Display device and drive method for display device

Abstract

To provide a display device that has a simple circuit configuration and that can achieve low power consumption; and a drive method for the display device.SOLUTION: Control is performed so that: rectangular wave voltage that varies between 0 volts and positive voltage V11 is applied to a facing electrode 26 of a reverse mode screen 21 at a period that is two times of a period that cause an optical state of an optical layer to change; and pulse voltage is applied to a control electrode 27 to achieve potential difference between electrodes that causes the optical state to change at timing that causes the optical state of the optical layer 25 to change, the pulse voltage being applied in a superimposed manner to rectangular wave voltage that varies between 0 volts and positive voltage V12 having the same period and phase as the rectangular voltage that is applied to the facing electrode 26 and having the same value as the V11.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a display device for displaying images and a method for driving the display device. [Background technology]

[0002] 2. Description of the Related Art Display devices that project an image from a light source such as a projector onto a screen (projection surface) to display the image have been known.

[0003] Liquid crystal elements are sometimes used in the screens of this type of display device. Liquid crystal elements (liquid crystal displays) generally have a short lifespan when driven by direct current, so they are driven by applying an alternating current voltage, i.e., they are driven by an alternating current voltage. Methods for applying this alternating current include frame inversion driving, row line inversion driving, column line inversion driving, and dot inversion driving (see Patent Document 1 for an example of a screen using an AC-driven liquid crystal).

[0004] Furthermore, Patent Document 2 proposes using a transmissive liquid crystal display panel as a screen, controlling the transmittance of the screen to alternate between a transparent state and an opaque state, photographing viewers with a camera installed behind the screen when it is in the transparent state, and displaying images as a display when it is in the opaque state. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 4229161 [Patent Document 2] Patent No. 4490357 Summary of the Invention [Problem to be solved by the invention]

[0006] The transmissive liquid crystal display described in Patent Document 2 also uses AC driving as its driving method. However, in the case of AC driving, in order to increase the degree of freedom in the timing of changing the optical state, one electrode may be fixed at 0 volts and a bipolar AC voltage waveform may be applied to the other electrode. In this case, two power supplies, one for a positive predetermined voltage +V and one for a negative predetermined voltage -V, are required, which results in a problem of complex power supplies and driving circuits. In addition, the amplitude of the driving voltage also becomes large, which results in a problem of increased power consumption.

[0007] In view of the above-mentioned problems, an object of the present invention is to provide a display device and a method for driving the display device that have a simple circuit configuration and can achieve low power consumption, for example. [Means for solving the problem]

[0008] In order to solve the above problem, the display device described in claim 1 comprises a screen having an optical layer whose optical state changes when a voltage is applied thereto, and first and second electrodes arranged opposite each other across the optical layer to apply a voltage to the optical layer, and a control unit that applies a predetermined voltage to the first and second electrodes during a projection period of image light projected onto the screen, and switches the screen between a predetermined image state in which the image light is scattered and a non-image state which is an optical state different from the image state, characterized in that the control unit applies to the first electrode a first rectangular wave voltage having a period that is an integer multiple of the period for changing the optical state, and applies to the second electrode a second rectangular wave voltage having the same period and the same phase as the first rectangular wave voltage or the same period and a phase shifted by half a period, superimposed with a pulse voltage which is an inter-electrode potential difference that changes the optical state at the timing for changing the optical state. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic configuration diagram of a display device according to a first embodiment of the present invention. [Figure 2]FIG. 2 is a schematic cross-sectional view of the screen shown in FIG. [Figure 3] FIG. 2 is an explanatory diagram of a projector that projects in synchronization with the optical characteristics of the screen shown in FIG. [Figure 4] 2 is an explanatory diagram of a display state in which an image formed by image light in the display device shown in FIG. 1 overlaps with the background of the screen. FIG. [Figure 5] 2 is a timing chart showing a driving voltage waveform when the screen shown in FIG. 1 operates in a reverse mode. [Figure 6] 2 is a timing chart showing a driving voltage waveform when the screen shown in FIG. 1 operates in a normal mode. [Figure 7] 10 is a timing chart showing a driving voltage waveform when the screen of the display device according to the second embodiment of the present invention operates in reverse mode. [Figure 8] 8 is a timing chart showing a driving voltage waveform when the screen shown in FIG. 7 operates in a normal mode. [Figure 9] 10 is a timing chart showing a driving voltage waveform when the screen of the display device according to the third embodiment of the present invention operates in reverse mode. [Figure 10] 10 is a timing chart showing a driving voltage waveform when the screen of the display device according to the fourth embodiment of the present invention operates in reverse mode. [Figure 11] 6 is a graph showing the relationship between the element voltage of the optical layer and the parallel light transmittance for the driving voltage waveform shown in FIG. 5. [Figure 12] 11 is a graph showing the relationship between the element voltage of the optical layer and the parallel light transmittance for the driving voltage waveform shown in FIG. 10. [Figure 13] 10 is a timing chart showing a driving voltage waveform when the screen of the display device according to the fifth embodiment of the present invention operates in reverse mode. [Figure 14] FIG. 10 is a schematic cross-sectional view of a screen according to a sixth embodiment of the present invention. [Figure 15]FIG. 15 is a schematic front view of the screen showing the arrangement of a plurality of control electrodes shown in FIG. 14. [Figure 16] FIG. 15 is an explanatory diagram of synchronous control of scanning and driving of the screen shown in FIG. 14. [Figure 17] FIG. 15 is an explanatory diagram of a projector that scans the screen shown in FIG. 14. [Figure 18] 15 is a schematic timing chart of scanning and driving of the screen shown in FIG. 14. [Figure 19] 15 is a timing chart showing a case where the drive voltage waveform shown in FIG. 14 is applied to a plurality of regions. [Figure 20] 13 is a timing chart showing a driving voltage waveform when the screen of the display device according to the seventh embodiment of the present invention operates in reverse mode. DETAILED DESCRIPTION OF THE INVENTION

[0010] A display device according to one embodiment of the present invention will be described below. This display device includes a screen having an optical layer whose optical state changes upon application of a voltage, and first and second electrodes disposed opposite each other across the optical layer for applying a voltage to the optical layer. The first electrode of the screen is driven by a controller to apply a first rectangular wave voltage having a period that is an integer multiple of the period for changing the optical state of the optical layer. The second electrode is driven by a controller to apply a second rectangular wave voltage having the same period and the same phase as the first rectangular wave voltage or a phase shifted by a half period from the first rectangular wave voltage, superimposed with a pulse voltage that generates an inter-electrode potential difference that changes the optical state at the timing for changing the optical state of the optical layer. Therefore, for example, by setting the amplitudes of the first and second rectangular wave voltages and the pulse voltage to an inter-electrode potential difference that achieves desired optical characteristics, a circuit can be configured that can be driven with a small power supply (one power supply), thereby achieving a simple circuit configuration. Furthermore, the voltage can be reduced, thereby reducing power consumption. Furthermore, the optical characteristics of the elements constituting the optical layer can be easily changed at any desired timing.

[0011] Furthermore, the first square wave voltage applied to the first electrode and the second square wave voltage applied to the second electrode may be composed of a positive or negative voltage and a voltage indicating 0 volts, and the amplitude of the second square wave voltage applied to the second electrode may be the same as or smaller than the first square wave voltage. This allows the two square wave voltages to be unipolar, thereby reducing the amplitude of the voltage applied to one electrode. Furthermore, by setting the amplitudes of the first and second square wave voltages to the inter-electrode potential difference at which the optical state changes, a circuit can be configured that can be driven with a single power supply, thereby simplifying the circuit configuration. Furthermore, by reducing the amplitude of the second square wave voltage, the device can be driven with a low voltage except when the pulse voltage is applied, thereby reducing power consumption.

[0012] Furthermore, the first square wave voltage applied to the first electrode and the second square wave voltage applied to the second electrode may be bipolar AC voltages, and the amplitude of the second square wave voltage applied to the second electrode may be the same as or smaller than the first square wave voltage. This allows the two square wave voltages to be bipolar, thereby reducing the absolute value of the voltage applied to one electrode. When the first square wave voltage and the second square wave voltage have the same amplitude, a common power supply can be used, resulting in a simple circuit configuration. Furthermore, when the amplitude of the second square wave voltage is small, even when a bipolar AC voltage is used, the amplitude of the first square wave voltage and the second square wave voltage can be reduced compared to conventional methods to generate an inter-electrode potential difference that changes the optical state, thereby reducing power consumption.

[0013] Alternatively, the first square wave voltage applied to the first electrode may be a biased AC voltage, the second square wave voltage applied to the second electrode may be composed of a positive or negative voltage and a voltage indicating 0 volts, and the amplitude of the second square wave voltage applied to the second electrode may be smaller than that of the first square wave voltage, and the median value of the amplitude may be the same as that of the first square wave voltage. In this way, even if the amplitude of the second electrode is reduced by the amount of biasing the first electrode, an inter-electrode potential difference that changes the optical state can be generated, thereby reducing power consumption.

[0014] Furthermore, the pulse voltage may be the same as at least one of the two voltage values ​​constituting the second rectangular wave voltage. This allows the power supply that generates the second rectangular wave voltage and the power supply that forms the pulse voltage to be common, thereby simplifying the circuit configuration of the drive circuit.

[0015] Furthermore, the pulse voltage may be the same as at least one of the two voltage values ​​constituting the first rectangular wave voltage. This allows the power supply that generates the first rectangular wave voltage and the power supply that forms the pulse voltage to be common, thereby simplifying the circuit configuration of the drive circuit.

[0016] Furthermore, the amplitude of the pulse voltage may be smaller than the amplitude of the first rectangular wave voltage, which allows the generation of an inter-electrode potential difference that changes the optical state even when the amplitude of the pulse voltage is small, thereby reducing power consumption.

[0017] Furthermore, the pulse voltage may be configured with a plurality of voltage values ​​in a half cycle of the second rectangular wave voltage, and by doing so, for example, by changing one pulse voltage in a stepwise manner, it is possible to optimize the transient characteristics of the elements constituting the optical layer.

[0018] Furthermore, the period of the first rectangular wave voltage may be an even multiple of the period for changing the optical state of the optical layer, which makes it easier to generate the waveform of the first rectangular wave voltage from a video synchronization signal or the like, thereby simplifying the circuit configuration.

[0019] Furthermore, the first square wave voltage and the second square wave voltage may be voltages that do not change the element characteristics of the elements that make up the optical layer due to the potential difference generated by these two square wave voltages, and that result in the same optical state as when no voltage is applied, thereby reducing deterioration of the elements over time.

[0020] Alternatively, the second electrode may be divided into a plurality of regions, and the control unit may sequentially switch the timing of applying the pulse voltage so as to sequentially switch the timing of changing the optical state of the optical layer corresponding to each region of the divided second electrode. This allows for a simple circuit configuration and reduced power consumption in a screen in which the second electrode is divided into a plurality of regions.

[0021] The second electrode may also be divided into strips, which allows the screen to be arranged vertically along the scanning direction, and the optical state to be changed sequentially in the scanning order of the video.

[0022] The control unit may also control the application time of the pulse voltage applied to the second electrode divided into multiple regions so that the application times of the pulse voltages in adjacent regions overlap each other. This allows the image to be displayed after the optical state of the next region has stabilized, thereby preventing display degradation such as uneven brightness.

[0023] The control unit may also change the time during which the desired optical properties of the optical layer are maintained by increasing or decreasing the application time of the pulse voltage applied to the second electrode, thereby allowing the retention time rate of the optical state of each region to be changed as desired, thereby changing the transparency of the screen.

[0024] Furthermore, when the control unit detects a second electrode, among the plurality of second electrodes, whose optical property maintenance time spans the timing of the voltage value change of the first rectangular wave voltage, the control unit may control the pulse voltage to be applied to the second electrode to change to a voltage value that inverts with respect to the median value of the second rectangular wave voltage at the timing of the voltage value change. In this way, even in the plurality of regions, whose optical property maintenance time overlaps the timing of the voltage value change of the first rectangular wave voltage, the optical properties can be maintained for a set time, and display degradation such as unevenness due to differences in the optical property maintenance time depending on the region can be prevented.

[0025] Furthermore, a method for driving a display device according to one embodiment of the present invention includes a screen having an optical layer whose optical state changes upon application of a voltage, and first and second electrodes disposed opposite each other across the optical layer for applying a voltage to the optical layer. The method applies a first rectangular wave voltage to the first electrode, the first rectangular wave voltage having a period that is an integer multiple of the period for changing the optical state of the optical layer, and the second rectangular wave voltage having the same period and the same phase as the first rectangular wave voltage or a phase shifted by a half period from the first rectangular wave voltage, and a pulse voltage superimposed on the second electrode, the pulse voltage being a potential difference between the electrodes that changes the optical state at the timing for changing the optical state of the optical layer. Therefore, for example, by setting the amplitudes of the first and second rectangular wave voltages and the pulse voltage to a potential difference between the electrodes that achieves the desired optical characteristics, a circuit can be configured that can be driven with a small power supply (one power supply), thereby achieving a simple circuit configuration. Furthermore, the voltage can be reduced, thereby reducing power consumption. Furthermore, the optical characteristics of the elements constituting the optical layer can be easily changed at any desired timing. [Example]

[0026] A display device 1 according to a first embodiment of the present invention will be described with reference to Fig. 1 to Fig. 6. As shown in Fig. 1, the display device 1 includes a projector 11, a screen 21, and a synchronization control unit 31. The display device 1 is a transmissive projection device in which the image light from the projector 11 is transmitted and scattered by the screen 21.

[0027] Projector 11 can use a transmissive or reflective liquid crystal light valve that sequentially shifts the black state (state where no projection light is emitted) on screen 21 during a scanning period, but other elements can also be used. Projector 11 can also be one that performs raster scanning during the image scanning period and projects image light dot-sequentially onto the display surface of screen 21. This projector 11 can be, for example, a laser projector that deflects the irradiation direction of an intensity-modulated light beam by reflecting it with a movable mirror. This projector 11 can be considered as one in which the irradiation position of image light is sequentially scanned in one direction on screen 21.

[0028] Projector 11 may be any device capable of projecting image light modulated by image information onto screen 21. The image information is obtained from a video signal input to projector 11. The video signal may be an analog video signal such as the NTSC (National Television Standards Committee) system or the PAL (Phase Alternation by Line) system, or a digital video signal such as the MPEG-TS (Moving Picture Experts Group - Transport Stream) format or the HDV (High-Definition Video) format. In addition to video signals of moving images, still image signals such as JPEG (Joint Photographic Experts Group) may also be input to projector 11. In this case, projector 11 may repeatedly scan screen 21 with the same image light for displaying still images.

[0029] The screen 21 may be any type that can change its optical state by applying a voltage. The optical state of the screen 21 is a scattering state, which is the imaging state, and a transparent transmitting state, which scatters incident light less and has a high transmittance for parallel light, which is the non-imaging state.

[0030] The screen 21 may be, for example, a light-controlling screen that uses a liquid crystal material and changes between a scattering state and a transparent, transmitting state in which incident light is little scattered. Light-controlling screens include those that use liquid crystal elements such as polymer-dispersed liquid crystals, and those that use elements that control the scattering state and the transparent, transmitting state in which incident light is little scattered by moving white powder in a transparent cell.

[0031] Fig. 2 shows a schematic cross-sectional view of a screen 21 whose optical state can be controlled. The screen 21 shown in Fig. 2 has an optical layer 25 in which a composite material containing liquid crystal is sandwiched between a pair of transparent glass plates 23 and 24. A counter electrode 26 is formed over the entire surface of one of the glass plates 24 facing the optical layer 25. A control electrode 27 is disposed over the entire surface of the other glass plate 23 facing the optical layer 25. An intermediate layer made of an insulator may be formed between the electrodes 26, 27 and the optical layer 25.

[0032] The counter electrode 26 and the control electrode 27 are formed as transparent electrodes using, for example, ITO (indium tin oxide). The optical layer 25 is disposed between the control electrode 27 and the counter electrode 26.

[0033] A voltage is applied to the screen 21 so as to generate a potential difference between the control electrode 27 and the counter electrode 26. The drive waveform (drive voltage waveform) described below indicates a waveform (voltage) applied to the control electrode 27 as the second electrode and the counter electrode 26 as the first electrode. The alignment state of the liquid crystal in the optical layer 25 changes depending on the voltage applied to the counter electrode 26 and the control electrode 27.

[0034] The synchronization control unit 31 as a control unit controls the screen 21 onto which an image is projected to a state in which the projected image light is scattered, and to a transparent state when no image is being projected. As shown in FIG. 1, the synchronization control unit 31 is connected to the projector 11 and the screen 21. The synchronization control unit 31 controls the optical state of the screen 21 in synchronization with the projection of the image light from the projector 11. The synchronization signal input from the projector 11 to the synchronization control unit 31 can be, for example, a synchronization signal synchronized with the scanning cycle of the projector 11.

[0035] Next, a projection method of the projector 11 that projects in synchronization with the optical characteristics of the screen 21 in the display device 1 according to this embodiment will be described with reference to FIG. 3. FIG. 3 is an explanatory diagram of a method in which the projector 11 projects image light at intervals. In this case, as shown in FIG. 3(B), image light is projected onto the screen 21 for a short period during a part of the scanning cycle. The screen 21 may be in a scattering state during that part of the period, as shown in FIG. 3(C).

[0036] If the optical state of the screen 21 is controlled so as to increase the parallel light transmittance of the screen 21 during periods other than this part, the see-through characteristics of the screen 21 can be obtained without causing a decrease in the brightness of the image during the scanning cycle. Compared to steadily projecting image light, to obtain the same brightness, projection light with an intensity roughly the reciprocal of the duty (duty: a in the figure) equivalent to the time during which the screen is in the scattering state for one scanning cycle is required. Therefore, to obtain high see-through characteristics, a powerful pulsed projection light output is required.

[0037] By controlling the projector 11 and the screen 21 in this way, the screen 21 can scatter and transmit image light with brightness equivalent to that of a screen in a constant scattering state, while retaining transparency that allows objects behind it to be recognized. In other words, it is possible to achieve both see-through properties that allow background objects to be recognized and high image visibility.

[0038] Information on the switching timing for synchronous control of the projector 11 and the screen 21 is sent as a synchronization signal from the projector 11 to the synchronization control unit 31. Note that the projector 11 and the synchronization control unit 31 may be capable of wireless communication using electromagnetic waves such as microwaves and infrared rays, and information for achieving synchronization between them may be transmitted and received via a wireless signal.

[0039] With the display device 1, for example, in the installation environment of Fig. 1, an image can be viewed as shown in Fig. 4. Fig. 4 is an explanatory diagram of a display state in which an image formed by the image light is superimposed on the background of the screen 21. In Fig. 4, an image of a person 41 formed by the image light is projected on the right side of the screen 21, and a tree 42 can be seen on the left side as the background on the other side of the screen 21.

[0040] Next, we will explain how to drive the display device 1 that uses the screen 21 operating in reverse mode. When the screen 21 operates in reverse mode, it is in a transparent, transmissive state in the normal state where no voltage is applied. When a voltage is applied, it enters a scattering state with a scattering rate of parallel light corresponding to the applied voltage. The optical state of the screen 21 is such that a predetermined scattering state corresponds to the video state, and a transparent, transmissive state with a higher transmittance of parallel light corresponds to the non-video state.

[0041] 5 shows a timing chart illustrating an example of the relationship between the optical state and the drive voltage waveform applied by the synchronization control unit 31 to the screen 21 in reverse mode according to this embodiment. The horizontal axis of the drive voltage waveform represents time, and the vertical axis represents voltage. The light transmittance represents the optical state of the optical layer 25. The horizontal axis of the light transmittance represents time, and the vertical axis represents the transmittance of parallel light. A low parallel light transmittance indicates strong scattering.

[0042] In FIG. 5, a square wave voltage having a period (2T) twice the repetition period (T) for changing the element characteristics is applied to the counter electrode 26 as a first square wave voltage. This repetition period for changing the element characteristics is, for example, one frame period. The square wave voltage applied to the counter electrode 26 is controlled to change between two voltages: 0 volts and a positive voltage of +V11 volts. Furthermore, the period of the square wave voltage applied to the counter electrode 26 is not limited to twice the repetition period for changing the element characteristics, but is preferably an even multiple such as four times.

[0043] On the other hand, the second electrode, which is the second square wave voltage, has a square wave voltage with the same period and phase (the voltage changes at the same timing) as the counter electrode 26, and a pulse voltage superimposed on it at the timing to change the optical state. That is, at the timing to change the optical state (T11 and T12 in FIG. 5), the potential difference between the counter electrode 26 and the control electrode 27 is increased to the potential difference at which the optical state of the optical layer 25 changes (from the transmissive state to the scattering state). In FIG. 5, the transmissive state is changed to the scattering state at T11 and T12. That is, from this timing, during the application time of the pulse voltage, the area corresponding to the control electrode 27 is in the video state, and an image is projected.

[0044] Furthermore, the square wave voltage and pulse voltage applied to control electrode 27 are controlled to change between two voltages, 0 volts and a positive voltage of +V12 volts. That is, a pulse voltage of +V12 volts is applied at T11, and a pulse voltage maintained at 0 volts is applied at T12, so the pulse voltage has the same value as the two voltage values ​​(+V12, 0) of the square wave voltage applied to control electrode 27.

[0045] It is desirable that the amplitudes V11 and V12 of the drive voltage waveforms applied to the counter electrode 26 and the control electrode 27 are the same (V11=V12) so as not to apply a DC bias component to the elements constituting the optical layer 25, but even if they are different, they may be appropriately set based on the potential difference that maintains the transparent state and the potential difference that results in the scattering state, as long as it does not affect the characteristics of the elements constituting the optical layer 25. For example, the amplitude of the square wave voltage applied to the control electrode 27 may be smaller than the square wave voltage applied to the counter electrode, and a pulse voltage may be applied that results in a potential difference that can change the optical state.

[0046] 5, the pulse voltage is applied to the control electrode 27 once per frame period (the repeating period for changing the element characteristics), but it may be applied two or more times to change the state to the scattering state multiple times per frame period. Of course, in the second and subsequent examples described below, the pulse voltage may also be applied multiple times in the repeating period for changing the element characteristics.

[0047] According to this embodiment, the synchronization control unit 31 controls the application of a rectangular wave voltage that varies between a positive voltage V11 and 0 volts at a period twice the period for changing the optical state of the optical layer to the counter electrode 26 of the screen 21 in the reverse mode, and a rectangular wave voltage that varies between a positive voltage V12 and 0 volts, the same value as the rectangular wave voltage V11, and the same period and phase as the rectangular wave voltage applied to the counter electrode 26, and the control electrode 27. The rectangular wave voltage V12 is superimposed on the rectangular wave voltage V12, and the inter-electrode potential difference that changes the optical state at the timing for changing the optical state of the optical layer 25. Therefore, for example, if the amplitudes V11 and V12 of the two rectangular wave voltages and the pulse voltage are the same value, and the inter-electrode potential difference is set to a value that achieves the desired optical characteristics, a circuit that can be driven by a single power supply can be configured, thereby achieving a simple circuit configuration. Furthermore, the applied voltage can be reduced, thereby reducing power consumption.

[0048] Furthermore, because a square-wave voltage is applied, AC drive is essentially performed. Furthermore, because the two square-wave voltages can be unipolar, the amplitude of the voltage applied to each electrode can be reduced. Furthermore, because the period of the square-wave voltage is twice the period for changing the optical state of the optical layer 25, which is an even multiple, it is easy to generate the waveform of the square-wave voltage from a video synchronization signal, etc. In other words, by synchronizing the square-wave voltage of the counter electrode 26 with the frame period of the projector 11, the modulation timing for changing to the scattering state, including its duration, can be made more flexible by adjusting the waveform of the control electrode 27, making it possible to easily change the optical properties of the elements constituting the optical layer 25 at any timing.

[0049] Furthermore, since the pulse voltage is the same value as one of the two voltage values ​​(+V12, 0) that make up the rectangular wave voltage applied to the control electrode 27, the power supply that generates the voltage to be applied to the control electrode 27 can be reduced to one, resulting in a simple circuit configuration.

[0050] In the above-described embodiment, the voltages applied to counter electrode 26 and control electrode 27 are 0 volts and a positive voltage, but they may also be 0 volts and a negative voltage. Also, the pulse voltage is set to the same value as one of the two voltage values ​​constituting the square wave voltage applied to control electrode 27, but it may also be set to the same value as one of the two voltage values ​​(+V11, 0) constituting the square wave voltage applied to counter electrode 26.

[0051] Furthermore, although the above-described embodiment has been described with reference to the screen 21 operating in the reverse mode, the screen 21 may also operate in the normal mode, as shown in Fig. 6. In the screen 21 operating in the normal mode, the screen 21 is in a scattering state in the normal state where no voltage is applied. When a voltage is applied, the screen 21 enters a transparent transmitting state with a parallel light transmittance according to the applied voltage.

[0052] In the normal mode, as shown in the timing chart of Fig. 6, a square wave voltage having a period twice the period for changing the optical state of the optical layer 25 is applied to the counter electrode 26, and a pulse voltage is superimposed on the square wave voltage having the same period but opposite phase (the period is shifted by half a period) as that of the counter electrode 26 to the control electrode 27 at the timings (T11, T12) for changing the optical state. In other words, at the timings (T11, T12 in Fig. 6) for changing the optical state, the potential difference between the counter electrode 26 and the control electrode 27 is reduced to the potential difference at which the optical state of the optical layer 25 changes (from the transmissive state to the scattering state). Here, V11 and V12 in Fig. 6 have the relationship V1 = V2, just like V11 and V12 shown in Fig. 6. [Example]

[0053] Next, a display device according to a second embodiment of the present invention will be described with reference to Figures 7 and 8. Note that the same parts as those in the first embodiment described above are given the same reference numerals and descriptions thereof will be omitted.

[0054] The display device according to this embodiment has the same configuration as that of the first embodiment, but differs in the drive voltage waveforms applied to the counter electrode 26 and the control electrode 27. This will be described with reference to the timing chart shown in FIG. 7, which shows an example of the drive voltage waveform according to this embodiment.

[0055] Fig. 7 shows an example of screen 21 operating in reverse mode. In Fig. 7, a square wave voltage having a period (2T) twice the repetition period (T) that changes the element characteristics is applied to counter electrode 26, which is a first square wave voltage. The square wave voltage applied to counter electrode 26 is a rectangular AC voltage that varies between +V21 volts and -V21 volts with 0 volts as the center.

[0056] On the other hand, a pulse voltage is superimposed on the control electrode 27 at a timing to change the optical state on a square wave AC voltage, which is a second square wave voltage and has the same period and phase as the counter electrode 26. The square wave AC voltage applied to the control electrode 27 varies between +V22 volts and −V22 volts with 0 volts as the center, and the pulse voltage applied is also −V22 volts or +V22 volts. In other words, this embodiment differs from the first embodiment in that the drive voltage waveforms applied to the counter electrode 26 and the control electrode 27 are bipolar AC voltages.

[0057] In this way, at the timing when the optical state is changed (T21, T22 in Figure 7), the potential difference between the counter electrode 26 and the control electrode 27 is increased to the potential difference at which the optical state of the optical layer 25 changes.

[0058] In this embodiment, the square wave voltage applied to the counter electrode 26 is a bipolar AC voltage, and the amplitude of the square wave voltage applied to the counter electrode 26 is the same as that of the square wave voltage applied to the control electrode 27 (V21=V22). Therefore, the voltage applied to the counter electrode 26 and the control electrode 27 is substantially half that of the first embodiment, and it becomes possible to change the optical characteristics of the element at any timing with a small voltage.

[0059] According to this embodiment, the rectangular wave voltage applied to the counter electrode 26 is a bipolar AC voltage, the amplitude of the rectangular wave voltage applied to the control electrode 27 is the same as that of the rectangular wave voltage applied to the counter electrode 26, and the median value of the amplitude is the same (0 volts). Therefore, even when a bipolar AC voltage is used, it is possible to generate an inter-electrode potential difference that changes the optical state while reducing the absolute value of the rectangular wave voltage (a value from 0 volts to two voltage values ​​that form a rectangular wave) more than before, thereby reducing power consumption.

[0060] Although the above-described embodiment has been described with reference to the screen 21 operating in reverse mode, the screen 21 may also operate in normal mode, as shown in FIG. 8. In normal mode, as shown in the timing chart of FIG. 8, a square-wave AC voltage having a period twice the period for changing the optical state of the optical layer 25 is applied to the counter electrode 26, and a pulse voltage is superimposed on the square-wave AC voltage having the same period and opposite phase as that of the counter electrode 26 to the control electrode 27 at the timings (T21, T22) for changing the optical state. In this way, at the timings (T21, T22 in FIG. 8) for changing the optical state, the potential difference between the counter electrode 26 and the control electrode 27 is reduced to the potential difference at which the optical state of the optical layer 25 changes. Here, V21 and V22 in FIG. 8 have the same relationship of V21 = V22 as V21 and V22 shown in FIG. 7. [Example]

[0061] Next, a display device according to a third embodiment of the present invention will be described with reference to Fig. 9. Note that the same parts as those in the first and second embodiments described above will be given the same reference numerals and descriptions thereof will be omitted.

[0062] The display device according to this embodiment has the same configuration as that of the first embodiment, but differs in the drive voltage waveforms applied to the counter electrode 26 and the control electrode 27. This will be described with reference to the timing chart shown in FIG. 9, which shows an example of the drive voltage waveform according to this embodiment.

[0063] FIG. 9 shows an example of a screen 21 operating in reverse mode, but the same principle can be applied to normal mode. In FIG. 9, a first rectangular wave voltage, a rectangular wave voltage with a period (2T) twice the repetition period (T) that changes the element characteristics, is applied to the counter electrode 26. The rectangular wave voltage applied to the counter electrode 26 is biased by a negative voltage of −V311 volts. In other words, it is a biased AC voltage. In addition, the positive voltage +V312 of the rectangular wave voltage is a voltage that is V311 volts greater than the amplitude V32 of the rectangular wave voltage and pulse voltage of the control electrode 27, which will be described later.

[0064] On the other hand, a pulse voltage is superimposed on the control electrode 27 at a timing that changes the optical state, on a square wave voltage having the same period and phase as the counter electrode 26, which is a second square wave voltage. The square wave voltage applied to the control electrode 27 is controlled to change between two voltages, 0 volts and a positive voltage of +V32 volts. The median value of the amplitude of the control electrode 27 and the median value of the amplitude of the counter electrode 26 are the same value (V32 / 2). That is, this embodiment differs from the first embodiment in that the square wave voltage applied to the counter electrode 26 is a biased AC voltage, the amplitude of the square wave voltage applied to the control electrode 27 is smaller than the amplitude of the square wave voltage applied to the counter electrode, and the medians of both are the same value.

[0065] In this way, at the timings (T31 and T32 in FIG. 9) when the optical state is changed, the potential difference between the counter electrode 26 and the control electrode 27 is increased to a potential difference that changes the optical state of the optical layer 25. That is, the optical state changes depending on the potential difference from +V32 to −V311 at T31, and from +V312 to 0 volts at T32.

[0066] 9 shows an AC voltage biased to a negative voltage, but an AC voltage biased to a positive voltage may also be used. In that case, the polarity of the waveform in FIG. 9 is reversed, and a negative voltage is applied to control electrode 27.

[0067] According to this embodiment, a negative voltage bias of -V311 volts is applied to the counter electrode 26, so that the square wave voltage or pulse voltage applied to the control electrode 27 can be reduced by the amount of the bias voltage, thereby reducing the cost and power consumption of the control electrode drive circuit. This is particularly effective when the control electrode 27 is divided and scanned, as described below. [Example]

[0068] Next, a display device according to a fourth embodiment of the present invention will be described with reference to Figures 10 to 12. Note that the same parts as those in the first to third embodiments described above will be given the same reference numerals and descriptions thereof will be omitted.

[0069] The display device according to this embodiment has the same configuration as that of the first embodiment, but differs in the drive voltage waveforms applied to the counter electrode 26 and the control electrode 27. This will be described with reference to the timing chart shown in FIG. 10, which shows an example of the drive voltage waveform according to this embodiment.

[0070] Fig. 10 shows an example of a screen 21 operating in reverse mode, but it can be applied in a similar manner to normal mode. In Fig. 10, a square wave voltage having a period (2T) twice the repetition period (T) that changes the element characteristics is applied to the counter electrode 26, which is a first square wave voltage. The square wave voltage applied to the counter electrode 26 is a rectangular AC voltage that varies between +V41 volts and -V41 volts with 0 volts as the center.

[0071] On the other hand, a pulse voltage is superimposed on the control electrode 27 at a timing to change the optical state on a square wave AC voltage having the same period and phase as the counter electrode 26, which is a second square wave voltage. However, since the square wave AC voltage applied to the control electrode 27 has an amplitude of 0, a DC voltage of 0 volts is essentially applied. The pulse voltage applied is a voltage of -V42 volts or +V42 volts. In other words, this embodiment differs from the first embodiment in that the square wave voltage applied to the counter electrode 26 is a bipolar AC voltage, the amplitude of the square wave voltage applied to the control electrode 27 is 0 (zero), and the medians of both amplitudes are the same.

[0072] In this way, at the timing when the optical state is changed (T41, T42 in Figure 10), the potential difference between the counter electrode 26 and the control electrode 27 is increased to the potential difference at which the optical state of the optical layer 25 changes.

[0073] According to this embodiment, the square wave voltage applied to the counter electrode 26 is a bipolar AC voltage, and a pulse voltage is applied to the control electrode 27 in addition to the square wave AC voltage of zero amplitude at a timing that changes the optical state of the optical layer 25. Therefore, it is possible to generate an inter-electrode potential difference that changes the optical state of the control electrode 27 stably and with a small amplitude only when necessary, thereby reducing power consumption.

[0074] In this embodiment, the square wave voltage applied to the control electrode 27 does not have to have an amplitude of 0, but may have an amplitude smaller than that of the square wave AC voltage applied to the counter electrode 26. Even in this case, the optical state of the optical layer 25 can be changed by adjusting only the pulse voltage to a required amplitude.

[0075] Furthermore, in this embodiment, an element can be used for the optical layer 25, in which the relationship between the voltage change and the parallel light transmittance changes sharply when switching between the transmitting state and the scattering state. The relationship between the voltage change and the parallel light transmittance will be explained with reference to Figures 11 and 12. Figure 11 is a graph showing the relationship between the voltage change and the parallel light transmittance in the drive voltage waveform shown in the first embodiment (Figure 5), and Figure 12 is a graph showing the relationship between the voltage change and the parallel light transmittance in the drive voltage waveform shown in this embodiment (Figure 10).

[0076] In the case of the driving voltage waveform shown in Figure 5, it is necessary to change between a transparent state at an element voltage of 0 volts and a scattering state at V 12 volts. At this time, the change from the transparent state to the scattering state occurs via an intermediate state in which the parallel light transmittance changes, so if the length (slope) of this intermediate state is long, the voltage used for the change must also be large.

[0077] Therefore, by using an element in which the length of this intermediate state is short (the slope is steep), the element voltage in the transmitting state can be increased to V41 volts, as shown in Figure 12, and the pulse voltage V42 applied to the control electrode 27 required to change the voltage to the scattering state voltage (V41 + V42) volts can be reduced.

[0078] 12, a voltage of +V41 or -V41 is applied to the elements constituting the optical layer 25 except when the pulse voltage is applied. This voltage is preferably set to a level that does not cause deterioration of the optical characteristics of the elements due to changes over time. That is, the voltage is set to a level that does not cause the element characteristics of the elements constituting the optical layer 25 to change due to the potential difference generated by the two square wave voltages (a voltage that does not cause the element characteristics of the elements constituting the optical layer 25 to change, and results in the same optical state as when no voltage is applied). This deterioration refers to, for example, a change over time in which the length of the intermediate state becomes longer (a steeper slope), or a change in the direction of decreasing the voltage at which the transparent state is achieved. [Example]

[0079] Next, a display device according to a fifth embodiment of the present invention will be described with reference to Fig. 13. Note that the same parts as those in the first to fourth embodiments described above will be given the same reference numerals and descriptions thereof will be omitted.

[0080] The display device according to this embodiment has the same configuration as that of the first embodiment, but differs in the drive voltage waveforms applied to the counter electrode 26 and the control electrode 27. This will be described with reference to the timing chart shown in FIG. 13, which shows an example of the drive voltage waveform according to this embodiment.

[0081] Fig. 13 shows an example of a screen 21 operating in reverse mode, but it can also be applied to normal mode for the same purpose. In Fig. 13, a square wave voltage having a period (2T) twice the repetition period (T) that changes the element characteristics is applied to the counter electrode 26, which is a first square wave voltage. The square wave voltage applied to the counter electrode 26 is controlled to change between two voltages, 0 volts and +V51 volts, as in the first embodiment.

[0082] On the other hand, a pulse voltage is superimposed on the second electrode, which is a square wave voltage having the same period and phase as the counter electrode 26, at a timing that changes the optical state. The pulse voltage applied to T51 in FIG. 13 is composed of two voltages, +V52 volts and +V53 volts. +V52 volts is applied first, and +V53 volts is applied after a predetermined time has elapsed. That is, the pulse voltage is composed of multiple voltage values ​​over a half cycle of the square wave voltage applied to the control electrode 27. Note that V52 > V53, and V53 is a voltage that can maintain the scattering state. The pulse voltage applied to T52 in FIG. 13 is a waveform resulting from a change in the voltage value of the square wave voltage applied to the control electrode 27. It is composed of two voltages, 0 volts and +V54 volts. 0 volts is applied first, and +V54 volts is applied after a predetermined time has elapsed. As described above, the pulse voltage applied to T51 and the pulse voltage applied to T52 have a relationship V52-V53=V54 because the voltage value of the rectangular wave voltage applied to control electrode 27 changes.

[0083] In this way, at the timing when the optical state is changed (T51, T52 in Figure 13), the potential difference between the counter electrode 26 and the control electrode 27 is increased to the potential difference at which the optical state of the optical layer 25 changes.

[0084] In particular, as shown in FIG. 13, by first applying a large voltage (V52) and then applying a lower voltage (V53), the response time can be made faster while stabilizing the device in an appropriate scattering state.

[0085] The voltages constituting the pulse voltage are not limited to two but may be three or more voltage values, and the combination of multiple voltages may be changed appropriately depending on the characteristics to be changed, rather than being changed in descending order.

[0086] According to this embodiment, the pulse voltage applied to the control electrode 27 is composed of multiple voltage values, and therefore, by changing one pulse voltage in stages, the transient characteristics of the elements constituting the optical layer 25 can be optimized. [Example]

[0087] Next, a display device according to a sixth embodiment of the present invention will be described with reference to Figures 14 to 19. Note that the same parts as those in the first to fifth embodiments described above will be given the same reference numerals and descriptions thereof will be omitted.

[0088] In the display device of this embodiment, the screen 21 is divided into multiple regions, and each region (divided region) can be switched between a transparent transmitting state, which causes little scattering of incident light, and a scattering state at an independent timing.

[0089] Fig. 14 shows a schematic cross-sectional view of a screen 21 capable of controlling the optical state for each divided region, and Fig. 15 shows a schematic front view of the screen showing the arrangement of multiple control electrodes in the screen 21 shown in Fig. 14. The screen 21 shown in Fig. 14 has an optical layer 25 in which a composite material containing liquid crystal is sandwiched between a pair of transparent glass plates 23 and 24. A counter electrode 26 is formed over the entire surface of one of the glass plates 24 facing the optical layer 25. A plurality of control electrodes 27 are arranged side by side on the other glass plate 23 facing the optical layer 25. An intermediate layer made of an insulator may be formed between the electrodes 26 and 27 and the optical layer 25.

[0090] The plurality of control electrodes 27 divide the area of ​​the screen 21 onto which the image light is irradiated into strips in one direction (for example, the scanning direction) (see FIG. 15). The plurality of control electrodes 27 are individually connected to a synchronization control unit 31, and voltages can be applied to them individually. Adjacent control electrodes 27 are arranged spaced apart from each other. Note that while the strip-shaped areas are arranged vertically in FIG. 15, they may also be divided horizontally to form a matrix of areas.

[0091] Furthermore, the optical layer 25 of this embodiment can be adjusted for each divided region between a transparent transmitting state in which incident light is little scattered and a scattering state in which incident light is scattered.

[0092] The width of the gap region in the optical layer 25, which corresponds to the region between the control electrodes 27 and where the control electrodes 27 are not formed, is preferably about 5 to 100 micrometers, and is as narrow as possible. The thickness of the optical layer 25 is several to several tens of micrometers, and is determined in consideration of the optical characteristics and the driving voltage.

[0093] Next, the basic operating principle of the display device 1 configured as described above will be explained. Fig. 16 is an explanatory diagram of the synchronous control of the scanning and driving of the screen 21. The projector 11 vertically scans the screen 21 from top to bottom with image light modulated with image information. The projector 11 vertically scans the screen 21 from top to bottom for each scanning repetition period (hereinafter also referred to as the scanning period).

[0094] 16(A) to 16(E) show the scanning state at each point in time during one scanning cycle in the scanning order. The screen 21 in Fig. 16 has five divided areas 22. The five divided areas 22 are arranged vertically along the scanning direction of the image light.

[0095] The synchronization control unit 31 individually controls the optical states of the five divided areas 22 in synchronization with one-dimensional vertical scanning of the screen 21 by the projector 11. When no image light is projected, each divided area 22 is controlled to a non-image state, i.e., a transparent transmission state in which incident light is little scattered.

[0096] When scanning of the imaging light is started, the scanning light of the projector 11 is first irradiated onto the uppermost divided region 22 of the screen 21, as shown in FIG. 16(A). In the following description, the divided region 22 irradiated with the scanning light will be referred to as 221 to distinguish it from the other divided regions 22 that are not scanned. The synchronization control unit 31 specifies the period during the scanning cycle during which the uppermost divided region 221 is scanned, based on a synchronization signal from the projector, and controls the uppermost divided region 221 to an imaging state. The imaging light scanning the uppermost divided region 221 is scattered by the divided region 221 in the scattering state and passes through the screen 21.

[0097] Next, as shown in FIG. 16(B), the scanning of the imaging light moves to the second divided region 221 from the top of the screen 21. The synchronization control unit 31 identifies the period during the scanning cycle during which this second divided region 221 from the top is scanned, and controls the second divided region 221 from the top to be in the imaging state. The imaging light scanning the second divided region 221 from the top is scattered by the divided region 221 in the scattering state and passes through the screen 21. After controlling the second divided region 221 from the top to be in the imaging state, the synchronization control unit 31 controls the topmost divided region 22 to be in the non-imaging state. Thereafter, as shown in FIGS. 16(C) to 16(E), the synchronization control unit 31 controls the divided region 221 scanned by the scanning light to be in the imaging state, and controls the other divided regions 22 to be in the non-imaging state.

[0098] By the above-described synchronization control, the portions of the screen 21 that are irradiated with the scanning light are maintained in an imaging state. As a result, the imaging light that scans the screen 21 is scattered by the screen 21, which is in a scattering state. Furthermore, the portions of the screen 21 that are not irradiated with the scanning light are controlled to a non-imaging state. Each divided area 22 is controlled to a transparent, transmissive state in the non-imaging state for most of the time that it is not scanned with the scanning light. During the projection period of the imaging light, the see-through characteristics of the screen 21 are obtained while maintaining the visibility of the image.

[0099] FIG. 17 illustrates a projection method of the projector 11 according to this embodiment. FIG. 17 is an explanatory diagram of the projector 11 scanning the screen 21. FIG. 17(A) is an explanatory diagram of a projection method in which the projector 11 scans the screen 21. In this case, image light is constantly projected onto the screen 21 during a scanning cycle. However, when focusing on each part of the screen 21, image light is projected during part of the scanning cycle, as shown in FIG. 17(B). Therefore, as shown in FIG. 17(C), each part of the screen only needs to be in a scattering state during the partial scanning period TP during which each part is scanned. Furthermore, if each part of the screen 21 is controlled to have a high parallel light transmittance during periods other than the partial scanning period TP, the see-through characteristics of the screen 21 can be obtained during the scanning cycle without causing a decrease in image brightness.

[0100] When the screen 21 is divided into strips in one direction, as in the screen 21 shown in FIG. 15, the projection light of the projector 11 is scanned sequentially in the division direction of the screen 21. Based on a synchronization signal from the projector 11, the synchronization control unit 31 controls the multiple divided regions 22 to change from a transparent transmissive state to a scattering state in the scanning order so that the area irradiated with the projection light of the projector 11 is maintained in an imaging state (in this embodiment, a scattering state). Through this synchronization control, each divided region 22 of the screen 21 is in the scattering state as an imaging state during a period Ton (see FIG. 18) that includes an imaging period during which the projection light is irradiated onto that region. Furthermore, during a non-imaging period Toff (see FIG. 18) during which the projection light is not irradiated, the divided regions 22 are in the transparent transmissive state as a non-imaging state.

[0101] Therefore, the screen 21 has a transparency that allows objects behind it to be recognized, while scattering and transmitting image light with brightness equivalent to that of a screen in a constant scattering state. In other words, it is possible to achieve both see-through properties that allow background objects to be recognized and high visibility of the image.

[0102] 18 is a schematic timing chart of scanning and driving of the screen 21. The horizontal axis represents time, and the vertical axis represents the vertical position on the screen, which corresponds to the multiple divided areas 22 on the screen 21.

[0103] Each divided region 22 of the screen 21 is controlled to change from a transparent transmitting state to a scattering state before the image light starts scanning the respective region. Furthermore, a divided region 22 in the scattering state is controlled to change from the scattering state to a transparent transmitting state after scanning of the region is completed.

[0104] The divided regions 22 are controlled to a video state (scattered state) in synchronization with the partial scanning periods TP during which the video light is irradiated onto each region by scanning, and are sequentially switched to the video state with a time lag in the scanning order. The video light scanning the screen 21 is efficiently scattered by the portions maintained in the video state, resulting in bright and highly visible images. Note that the video light scanning in FIG. 18 is indicated by three arrows, which represent the video light corresponding to each of the three primary colors of light: red, green, and blue.

[0105] As in the first embodiment, switching timing information for this synchronization control is sent from the projector 11 to the synchronization control unit 31 as a synchronization signal. The synchronization control unit 31 preferably controls the voltages applied to the counter electrode 26 and each control electrode 27 so that projection light is emitted during a period in which the optical state of each divided region 22 is stable in a predetermined scattering state. The optical state of each divided region 22 is switched depending on the signal waveform of the voltage applied to the control electrode 27. In particular, the switching timing information output by the projector 11 to the synchronization control unit 31 should preferably include information on the timing at which the projector 11 starts scanning each frame and the scanning speed (scanning delay / shift). This allows for a good see-through display without image distortion even when the frame frequency changes. Note that the projector 11 and synchronization control unit 31 may be capable of wireless communication using electromagnetic waves such as microwaves or infrared rays, and information for achieving synchronization between them may be exchanged via wireless signals.

[0106] By the above-mentioned synchronization control, the synchronization control unit 31 of this embodiment switches the optical state of the multiple divided areas 22 in the scanning period T of the image light in synchronization with the scanning of the image light by the projector 11, and sets the optical state of the part of the screen 21 onto which the image light is projected to the image state.

[0107] Therefore, the screen 21 can display an image because the area irradiated with the image light is maintained in a scattering state during the period Ton including the timing at which the image light is irradiated.

[0108] Moreover, since each portion of the screen 21 is controlled to a transparent transmitting state at times other than the period Ton during the projection period of the image light, it is possible to see through the screen 21. Since the light transmitted through the screen 21 appears averaged (integrated) to the human eye, a see-through characteristic can be obtained in which no flicker is perceived if the scanning cycle is sufficiently short.

[0109] Next, driving of the display device 1 using the screen 21 operating in reverse mode will be described. Fig. 19 shows a timing chart showing an example of the relationship between the drive voltage waveform applied by the synchronization control unit 31 and the optical state. Fig. 19 shows an example in which the drive voltage waveform of the first embodiment (Fig. 5) is applied, but the drive voltage waveforms shown in the second to fifth embodiments may also be applied.

[0110] As shown in FIG. 19, the pulse voltage is applied so as to shift sequentially at equal time intervals starting from region 1, which is the beginning of the scan, during the period T, thereby changing from the transmissive state to the scattering state. Note that in FIG. 19, the application of the pulse voltage partially overlaps in time between adjacent regions, but it may also be completely overlapping (i.e., multiple regions may change simultaneously), or may not overlap. Furthermore, the application time of the pulse voltage may not be the same (equal time intervals) but may be different (unequal intervals). The time for which the optical properties of the optical layer 25 are maintained can be changed by increasing or decreasing the application time of the pulse voltage.

[0111] That is, the timing for changing the optical state of the optical layer 25 corresponding to each region of the divided control electrode 27 is switched sequentially, and the timing for applying the pulse voltage is switched sequentially based on this switching.

[0112] According to this embodiment, the control electrode 27 is divided into a plurality of rectangular regions, and the synchronization control unit 31 sequentially switches the timing of application of the pulse voltage so as to sequentially switch the timing of changing the optical state of the optical layer 25 corresponding to each region of the divided control electrode 27. This allows for a simple circuit configuration and reduced power consumption in a screen 21 in which the control electrode 27 is divided into a plurality of regions. Furthermore, by dividing the control electrode 27 into rectangular regions, the screen 21 can be arranged vertically along the scanning direction, for example, and the optical state can be changed sequentially in the order in which the image is scanned. [Example]

[0113] Next, a display device according to a seventh embodiment of the present invention will be described with reference to Fig. 20. Note that the same parts as those in the first to sixth embodiments described above will be given the same reference numerals and descriptions thereof will be omitted.

[0114] The display device according to this embodiment has the same configuration as the sixth embodiment, but differs in the drive voltage waveforms applied to the counter electrode 26 and the control electrode 27. This will be described with reference to the timing chart shown in FIG. 20, which shows an example of the drive voltage waveform according to this embodiment.

[0115] In this embodiment, in a screen 21 divided into a plurality of regions as shown in the sixth embodiment, in order to extend the time for which the optical characteristics of each region are maintained, the length of the pulse voltage (voltage holding time) is extended, and when the voltage holding time of the pulse voltage overlaps with the time when the voltage value of the rectangular wave voltage applied to the counter electrode 26 or the control electrode 27 changes, the voltage waveform of the pulse voltage is also changed so as to maintain the potential difference necessary to maintain the optical state.

[0116] In the case of Figure 20, if the voltage holding time is ΔT and the time when the voltage value of the square wave voltage changes is T7, the voltage holding time of region N will exceed (cross over) T7. Therefore, during the voltage holding time of region N, the voltage value is changed from 0 volts to +V12 volts after T7 so that the scattering state can be maintained. For this type of control, since the voltage holding time (time to maintain optical characteristics) and the period of the square wave voltage are known in advance, it is possible to predict in which region the time T7 at which the voltage value changes will be crossed, and when T7 is reached, the voltage value of that region is changed (changed to a voltage that is inverted with respect to the median value of control electrode 27).

[0117] In this embodiment, the driving voltage waveform shown in the first embodiment has been described as the driving voltage waveform, but it goes without saying that the driving voltage waveforms shown in the other embodiments may also be used.

[0118] According to this embodiment, if there is an electrode among the plurality of control electrodes 27 whose voltage holding time of the pulse voltage overlaps the timing of the voltage value change of the rectangular wave voltage, the pulse voltage applied to that control electrode is controlled so that its voltage value changes at the timing of the voltage value change of the rectangular wave voltage. Therefore, even in the plurality of regions whose voltage holding time overlaps the timing of the voltage value change of the rectangular wave voltage, the optical characteristics can be maintained for a set period of time, and display degradation such as unevenness in display due to differences in the optical characteristics maintenance time depending on the region can be prevented.

[0119] In the above-described embodiments, the amplitude of the pulse voltage is equal to or smaller than the square wave voltage applied to the counter electrode 26, but it may be larger than the square wave voltage applied to the counter electrode 26. In this way, the optical state of the optical layer 25 can be changed even if the amplitude of the square wave voltage applied to the counter electrode 26 is reduced.

[0120] According to the above-described embodiment, the following display device 1 and driving method for the display device 1 are obtained.

[0121] (Note 1) A screen 21 having an optical layer 25 whose optical state changes when a voltage is applied, and a counter electrode 26 and a control electrode 27 that are arranged opposite each other across the optical layer 25 in order to apply a voltage to the optical layer 25; a synchronization control unit 31 that applies a predetermined voltage to the counter electrode 26 and the control electrode 27 during a projection period of the image light projected onto the screen 21, and outputs a switching signal that switches the screen 21 between a scattering state in which the image light is scattered and a transmission state that is a different optical state; In a display device 1 having A display device 1 characterized in that a synchronization control unit 31 applies to the opposing electrode 26 a first rectangular wave voltage having a period that is an integer multiple of the period at which the optical state of the optical layer 25 is changed, and applies to the control electrode 27 a second rectangular wave voltage having the same period and the same phase as or shifted by half a period from the first rectangular wave voltage, superimposed with a pulse voltage that becomes an inter-electrode potential difference that changes the optical state at the timing at which the optical state of the optical layer 25 is changed.

[0122] (Note 2) A screen 21 having an optical layer 25 whose optical state changes when a voltage is applied, and a counter electrode 26 and a control electrode 27 that are arranged opposite each other across the optical layer 25 in order to apply a voltage to the optical layer 25; a synchronization control unit 31 that applies a predetermined voltage to the counter electrode 26 and the control electrode 27 during a projection period of the image light projected onto the screen 21, and outputs a switching signal that switches the screen 21 between a scattering state in which the image light is scattered and a transmission state that is a different optical state; In a method for driving a display device 1 having A method for driving a display device (1), characterized in that a synchronization control unit (31) applies to the opposing electrode (26) a first rectangular wave voltage having a period that is an integer multiple of the period at which the optical state of the optical layer (25) is changed, and applies to the control electrode (27) a second rectangular wave voltage having the same period and the same phase as the first rectangular wave voltage or a phase shifted by half a period, superimposed with a pulse voltage that becomes an inter-electrode potential difference that changes the optical state at the timing at which the optical state of the optical layer (25) is changed.

[0123] According to this display device 1 and the driving method of the display device, for example, by setting the amplitude of the first and second rectangular wave voltages and the pulse voltage as the inter-electrode potential difference at which the optical state changes, a circuit can be configured for driving with a small power supply, thereby achieving a simple circuit configuration. Furthermore, since the voltage can be reduced, power consumption can be reduced. Furthermore, the optical properties of the elements constituting the optical layer can be easily changed at any timing.

[0124] The above-described embodiments merely show typical embodiments of the present invention, and the present invention is not limited to these embodiments. In other words, the present invention can be implemented in various modifications without departing from the gist of the present invention. [Explanation of symbols]

[0125] 1 Display device 11 Projector 21 screens 25 Optical layer 26 Counter electrode (first electrode) 27 Control electrode (second electrode) 31 Synchronization control unit (control unit)

Claims

[Claim 1] a screen including an optical layer whose optical state changes when a voltage is applied, and a first electrode and a second electrode disposed opposite each other across the optical layer for applying a voltage to the optical layer; a control unit that applies a predetermined voltage to the first electrode and the second electrode during a projection period of the image light projected onto the screen, and switches the screen between a predetermined image state in which the image light is scattered and a non-image state that is an optical state different from the image state; In a display device having A display device characterized in that the control unit applies to the first electrode a first rectangular wave voltage having a period that is an integer multiple of the period at which the optical state is changed, and applies to the second electrode a second rectangular wave voltage having the same period and the same phase as the first rectangular wave voltage or the same period and a phase shifted by half a period, superimposed with a pulse voltage which is the potential difference between the electrodes that changes the optical state at the timing at which the optical state is changed.

Images