JPH0458610B2 - - Google Patents

Description

[Detailed Description of the Invention] [Summary] Using the light wavelength selectivity of liquid crystal cells, cells with different selected wavelengths are stacked, white light is projected onto this, and the passing light is focused by a condensing lens to form a screen. The present invention provides a color display device for projecting images onto the screen.

[Industrial application field]

The present invention relates to a projection type color display device that utilizes the wavelength selectivity of a cholesteric liquid crystal having a helical structure.

Until now, TN display using the twisted nematic effect has been commonly used as a liquid crystal display method, but since it requires a polarizing plate, projection display has a large optical loss and requires a bright display. Hard to obtain.

Color display devices using cholesteric-nematic phase change liquid crystals are known, but these are not projection type display devices but direct view type display devices (Japanese Patent Application Laid-open No. 59-116614, JP-A No. 59-116680, JP-A No. 59-116680, 52−
133781, JP-A-61-32801, JP-A-60-29091, JP-A-59-18925, JP-A-57-124712, JP-A-57-
111514, FJKahn”ELECTRIC−FIELD−
INDUCED COLOR CHANGES AND PITCH
DILATION IN CHOLESTERIC LIQUID
CRYSTALS” PHYSICAL
REVIEWLETTERS Vol.24, No.5, 1970, p209
−212). Furthermore, the coloring principle of these color display devices is different from the coloring principle of the multicolor color display device according to the invention. Further, Japanese Patent Application Laid-Open No. 57-201218 discloses a liquid crystal display device in which two liquid crystal layers each containing a nematic liquid crystal and a cholesteric liquid crystal as main components are laminated. Utilizing the Grange Yuan structure in which the helical axes of liquid crystal molecules are aligned perpendicular to the wall surface, a voltage of _V4 or more is applied to obtain a transparent state h, and then the voltage is gradually reduced to a light scattering state i and then to a first memory state j. Alternatively, the electric power is rapidly reduced to a transparent state g (second memory state). This is a display device that displays a light scattering state (focalconic) and a transparent state (grunge) when the applied voltage is turned off, and improves the contrast ratio. This differs from those using phase-transition liquid crystals that take a bistable state between Also, nothing is said about color display.

On the other hand, the nematic-cholesteric phase change type liquid crystal display is characterized in that it is bright and easy to view because it does not use a polarizing plate, and that it can display a large capacity with a simple matrix structure.

Figure 3 shows the display principle of a projection display using a phase change liquid crystal. percentage).

That is, when the applied voltage is low, the liquid crystal assumes a cholesteric phase in which the molecules are arranged in a helical state, and the light is scattered and scattered outside the projection condensing lens 18, resulting in a dark area on the screen.

Furthermore, when the voltage is high, the liquid crystal becomes a nematic phase in which molecules are aligned in the direction of the electric field, and light enters the lens without being scattered, resulting in a bright area on the screen.

In this way, a liquid crystal composition in which cholesteric liquid crystal is mixed with nematic liquid crystal exhibits a cholesteric phase when no voltage is applied, but as the electric field increases, it changes to the nematic phase, and on the other hand, as the electric field decreases, it changes from the nematic phase to the cholesteric phase. It is known that a phase transition occurs in the cholesteric phase, and the phase change at this time draws a hysteresis curve with respect to the applied voltage.

This shows that optically different bistable states can be achieved with the same driving voltage, and this memory effect is used to perform large-capacity displays.

[A.Mochizuki etc. “New Nematic−
Cholesteric LCD Using Hysteresis Behavior”
(Display method using hysteresis characteristics of nematic-cholesteric liquid crystal) Proceedings of the
Society for Information Display (SID) 26, No.
4, p. 243-248, 1985] The present invention relates to a projection type color display using such a nematic-cholesteric phase change liquid crystal.

[Conventional technology]

Until now, the following two methods have been known as projection-type color display methods, but neither has yet been put into practical use.

How to use an RGB (Red-Green-Blue) filter.

A method in which light that passes through different liquid phase layers is mixed and projected onto a screen using an optical system.

Here, the method is to project on the screen,
There is a problem in that the amount of projected light is less than 1/3 of that for black and white images.

That is, when a projection display is used for a meeting or for displaying to the public, it is rarely used in the dark, and therefore, if the amount of projected light is small, the projected image will be hidden by external light.

Further, the method (2) has the problem that the apparatus becomes large and expensive.

[Problem that the invention seeks to solve]

As described above, projection-type color displays using liquid crystals have performance and cost problems.

Therefore, it is an issue to realize a bright, high-resolution, and compact projection-type liquid crystal display device using a phase-transition liquid crystal display method that allows large-capacity display.

[Means for solving problems]

The above purpose is to provide a liquid crystal panel having multiple layers of cholesteric-nematic phase change liquid crystal having positive dielectric constant anisotropy, to make white light almost perpendicularly incident on the liquid crystal panel, and to The phase change type liquid crystal has an optical system that forms an image on a screen by selecting a homeotropic state and a focal conic state. It is a phase change type liquid crystal in which the plurality of liquid crystal layers have different thicknesses and/or refractive index anisotropy of the liquid crystal material, so that the wavelength dependence of the transmittance in the focal conic state can be improved. This is achieved by projection type color display devices each having different settings.

In the projection type color display device of the present invention, the cholesteric-nematic phase transition liquid crystal has focalonic (cholesteric) and homeotropic (nematic) structures. In the focal conic structure, the helical axes of liquid crystal molecules are aligned parallel to the liquid crystal panel. Light incident perpendicularly to the helical axis of the liquid crystal molecules in the focal conic state undergoes scattering with wavelength selectivity. Prior to the present invention, there was no known phenomenon in which wavelength-selective scattering occurs when light is incident perpendicularly to the helical axis of liquid crystal molecules in the cholesteric phase. In the method of the invention, the wavelength or hue of the transmitted light or the displayed color depends only slightly on the angle of incidence and significantly on the cell thickness. On the other hand, it is known that when light is incident parallel to the helical axis of cholesteric liquid crystal molecules, reflection with wavelength selectivity occurs (see above, FJ
Kahn). Also, JP-A-59-116614, JP-A-52-
In 133781, the cholesteric liquid crystal layer is used as a color filter, but the cholesteric liquid crystal layer has a grunge texture, and the helical axes of the liquid crystal molecules are aligned perpendicular to the liquid crystal panel or substrate. In these known wavelength-selective scattering methods,
The wavelength of the scattered light does not depend on the cell length, but on the helical pitch and incidence angle.

[Effect]

The present invention was made as a result of research into light scattering of cholesteric-nematic phase transition type liquid crystals.

That is, the inventors focused on the fact that light scattering in the cholesteric phase in a bistable state originates from the helical structure, and that light is diffracted by refractive index modulation corresponding to the helical pitch.

In other words, liquid crystal molecules have an elongated structure and have anisotropy in refractive index, so the refractive index is different between where the liquid crystal molecules are aligned perpendicular to the substrate and where they are aligned horizontally. .

Therefore, there is refractive index modulation corresponding to the helical pitch, and a volume phase type diffraction grating is formed.
The diffraction efficiency η is H at Bragg angle incidence.
Kogelnic's formula (THE BELLSYSTEM)
TECHNICAL JOURNAL, Vol.48, No.9,
p2909-2947, 1969).

η _nax = Sin ² (πδnd/2λcosθ) In the formula, δn is the refractive index modulation width, which has a value smaller than the refractive index anisotropy Δn of the liquid crystal due to disturbances such as thermal fluctuation. λ is the wavelength, d is the thickness of the diffraction grating,
Here it corresponds to the thickness of the cholesteric phase. Further, θ is the Bragg angle, and in the case of a cholesteric liquid crystal (focal conic), cos θ≈1.
In the cholesteric phase, there is some variation in the helical pitch and the direction of the helical axis is random within the plane, so light is scattered in wide concentric circles.

From this, it can be seen that η _nax fluctuates periodically as the cell thickness increases.

Figure 1 shows the relationship between the cell thickness and the non-scattering transmittance Ψ _nio for a phase change type liquid crystal with a wide hysteresis width as shown in Figure 3. ) and helium-neon (He-Ne) laser 2 (wavelength 633n)
The results of measurements using m) are shown.

Here, _Ψnio is the ratio of the amount of transmitted light in the cholesteric state 4 to the amount of unscattered transmitted light in the nematic state 3 when the driving voltage vd, which is the best operating condition, is applied in the hysteresis loop shown in FIG.

From FIG. 1, it can be seen that Ψ _nio changes periodically as the cell thickness increases, and that the shorter the measurement wavelength, the shorter the period.

From the same figure, to display blue color, the cell thickness should be 7 μm.
It can be seen that it is best to set the cell thickness to 3 μm in order to display a red color.

In other words, when the cell thickness is 7 μm, the He-Ne laser beam 2 has a high scattering rate and a low unscattered transmittance (ψ _nio ), while the Ar laser beam 1 has a low scattering rate and a low unscattered transmittance (ψ nio ). ψ _nio ) is high, so
The transmitted light corresponds to the non-scattered transmittance of the He--Ne laser beam 2 and the non-scattered transmittance of the Ar laser beam 1, and as a result, blue light is projected onto the screen.

In addition, in the case of 3 μm, the Ar laser beam 1 has a high scattering rate and the non-scattered transmittance (ψ _nio ) is low;
Since the He-Ne laser beam 2 has a low scattering rate and a high non-scattered transmittance (ψ _nio ), the transmitted light depends on the unscattered transmittance of the He-Ne laser beam 2 and the non-scattered transmittance of the Ar laser beam 1. As a result, red light is projected and a red display is obtained.

Note that in order to obtain more vivid coloring, it is effective to use dichroic dyes as an auxiliary means.

Since a liquid crystal cell using a cholesteric-nematic phase transition type liquid crystal has wavelength selectivity as shown in FIG. It displays color by projecting light onto the screen.

Figure 4 is an example of two liquid crystal cells stacked on top of each other.
A structure in which a first glass substrate 6 with transparent electrodes 5 provided on both sides is provided with second glass substrates 8 and 9 provided with transparent electrodes 7 on both sides, and liquid crystal 11 is sealed with a separator 10 at a constant distance. doing.

Here, the thickness of the two liquid crystal cells is set so that the non-scattering transmittance is high for the wavelength of light corresponding to the display color, and display is performed by subtractive color mixing for white light. .

In other words, when both layers are in the nematic phase, the display is white, and when only one layer is in the cholesteric phase, the color corresponds to the cell thickness condition.
When both are in the cholesteric phase, they exhibit a subtractive mixture of the two colors.

〔Example〕

FIG. 2 shows a cross-sectional view of the projection display device used in the present invention, in which the light from the light source 13 is made into parallel light by the first Fresnel lens 14, and then passes through the ultraviolet cut filter 15 and hits the liquid crystal panel 16 almost perpendicularly. The transmitted light is focused on a lens 18 by a second Fresnel lens 17, reflected by a mirror 19, and projected onto a screen 20.

In this embodiment, the liquid crystal panel 16 is composed of two liquid crystal cells, and the composition of this liquid crystal is a mixed liquid crystal containing ethane, bicyclohexane, and ester as main components as a nematic liquid crystal. Chiral nematic liquid crystal having two asymmetric carbon atoms was used in an amount of 13% by weight as a cholesteric liquid crystal, and this was mixed above the isotropic phase transition temperature to produce a phase transition type liquid crystal.

This liquid crystal is injected into liquid crystal cells with different cell thicknesses,
The relationship between cell thickness and non-scattered transmittance was measured using a He-Ne laser (633 nm) and an Ar laser (476 nm), and the results shown in FIG. 1 were obtained.

From this result, the thickness of the cell for blue display is 7.0 μm.
In addition, 3.0 μm was selected as the thickness of the cells for red display, and display panels of 80×120 dots were formed for each cell.

These display panels were injected with the above phase change type liquid crystal and sealed. At that time, a blue dichroic dye was added to the liquid crystal cell for blue display, and a red dichroic dye was added to the liquid crystal cell for red display. Color pigments were added in 1% by weight portions.

The display panel 16 consisting of two layers of liquid crystal cells is set in the display device as shown in FIG.
A 650W halogen lamp was used as the light source.

In the case of blue color display, the liquid crystal cell for red display is kept in a transparent state as a nematic phase, and an image is formed on the liquid crystal cell for blue display.

Similarly, when displaying red color, the liquid crystal cell for blue display is kept in a transparent state, and the liquid crystal cell for red display is displayed.

Also, to display black (actually a little purple), both cells should be displayed at the same time.

By doing this, it was possible to display four colors: blue, red, black, and white (background area).

It should be noted that the layered structure of the liquid crystal cell seems to cause greater optical loss than the case of a single layer, but when the liquid crystal cell takes a nematic phase, the cell thickness is thin, so the transmittance is good and the light loss is large. Absorptive effects can be ignored.

〔Effect of the invention〕

As described above, by forming cholesteric-nematic phase change liquid crystal cells with wavelength selectivity and stacking them, a plurality of color displays can be performed.

Note that for conferences and public displays, only the parts that should attract attention are colored, so a two-layer structure is usually sufficient, but a three-layer structure allows for even more color display.

A cholesteric-nematic phase transition type liquid crystal with positive dielectric anisotropy and a bistable state of a cholesteric phase (focalconic state) and a nematic phase (homeotropic state) at the same voltage, and a phase transition history with respect to voltage. A projection type display device that uses
It has the advantage of enabling bright, large-capacity information displays.

[Brief explanation of drawings]

Figure 1 is a diagram of the relationship between non-scattered transmittance (ψ _nio ) and cell thickness, Figure 2 is a cross-sectional view of the projection display device used in the present invention, and Figure 3 is the non-scattered transmittance of phase change type liquid crystal. FIG. 4 is a cross-sectional view of the liquid crystal panel used in the present invention. It is. In the figure, 3 is a nematic state, 4 is a cholesteric state, 11 is a liquid crystal, 13 is a light source, and 16 is a cholesteric state.
is a liquid crystal panel, and 20 is a screen.

Claims (1)

[Claims] 1. Cholesteric material with positive dielectric anisotropy
A liquid crystal panel having multiple layers of nematic phase change type liquid crystal, and a liquid crystal panel formed by making white light almost perpendicularly incident on the liquid crystal panel and selecting a homeotropic state or a focal conic state for each pixel in the liquid crystal panel. The above-mentioned phase change type liquid crystal is a phase change type liquid crystal that takes a homeotropic and focal conic bistable state at the same voltage, and the above-mentioned plurality of liquid crystal layers are liquid crystal layers. A projection type characterized in that the wavelength dependence of transmittance in a focal conic state is set to be different by setting the thickness of the liquid crystal material and/or the refractive index anisotropy of the liquid crystal material to be different. Color display device.