JP2008225098A - Liquid crystal device and electronic equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress variation in gradation of a digital drive type liquid crystal device performing gradation display using sub-fields, due to temperature variation without increasing device constitution nor processing. <P>SOLUTION: The liquid crystal device performs gradation display by applying an on voltage or off voltage to a plurality of pixels arrayed in matrix for each sub-field period while a one-field period is divided into a plurality of sub-field periods differing in period length is characterized in that the dielectric constant of liquid crystal is 6.3 to 7.7 and the on voltage is 2.7 to 3.3 V. Consequently, response speed variation of liquid crystal due to temperature variation can be made equal when a voltage pulses rises and falls, so that time required for the liquid crystal requires to have 100% transmissivity when the on voltage is applied and time required for the liquid crystal to have 0% transmissivity when the off voltage is applied cancel each other and gradations do not change even when the temperature varies. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a liquid crystal device and an electronic apparatus.

  The liquid crystal device displays pixels by arranging pixels in a matrix and controlling a voltage applied to a pixel electrode provided in each pixel. One of the methods for realizing gradation display in a liquid crystal device is a digital driving method. In the digital driving method, one field is divided into a plurality of subfields, and the applied voltage level “H” or “L” to the pixel electrode is selected for each of the subfields. The effective applied voltage in one field is controlled by the number of subfields for selecting the “H” level, and an intermediate gray level is obtained.

  The response speed when a voltage is applied to the liquid crystal varies depending on the temperature. The response speed increases as the temperature increases, and conversely, the response speed decreases as the temperature decreases. That is, when a voltage level of “H” or “L” is applied, the time until the transmittance of the liquid crystal becomes maximum or minimum varies depending on the temperature. For this reason, in the digital driving method, even if the effective applied voltage in one field is the same, the gradation changes depending on the temperature.

  In order to solve this problem, for example, in the liquid crystal device disclosed in Patent Document 1, a measurement unit for measuring the temperature of the device is provided, and an optimum sub-data is prepared from data prepared in advance based on the measured temperature. The time width of the field is selected.

JP 2001-100180 A

  However, in the method of Patent Document 1, it is necessary to provide a temperature measurement unit, and it is necessary to select a time width of a subfield based on the measured temperature, so that an extra device configuration and processing are required.

  An object of the present invention is to suppress a gradation change accompanying a temperature change in a digital drive type liquid crystal device that performs gradation display using subfields without increasing the device configuration or processing.

  In the liquid crystal device according to the present invention, one field period is divided into a plurality of subfield periods each having a different length, and a gradation display is performed by applying an on voltage or an off voltage for each subfield period. In the device, the dielectric constant of the liquid crystal is selected so that the rise response time and fall response time of the liquid crystal when an on-voltage is applied are substantially constant.

The present invention pays attention to the fact that the response speed of the liquid crystal when the on-voltage is applied depends on the dielectric constant of the liquid crystal, whereas the response speed when the off-voltage is applied does not depend on the dielectric constant. The dielectric constant of the liquid crystal was selected so that the rise response time and the fall response time were substantially constant. In other words, since the dielectric constant of the liquid crystal is selected so that the difference in average transmittance between the rising response period and the falling response period when the on-voltage is applied is always the same with an error of 0% to 20%. Even if the time required for the transmittance of the liquid crystal to be maximized when the on-voltage is applied due to the temperature rise is shortened, the time required for the transmittance to be minimized when the off-voltage is applied is also shortened. The key is kept almost constant. As described above, according to the present invention, it is possible to suppress gradation change due to temperature without increasing the number of extra circuits, temperature sensors, and the like.
In addition, the value of a dielectric constant can be 6.3-7.7, for example.

  In the liquid crystal device according to the present invention, one field period is divided into a plurality of subfield periods each having a different length, and gradation display is performed by applying an on voltage or an off voltage for each subfield period. In the liquid crystal device, the on-voltage is selected so that the rise response time and the fall response time of the liquid crystal when the on-voltage is applied are substantially constant.

The present invention pays attention to the fact that the response speed of the liquid crystal when the on-voltage is applied depends on the drive voltage, whereas the response speed when the off-voltage is applied does not depend on the drive voltage, and the rise response of the liquid crystal when the on-voltage is applied The on-voltage of the liquid crystal was selected so that the time and the fall response time were almost constant. In other words, the on-voltage is selected so that the difference in average transmittance between the rising response period and the falling response period of the liquid crystal when the on-voltage is applied is an error of 0% to 20%. Even if the time required for the transmittance of the liquid crystal to be maximized when applied is reduced, the time required for the transmittance to be minimized when the off-voltage is applied is also reduced, resulting in a substantially constant gradation. Kept. Thus, according to the present invention, it is possible to suppress gradation change due to temperature without increasing the number of extra circuits, temperature sensors, and the like.
Note that the value of the on-voltage can be set to 2.7 V to 3.3 V, for example.

  In the liquid crystal device according to the present invention, one field period is divided into a plurality of subfield periods each having a different length, and gradation display is performed by applying an on voltage or an off voltage for each subfield period. In the liquid crystal device, the dielectric constant of the liquid crystal or the on-state electricity is selected so that the difference in average transmittance between the rise response period and the fall response period of the liquid crystal when an on-voltage is applied is 20% or less. Is.

In the present invention, it is preferable to further include a phase difference compensation layer for compensating for the phase difference of the liquid crystal that changes with a temperature change. In particular, it is desirable that the difference between the phase difference value of the phase difference compensation layer and the phase difference value of the liquid crystal, which change with temperature changes, is always within 10% of the liquid crystal phase difference value. In other words, even when the retardation value of the retardation compensation layer and the retardation value of the liquid crystal change as the temperature changes, the difference between the retardation value of the retardation compensation layer and the retardation value of the liquid crystal after the change. The difference is desirably within 10% when the retardation value of the liquid crystal is 100%.
Thereby, the contrast of a display image can be improved and the contrast of an image can be kept high irrespective of a temperature change.

Further, the liquid crystal device according to the present invention can be used as a light valve of a liquid crystal projector of a transmissive type or a reflective type.
In addition, the liquid crystal device according to the present invention can be applied to electronic devices. Here, the electronic apparatus refers to a general apparatus having a certain function provided with the liquid crystal device according to the present invention, and the configuration thereof is not particularly limited. For example, a projection display device, an IC card, a mobile phone, a video camera, A personal computer, a head-mounted display, a fax machine with a display function, a digital camera finder, a portable TV, a DSP device, a PDA, an electronic notebook, an electric bulletin board, a display for advertising, and the like are included.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Embodiment 1 FIG.
FIG. 1 is a block diagram showing a configuration of a drive unit of liquid crystal device 100 according to Embodiment 1 of the present invention. As shown in the figure, the display unit A of the liquid crystal device 100 has a plurality of pixels 6 arranged in a matrix, and along the matrix, data lines 4 are arranged in the vertical direction and scanning lines 5 are arranged in the horizontal direction. ing. A data driver 2 is connected to each data line 4, and a scanning driver 3 is connected to each scanning line 5. A field / subfield timing signal generation circuit 13 is connected to the data driver 2 and the scan driver 3, and a reference clock generation circuit 11 and a data coding circuit 14 are connected to the field / subfield timing signal generation circuit 13. A data coding circuit 14 is connected to the data driver 2.

  The pixel 6 is scanned by the scan driver 3 from top to bottom in each subfield (subdisplay period) within one field (vertical display period), and a voltage of “H” level is sequentially applied to the scan line 5. In synchronization with this scanning, the data driver 2 applies a voltage of “H” or “L” to the data line 4 based on the input data developed by the data coding circuit 14. Each pixel 6 is provided with a switching element realized by a TFT (thin film transistor). This switching element is turned on only when the connected scanning line 5 is “H”, and the connected data line is connected to the pixel 6. When a potential of 4 is applied to the pixel electrode and the scanning line 5 is “L”, the pixel electrode is turned off.

  The liquid crystal device 100 is driven by a digital driving method. FIG. 2 is a diagram for explaining the principle of the digital drive method. FIG. 2A shows a pulse signal representing field timing, and one field is from one pulse to the next. In FIG. 2B, this one field is divided into 63 subfields SF1 to SF63, and an “H” level voltage (ON voltage) is applied to the pixel electrode in only five subfields SF5 to SF9. In the sub-field, the case of applying the “L” level voltage (off voltage) is shown. In this case, the pixel has a brightness of about 5/63 when the voltages are applied to all the subfields SF1 to SF63. In this way, by selecting the voltage for each subfield, it is possible to express 64 levels of gradation from 0/63 to 63/63.

  FIG. 2C shows the reaction timing of the liquid crystal when voltage is applied in the pattern shown in FIG. As shown in the figure, a voltage of “H” level is applied to the subfields SF5 to SF9. However, the response of the liquid crystal to the subfields SF5 to SF9 has a slow rise, and it takes time to reach a steady state, and “H”. Similarly, it takes time when the pulse level decreases from “L” to “L”. As a result, an error occurs in gradation at the rising and falling portions.

  In addition, the reaction rate of the liquid crystal changes depending on the temperature, and the reaction rate tends to increase when the temperature is high, and slow when the temperature is low. FIG. 2 (d) shows the reaction of the liquid crystal when the temperature is lower than that in FIG. 2 (c), and the time to reach the steady state is longer at both the rise and fall of the voltage pulse. Also, the error amount of the gradation to be displayed changes.

The operation of the liquid crystal device 100 will be described with reference to FIG.
The reference clock generation circuit 11 generates reference clock pulses at regular time intervals. The field / subfield timing signal generation circuit 13 generates a field timing signal and a subfield timing signal while counting the reference clock.

  The data coding circuit 14 temporarily holds image data representing the luminance in the current field of all the pixels 6 and counts subfield addresses (0 to 63) according to the input subfield timing signal. A counter is provided. In this apparatus, the application of the “H” voltage to the pixel starts from the subfield SF1, ends in the subfield corresponding to the data of each pixel, and returns to “L”. The data coding circuit 14 Based on the image data, a voltage of “H” or “L” is output for each pixel and for each subfield.

  Upon receiving the input of the subfield timing signal, the scanning driver 3 sequentially outputs “H” voltage to the scanning line 5 in accordance with the signal. Further, the data driver 2 sequentially applies the “H” or “L” voltage output from the data coding 14 to the data line 4 in synchronization with the vertical scanning. In this way, the gradation display of each pixel is realized by receiving the application of the voltage according to the subfield configuration.

  Here, as described above, in the case of the digital drive type liquid crystal device 100 that performs gradation display using subfields, the response speed of the liquid crystal changes depending on the temperature, so that the gradation changes according to the temperature change. As shown in FIGS. 2 (c) and 2 (d), the response speed increases as the temperature increases at both the rise and fall times.

  The gradation is expressed by changing the alignment of the liquid crystal when the voltage “H” is applied for a certain period of time, thereby changing the light transmittance. FIG. 3 is a graph showing the change over time of the transmittance of the liquid crystal when a voltage is applied. The gradation is expressed by an integral value of transmittance in one field, that is, an area surrounded by the curve and the time axis in the graph of FIG. Therefore, if the integral value of the transmittance is constant, the gradation does not change.

  FIG. 4 is a diagram for explaining the principle of suppressing gradation changes accompanying temperature changes. In order to make the integral value of the transmittance constant, as shown in FIG. 4, the change in the integral value of the transmittance at the time of rising (area A in the figure) and the integral value of the transmittance at the time of falling It suffices if the amount of change (area B in the figure) is the same. In other words, if the change in response speed due to temperature change is almost the same at the rise and fall, even if the response speed at the rise increases due to the temperature rise and the integral value of transmittance increases, The response speed at the time of falling also increases, and the integral value of the transmittance decreases accordingly, so that the change in the integral value of the transmittance is canceled as a result, and the gradation is kept almost constant.

In general, the response speed at the rise and the response speed at the fall are expressed by different equations. For example, the response speed τr at the time of rising and the response speed τd at the time of falling of the nematic liquid crystal such as the TN mode, the GH mode, and the ECB mode are expressed by the formulas (1) and (2), respectively.
τr = ηid ² / (ε0 | Δε | V ² -kiiπ ² ) (1)
τd = ηid ² / π ² kii (2)
Here, ηi is a viscosity parameter of the liquid crystal, kii is an elastic parameter, Δε is a dielectric constant parameter, and V is an applied voltage parameter.

As is clear from the equations (1) and (2), the response speed τr at the time of rising and the response speed τd at the time of falling are expressed by different equations, and the response speed τr at the time of rising is expressed by the dielectric constant Δε of the liquid crystal and the applied voltage. Whereas it varies depending on V, the response speed τd at the time of falling does not vary depending on these parameters. Therefore, by adjusting these two parameters, the change in the response speed at the rise and the change in the response speed at the fall can be made the same.

FIG. 5 is a graph showing a change in the response speed of the liquid crystal accompanying a change in temperature when the dielectric constant of the liquid crystal is changed. Here, the liquid crystal uses a TN mode, and the thickness of the liquid crystal layer is 3 μm. Moreover, the liquid crystal material is a fluorine-based liquid crystal, the refractive index is 0.15, and the rotational viscosity is 300 to 350 m · Pa · s. The drive voltage (applied voltage) is 5V.
The graphs of the change in response speed at the rise and the change in response speed at the fall when the dielectric constant of the liquid crystal is 5, 7, and 10 are shown. Although the response speed change with respect to temperature is almost constant even when the rate is changed, the response speed changes depending on the value of the dielectric constant at the time of rising, and is almost the same as the response speed change at the time of falling when the dielectric constant is 7. become.

  From the above, by setting the dielectric constant of the liquid crystal to 7 or 6.3 to 7.7, the change in the response speed accompanying the temperature change is made the same at the rise and fall, and the gradation of the temperature change Change can be suppressed.

FIG. 6 is a graph showing a change in the response speed of the liquid crystal accompanying a change in temperature when the drive voltage is changed. Here, the liquid crystal uses the TN mode, and the thickness of the liquid crystal layer is 3 μm. The liquid crystal material is a fluorine-based liquid crystal having a refractive index of 0.15 and a rotational viscosity of 330 m · Pa · s. The dielectric constant is 10.
The graph shows the change in response speed at the rise and the change in response speed at the fall when the drive voltage is 3V, 5V, and 7V, respectively. Even if it is changed, the response speed change with respect to the temperature is almost constant, whereas the response speed changes with the drive voltage at the time of rise, and is closest to the change in response speed at the time of fall when the voltage is 3V.

  From the above, by setting the drive voltage to 3 V, or 2.7 V to 3.3 V, it is possible to make the change in the response speed accompanying the temperature change closer at the rise time and the fall time, and the gradation due to the temperature change. Can be suppressed.

  Note that the contrast of the display image can be improved by applying a retardation compensation layer for compensating for a change in the retardation of the liquid crystal layer due to a temperature change to the liquid crystal device 100. By making the change of the phase difference value of the phase difference compensation layer substantially the same as the change of the phase difference value of the liquid crystal device 100, the contrast of the image can be kept high regardless of the temperature change.

  In the first embodiment, 64 levels of gradation are reproduced by subfields obtained by dividing one field into 63. However, the number of subfields is not limited to this.

  In the first embodiment, only two types of voltage levels “H” or “L” (common potential) are applied to the pixel electrode through the data line 4, but other voltage levels can be selected. A configuration in which gradation control is performed in fine steps may be employed.

  The liquid crystal device 100 according to the present invention can be applied to all types of liquid crystal such as TN mode, VA mode, IPS mode, FFS mode, OCB mode, and ECB mode.

Liquid Crystal Projector FIG. 7 is a diagram showing a configuration of a projection type liquid crystal projector 10 provided with a liquid crystal device according to the present invention. The liquid crystal projector 10 includes a light source device 101, dichroic mirrors 112 and 114, mirrors 116, 122, and 124, liquid crystal panels 200R, 200G, and 200B according to the present invention, a cross dichroic prism 160, and a projection lens system 180. The liquid crystal panels 200R, 200G, and 200B correspond to the liquid crystal device according to the present invention.

  Illumination light emitted from the light source device 101 enters the dichroic mirror 112. The dichroic mirror 112 transmits only the red light component (R) of the illumination light and reflects the green light component (G) and the blue light component (B). The red light transmitted through the dichroic mirror 112 is reflected by the mirror 116 and reaches the liquid crystal panel 200R for red light. Of the blue light and green light reflected by the dichroic mirror 112, the green light is reflected by the dichroic mirror 114 and reaches the liquid crystal panel 200G for green light. On the other hand, the blue light passes through the dichroic mirror 114, is reflected by the mirrors 122 and 124, and reaches the blue light liquid crystal panel 200B.

  The liquid crystal panels 200R, 200G, and 200B modulate the three color lights according to the given image information (image signal). The cross dichroic prism 160 forms a color image by combining the three color lights modulated by the three liquid crystal panels 200R, 200G, and 200B. The combined light generated by the cross dichroic prism 160 is emitted in the direction of the projection lens system 180. The projection lens system 180 enlarges and projects the combined light generated by the cross dichroic prism 160 on the projection screen SC and displays a color image.

  Further, the liquid crystal device 100 according to the present invention is not limited to the projection type liquid crystal projector 10 as described above, but can be applied to a reflection type liquid crystal device (LCOS) or the like.

Electronic Device Next, a specific example of an electronic device including the above-described liquid crystal device 100 will be described.
FIG. 8 is a perspective view illustrating a specific example of an electronic apparatus including the liquid crystal device 100. FIG. 8A is a perspective view illustrating a mobile phone which is an example of an electronic device. The cellular phone 1000 includes a display unit 1001 configured using the liquid crystal device 100 according to the present invention. FIG. 8B is a perspective view illustrating a wrist watch that is an example of an electronic apparatus. The wristwatch 1100 includes a display unit 1101 configured using the liquid crystal device 100 according to the present invention. FIG. 8C is a perspective view illustrating a portable information processing device 1200 which is an example of an electronic device. The portable information processing apparatus 1200 includes an input unit 1201 such as a keyboard, a main body unit 1202 in which arithmetic units and storage units are stored, and a display unit 1203 configured using the liquid crystal device 100 according to the present invention. Yes.
In addition to the above, the liquid crystal device 100 can be used as a display unit of a device such as a display device, a television device, electronic paper, a clock, or a calculator.

It is a block diagram which shows the structure of the drive part of the liquid crystal device by Embodiment 1 of this invention. 2A to 2D are timing charts showing the principle of the digital driving method. It is a graph which shows the time change of the transmittance | permeability of the liquid crystal at the time of applying a voltage. It is a figure explaining the principle of suppression of the gradation change accompanying the temperature change of the liquid crystal device by this invention. It is a graph which shows the change of the response speed of the liquid crystal accompanying a temperature change when changing the dielectric constant of a liquid crystal. It is a graph which shows the change of the response speed of the liquid crystal accompanying a temperature change when a drive voltage is changed. It is a figure which shows the structure of the liquid crystal projector using the liquid crystal device by this invention. It is a figure which shows the example of the electronic device using the liquid crystal device by this invention.

Explanation of symbols

  2 data drivers, 3 scan drivers, 4 data lines, 5 scan lines, 6 pixels, 11 reference clock generation circuit, 13 field / subfield timing signal generation circuit, 14 data coding circuit, 100 liquid crystal device, 10 liquid crystal projector, 101 light source Apparatus, 112, 114 dichroic mirror, 116, 122, 124 mirror, 200, 200R, 200G, 200B liquid crystal panel, 160 cross dichroic prism, 180 projection lens system, 1000 mobile phone, 1001, 1101, 1203 display unit, 1100 wristwatch, 1200 portable information processing apparatus, 1201 input unit, 1202 main unit

Claims (7)

A liquid crystal device in which one field period is divided into a plurality of subfield periods each having a different length, and gradation display is performed by applying an on voltage or an off voltage for each subfield period,
A liquid crystal device, wherein a dielectric constant of the liquid crystal is selected so that a rise response time and a fall response time of the liquid crystal when an ON voltage is applied are substantially constant.   The liquid crystal device according to claim 1, wherein the dielectric constant is 6.3 to 7.7. A liquid crystal device in which one field period is divided into a plurality of subfield periods each having a different length, and gradation display is performed by applying an on voltage or an off voltage for each subfield period,
A liquid crystal device, wherein the on-voltage is selected so that a rise response time and a fall response time of the liquid crystal when an on-voltage is applied are substantially constant.   The liquid crystal device according to claim 3, wherein the ON voltage is 2.7 V to 3.3 V.   5. The liquid crystal device according to claim 1, further comprising a phase difference compensation layer for compensating for a phase difference of the liquid crystal that changes with a temperature change. A liquid crystal device in which one field period is divided into a plurality of subfield periods each having a different length, and gradation display is performed by applying an on voltage or an off voltage for each subfield period,
The dielectric constant of the liquid crystal or the on-state electricity is selected so that the difference in average transmittance between the rising response period and the falling response period when the on-voltage is applied is 20% or less. Liquid crystal device.   An electronic apparatus comprising the liquid crystal device according to claim 1.

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