KR19990036039A - Optical Modulation Using Asymmetric Liquid Crystals - Google Patents

Abstract

The optical modulator for display comprises a cyclic polarization source 1, 5, 23 and a modulator 34 composed of cholesteric liquid crystal material and whose light is directed to the liquid crystal material. The modulator is governed by the properties of the cholesteric liquid crystal that blocks cyclic polarized monochromatic light in the same direction as in 26. When a voltage is applied, the cholesteric properties of the liquid crystal are destroyed and light is transmitted 20. The transmitted light can be used, for example, to drive phosphors for displays. Cholesterics can block light directly, eliminating the need for polarizers or other filters.

Description

Optical Modulation Using Asymmetric Liquid Crystals

The present invention uses an asymmetric liquid crystal, that is, a liquid crystal having a helical structure. Such asymmetric liquid crystals have asymmetric nematic phases (cholesteric or N *, where * represents achirality) and asymmetric smectic phases such as asymmetric smectic C phase (SmC *). Illustrated by These phases have a helical structure. The helix is slightly thread-shaped and has a pitch P associated with the repetition distance along the helix. The repetition distance corresponds to a rotation around the helix of 2π radians.

The color display device can emit light above a bandwidth of 380 to 780 nanometers (nanometres). This wide range of electromagnetic radiation causes many disruptions and inconveniences for designers of thin liquid crystal display devices for use in products such as laptop-top computers and televisions. In addition, it is not surprising that only about 4% of the energy input can be viewed since another problem is associated with the white light filtering system used to generate red, blue and green pixel elements.

To address this energy loss, attempts have been made to increase the light directed towards the viewer using cholesteric liquid crystals. A promising approach is to use a "cholesteric mirror", and when used with polarized light, the use of cholesteric mirrors is reflected by the reflective and transmissive properties of cholesteric liquid crystals.

In the basic cholesteric mirror approach shown in FIG. 1, the non-polarization from source 1 is directed towards layer 23 of cholesteric liquid crystal. Such liquid crystals pass light of a wavelength slightly different from the pitch (in consideration of the reflectance of the material), and also pass light of a wavelength that is polarized close to the pitch and circulated in the opposite direction. Light of the correct wavelength circulated in the same direction and polarized is reflected. Thus, layer 23 acts as a filter.

As shown, monochromatic light including left and right polarized light (non-polarized light) hits the asymmetric filter substrate 23, and the same asymmetric light in the left direction, such as the asymmetric liquid crystal, is almost entirely reflected and the monochromatic light in the opposite direction is almost entirely. Cross the filter. To enhance the intensity of the light, reflector 11 (diffuse or specular reflection) can be positioned behind the filter in front of or behind the light emitter. The cyclic polarization is then transmitted by the filter after facing a phase change of π in the reflector that effectively reverses one side. Circulating polarization can be used to enhance the overall transmitted light. For this arrangement, see B. Kerllenevich & A. Coche, conference paper for SID France, August, 31-September 3, 1993.

Rightward cyclic polarization across the filter 23 can be modulated. In the conference paper, a twisted nematic (TN) liquid crystal layer 12 is used, and a voltage can be applied to the liquid crystal layer by electrodes on each surface. This liquid crystal layer is made of a material and thickness corresponding to a half wave plate when no voltage is applied, and instead of a TN liquid crystal layer, a supertwisted nematic (STN) liquid crystal layer is formed in which the same operation is performed for the present purpose. Can be used. The effect of the half wave plate is to reverse one direction of the incident light, as shown. On the other hand, when a voltage is applied to the liquid crystal, the polarization in the right direction passes through unchanged. Thus, by arranging the cholesteric mirror in the right direction on the other side of the TN liquid crystal layer, light is blocked or passed depending on whether voltage is applied.

This approach has the great advantage of making monochrome display devices work well enough and does not require polarizers that are not 100% efficient, but color display devices are needed because some cholesteric layers are needed to cover a range of wavelengths. Not suitable for In this regard, cholesteric liquid crystals are not used much in display devices.

In a broad sense, the present invention uses cholesteric liquid crystals to block or transmit circulating polarization and can be used to fabricate color display devices. In one aspect, the invention is achieved by a system comprising means for providing circulating polarization and a modulator made of a cholesteric liquid crystal material to which light is sent and selectively driven to pass or block cyclic polarization. Is achieved.

In essence, such an arrangement provides a monochromatic output suitable for several purposes, but is particularly effective when the light is substantially monochromatic, especially in ultraviolet light, and once modulated light is used to drive materials capable of emitting visible light. Or to change the reflective properties or absorbency of a material or to generate an output. This type of material is exemplified by the phosphor when there is emission power. Reference may be made to WO95 / 27920 concerning display devices of this kind. When the present invention is incorporated in a display device, a monochrome display screen and a full-color display screen can be formed.

Input cyclic polarization can be generated by a conventional "cholesteric mirror" arrangement as described above.

The present invention can provide a low loss liquid crystal modulator that does not require a polarizer but, in the simplest case, requires only a filter and a modulator, and actually applies anything similar to an analyzer. This is because the liquid crystal can directly block light rather than simply change the polarization state for continuous blocking by another optical element, as is done with a conventional TN type liquid crystal display device. The liquid crystal can be, for example, a predetermined pitch of asymmetric liquid crystals used with monochromatic or near monochromatic light, which is exemplified by sub-visible, short wavelength electron radiation, also referred to as ultraviolet.

Preferably, the process uses ultraviolet light with a wavelength of 365 nm which requires an asymmetric liquid crystal with a pitch of 230 nm with a typical average reflectance of 1.6. Such pitch is known for asymmetric nematic liquid crystal (N *) and asymmetric smectic (SmC *) materials.

In order to make the most of the process, collimation of the input unpolarized light is desirable. This collimation can be performed using a lenslet system. However, since the embodiment of the present invention does not require a polarizer, in the embodiment using the phosphor type output element, these elements may be disposed inside the liquid crystal cell. This greatly reduces the need for collimation and makes the arrangement considerably simpler.

As the secondary emitter in the form of a phosphor, ultraviolet rays having a wavelength of 365 nm are ideal, and a single pitch asymmetric liquid crystal can be used. For the selected wavelength 365nm, typical pitch values at μm are as follows.

Expression (1)

However, another possibility is to use smaller wavelengths of light, for example blue light, to drive phosphors that emit longer wavelengths, for example red or green. This will prevent the use of UV.

In another application of the invention, the cholesteric mirror system can be used to generate polarization linearly, for example by adding a quarter wave plate, without using a conventional polarizer. This linear polarization can be optically switched by liquid crystal electron optics that require linear polarization, as used in conventional twisted nematic display devices and supertwist nematic display devices, such as bi-refringence. For example, it is applicable with a number of ferroelectric effects, electroclinic effects and other electro-optic effects based on simple (untwisted) nematic planner homeotropic switching. For example, with a twist-free nematic liquid crystal and a ferroelectric liquid crystal, some dichroic effects can be utilized to the maximum by aligning the polarization direction and the absorption direction.

In another aspect, the present invention provides a substantially monochromatic ultraviolet source, a first cholesteric mirror that transmits light circulating in one direction, a modifiable liquid crystal layer that selectively changes the polarization of the transmitted light, and optionally Provided is an optical modulation device comprising means such as a second cholesteric mirror that passes or blocks selected polarizations. In one variation, a quarter wave plate is provided between the cholesteric mirror and the liquid crystal modulator so that the modulator can be a conventional TN, STN or other double refractive liquid crystal, with the intermediate light being linearly polarized.

In other words, the transmitted light can be used to drive a second emitter, such as a phosphor, in particular in a color display device. Thus, the light source of the present invention can be used to make ultraviolet current pass, fluoresce and produce an emission display device. This solves the problem with the known arrangement of FIG. 1 in relation to color displays.

FIELD OF THE INVENTION The present invention relates to the use of asymmetric liquid crystals for the generation and modulation of polarized light, and more particularly to the use of asymmetric liquid crystals in display devices. One of the major problems with color liquid crystal display screens is the loss of light in polarizers and filters, resulting in inefficient use of input energy. The present invention solves this problem despite application to monochrome display devices.

1 shows the principle of known cholesteric mirrors.

Figure 2 is a block diagram showing a first embodiment of the present invention using two cholesteric mirrors in series, one polarizing filter and the other modulator.

3 is a block diagram showing a second embodiment of the present invention using cyclic and linear polarization associated with a conventional polarizer or an asymmetric polarizer.

4 is another schematic diagram showing the arrangement of FIG. 2 over a larger area as the display portion.

In FIG. 2, source 1 produces unpolarized, non-colimate electron radiation, which in this example produces substantially monochromatic or narrow band ultraviolet radiation. Light passes through a lens 5 which may be a component of the lenslet array from which light is emitted, such as unpolarized and collimated light 9 in display applications. Circularly polarized light in the left direction is represented by the symbol 15 and circularly polarized light in the right direction is represented by 19. The light strikes the surface 21 of the cholesteric mirror 23 comprising the asymmetric liquid crystal material in the right direction. Light with polarization 19 in the right direction is reflected from the surface 21 as indicated by 6, while light with polarization 15 in the left direction traverses the cholesteric mirror. Thus, the cholesteric mirror acts as a kind of filter. The helical characteristic of the liquid crystal is indicated by 8.

For display purposes the arrangement consists essentially of the cyclic polarization filter 23 and the asymmetric modulator 34. The modulator helix must be in the same direction as the light it contacts. The cyclic polarization filter does not need to be switched at this stage unless there is a requirement for modulation. The wavelength λ of light must satisfy the following equation.

λ = n × P ..... Equation (2)

Where n is the average reflectance and P is the pitch of the asymmetric material in the filter and modulator.

If no voltage is applied to the modulator, the light in that direction is almost totally reflected and there is little light across the modulator. When an electric field is applied to the modulator, the pitch is unwound above the threshold given by the following equation.

........... Formula (3)

Where K ₂₂ is a twist elastic constant, Δε is allowable anisotropy, and E ₀ is free space allowance. Then, light is transmitted. 2 shows a cell area in which light is not transmitted since no electric field is applied.

If the pixel cell is off, the helical structure remains as in 34a, and when an electric field is applied across the cholesteric cell, the helix is repeated and the liquid crystal enters the nematic phase as in 34b. For pixel 34a, the leftward polarization is reflected as indicated by 26 while the light entering the nematic state pixel 34b traverses the cholesteric mirror as in 30 with the leftward polarization indicated by 15.

The cyclic polarization filter 23 has a helix in an appropriate direction with respect to the direction of the modulator and a pitch length satisfying Equation (2), and may be formed of a convenient asymmetric material, for example a polymeric material.

Basically, such a system is a cyclic analog of a standard display system using twisted nematic liquid crystals and linear polarization with the advantage that an analyzer is not required. The arrangement shown emits a monochromatic circularly polarized light 30 that can be used directly, for example for display purposes. However, if this monochromatic light is used to drive an output element (not shown) which is essentially a phosphor that alternately emits light to the viewer, another advantage can be obtained. In this case, the phosphor may be an RGB phosphor similar to the phosphor used in the CRT display device, which means that an efficient color display device can be manufactured. Such configurations and advantages are described, for example, in WO 95/27920 (Crossland et al.).

3 shows an embodiment where cyclic polarization shows conversion to linear polarization and the modulator responds to linear polarization. Here, as in FIG. 1, the light source 1 produces narrow band unpolarized, non-colimate light that inputs the lens 5 shown at 3, wherein the lens 5 is a unit part of the lenslet array. . A collimated, unpolarized light, labeled 9, is produced. Circularly polarized light in the left direction is represented by 15 and circularly polarized light in the right direction is represented by 10. Thus, the collimated light strikes the base 21 of the cholesteric mirror 23. This mirror comprises a cholesteric liquid crystal with asymmetry in the right direction (twist in the right direction). The component polarized in the right direction of the collimated, non-polarized light is reflected from the base 21 of the cholesteric mirror 23, and the polarized component in the left direction crosses the mirror indicated by 15 as in 27. Thus, the arrangement corresponds to the arrangement of FIG.

In this embodiment, the collimated, left polarization component crosses the quarter-wave substrate indicated by 31 and appears as linearly polarized light 35, with the polarization represented by 39. The linearly polarized light 35 enters a half-wave plate (optimum condition) that modulates the substrate 43, which may be constructed in a conventional (eg, TN) design and traverses multiple pixels. Two of the plurality of pixels are represented by 43a and 43b. In Fig. 3, pixel 43a is switched off and pixel 43b is switched on. The polarization across the pixel 43a remains unchanged while the polarization across the pixel 43b rotates 90 degrees. Thus, the linear polarization changes. The original polarization is represented by 39 and the changed polarization is represented by 47. Light in the form of linearly polarized light is represented by 35 and 45, respectively.

Light across the device may be treated in one of two ways, shown generally at 44 and labeled 44a and 44b.

Once the process 44a is selected, the linear polarizations of orientations 35 and 45 are represented by 53 and after passing through a quarter wave plate exactly oriented with a fast axis, both forms of linear polarization are transformed into cyclic polarizations. The unmodulated linear 35 is transformed into a circularly polarized light 54 in the left direction with 15, and the linear 45 is transformed into a circularly polarized light 56 in the right direction with 19. Two types of cyclic polarization hit the base 65 of another cholesteric mirror 59. The cholesteric mirror comprises a liquid crystal of asymmetry (twist) in the left direction. Circularly polarized light in the left direction is reflected from the base 65 of the mirror and circularly polarized light in the right direction crosses the cholesteric mirror 59. The light shown is shown as in 61 and the cyclic polarization in the right direction is indicated at 19. Of course, the mirror 59 may be in the right direction when the image is negative. This action is related to the Kerllenevich & Coche design (Figure 1).

When option 44b is selected, the conventional linear polarizer shown at 55 removes polarization 35 having an orientation indicated by 39, while polarization 45 having an orientation indicated at 47 is applied to a polarizer (as in 61 '). 55, it becomes possible to be seen by the viewer 70. In this option, some have a simpler structure but retain some of the advantages of having a cholesteric mirror instead of the first polarizer used in normal TN display devices. 4 shows a portion of the display device along the line of FIG. 2, for which light 15 should be collimated for best results, but for clarity this is not shown. The transmissive pixels, ie the areas where voltage is applied, in the modulator 64 pass cyclic polarization 63 for monochrome display or further processing.

As described above, in the operation display device, the output light 59, 61, 63 touches the array of the secondary light emitting elements. Although the secondary light emitting elements are not shown, the international application number PCT / GB / 95/770 ( Crossland et al., That is, along the lines shown in the application for RGB phosphor dots, a secondary light emitting device can also be a device whose absorption and reflection properties are changed by the action of UV light. Together with the proper modulation of UV light, these secondary light emitting devices produce a preferred color display device, which solves the problem of producing a color display device using a high wavelength selective cholesteric liquid crystal. It is not limited to, and is not limited to the use of ultraviolet rays.

Clearly, many variations of the arrangement shown are possible. The collimator does not need to be used as a lens or lenslet array, but other applications besides the display are conceivable.

Claims (12)

An optical modulation device comprising a modulator and means for providing cyclic polarization, the optical modulation device comprising: And a modulator comprising a cholesteric liquid crystal material is light directed towards the liquid crystal material and operable to selectively pass or block cyclic polarization. The light modulator of claim 1, wherein the cholesteric liquid crystal is adapted to switch between reflective and clear states. 3. An optical modulation device according to claim 1 or 2, wherein the source of cyclic polarization comprises a cholesteric filter for passing circulating light in one direction and reflecting light in the other direction. 4. The light source according to claim 3, wherein the light source comprises a rear mirror, wherein the cyclic polarization reflected by the cholesteric filter is reflected from the mirror and transmitted behind it towards the pi-phase changed and switched liquid crystal modulator. Modulation device. The light modulating device according to claim 1, wherein the light is at least partially collimated. The optical modulation device according to any one of the preceding claims, wherein the light is ultraviolet or short wavelength visible light. A narrow band ultraviolet source, a first cholesteric mirror that transmits light in one direction, a modifiable liquid crystal layer that selectively changes the polarization of the transmitted light, and, optionally, selection means for passing or blocking the changed polarization. Optical modulation device comprising a. 8. An optical modulation device according to claim 7, wherein said selection means comprises a second cholesteric mirror. An optical modulation device comprising a narrow band or single wavelength electron radiation source circularly polarized along a quarter wave plate for conversion to linear polarization, and a switchable liquid crystal cell to modulate linear polarization. 10. An optical modulation device according to claim 9, comprising an asymmetric liquid crystal filter with another quarter-wave plate to restore cyclic polarization. And a secondary emitter means for generating a display output when impinged by the light transmitted by the modulator. A substantially monochromatic light source, a first cholesteric mirror that transmits light circulating in one direction, a modifiable liquid crystal layer that selectively changes the polarization of the transmitted light, and optionally passes or blocks the changed polarization And a secondary emitter means for generating a display output when impinged by light transmitted by the modulator with an optical modulation device comprising selection means for causing the display device to change.