JPH041888B2 - - Google Patents

Description

[Detailed Description of the Invention] The nematic liquid crystal shutter of the present invention was developed in May 1973.
It is suitable, but not necessarily limited, for use in field effect liquid crystal light shutter displays such as those shown in U.S. Pat.

This type of light shutter comprises a layer of nematic liquid crystal material sandwiched between a pair of parallel transparent plates, with a transparent conductive material coated in selected areas of the plates to form a display of alpha numerals. has been done. The surfaces of the transparent plates in contact with the liquid crystal material are rubbed at right angles to each other, creating a twisted nematic structure in the liquid crystal material. The nematic structure is rotated or untwisted by applying an electrical potential between the transparent conductive material coatings. By providing polarizers on both sides of the display, polarized light is allowed to pass through the nematic structure or is blocked depending on whether the polarizers cross or are parallel to each other.

When a liquid crystal display of the type described above is viewed along the axis of the display (i.e. at right angles to the aforementioned transparent plate), the display formed by the α-nematic display will rotate said display 360°.
It can be easily viewed even when rotated, but since liquid crystals are generally birefringent, the off-axis performance of the display is not uniform throughout the 360° rotation of the display. . For example, 45° to the display axis
Assume that the display device is viewed at an angle of . Now, if you rotate the device 360°, the display will look good in off-axis directions that are parallel and perpendicular to the axis of the front polarizer on the display, but it will look better in the middle of these positions, especially when viewing perpendicular or parallel to the axis of the polarizer. When the display is rotated 45 degrees from its angular location, the image becomes more or less blurred, making viewing very difficult.

It has been determined that this effect is caused by the birefringence of the liquid crystal material oriented perpendicular to the surface of the aforementioned transparent plate. This effect is very disturbing in displays of moderate thickness. This effect can be reduced by making thinner displays, but such thinner displays tend to cause light scattering due to the different refractive indexes of different wavelengths, which can cause color shift when the display is not activated. becomes.

The present invention also relates to linearly aligned liquid crystal cells (also known as birefringent liquid crystal cells) and light control devices, such as light shutter modulators and displays, made from the liquid crystal cells. In devices of this type, the cell surfaces are treated (eg, rubbed) parallel to each other to produce a nearly parallel alignment of the liquid crystals. When an electric field is applied, the liquid crystal aligns along the electric field.

The present invention is based on the discovery that color scattering can be reduced or eliminated and uniform retardation can be improved in high speed light modulators using linearly aligned liquid crystal devices. The liquid crystal cell is oriented so that its surfaces are rubbed parallel to each other, and polarized light is directed toward a liquid crystal device that maintains the orientation of the liquid crystal in the relative center of the device by means of an electric field or the like. When driven to change the orientation of the liquid crystal near the surface of the device, for example, the polarization direction of the polarized light transmitted through the device will depend uniformly and at least substantially without color scattering on the orientation of the liquid crystal at the surface. Become.

The observation hole of the cell is made by placing two wave plates (retardation plates) equal to the retardation (phase difference) of the nematic liquid crystal next to the cell at right angles to each other and parallel to two orthogonal polarizing plates. Therefore, it can be expanded significantly. Furthermore, by adding a third wave plate oriented perpendicularly to the direction in which the liquid crystal is rubbed and oriented at an angle of 45° to the polarizing plate, zero-order operation is not possible. The characteristics of the octopus type device can be further modified such that the birefringence of the liquid crystal at the desired average drive level is equal to the retardation of the attached wave plate.

The present invention provides a liquid crystal light control device comprising: a layer of a nematic liquid crystal material having positive dielectric anisotropy; and a pair of generally parallel liquid crystal opposing layers disposed with the layer of liquid crystal material sandwiched therebetween and supporting the layer. a pair of generally parallel liquid crystal facing surfaces for orienting a liquid crystal material adjacent the liquid crystal facing surfaces, the layer of liquid crystal material having a core and a surface boundary, and the layer of liquid crystal material having a core and a surface boundary; are between the surface boundaries, each of the surface boundaries extending toward the core from each of the liquid crystal opposing surfaces in contact with the respective surface boundaries, and the pair of liquid crystal opposing surfaces are the pair of liquid crystal materials are treated to orient the liquid crystal material at each surface boundary substantially parallel to the respective liquid crystal opposing surfaces, and are operable to cooperate with the liquid crystal material at the surface boundary; The liquid crystal facing surfaces are disposed in parallel with the generally linear alignment direction of at least some of the liquid crystal material in the respective surface boundaries, and the apparatus further comprises: , comprising means for rapidly changing the retardation properties of the layer of liquid crystal material in the zero order direction by changing the orientation of the liquid crystal structure primarily at the surface boundaries of the layer of liquid crystal material in response to the application of an electric field; The invention relates to a liquid crystal light control device whereby the optical retardation properties of a layer of liquid crystal material vary according to the orientation of the surface boundaries of said layer; two generally parallel liquid crystal facing surfaces disposed across the layer for supporting the layer, the liquid crystal facing surfaces being treated to orient the liquid crystal material parallel to the liquid crystal facing surfaces; A method for controlling polarization by a light modulator according to the present invention, wherein the method comprises: (a) adding one first surface to each first surface boundary of said liquid crystal material by each said liquid crystal facing surface;
(b) orienting a second portion of the liquid crystal material between said first surface boundaries in a direction different from said first parallel orientation; applying a working electric field to the layer of liquid crystal material to rapidly change the orientation of each first surface boundary of the liquid crystal material;
A method for controlling the polarization of light by a light modulation device, comprising thereby changing the retardation properties of a layer of liquid crystal material in the zero-order direction.

The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, which form a part of this specification.

Referring now to the drawings, and particularly to FIG. 1, there is shown a field effect light shutter type liquid crystal cell described in US Pat. No. 3,731,986. It comprises a pair of transparent plates 10 and 12 separated by a gasket 14 which separates the pair of transparent plates approximately 0.0005 inches apart. Gasket 14 between transparent plates 10 and 12
A layer of liquid crystal material is disposed in the space surrounded by. For purposes of this discussion, it will be assumed that the liquid crystal material is of the nematic type with positive dielectric anisotropy as taught in U.S. Pat. No. 3,918,796 to Ferguson. As shown in FIG. 1, a pattern of a film of a transparent conductive material, such as tin oxide or indium oxide, is formed on opposing surfaces of transparent plates 10 and 12.
The transparent plate 12 is provided with four patches 16, 18, 20 and 22 of transparent conductive material, the other transparent plate 10 is generally designated by the reference numerals 24, 26, 28.
The four pairs shown at 30 and 30 are provided with insulated strips of transparent conductive material between them. When plates 10 and 12 are bonded to opposite sides of gasket 14, transparent conductive patches 16-22 are positioned to align with the four sets of strips 24-30 on transparent plate 10. Point or decimal point 32 is board 10
Each of the upper sets of strips is provided and these are aligned with corresponding points 34 on plate 12.

The operation of the liquid crystal cell will be explained below. However, it will be appreciated that if the entire strip of patch 24 were made opaque, for example, and the area around it transparent, the resulting image would represent the number "8". Similarly, for example, patch (set)
Selected strips of the 24 strips are rendered opaque, thereby allowing the numbers 1 to 0 to be visible.

The various electrically conductive strips insulated from one another in sets 24-30 are made of a transparent electrically conductive material 36.
is mounted for connection to external conductors (not shown) through a number of mutually insulated strips. In this regard, in order to connect the conductive strip 36 to an external electrical circuit, the lower end of the plate 10 with the strip 36 extends below the liquid crystal cell so that a suitable electrical contact can slide under the plate 10. I hope you understand that. Strip 36A
Note that it extends from the bottom of the plate 10 to the horizontal portion 40 at its top. This horizontal section 40 is directly opposite a corresponding horizontal section 42 connected to the patch 16 of conductive material on the plate 12. A conductive epoxy or similar material is filled into holes 44 in gasket 14 to connect horizontal sections 40 and 42. Because of this arrangement, one terminal of the potential source can be connected to the conductive strip 36A, and thus to the conductive patch on one side of the liquid crystal material, and on the other hand select strips of the remaining conductive strips 36 can be connected to the same potential. can be connected to the other terminal of the potential source and, depending on which of the strips 36 (i.e. the strip connected to the other terminal of the potential source) is energized, creates a potential gradient across the liquid crystal material in a selected area. Generates an electric field.

In making a liquid crystal unit, a layer of transparent conductive material in contact with a nematic phase liquid crystal material is
Cotton cloth in one direction (i.e. arrow 1 in Figure 1)
0a and 12a). The effect is to create a nematic structure oriented parallel to the liquid crystal material interposed between the transparent plates. The first polarizing plate 4 is in contact with the plate 10.
6, and on the back side of the plate 12 is a second polarizing plate 48. The polarization planes of these two polarizing plates 46 and 48 are orthogonal to each other, and the polarization plane of the first polarizing plate 46 is
45° to the rubbing direction of the transparent conductive material on top,
The plane of polarization of the second polarizing plate 48 is 45° in the direction of rubbing on the plate 12. Finally, behind the second polarizer 48 is a reflector 50, which is the subject of US Pat. No. 3,881,809, issued May 6, 1975. In short, the reflector scatters polarized light from in front of the display through the liquid crystal film without depolarization so that the display can be viewed with ambient light scattered from the reflector 50. Alternatively, the reflector 50 may be omitted and the display may be illuminated by a backside light source as is well known in the industry.

In operation of this device, ambient light impinging on the front surface of the first polarizer 46 passes through the plate 10 as polarized light in the direction of the rubbing on the transparent conductive material of the plate 10. This polarization direction is indicated by arrow 51 in FIG. The plane of polarization of the second polarizer 48 is at a 90° angle to the plane of polarization of the first polarizer 46 and is indicated by arrow 53 in FIG. Therefore, board 1
When no potential is applied between the conductive material films on 0 and 12, the polarized light passes through the front surface of the liquid crystal cell, is scattered from the reflector 50, and is reflected by the second polarizer 48,
It passes through the liquid crystal cell and the first polarizing plate 46 again. Under these circumstances, the display appears virtually all white.

Now, when a potential of the order of 5 volts or more is applied between the films of conductive material on plates 10 and 12, the second polarizer plate 48 blocks light in the area where the potential is applied and attaches a The applied strips appear dark on the white ground. Any number from 1 to 0 can be made visible depending on the energized strip.

Instead of providing crossed polarizers as in this mode of operation just described, it is also possible to provide parallel polarizers, in which case the light is blocked when no potential is applied through the liquid crystal layer. When an electric potential is applied, white numbers can be seen on a black background.

As explained above, the liquid crystals used in displays such as that shown in FIG. 1 are birefringent. The effect of this is to produce a pattern depending on the viewing quadrant. This is illustrated for example in FIG. In the figure, the liquid crystal cell is indicated by reference number 54. The polarization axis of the front polarizer is reference number 56
It is shown by. It is assumed that the numbers on the α-number display are viewed from a wide viewing location 58 at an angle of 45° to the surface of the front polarizer, and
The liquid crystal cell (display) 54 is at its center point 60
Assuming rotation about the Notice what you see. However, between these points in the four quadrants of the display the numbers become blurry and almost invisible, the worst case being at an angle of 45° with respect to the polarization axis 56. As a result, the viewing angle α with respect to the polarization axis 56 at the 45° position is typically not greater than 20°.

According to the present invention, the above-mentioned condition is alleviated by compensating for the vertical component of birefringence when the display is on. This means that the two wavelength plates 64 and 66 (the first
(see figure). Wave plate 64
and 66 have mutually orthogonal slow optical axes, one axis being indicated by arrow 68, which is connected to polarizer 46.
The other wave plate 64 has a slow optical axis 70 that is parallel to the polarization direction of the second polarizer 48 . Arrows 68 and 70 are parallel to the direction of friction on the respective transparent plates 10 and 12. The net retardation of each of wave plates 64 and 66 is less than or equal to the net retardation of the liquid crystal cell. This has the effect of increasing the viewing angle shown in FIG. 2 from less than 20° to at least 45°. The net effect is a display that can be viewed from a much wider angle. Since each wave plate is parallel to one polarizer and perpendicular to the other, there is no wave plate effect for normally incident light, resulting in a substantially increased performance display in terms of viewing angle.

The waveplates 64 and 66 are, for example, two
A sheet of oriented plastic film (eg polyvinyl alcohol). However, since the wave plates are relatively thin, they are separated from the front polarizing plate (first polarizing plate) 46.
For example, by laminating it into layers and processing it as part of it, the change in overall thickness of a display device can be reduced to 0.025 mm (1 mm).
mill) or less. Materials that can be used for this purpose are linearly oriented polyvinyl butyrol, oriented vinyl alcohol, oriented polyesters such as mylar (manufactured by Dupont, trade name for polyethylene glycol terephthalate film), cellulose acetate, and An oriented film of cellulose butyrate, oriented polypropylene, polycarbonate, or any oriented film whose refractive index in the direction of polarized light perpendicular to the film is less than the refractive index of polarized light in the direction of orientation of the film. In most cases optically positive uniaxial films are used. However, materials such as Mylar, whose film is biaxially oriented, can also be used as long as they are properly oriented with respect to the polarizing plates of the liquid crystal cell.
A compensation effect can also be obtained by using a material with negative optical sign oriented with its optical axis perpendicular to the plate.

The principles described above for the device of FIG. 1 can be used to create a relatively wide angle, high speed modulator and display 99, shown in FIG. 3, using a linearly oriented (birefringent) liquid crystal device. Dashed (') reference numerals have been used in FIG. 3 to indicate parts corresponding to FIG. If the liquid crystal cell 100 shown in FIG. 3 is oriented so that the liquid crystal facing surfaces 101 and 102 of the conductive transparent plates 10' and 12' are rubbed parallel to each other, the cell 100 will have a relatively thick thickness of 0.05 to 0.075. mm (2 to 3 mils), and a high voltage can be applied by a power supply 103 across a layer 104 of liquid crystal disposed between the plates 10' and 12'. In this case, the thickness of the display becomes less important and the electric field interaction acts mainly at the surface. If a relatively high birefringence liquid crystal material is used, the effective thickness of the light modulator or display will be as if the liquid crystal facing surface 1
Since only the interaction surface thickness of the liquid crystal at and near 01,102 acts as
Because such effective surfaces are relatively thin, the liquid crystal cell is able to change the polarization of light without substantially color scattering. Additionally, the response of the optical modulator or display becomes extremely rapid, responding in short periods of time, such as 10 to 100 microseconds. In one illustrative embodiment,
This means that the liquid crystal facing surface 10 of the plates 10' and 12'
1,102, and the liquid crystal facing surface 10
A fixed bias can be applied to the display to orient at least some of the liquid crystal in the core of the liquid crystal material layer 104 between the surface boundaries of the liquid crystal layer near the surface boundaries of the liquid crystal layer 102 near the light modulator or the audio modulation of the display. Any modulation means can be easily superimposed on the fixed bias to determine or change the orientation of the liquid crystal at the liquid crystal facing surface. Since the main optical effect is obtained in the thin surface layer of the liquid crystal layer 104, the operation is rapid, uniform, and virtually free of color scattering. The effect of alignment as described here is due to the high dielectric constant that is approximately parallel to the applied electric field at the core of the liquid crystal cell (this means that the voltage at the liquid crystal surface area or boundary where the liquid crystal is oriented according to the liquid crystal facing surfaces) is approximately parallel to the applied electric field. lowering). The power supply 103 is any power supply that operates or drives the liquid crystal cell 100 as described above, i.e., generally maintains the alignment of the liquid crystal in the core of the liquid crystal layer 104, but switches the orientation of the liquid crystal at the surface as described above. It can be any power source that can be used.

However, display device 99 of the type described above has two drawbacks. The first drawback is that a ray of light incident perpendicularly cannot be of zero order (the molecules at the liquid crystal facing surfaces 101, 102 are never perpendicular to the surface of the conductive transparent plate, regardless of the applied voltage). Because it's impossible).

The second drawback is that the birefringence in the center of the cell perpendicular to the liquid crystal cell surface is relatively large, which results in a large phase shift with respect to angle, which is caused by a very small hole or viewing angle (i.e., yielding a device with an angle α) at . This hole can be significantly enlarged by applying the compensation principle described above (ie using a wave plate). In this way, two retardation plates (wavelength plates) 64', 6 equal to the retardation (phase difference) of the nematic liquid crystal are
6' next to the liquid crystal cell, at right angles to each other, and front (first) and rear (second) polarizing plates 46', 48'.
By placing them parallel to each other, a very wide-angle or large-aperture, very rapid shutter is obtained. This device can be driven, for example, by photoconductors, thin film transistors, or many similar devices, resulting in very high speed operation. Due to the large nonlinearity, this device can be adapted to multiplexing.

Another drawback to linearly aligned liquid crystal devices is their inability to operate in zero order, as discussed above. This is corrected by using a third wave plate so that the birefringence of the liquid crystal at the desired average drive level is equal to the retardation of the attached wave plate. This allows the display to be used for white and black colors. The third wave plate is oriented perpendicular to the rubbing direction 10a, 12a of the conductive transparent plates 10', 12' of the liquid crystal cell.
On the other hand, for example, the transparent plates 10' and 12' in FIG. 3 are oriented in a rubbing direction of 45° to the polarizing plate. Alternatively, the third wave plate may have a slow axis running parallel to arrow 51', for example, with plates 10' and 1
2′ to their top and bottom surfaces.
It is rubbed at a 45° angle. The entire system for a wide-angle display can be as follows (referring to Figure 3): Front polarizer (first polarizer)
46', two wave plates 64' and 66', where one plate 66' has an axis oriented parallel to the polarization direction 51' of polarizer 46', and the other wave plate has an axis oriented parallel to the polarization direction 51' of polarizer 46'.
1'), with retardation equal to the average modulation level, and with arrow 5
a third wave plate 106 oriented at 45° to 1′;
(Not shown in Figure 1 but shown in Figure 3)
A liquid crystal cell oriented at 45 degrees to polarizer 46' and perpendicular to a third phase plate (not shown in FIG. 1) and a second polarizer oriented perpendicular to polarizer 46'. This device can be multiplexed to a higher level (ie, at a higher speed) than the twisted nematic device described above.

The present invention can be summarized as follows.

The present invention is a nematic liquid crystal light shutter or modulator with improved viewing angles due to phase differences due to the birefringent liquid crystal material itself, reduced or eliminated color scattering, and improved uniformity. The liquid crystal material itself is oriented roughly linearly between parallel plates rubbed parallel to each other, and in response to an electric field maintains its orientation with respect to said parallel plates in the liquid crystal core and changes its orientation near the surfaces of said plates. nematic liquid crystal shutters or modulators in which the direction of the transmitted polarized light can be varied as a function of the applied voltage and thus the phase difference caused by the orientation of the liquid crystal material at the surface.

Although the invention has been described with respect to certain specific embodiments, it will be apparent to those skilled in the art that modifications may be made in the shape and arrangement of the components without departing from the spirit and scope of the invention. In this regard, since the nature of the wave plates is independent of the direction of light propagation, it is clear that they can be placed behind as well as in front of the liquid crystal cell.

[Brief explanation of drawings]

FIG. 1 is an exploded view of the device of an embodiment of the present invention when applied to a parallel oriented nematic cell structure, and FIG. 2 illustrates the off-axis properties of a conventional liquid crystal cell as modified by the present invention. FIG. 3 is an exploded view of an embodiment of the invention using a linearly oriented liquid crystal cell structure. In the figure, 10...Transparent plate (plate), 12...Transparent plate (plate),
10a, 10b, 12a...rubbing direction, 14...
...Gasket, 16, 18, 20, 22...Patch (of transparent, conductive material), 24, 26, 28,
30...(transparent, conductive material) strip, 3
2, 34... point, 36... transparent conductive material (strip), 40, 42... horizontal part, 44...
Hole (of gasket 14), 46... First polarizing plate (front polarizing plate), 48... Second polarizing plate (rear polarizing plate), 50... Reflecting plate, 51... Polarizing direction, 53
... Polarization direction, 54 ... Liquid crystal cell, 56 ... Front polarizing plate polarization axis, 58 ... Viewpoint, 60 ... Center point,
64, 64', 66, 66'... Wave plate, 99...
Display, 101, 102...liquid crystal opposing surface, 103...power supply, 104...layer of liquid crystal material.

Claims (1)

[Scope of Claims] 1. A liquid crystal light control device comprising: a layer of a nematic liquid crystal material having positive dielectric anisotropy; and a pair of main bodies arranged to sandwich the layer of the liquid crystal material and supporting the layer. a pair of generally parallel liquid crystal facing surfaces for orienting a liquid crystal material adjacent to the liquid crystal facing surfaces, the layer of liquid crystal material having a core and a surface boundary; , the core is between the surface boundaries, each of the surface boundaries extending toward the core from each of the liquid crystal facing surfaces in contact with the respective surface boundaries, and the one liquid crystal facing the surfaces are treated and operable to cooperate with the liquid crystal material at the respective surface boundaries to orient the liquid crystal material at the respective surface boundaries substantially parallel to the respective liquid crystal facing surfaces; and the pair of liquid crystal opposing surfaces are arranged in parallel in a substantially linear alignment direction of at least some of the liquid crystal material in the respective surface boundaries, the apparatus further comprising: rapidly retardating the core of the layer of liquid crystal material; for rapidly changing the retardation properties of a layer of liquid crystal material in the zero-order direction by changing the orientation of the liquid crystal structure primarily at the surface boundaries of the layer of liquid crystal material in response to the application of an electric field without changing the retardation properties of the layer of liquid crystal material; A liquid crystal light control device comprising means by which the light retardation properties of a layer of liquid crystal material vary according to the orientation of the surface boundaries of the layer. 2. Two approximately parallel liquid crystal facing surfaces disposed with a layer of a liquid crystal material having positive dielectric anisotropy and a layer for supporting the layer sandwiched therebetween, wherein the liquid crystal material is placed on the liquid crystal facing surfaces. a method for controlling polarization by a light modulating device comprising liquid crystal facing surfaces treated to align in parallel; one first by the surface
(b) orienting a second portion of the liquid crystal material between said first surface boundaries in a direction different from said first parallel orientation; applying a working electric field to the layer of liquid crystal material to rapidly change the orientation of each first surface boundary of the liquid crystal material;
A method of controlling the polarization of light by means of a light modulation device, which consists in thereby changing the retardation properties of a layer of liquid crystal material in the zero-order direction.