JP3998954B2 - Image shift element and image display device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image shift element suitably used for a head-mounted display (hereinafter referred to as “HMD”), a projection image display apparatus (projector), and the like, and an image display apparatus including the image shift element. Yes.
[0002]
[Prior art]
The liquid crystal display element includes a pair of substrates and a liquid crystal layer sandwiched between the substrates. The substrate has a plurality of pixel electrodes regularly arranged in rows and columns (matrix), and a driving voltage corresponding to an image signal is applied to each of the pixel electrodes. Application of this voltage changes the optical characteristics (light transmittance and reflectance) of the liquid crystal layer for each pixel, so that images, characters, and the like can be displayed.
[0003]
There are a “simple matrix method” and an “active matrix method” as methods for applying an independent drive voltage to each pixel electrode on the substrate.
[0004]
In the case of the active matrix method, switching elements corresponding to the pixel electrodes are arranged on the substrate. A substrate on which such switching elements are arranged is called an active matrix substrate. The switching element on the active matrix substrate functions to switch an electrical conduction / non-conduction state between the corresponding pixel electrode and the signal wiring. As such a switching element, a metal-insulator-metal (MIM) element, a thin film transistor (TFT), or the like is preferably used.
[0005]
The switching element is required to exhibit as high an electrical resistance as possible when in a non-conducting state. However, when strong light is incident on a switching element that is in a non-conducting state, the electrical resistance of the switching element is reduced and a leakage current is generated, so that the charge stored in the pixel electrode is discharged. Arise. In addition, a driving voltage of an appropriate level is not applied to the pixel electrode, the original display operation is not performed, and there is a problem that light leaks even in a black state and the contrast ratio is lowered.
[0006]
When the liquid crystal display element is a transmissive type, a light shielding layer called a black matrix is disposed on the active matrix substrate or on a counter substrate facing the active matrix substrate with a liquid crystal layer interposed therebetween in order to solve the above problem. . The presence of this black matrix reduces the area ratio (opening ratio) of the pixel opening. To achieve high definition by reducing the area occupied by the black matrix, the switching elements and wirings can be miniaturized. However, miniaturization of the switching elements and wirings leads to a decrease in driving force and an increase in wiring resistance. It will be. In addition, it is difficult to miniaturize switching elements and wiring due to restrictions on manufacturing technology.
[0007]
US Pat. No. 4,984,091 discloses a technique for optically moving a display image by a pixel pitch for the purpose of achieving high definition using a non-display area on a black matrix. According to this technique, an image corresponding to the moved pixel position is displayed in synchronization with the movement of the pixel. As a result, the apparent number of pixels increases, so that even when a display element with a low resolution is used, the same display as when a high-definition display panel is used is possible.
[0008]
US Pat. No. 6,061,103 discloses a method in which red, green, and blue (hereinafter referred to as “RGB”) pixels are optically sequentially shifted by a shift element, and the shifted pixels are superimposed and displayed. Is disclosed. In this method, each pixel of RGB is displayed in a time division manner in an area corresponding to one pixel. As a result, the apparent resolution can be improved by a factor of three without reducing the pixel pitch on the display panel.
[0009]
The above-mentioned US Pat. No. 6,061,103 discloses an image shift element in which a liquid crystal element and a birefringence element are combined as means for optically shifting an image. The birefringent element is formed of a material whose light refraction direction changes depending on the polarization direction of incident light. If the polarization direction of the light incident on the birefringent element is changed by the liquid crystal element, the optical axis of the light emitted from the birefringent element can be shifted.
[0010]
FIG. 1 shows a known image shift element. The image shift element includes a liquid crystal element 7 and a birefringent element 11 arranged in series along the light propagation direction. The liquid crystal element 7 changes the polarization state between a state in which an incident electric field vector oscillation plane (hereinafter referred to as a “polarization plane”) of linearly polarized light that is incident is rotated by 90 ° and a state that is transmitted without being rotated. Switch. The birefringent element 11 can shift the light beam according to the direction of the polarization plane of the linearly polarized light that has entered.
[0011]
In the example shown in FIG. 1, the electric field vector direction (polarization direction) of the light incident on the liquid crystal element 7 is perpendicular to the paper surface. Since the liquid crystal element 7 uses a TN mode liquid crystal (TN liquid crystal) having a positive refractive index anisotropy Δε, a voltage is not applied to the liquid crystal layer of the liquid crystal element 7 (when the voltage is in an OFF state). The liquid crystal molecules are twisted by 90 °, and the polarization plane of the incident light rotates by 90 ° due to its optical rotation. On the other hand, when a voltage of a predetermined level or higher is applied to the liquid crystal layer of the liquid crystal element 7 (when the voltage is in an ON state), the liquid crystal molecules are aligned with the direction of the electric field. The light is emitted while being perpendicular to the paper surface. The birefringent element 11 shown in the figure transmits light whose polarization plane is perpendicular to the paper surface as it is, but can shift light parallel to the paper surface.
[0012]
The liquid crystal element 7 in the image shift element as shown in FIG. 1 emits a first linearly polarized light and a second straight line having a polarization plane perpendicular to the state according to the magnitude of the applied voltage. It is required to switch the state appropriately and quickly between the state of emitting polarized light.
[0013]
As described above, in the case of the liquid crystal element using the above TN liquid crystal, when no voltage is applied to the TN liquid crystal, the linearly polarized light incident on the liquid crystal element is emitted as linearly polarized light whose polarization plane is rotated by 90 °. When a voltage is applied to the liquid crystal, the orientation of the liquid crystal molecules changes rapidly due to the electric field, and transitions to a state where the polarization of incident light is not changed. On the other hand, when the application of voltage to the TN liquid crystal is stopped, the liquid crystal molecules transition (relax) to the original state, but the speed is slow.
[0014]
Thus, between the case where the voltage applied to the liquid crystal layer is changed from Low (typically 0 volts) to High (for example, 5 volts) and the case where the applied voltage is changed from High to Low, The speed at which the orientation of liquid crystal molecules changes is different. In order to evaluate this response speed, a pair of orthogonal polarizers may be disposed before and after the liquid crystal layer, and the temporal change in light transmittance may be measured. FIG. 2 shows the change in transmittance when the applied voltage is changed from High to Low after a predetermined time has elapsed after the applied voltage is changed from Low to High. Here, the time for the transmittance to fall from the maximum value to zero is referred to as “liquid crystal rise response time τr”, and the time for the transmittance to rise from zero to the maximum value is referred to as “liquid crystal fall response time τd”. I will do it. The rise response time τr of the liquid crystal is relatively short, but the fall response time τd is relatively long. When the falling response time τd of the liquid crystal is long, the image cannot be shifted in synchronization with the switching timing of the image displayed by the image display element. Before describing this problem, first, the image switching speed in the image display device will be described.
[0015]
Usually, either an interlaced drive or a non-interlaced drive is generally used as a driving method of the image display element. Interlaced driving is a display method in which only odd and even rows are selected for each field, and one image is completed in odd and even fields. The selection time for each field is usually 16.6 milliseconds (60 Hz). ). On the other hand, non-interlaced driving is a display method of sequentially selecting regardless of odd and even rows of display elements, and the selection time of each field is normally 16.6 milliseconds (60 Hz) as in interlaced driving. . Here, the field refers to a period between vertical synchronizations of images regardless of interlace driving or non-interlace driving. In the liquid crystal display element, a scanning period including blanking time corresponds to a field period.
[0016]
In the method described in the above-mentioned US Pat. No. 6,061,103, one field period is divided according to the R, G, and B shift positions, and the divided period (hereinafter referred to as “subfield period”). .) Different images (“subfield images”) are displayed on the image display element. Since the subfield period in this case is about 5 milliseconds, the image shift element needs to shift the image at a short time interval of about 5 milliseconds. Furthermore, since the image shift by the image shift element needs to synchronize the subfield switching and timing, the state can be changed at high speed in response to the voltage applied to the liquid crystal element simultaneously with the subfield switching. Desired.
[0017]
[Problems to be solved by the invention]
However, in the case of an actual liquid crystal element, it is difficult to change the state at high speed in response to voltage application. For example, in the case of a TN mode liquid crystal, as shown in FIG. 2, the rise response time τr is relatively short, but the fall response time τd is usually about a dozen milliseconds, which is longer than the subfield selection period. .
[0018]
The reason for the difference in response time is that the “rise” of the curve in FIG. 2 is performed by applying a voltage to the liquid crystal and forcibly aligning the liquid crystal molecules in one direction. The “falling” is performed by stopping the application of voltage to the liquid crystal and naturally relaxing the orientation of the liquid crystal molecules to the original state.
[0019]
If a liquid crystal having a long fall response time τd is used as described above, there is a problem that polarization cannot be switched appropriately. This problem will be described with reference to FIG. As shown in FIG. 1, when the voltage application to the liquid crystal cell 7 is changed from the “ON” state to the “OFF” state, the polarization plane of the light emitted from the liquid crystal element 7 is rotated by 90 °, resulting in birefringence. The optical axis of the light emitted from the element 11 is shifted from the position B to the position A. At this time, if the falling response time τd is too long, the linearly polarized light becomes elliptically polarized at the transitional stage of the falling, and the same image is displayed twice at both the position A and the position B, so the resolution decreases. Will do.
[0020]
In addition, if there is a large difference between the falling response time τd and the rising response time τr, there is a difference in the generation level of the double image between when the image is shifted from position A to position B and vice versa. Will be recognized as flicker.
[0021]
In JP-A-2000-199901, in a TN mode type liquid crystal display device, by adjusting the concentration of the chiral agent added to the liquid crystal, the twist angle inherent to the liquid crystal is increased, whereby the voltage is turned on. The liquid crystal response speed when changing from "" to "OFF state" can be improved. However, in the case of a liquid crystal display device, when the concentration of the chiral agent is increased in this way, the magnitude of the voltage to be applied to the liquid crystal layer in the voltage “ON” state needs to be increased compared to the conventional case. Considering the ability of a semiconductor thin film transistor that functions as a switching element in the display region, it was difficult in practice. Further, as the twist angle inherent to the liquid crystal is increased, the liquid crystal in the voltage “OFF” state becomes unstable, and the 90 ° twist can be maintained only for a short time. For this reason, the above technique has not been applied to an actual liquid crystal element.
[0022]
The present invention has been made in view of the above circumstances, and a main object of the present invention is to generate a double image due to a response delay of liquid crystal when voltage application is changed from an “ON” state to an “OFF” state. It is an object of the present invention to provide an image shift element in which the image shift is suppressed and an image display device using the image shift element.
[0023]
[Means for Solving the Problems]
The image shift element according to the present invention is an image shift element including at least one image shift unit that periodically shifts the position of the optical axis, and the image shift unit is configured to be in accordance with High / Low of the applied voltage. A liquid crystal cell that switches a polarization direction of light between two directions orthogonal to each other; and a birefringent element having a refractive index different depending on the polarization direction of the light. The liquid crystal cell and the birefringent element are arranged in this order. The TN mode is arranged so as to transmit light, and the liquid crystal layer included in the liquid crystal cell satisfies a relational expression of 0.25 <d / p <1, where p is a natural pitch length and d is a cell gap. It is formed of a liquid crystal layer and is arranged so as to be twisted by 90 ° in the liquid crystal cell.
[0024]
In a preferred embodiment, the natural pitch length p of the liquid crystal layer is adjusted by adding a chiral agent.
[0025]
In a preferred embodiment, the liquid crystal layer satisfies a relational expression of 0.5 <d / p.
[0026]
In a preferred embodiment, the liquid crystal layer satisfies a relational expression of d / p <0.75.
[0027]
In a preferred embodiment, the pretilt angle of the liquid crystal layer is adjusted to be less than 5 °.
[0028]
In a preferred embodiment, the number of the image shift units is plural.
[0029]
An image shift element module according to the present invention includes any one of the image shift elements described above and a drive circuit that supplies a voltage to the liquid crystal cell of the image shift element, and the drive circuit performs switching timing of an image to be shifted. The voltage is changed in synchronization.
[0030]
In a preferred embodiment, the driving circuit applies a voltage of 7 volts or more to substantially the entire liquid crystal layer included in the liquid crystal cell of the image shift element for a selected period.
[0031]
The selected period has a length equal to or longer than the subfield period of the image.
[0032]
An image display device according to the present invention is an image display device that includes an image display unit that displays an image and any one of the image shift elements described above. The received light is shifted while being synchronized with the display of the image display unit.
[0033]
In a preferred embodiment, a drive circuit for supplying a voltage to the liquid crystal cell of the image shift element is further provided.
[0034]
In a preferred embodiment, the driving circuit applies a voltage of 7 volts or more to substantially the entire liquid crystal layer included in the liquid crystal cell of the image shift element for a selected period.
[0035]
In a preferred embodiment, the selected period has a length equal to or longer than a subfield period of the image.
[0036]
In a preferred embodiment, the light exiting from the image display unit and entering the image shift element is linearly polarized light.
[0037]
In a preferred embodiment, the pixels constituting the image are superimposed in a time division manner by the shift of the image.
[0038]
Another image display device according to the present invention includes a light source, an image display panel having a plurality of pixel regions each capable of modulating light, and light from the light source in the plurality of pixel regions according to a wavelength region. An image display device comprising: a light control unit that focuses light on a corresponding pixel region; and an optical system that forms an image on a projection surface by light modulated by the image display panel, A plurality of sub-frame image data is generated from each frame image data, and the image display panel displays the plurality of sub-frame images in a time-division manner; and the plurality of sub-frame images displayed by the image display panel The image shift element according to any one of the above, wherein the sub-frame image selected from the sub-frame images is shifted on the projection surface; Same area on the projection surface by the light belonging to different wavelength ranges that have been modulated by comprising a pixel region sequentially irradiating.
[0039]
DETAILED DESCRIPTION OF THE INVENTION
The image shift element according to the present invention includes at least one image shift unit that periodically shifts the position of the optical axis. This image shift unit includes a liquid crystal cell and a birefringence element, similarly to the conventional image shift element shown in FIG. One of the features of the present invention is that the liquid crystal layer included in the liquid crystal cell has a TN mode liquid crystal layer satisfying a relational expression of 0.25 <d / p <1, where p is a natural pitch length and d is a cell gap. And is arranged so as to be twisted by 90 ° in the liquid crystal cell.
[0040]
The value indicated by d / p can be controlled by the type and concentration of the chiral agent added to the liquid crystal layer. Assuming that the chiral agent HTP (Herical Twisting Power) is x and the concentration of the chiral agent is c, the natural pitch p of the liquid crystal is expressed by the following formula by x and c.
[0041]
p = 1 / (c · x)
[0042]
Since x is a value inherent to the combination of the liquid crystal and the chiral agent, a desired natural length pitch p can be obtained by adjusting the concentration c of the chiral agent.
[0043]
Here, in the liquid crystal layer having the cell gap d (= the thickness of the liquid crystal layer), the natural twist angle φ is expressed by the following equation.
[0044]
φ = (d / p) · 360 °
[0045]
The natural twist angle φ is a natural twist angle when the orientation of the liquid crystal molecules on the upper and lower surfaces of the liquid crystal layer is not restricted by an alignment film or the like.
[0046]
In the present invention, by adjusting the concentration c of the chiral agent, p is reduced and d / p satisfies the relational expression of 0.25 <d / p <1. In this case, the natural twist angle φ becomes a size satisfying 90 ° <φ <360 °. On the other hand, in the present invention, the twist angle of the liquid crystal layer is set to 90 ° by regulating the twist angle with an alignment film or the like.
[0047]
As described above, in the present invention, the natural twist angle φ is set to exceed 90 ° by adding a chiral agent having a higher concentration than that of a liquid crystal display device in practical use to the liquid crystal layer of the image shift element. ing. For this reason, in a state where a 90 ° twist is applied to the liquid crystal layer by an alignment film or the like, the liquid crystal layer is distorted. By introducing such distortion, it is possible to improve the response speed when the voltage applied to the liquid crystal layer is changed from the ON state to the OFF state.
[0048]
From the viewpoint of increasing the response speed, it is preferable to satisfy the relational expression of 0.5 <d / p. However, since it is necessary to increase the applied voltage in the ON state as d / p increases, it is preferable to satisfy d / p <0.75.
[0049]
In the present invention, a liquid crystal layer having a high d / p is used in a liquid crystal cell in an image shift element, not in a liquid crystal display device. The liquid crystal cell in the image shift element does not require halftone display and is required to perform binary switching between a state where the polarization plane of incident light is rotated and a state where the plane of polarization is not rotated. Further, in a liquid crystal cell of an image shift element, it is usually necessary to shift an image not by line sequential scanning but by plane switching. For this reason, in the liquid crystal cell of the image shift element, a voltage is applied to a wide region (typically substantially the entire region) of the liquid crystal layer, and the voltage application state is continued for a subfield period or longer. That is, in the liquid crystal cell of the image shift element, static driving can be performed instead of active matrix driving, and the unit region where the state of the liquid crystal layer transitions is not as large as a pixel, for example, several square centimeter mail. It has the above width.
[0050]
On the other hand, in the liquid crystal display device, in order to form an image, it is necessary to change the voltage application state of the liquid crystal layer for each narrow region of a pixel unit. In the case of line sequential driving, for example, after a low voltage is applied to a liquid crystal region on a certain scanning line, a high voltage may be applied to the liquid crystal region on the next scanning line. In such a case, the liquid crystal state may become unstable as a result of interference of the liquid crystal state in adjacent pixel regions.
[0051]
In such a liquid crystal display device, when the magnitude of d / p is larger than 0.5, for example, the liquid crystal state is strongly influenced by the liquid crystal in the adjacent region unless the pretilt angle is set to 5 ° or more. There is a possibility that a light leakage area or disclination may occur, resulting in poor display quality, or a stable operation being disabled.
[0052]
On the other hand, in the liquid crystal cell of the image display device of the present invention, since a substantially uniform voltage is statically applied over a wide area of the liquid crystal layer, even if the magnitude of d / p is larger than 0.5, The stability of the liquid crystal is maintained, and the pretilt angle can be arbitrarily set. On the contrary, when the pretilt angle is increased in the present invention, an optical rotation by the liquid crystal layer in a state where no voltage is applied is abnormal, and a residual component is added to the polarized light, resulting in a deterioration in the degree of polarization. For this reason, in the present invention, it is preferable to reduce the pretilt angle. Therefore, in the present invention, the blur tilt angle is preferably set to less than 5 °, more preferably less than 4 °, and still more preferably less than 3 °.
[0053]
Thus, in the case of the image shift element of the present invention, even when d / p is large, the pretilt angle can be set to a sufficiently small value, so that the polarization characteristics of the liquid crystal can be switched sharply. .
[0054]
As described above, in the image shift element of the present invention, a predetermined voltage is maintained by a drive circuit (without a thin film transistor or the like) for substantially the entire liquid crystal layer over a subfield period or a longer period. Therefore, the liquid crystal state is stably maintained, and the polarization plane is switched with high accuracy. Therefore, even when d / p is set to a large value that is difficult to be practically used in a liquid crystal display device (for example, a value larger than 0.5), the liquid crystal state when the voltage is OFF is maintained with the pretilt angle kept sufficiently small. Since it can be stabilized, the occurrence of double images can be effectively prevented.
[0055]
Hereinafter, the relationship between the response speed of liquid crystal and d / p will be described with reference to FIGS.
[0056]
First, referring to FIG. FIG. 10 shows the relationship between “falling response” of liquid crystal and d / p. This relationship is obtained by computer simulation. As can be seen from FIG. 10, as d / p exceeds 0.25 and approaches 1, the falling response time of the liquid crystal becomes shorter.
[0057]
Further, FIG. 10 shows that when d / p is 0.25 or less, the transmittance (Transmittance) once decreases and then increases again to reach 100%. This is a phenomenon that occurs when the voltage applied to the liquid crystal layer is released, because the liquid crystal molecules in the approximate center of the liquid crystal layer receive a force in the opposite direction due to the viscosity of the liquid crystal located on the side close to the substrate (backflow). ). If the amount of chiral agent added is increased, this backflow can be suppressed and the response speed can be increased.
[0058]
FIG. 11 shows how the “voltage-transmittance curve” varies with d / p. This voltage-transmittance curve was also obtained by computer simulation. As can be seen from FIG. 11, the voltage (voltage high level) required to make the transmittance zero is required to increase as d / p exceeds 0.25 and approaches 1. According to experiments, the applied voltage is preferably 7 volts or more, and more preferably 10 volts or more.
[0059]
In the case of a liquid crystal display device, when active matrix driving is performed, since a voltage is applied to the liquid crystal layer through the switching element and the pixel electrode, it is possible to apply a large voltage exceeding 7 volts to the liquid crystal layer. In addition, the magnitude of the applied voltage tends to decrease with time. For this reason, in the liquid crystal display device, when d / p is increased, there arises a problem that the transmittance decreases or decreases with time. However, in the case of an image shift element, a predetermined voltage can be applied from the drive circuit to the pair of transparent electrodes of the liquid crystal cell. Can be applied to the layer.
[0060]
FIG. 12 shows the “falling” response curve of the liquid crystal obtained by actual measurement when d / p is 0.04, 0.26, and 0.51. As can be seen from FIG. 12, when d / p is 0.26, the above-described backflow is suppressed. It can be seen that the suppression of the backflow is significant when d / p exceeds 0.5.
[0061]
FIG. 13 is a graph showing the relationship between the measured value of the liquid crystal falling response time τd and d / p. As can be seen from FIG. 13, as d / p increases beyond 0.25, the falling response speed τd of the liquid crystal decreases. When d / p exceeds 0.5, the falling response speed τd is shortened to 50% or less of the falling response speed τd when d / p is 0.04 (comparative example). According to experiments, as d / p exceeds 0.5 and approaches 1, the 90 ° twisted state of the liquid crystal becomes unstable, and the risk of 270 ° twisting increases. For this reason, it is preferable to set d / p smaller than 0.75. The most preferable range of d / p is 0.5 <d / p <0.65.
[0062]
In this specification, in the case where the polarization plane of the light transmitted through the liquid crystal can take two different states by 90 ° depending on the voltage High / Low applied to the liquid crystal layer, a high voltage is applied to the liquid crystal layer, It is referred to as “the liquid crystal layer (or liquid crystal cell) is in the ON state” when it is in a state where it can emit predetermined linearly polarized light. Then, when a voltage sufficiently smaller than the magnitude (absolute value) of the voltage necessary to turn on the liquid crystal layer is applied to the liquid crystal layer, and as a result, the liquid crystal layer is in the “ON state” When light having a plane of polarization substantially orthogonal to the plane of polarization of the emitted light obtained in the above is emitted from the liquid crystal layer, it is referred to as “the liquid crystal layer (or liquid crystal cell) is in the OFF state”.
[0063]
In order to put the liquid crystal layer in the “OFF state”, the voltage (Low) applied to the liquid crystal layer may be set to zero. However, when the liquid crystal layer is set to the “OFF state”, a voltage (offset voltage) having a non-zero value (for example, 2.5 volts) may be applied.
[0064]
The applied voltage “High” used in this specification is a voltage at a level that can turn the liquid crystal layer “ON”, and the applied voltage “Low” is a voltage that can turn the liquid crystal layer “OFF”. It is. When a TN liquid crystal having a positive refractive index anisotropy Δε is used, the liquid crystal in the “ON state” has liquid crystal molecules aligned in the direction of the electric field, and the liquid crystal in the “OFF state” is twisted by about 90 °. Is in a state. On the other hand, when a TN liquid crystal having a negative refractive index anisotropy Δε is used, the liquid crystal in the “ON state” is twisted by about 90 °, and the liquid crystal in the “OFF state” has the liquid crystal molecules in one direction. Oriented. In the present specification, the invention will be described centering on the case where a liquid crystal having a positive refractive anisotropy Δε is used. However, the present invention can also be realized by using a liquid crystal having a negative refractive index anisotropy Δε.
[0065]
Hereinafter, preferred embodiments of an image display device according to the present invention will be described with reference to the drawings.
[0066]
(Embodiment 1)
First, referring to FIG. FIG. 3 is a schematic diagram of the image display apparatus of the present invention.
[0067]
The illustrated image display apparatus of the present embodiment includes a backlight 1, a liquid crystal display element 2, an image shift element 3, and an observation optical system 4. The backlight 1 is a light source that illuminates the transmissive liquid crystal display device 2. The liquid crystal display element 2 can receive a drive signal and a video signal from the drive circuit 5 and display an image having contents corresponding to the video signal. it can. The observation optical system 4 is an optical system for optically enlarging an image displayed on the liquid crystal display element 2. The observer can observe the image displayed on the liquid crystal display device 2 through the image shift element 3 and the observation optical system 4.
[0068]
In this embodiment, a transmissive liquid crystal display element that requires a backlight is used. However, as long as the element can display an image, not only a reflective liquid crystal display element but also an organic EL element or a plasma display panel ( A self-luminous display element such as a PDP can also be used.
[0069]
The operation of the image shift element 3 is controlled by a drive circuit 6 for the image shift element. The drive circuit 6 supplies a drive signal synchronized with the video display of the liquid crystal display element 2 to the image shift element 3. The drive circuit 6 has a voltage application unit for applying a High or Low voltage to the liquid crystal elements included in the image shift element 3. Such an image shift element 3 and a drive circuit constitute an image shift element module.
[0070]
Among the above-described constituent elements, the backlight 1, the liquid crystal display element 2, the observation optical system 4, and the drive circuit 5 have the same configuration as the elements and circuits used in the conventional image display apparatus. The characteristic point of the display device according to the embodiment is the configuration and operation of the image shift element 3. Hereinafter, the image shift element 3 will be described in detail with reference to FIGS.
[0071]
The illustrated image shift element 3 is manufactured using two liquid crystal elements 7 and three birefringent elements 8. Each of the two liquid crystal elements 7 includes a liquid crystal layer and a pair of transparent electrodes that sandwich the light incident surface and the light output surface of the liquid crystal layer. Each liquid crystal element 7 in the present embodiment has a state in which the polarization plane of incident light is rotated by about 90 ° (first state) and a state in which the polarization plane of incident light is emitted without being substantially rotated (second state). , And any one of the states can be selectively taken according to the magnitude of the applied voltage.
[0072]
The liquid crystal element 7 in the present embodiment is obtained by adding about 1.5 to 3% by mass of a chiral agent (S-811: manufactured by Merck) to, for example, ZLI-2293 liquid crystal manufactured by Merck. It is produced using a liquid crystal cell in which / p is set to about 0.55 to 0.75. The value of d / p is not limited to the above range, and is set to a necessary value within the range of 0.25 to 1. In the case of this embodiment, the preferable range of d / p is 0.5 to 0.65.
[0073]
The birefringent element 8 is made from a uniaxial crystal material (for example, quartz). The material used for the birefringent element 8 is not limited to quartz, and any material can be used as long as it is a uniaxial crystal. For example, lithium niobate, calcite, mica, rutile (TiO ₂ ), Chile nitrate (NaNO) _Three ) Or the like can be used. However, when it is necessary to reduce the total weight of the display device, such as an HMD, it is preferable to use lithium niobate or rutile having a relatively large refractive index anisotropy (Δn). If the material has a large Δn, the thickness of the birefringent element 8 necessary for obtaining a necessary image shift amount can be reduced, which is suitable for miniaturization and weight reduction.
[0074]
The birefringent element 8 has an optical axis inclined from the light incident surface of the birefringent element 8. The birefringent element 8 can separate incident light into ordinary light and extraordinary light within a plane including the traveling direction of the incident light and the optical axis (hereinafter referred to as “main cross section”). Therefore, if the polarization direction of the light incident on the birefringent element is perpendicular to the “main cross section”, the incident light becomes ordinary light for the birefringent element 8 and propagates straight in the main cross section. On the other hand, if the polarization direction of the light incident on the birefringent element 8 is parallel to the main cross section, the incident light becomes extraordinary light for the birefringent element 8 and is refracted in the main cross section.
[0075]
From the above, if the voltage level applied to the liquid crystal element 7 is changed so that the polarization direction of the incident light is switched in a direction perpendicular or horizontal to the main cross section of the birefringent element 8, the main Within the cross section, the incident light can be shifted. As a result, the image displayed on the display element 2 can be shifted in a direction perpendicular to the incident optical axis. In this embodiment, since the TN liquid crystal whose falling response speed is improved by adjusting d / p is used, the polarization direction can be switched at high speed. In this embodiment, the pretilt angle is set to about 2 °.
[0076]
In FIG. 4A, the light is guided to the position A on the virtual plane by setting the two liquid crystal elements 7 to the “OFF” state. In FIG. 4B, the light beams are guided to the position B on the virtual plane by setting the two liquid crystal elements 7 to the “ON” state and the “ON” state, respectively. In FIG. 4C, the light is guided to the position C on the virtual plane by setting the two liquid crystal elements 7 to the “ON” state and the “OFF” state, respectively. In FIG. 4D, the light is guided to the position D on the virtual plane by setting the two liquid crystal elements 7 to the “OFF” state.
[0077]
As described above, by controlling the voltages applied to the two liquid crystal elements 7, the image on the display element 2 can be shifted in the order of A → B → C → D → A →. Such a shift of the image is executed in synchronization with the switching timing of the image displayed on the display element 2.
[0078]
FIG. 5A shows a pixel array when the shift operation by the image shift element is not performed, and FIG. 5B shows a pixel array when the shift operation by the image shift element is performed. It can be seen that the number of pixels of the liquid crystal display element 2 is effectively increased fourfold by using the image shift element.
[0079]
In this specification, when a voltage for controlling the polarization plane is applied to the liquid crystal element 7 and the polarization plane of the light emitted from the liquid crystal element 7 is rotated by about 90 ° as compared with the case where no voltage is applied, Is in the ON state ". Then, a voltage sufficiently smaller than the magnitude (absolute value) of the voltage necessary for turning on the liquid crystal element is applied to the liquid crystal of the liquid crystal element 7, and as a result, the liquid crystal element 7 is turned on. When light having a polarization plane substantially orthogonal to the polarization plane of the emitted light obtained when the liquid crystal element 7 is emitted from the liquid crystal element 7, it is referred to as “the liquid crystal element is in an OFF state”.
[0080]
In order to put the liquid crystal element in the “OFF state”, a voltage (offset voltage) having a non-zero value (for example, 2.5 volts) may be applied.
[0081]
As described above, in this embodiment, the magnitude of the voltage applied to each liquid crystal element 7 in the image shift element is set to a relatively high level (for example, 8 volts or more) and a relatively low level (for example, 1 volt or less). Thus, one pixel is sequentially shifted to four different positions of A → B → C → D on a certain virtual plane.
[0082]
In this embodiment, the display pixels are sequentially shifted to four positions. However, the shift direction and the number of shift positions of the image shift element according to the present invention are not limited to this. Moreover, although the display apparatus of this embodiment can be used suitably as HMD, this invention is not limited to this. The present invention can also be applied to a projection type image display apparatus. In this case, a illuminating light source having directivity may be used instead of the backlight 1 that emits diffused light, and a projection lens may be used instead of the observation optical system.
[0083]
(Embodiment 2)
Next, another embodiment of the image display apparatus according to the present embodiment will be described with reference to FIG.
[0084]
In the image display apparatus according to the present embodiment, the light emitted from the light source 501 (light including at least RGB components) is reflected by the parabolic mirror 502 to be substantially parallel light, and then enters the fly-eye lens 503. The fly-eye lens 503 is used to illuminate the liquid crystal panel 504 uniformly. An aperture 505 that restricts the parallelism of illumination light incident on the liquid crystal panel 504 is installed on the exit side of the fly-eye lens 503. The aperture 505 has a rectangular opening, and the shape thereof is designed according to the pixel shape. The light that has passed through the aperture 505 passes through the lens 506 and is then separated into RGB light by the dichroic mirrors 507R, 507G, and 507B. The liquid crystal panel 504 is irradiated with the light substantially parallelized by the lens 506 and the lens 508, but each of the RGB lights separated by the dichroic mirror is incident on the liquid crystal panel 504 at different angles. In the present embodiment, a 0.9-type panel (768 × 1024 dots) is used as the liquid crystal panel 504.
[0085]
FIG. 7 is a cross-sectional view showing how RGB light enters the liquid crystal panel 508. As shown in FIG. 7, on the light incident side of the liquid crystal panel 504, an array of microlenses 509 is arranged at a ratio of one for three pixels (R pixel, G pixel, and B pixel). . Each microlens 509 causes RGB light incident at different angles to enter corresponding pixels. Since each pixel is driven independently, the RGB light is modulated independently.
[0086]
Reference is again made to FIG. The light modulated by being transmitted through the liquid crystal panel 504 enters the projection lens 511 through the image shift element 510. The light that has passed through the projection lens 511 forms an image on the screen. The screen image is periodically shifted by an image shift element.
[0087]
Next, the image shift element 510 of this embodiment will be described in detail with reference to FIG. As shown in FIG. 8, the image shift element 510 has a configuration in which two liquid crystal elements 10 and two birefringent elements 11 are alternately arranged in series, and a plane intersecting the optical axis (“ The image can be shifted to three different positions (A, B, C) within the "main section"). What are the combinations of the liquid crystal element 10 and the birefringent element 11? 3 has the same configuration as that shown in FIG. 3, and the liquid crystal element constituting the image shift element 510 is configured using TN liquid crystal having the same characteristics as the display device according to the first embodiment. Yes.
[0088]
FIG. 9 schematically shows how the sub-frame image shifted by the image shift element 510 shown in FIG. 8 moves up and down. As a result of the action of the image shift element 510, the sub-frame images constituting the image are sequentially shifted by one pixel pitch in the vertical direction (or the horizontal direction), so that the band of light that irradiates the same pixel area on the screen is B → G → R → B → G → R... By adopting such a configuration, a resolution equivalent to that of a three-plate projection type image display apparatus using three liquid crystal display panels can be realized even with a single-plate type.
[0089]
According to this embodiment, since the image shift element is configured using the liquid crystal layer having a fast fall response speed, it is possible to perform image shift at high speed while suppressing the generation of a double image.
[0090]
In this embodiment, a Philips UHP lamp having an output of 120 W and an arc length of 1.4 mm is used as the light source 501, but a halogen lamp, a xenon lamp, a metal halide lamp, or the like is also used as the light source. be able to.
[0091]
Note that the image display apparatus according to the present embodiment is a projection display apparatus that projects a display image on a screen, but can also be applied to an HMD by using an observation optical system.
[0092]
【The invention's effect】
According to the image shift element of the present invention, it is possible to increase the response speed of the liquid crystal when the applied voltage is changed from the ON state to the OFF state, and to shift the image at a high frequency while suppressing the generation of the double image. it can. By using such an image shift element, an image display device that displays a high-resolution image is provided.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a conventional configuration of an image shift element in which a liquid crystal element and a birefringent element are combined.
FIG. 2 is a graph showing liquid crystal applied voltage-transmittance characteristics when a pair of polarizing plates are arranged orthogonally and a liquid crystal element is arranged between the polarizing plates. The vertical and horizontal axes are arbitrary units.
FIG. 3 is a cross-sectional view showing a basic configuration of an image shift element module according to the present invention.
FIGS. 4A to 4D are diagrams illustrating the operation of the image shift element used in the first embodiment. FIGS.
FIGS. 5A and 5B are diagrams showing an increase in the resolution of an image by the image shift element of FIG. 3, in which FIG. 5A shows an image when the image shift element is not operating, and FIG. 5B is an image shift element operating; The image is shown when
FIG. 6 is a diagram showing a second embodiment of an image display device according to the present invention.
FIG. 7 is a cross-sectional view showing a liquid crystal display panel used in the second embodiment.
FIG. 8 is a cross-sectional view showing an image shift element used in the second embodiment.
FIG. 9 is a diagram schematically illustrating image shift.
FIG. 10 is a graph showing the relationship between “falling response” of liquid crystal and d / p (simulation). The horizontal axis is time (arbitrary unit).
FIG. 11 is a graph showing how the “voltage-transmittance curve” varies with d / p (simulation). The horizontal axis is voltage (arbitrary unit).
FIG. 12 is a graph showing measured values of “falling response” when d / p is 0.04, 0.26, and 0.51. The horizontal axis is time (arbitrary unit).
FIG. 13 is a graph showing a relationship between a fall response time τd (actual measurement value) and d / p. The vertical axis represents time (arbitrary unit).
[Explanation of symbols]
1 Backlight
2 LCD panel
3 Image shift element
4 Observation optical system
5 Liquid crystal element panel drive circuit
6 Image shift element drive circuit
7 Liquid crystal cell
11 Birefringent elements

Claims (14)

An image shift element including at least one image shift unit that periodically shifts the position of the optical axis,
The image shift unit includes:
A liquid crystal cell that switches the polarization direction of light between two orthogonal directions according to the applied voltage High / Low;
A birefringent element having a different refractive index depending on the polarization direction of light;
With
The liquid crystal cell and the birefringent element are arranged to transmit light in this order,
The liquid crystal cell has a liquid crystal layer and an alignment film in contact with the liquid crystal layer,
The liquid crystal layer included in the liquid crystal cell, when the natural pitch length p, the cell gap is d, is formed from a TN mode liquid crystal layer to satisfy the 0.5 <d / p <1 in equation, the alignment layer are arranged so twisted 90 ° in the liquid crystal cell by,
When the applied voltage is low, a pretilt of liquid crystal molecules occurs in the liquid crystal layer, and the pretilt angle of the liquid crystal layer is adjusted to be less than 5 °.   The image shift element according to claim 1, wherein a natural pitch length p of the liquid crystal layer is adjusted by adding a chiral agent. The image shift element according to claim 1 , wherein the liquid crystal layer satisfies a relational expression of d / p <0.75.   The image shift element according to claim 1, wherein the number of the image shift units is plural. An image shift element according to any one of claims 1 to 4,
A drive circuit for supplying a voltage to the liquid crystal cell of the image shift element;
With
Wherein the driving circuit, an image shifting device module for changing the voltage in synchronization with the switching timing of the image to be shifted.   6. The image shift element module according to claim 5, wherein the drive circuit applies a voltage of 7 volts or more to substantially the entire liquid crystal layer included in the liquid crystal cell of the image shift element for a selected period.   The image shift element module according to claim 6, wherein the selected period has a length equal to or longer than a subfield period of the image. An image display unit for displaying an image;
An image shift element according to any one of claims 1 to 4,
An image display device comprising:
An image display device that shifts light emitted from the image display unit in synchronization with the display of the image display unit by using the image shift element.   The image display device according to claim 8, further comprising a drive circuit that supplies a voltage to the liquid crystal cell of the image shift element.   The image display apparatus according to claim 9, wherein the driving circuit applies a voltage of 7 volts or more to substantially the entire liquid crystal layer included in the liquid crystal cell of the image shift element during a selected period.   The image display device according to claim 10, wherein the selected period has a length equal to or longer than a subfield period of the image.   The image display device according to claim 8, wherein light that exits from the image display unit and enters the image shift element is linearly polarized light.   The image display device according to claim 8, wherein pixels constituting the image are superimposed in a time division manner by the shift of the image. A light source;
An image display panel having a plurality of pixel regions each capable of modulating light;
Light control means for condensing light from the light source in a corresponding pixel region of the plurality of pixel regions according to a wavelength range;
An optical system for forming an image on a projection surface by light modulated by the image display panel;
An image display device comprising:
A circuit for generating data of a plurality of sub-frame images from data of each frame image constituting the image, and displaying the plurality of sub-frame images in a time division manner by the image display panel;
The image shift element according to any one of claims 1 to 4, wherein a subframe image selected from the plurality of subframe images displayed by the image display panel is shifted on the projection surface.
With
An image display device that sequentially irradiates the same region on the projection surface with light belonging to different wavelength regions modulated in different pixel regions of the image display panel.

Images