflective liquid crystal display is obtained by means of stacking the two display layers 7a, 7b supplied for a verification experiment, operations pertaining to the “writing operation,” the “adjustment voltage-applying operation (operation),” and the “second writing operation,” all of which pertain to the reflective liquid crystal display (including a photoconductive layer) driving method of the present invention, are sequentially performed. As a result, a sufficient operation margin is ensured for the upper and lower threshold values while sufficient reflectance is being imparted to the display layers, and stable threshold shift driving is implemented.
First, a bias voltage (a reset voltage)—which is lower than the upper threshold voltage of the display layer 7b and higher than the upper threshold voltage of the display layer 7a—is applied, in the form of a pulse waveform having a frequency of 50 Hz, to all the display layers. Concurrently, the light irradiation section 18 selectively exposes the OPC layer 10 to address light, thereby changing (lowering) the resistance value of the OPC layer 10 in the exposed areas. Thus, the potential division between the display layers 7a, 7b is increased. In the exposed areas, the voltage applied to the display layers 7a and 7b exceeds the upper threshold value, and the texture of the display layer 7b changes to a homeotropic texture.
The voltage applied to the unexposed portions of the display layer 7b is Vc, and the display layer 7b enters a focal conic texture.
Meanwhile, both the exposed and unexposed areas of the display layer 7a enter a homeotropic texture. However, in reality, the operation margins of the two display layers cannot necessarily be ensured sufficiently. As in the case of the first embodiment, the display layers 7a, 7b for which no operation margin is ensured are used even in the present embodiment as shown in
In the adjustment voltage-applying operation performed in succession to the writing operation, the adjustment voltage V2, or a d.c. voltage, is continually applied to the display layers 7a, 7b. Since the influence of the adjustment voltage V2 on the display layers 7a, 7b is as has been described by reference to
At the time of release of application of the voltage, the display layer 7a whose entirety is in the homeotropic texture and the areas of the display layer 7b remaining in the homeotropic texture are changed to a planar texture. The areas of the display layer 7b remaining in the focal conic texture remain unchanged. In short, in the display layer 7b, the textures selected by means of reflection realized by the planar texture and transmission realized by the focal conic texture in the writing operation are maintained intact.
Thus, the display layer 7b can be subjected to selective writing at the upper threshold value in the writing operation. In the adjustment voltage-applying operation, the entirety of the display layer 7a can be brought to a planar texture. The display layer 7a is subjected to selective writing at the lower threshold value in the second writing operation subsequent to the writing operation.
The following descriptions are provided by use of the graphs in
After operation pertaining to the writing operation and operation pertaining to the adjustment voltage-applying operation have been performed, a bias voltage (a select voltage)—which becomes the voltage for the period Va that is lower than the lower threshold voltage Vpfa of the display layer 7a—is applied, in the form of a pulse waveform having a frequency of 5 Hz, to the entire display layers. Concurrently, the exposure apparatus selectively exposes the display layers to light, thereby increasing the potential division between the display layers 7a, 7b in the exposed areas, as in the case of the writing operation. Thereby, in the exposed areas, the voltage applied to the display layer 7a and the display layer 7b exceeds the lower threshold value Vpfa. The texture of the display layer 7a changes to a focal conic texture. Moreover, the voltage applied to the unexposed areas is Va, and hence the display layer 7a maintains the planar texture.
Meanwhile, all of the exposed and unexposed areas of the display layer 7b maintain the planar texture or the focal conic texture.
In the second writing operation, a texture is selected from one area to another in the display layer 7a in the manner as mentioned above.
Operation pertaining to the “writing operation,” operation pertaining to the “adjustment voltage-applying operation,” and operation pertaining to the “second writing operation,” which have been described thus far, are performed sequentially. Exposure/nonexposure is selected for each area while voltage signals of two levels; namely, the upper and lower threshold values, are being applied to the display layers. According to the combination of exposure with nonexposure, an arbitrary one or both of the display layers 7a, 7b can be brought into a reflection state; or both of the display layers can be brought into a transmission state. Thus, the textures are selected, and writing (driving) of the reflective liquid crystal display is performed.
As has been described above, in the present embodiment, the liquid crystal composition of each of the display layers 7a, 7b is controlled in the optical-writing, stacked, reflective liquid crystal display, and an appropriate adjustment voltage is applied to the display layers in the adjustment voltage-applying operation in accordance with the control of liquid crystal composition. Accordingly, the operation margin at the upper threshold value can be substantially enlarged, thereby realizing stable threshold shift driving. Moreover, even in relation to the lower threshold value, the frequency of the write voltage in the second writing operation is reduced, thereby relaxing the capacitive division ratio to the resistive division ratio to thus enlarge the operation margin. As a result, stable threshold shift driving can be implemented.
Specifically, according to the present embodiment, even in an optical-writing, stacked, and reflective liquid crystal display, the operation margin at the upper and lower threshold values is enlarged by means of imparting sufficient reflectance to each of the display layers 7a, 7b and realizing stable threshold shift driving.
A third embodiment is an example applied invention (B) which is applied for the purpose of enhancing the contrast of a single-layer, optical-writing, and reflective liquid crystal display.
The following descriptions chiefly provide differences between the present embodiment and the first and second embodiments in terms of a configuration, operation, and advantages. Those elements having the same functions as those of the first and second embodiments are assigned the same reference numerals provided in the drawings, and their repeated explanations are omitted, as applicable.
A display medium of the present embodiment is an element which enables selective driving of a single liquid crystal layer (a selective reflection layer) upon exposure by an address light and receipt of an applied bias signal. Specifically, the display medium is a reflective liquid crystal display.
In the present embodiment, the display medium 1″ is a substance formed by means of stacking, in sequence from a display surface side, the transparent substrate 3; the transparent electrode 5; a display layer (a selective reflection layer) 7′; the laminate layer 8; the coloring layer (the light-shielding layer) 9; the OPC layer (a photoconductive layer) 10; the transparent electrode 6; and the transparent substrate 4. Specifically, the display medium 1″ has a structure in which the two-layer structure of the display medium 1′ formed from the display layers 7a, 7b in the second embodiment is changed to the single layer; namely, the display layer 7′. Since the display layer 7′ is intrinsically identical with the display layers 7a and 7b, detailed descriptions of the layer are omitted here for brevity.
The writing apparatus is also identical with its counterpart of the second embodiment in terms of, particularly, a configuration, except that a control circuit 16″ performs control operation in such a way that the operation unique to the present embodiment is performed. Hence, detailed descriptions of the writing apparatus are omitted here for brevity.
Specific control operations performed by the control circuit 16″ are made up of only two steps (operations); namely, the “writing operation (step)” and the “adjustment voltage-applying operation (operation),” both of which are features of the present invention. Details of the control operations will be described later.
The method for driving the reflective liquid crystal display and the apparatus for driving (operating) the reflective liquid crystal display, both of which pertain to the present embodiment, will be described hereunder by reference to a brief verification experiment.
First, in the writing operation, a bias voltage (a reset voltage) which is lower than the threshold voltage of the display layer 7′ is applied in the form of a pulse waveform having a frequency of 50 Hz pulse. Concurrently, the light irradiation section 18 selectively exposes the display layer to address light, thereby changing (lowering) the resistance value of the OPC layer 10 in exposed areas to thus increase potential division of the display layer 7′. Thereby, the voltage applied to the display layer 7′ in the exposed areas exceeds the threshold voltage, and the texture of the display layer 7′ changes to a homeotropic texture. The voltage applied to unexposed areas remains lower than the threshold voltage, and hence such areas enter a focal conic texture.
In succession to the writing operation, the adjustment voltage V2 in the form of a high-frequency short pulse is continually applied in the adjustment voltage-applying operation. In the present embodiment, immediately after application of a 50 Hz reset pulse in the writing operation, a DC pulse of 5 ms is applied as the adjustment voltage V2 in the adjustment voltage-applying operation. The wavelength of the 50 Hz reset pulse is 20 ms, but the adjustment voltage V2 is a considerably-short pulse.
After release of application of a voltage, the areas of the homeotropic texture change to a planar texture. The areas of the focal conic texture remain in their state.
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Thus far, the present invention has been described in detail by means of three exemplary embodiments. However, the present invention is not limited to these embodiments. The method and apparatus for driving the reflective liquid crystal display of the present invention provide adjustment voltage-applying operation; that is, a new reflectance control technique which is separate and different from and independent of drive operation for writing an image in the reflective liquid crystal display. By means of appropriately adjusting liquid crystal composition or requirements for applying an adjustment voltage, the influence of adjustment voltage-applying operation is controlled, thereby enhancing the diversity and controllability of driving of the reflective liquid crystal display. Therefore, any modification examples fall within the scope of the present invention, so long as the examples include an adjustment voltage-applying operation of applying a predetermined adjustment voltage subsequently to the writing operation.
The aspects (A) and (B) of the present invention, which are specific, effective utilization embodiments of the adjustment voltage-applying operation of the present invention, are also not limited to the above-described embodiments.
For instance, in relation to the aspect (A) of the present invention, only the reflective liquid crystal display—where the selective reflection layers (display layers) are double—is described as a specific example in the two embodiments; namely, the first and second embodiments. The number of selective reflection layers is not limited to two in the aspect (A) of the present invention but may be formed into three layers or more. So long as the selective reflection layers are formed into three layers to thus effect additive color mixture of coloring of the respective layers as blue, green, and red, a full-color image can be readily acquired by means of a simple drive technique embodied by the aspect (A) of the present invention.
In the case of a selective reflection layer of three-layer structure, the reflective liquid crystal display is configured by means of adjusting the upper and lower threshold values such that a difference arises in the upper and lower threshold voltages in each of the three layers. There will be briefly described operation performed when three selective reflection layers are arranged as a display layer “a,” a display layer “b,” and a display layer “c” in ascending sequence of threshold voltage.
In a general case irrelevant to the present invention, in order to drive a stacked, reflective liquid crystal element of three-layer configuration under the threshold shift method, a voltage is selected from seven voltages provided below, as required, in accordance with the intensity of light used for exposure, so that the respective reflection layer scan be switched from each other.
When the reflective liquid crystal display does not include a photoconductive layer, the voltage “e,” the voltage “f,” and the voltage “g” are selected and applied. Subsequently, the voltage “a,” the voltage “b,” the voltage “c,” and the voltage “d” are selected, as appropriate, and applied, to thereby select switching/non-switching of each of the selective reflection layers. Specifically, driving of the respective selective reflection layers can be selected by means of appropriately selecting the magnitude of a voltage to be applied from seven magnitudes. An image can be written simultaneously by means of a single signal.
Meanwhile, when the reflective liquid crystal display includes a photoconductive layer, the display element is exposed while being applied with the voltage “e” or “a,” thereby selecting switching or non-switching of each of the selective reflection layers. At this time, by means of selecting the intensity of exposure light from four types of light intensity, exposure intensity is designed so as to induce a texture change in the selective reflection layer of each desired threshold value.
By means of the above design, the switching states of three layers can be freely selected by means of single exposure. Specifically, exposure intensity is selected as appropriate from four types of exposure intensity levels while the applied voltage is maintained, thereby selecting driving of each of the selective reflection layers. An image can be written simultaneously by means of a single signal.
Since the above switching operation is performed at the respective upper and lower threshold values, operation pertaining to the writing operation (operation), operation pertaining to the adjustment voltage-applying operation, and processing pertaining to the second writing operation (operation) are performed as in the case of the first and second embodiments. A display layer (at least the display layer “a”; and the display layer “b” in some cases)—for which a texture state is not scheduled to be selected—must be in a homeotropic texture at the upper threshold value after operation pertaining to the writing operation (operation). When an operation margin cannot be sufficiently ensured, or the like, the configuration of the aspect (A) of the present invention is effective. A substantial operation margin can be ensured.
The following method is mentioned as a method for applying the aspect (A) of the present invention to drive the stacked reflective liquid crystal display of three-layer configuration by means of the threshold shift method.
The influence of adjustment voltage-applying operation on the display layer “a,” the display layer “b,” and the display layer “c” is appropriately controlled, to thus form a liquid crystal layer. Specifically, the display layers are made less susceptible to the influence of adjustment voltage-applying operation in sequence of the display layer “a,” the display layer “b,” and the display layer “c.”
First, in the writing operation (operation), there is applied a reset voltage which brings all of the display layers into a homeotropic texture or a focal conic texture. Immediately after application of the reset voltage, a DC pulse (adjustment voltage) is applied in the form of voltages of two magnitudes in the adjustment voltage-applying operation (operation).
When the homeotropic reset has been performed in the writing operation (operation), all of the display layers are brought into a planer texture regardless of application of the adjustment voltage.
When focal conic reset has been performed in the writing operation (operation), the display layers “a” and “b” enter a homeotropic texture at the time of application of a DC pulse of large voltage as the adjustment voltage, to thus finally enter a planar texture. The display layer “c” maintains a focal conic texture.
Similarly, when the focal conic reset is performed in the writing operation (operation) and when a DC pulse of small voltage is applied to the display layers, only the display layer “a” enters the homeotropic texture and finally enters a planar texture. The display layers “b” and “c” maintain a focal conic texture.
Specifically, in relation to the drive operation at the upper threshold value, the magnitude of an applied voltage is utilized in the adjustment voltage-applying operation featuring the present invention in place of selection of the texture of the common threshold shift method that encounters difficulty in ensuring an operation margin, thereby substantially enabling ensuring of the operation margin at the upper threshold value. Thereby, drive operation at the lower threshold value performed in the second writing operation (operation) is performed appropriately, and stable threshold shift driving can be implemented.
Particularly, in the case of a three-layer structure, an operation margin must be ensured among three layers. The freedom degree of selection of a material tends to become smaller. In order to ensure an operation margin at the upper threshold value as in the case of the applied invention, liquid crystal composition can be designed in consideration of solely the influence of the adjustment voltage-applying operation. The freedom degree of selection of a material is enlarged significantly, and a considerably wide operation margin can be ensured.
Specifically, as a matter of course, the aspect (A) of the present invention exhibits a superior advantage in spite of the two-layer structure. In the configuration of three or more layers where an operation margin among the respective layers is difficult to ensure, an excellent advantage can be said to be yielded.
No particular limitation is imposed on drive operation performed at the lower threshold value, and the essential requirement is to perform operation based on a general threshold shift method. However, when the operation margin at the lower threshold value is described to be ensured, the frequency of the applied voltage for the upper threshold value is made different from that for the lower threshold value, whereby the operation margin can be ensured at the lower threshold value without relying on capacitive division and by means of relaxing capacitive potential division to resistive potential division.
In this case, capacitive division is relaxed to resistive division, thereby liberating constraints on selection of a material imposed by a difference in dielectric constant or eliminating limitations. A material can be selected by means of a resistance ratio. Stable threshold shift driving is realized while all of the three layers are imparted with high reflectance. In short, in addition to the aspect (A) of the present invention, the frequency of an applied voltage for the upper threshold value is made different from that for the lower threshold value, thereby relaxing capacitive division to resistive division. By means of this, there is enlarged the range of choices of liquid crystal composition in a configuration of three or more layers where the operation margin among layers is difficult to ensure; particularly, in selective reflection layers. Further, controllability of driving operation is extended, and stable threshold shift driving can be implemented while high reflectance is imparted to the reflection layers.
In the aspect (A) of the present invention, the selective reflection layers are not limited to two layers or three layers. Even in the case of four or more selective reflection layers, switching of each of the selective reflection layers can be selected, as appropriate, by means of appropriately adjusting the magnitude of an applied voltage and the number of voltages (in a case where a photoconductor is not included) or appropriately adjusting the intensity of exposure and the number of light beams (in a case where the photoconductor is included). The greater the number of selective reflection layers, the more difficult the operation margin is to ensure. Accordingly, the aspect (A) of the present invention is particularly effective.
In addition, persons skilled in the art can alter, as appropriate, the present invention (including the aspects (A) and (B) of the present invention) in accordance with related-art, publicly-known knowledge. As a matter of course, any modifications fall within the scope of the present invention, so long as the modifications are equipped with the configuration of the present invention even after alterations have been made.
The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2006-169201 | Jun 2006 | JP | national |
