JPH03206425A - Projection type liquid crystal display element - Google Patents

Abstract

PURPOSE:To make distinct black and white displaying by adding a dichromatic dye to absorb light of the wavelength of the light which penetrates a liquid crystal in its scattering state to the liquid crystal. CONSTITUTION:The dichromatic dye to absorb the light of the wavelength of the light which penetrates through the liquid crystal by diffraction in its scattering state is added into the liquid crystal. While the dichromatic dye is transparent to the light in the major axis direction, the dichromatic dye has the property to absorb the light of the specific wavelength in the minor axis direction. For example, green light having the wavelength ranging 492 to 577nm is absorbed and the dependency of the transmitted light on the wavelength is eliminated if the dichromatic dye of an azo deriv. which is red in color tone and absorbs the wavelength of green is mixed with the phase transition type liquid crystal. The black and white display of a good contrast is attained in this way.

Description

[Detailed Description of the Invention] [Summary] Regarding a projection type liquid crystal display element, the purpose of this invention is to provide a clear black and white display.
In a projection type liquid crystal display that uses the hysteresis loop of applied voltage and light transmittance of cholesteric phase change type liquid crystal and changes the conditions for applying the driving voltage, it displays by selecting a transparent state or a scattering state at the same voltage. A projection type liquid crystal display device is characterized in that a dichroic dye is added to the liquid crystal, which absorbs light of a wavelength that is transmitted when the liquid crystal is in a scattering state.

[Industrial application field]

The present invention relates to a projection type liquid crystal display element capable of displaying clear black and white images.

With the progress of OA (Office Automation), opportunities to provide explanations while using projection-type displays at lecture halls, conferences, etc. are increasing.

Overhead projectors and slide projectors have conventionally been used for such applications.

However, demand for display devices that input information in the form of signals to a liquid crystal display element, rewrite the content at will, and display it on a screen is increasing, rather than a projection device that periodically replaces information and displays it on a screen. ing.

[Conventional technology]

The inventors have been researching nematic-cholesteric phase transition type liquid crystal displays for some time, and found that light transmittance becomes wavelength-dependent due to diffraction and scattering caused by the helical structure of the cholesteric phase in the scattering haze of liquid crystals. We are proposing a projection color display method that takes advantage of this phenomenon.

(Unexamined Japanese Patent Publication No. 63-49736, published on March 21, 1986) The outline of this is explained as follows.

Figure 2 shows the display principle of a nematic cholesteric phase change liquid crystal. transmittance).

In other words, when no voltage is applied to a liquid crystal composition made by mixing nematic liquid crystal and cholesteric liquid crystal, or when the applied voltage is low, the liquid crystal assumes a cholesteric phase with a spiral molecular arrangement, and light is scattered and used for projection. Since the light is reflected outside the condenser lens, the light transmittance is low, 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 the light enters the condenser lens without being scattered, resulting in a bright area on the screen.

The phase transition between the cholesteric phase and the nematic phase draws a hysteresis curve with respect to the applied voltage, as shown in the figure.

A necessary condition for such a phase change type liquid crystal display element is that the voltage width (hysteresis width) of the rising curve 1 and falling curve 2 of the hysteresis loop is sufficiently wide.

Here, the hysteresis width is the difference between the applied voltage value (VD9.) at which the transmittance is 90% on the falling curve 2 and the applied voltage value (V, 2.) at which the transmittance is 20% on the rising curve 1. This intermediate voltage value is the drive voltage (
V, ).

Does this mean optically with the same driving voltage (Va)? This phenomenon has been shown to be capable of achieving a bistable state, and large-capacity displays are performed using this phenomenon.

Here, the inventors believe that scattering in the cholesteric phase in a bistable state is caused by the helical structure of the liquid crystal, but refractive index modulation occurs in response to the helical pitch, which causes light to be diffracted. We focused on the fact that

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

Therefore, there is a refractive index modulation corresponding to the helical pitch, and a volume phase type diffraction grating is used.

However, since there is some variation in the helical pitch and the direction of the helical axis is not constant, the light is scattered on wide concentric circles.

Here, the scattering efficiency η■8 is approximated by the following equation under the condition of incidence at the Plagg angle.

77+sax=stn” (kπΔ, 1d/λ)
・(1) However, Δa...Refractive index anisotropy d...Cell thickness λ...Light wavelength As can be seen from equation (1), light transmittance (non-disruptive transmittance)
has wavelength and cell thickness dependence, and experimentally the light transmittance changes periodically in the range of about 40% to about 2% in proportion to the increase in cell thickness, and the repetition period changes depending on the measurement wavelength. do.

The projection-type color display method proposed by the inventor uses the wavelength dependence of light transmittance to color the image on the screen, and uses refractive index anisotropy. When the cell thickness increases, the display color becomes red→
It is clear that the color changes from purple to blue to green.

[Problem to be solved by the invention]

Although a display element using a phase change type liquid crystal can perform color projection display in this manner, it is not suitable for performing black and white display with good contrast.

On the other hand, since there is a demand for black-and-white display, the challenge is to use phase-change liquid crystals to provide projection display with good contrast.

[Means for solving the problem] The above problem can be solved by using the hysteresis loop of applied voltage and light transmittance of nematic-cholesteric phase transition type liquid crystal, and by changing the driving voltage application conditions, it is possible to achieve a transparent state with the same voltage. In a projection type liquid crystal display that selects and displays a cloudy state, the projection type liquid crystal display element is characterized in that a dichroic dye is added to the liquid crystal to absorb light of a wavelength that is transmitted in the scattered state of the liquid crystal. The problem can be solved by using force.

[Effect]

The present invention relates to a projection type liquid crystal display element, in which a dichroic dye is added to the liquid crystal, which absorbs light at a wavelength that is transmitted by a diffraction phenomenon in the scattering state of the liquid crystal, thereby making it possible to display black.

Note that dichroic dyes are transparent to light in the long axis direction, but have the property of absorbing light of a specific wavelength in the short axis direction.

〔Example〕

A phase change liquid crystal for green display is prepared by mixing 10% by weight of chiral nematic liquid crystal having two chiral centers with the nematic liquid crystal shown in Table 1 (refractive index anisotropy is 0.212) as a phase change liquid crystal. did.

Table 1 Then, 2% by weight of an azo derivative dichroic dye (product name M-6l8, Mitsui Toatsu Kagaku ■) having a red color tone and absorbing green wavelengths was mixed into this liquid crystal.

This liquid crystal has a pixel size of 350 x 350 μm and 6
The display device was encapsulated in a 12-inch panel with 40×400 pixels and a thickness of 6.3 μm.

Attach this to a projection display device, and measure the spectrum of the display light projected onto the screen using a spectrometer (product name: MC) at room temperature.
The measurement was performed using a PD-100 (Otsuka Electronics Co., Ltd.).

In addition, for comparison, the spectra of display light not mixed with dichroic dye were similarly measured.

FIG. 1 shows the results, with the horizontal axis representing the measurement wavelength and the vertical axis representing the absorbance.

In the figure, a broken line 3 is a spectrum of display light not mixed with a dichroic dye, and a solid line 4 is a spectrum of display light mixed with a dichroic dye.

It can be seen that in the broken line 3, green light having a wavelength in the range of 492 to 577r+n+ is not absorbed, but in the solid line 4, it is absorbed, which is why the displayed color is black.

In addition, Table 2 shows the display colors of display elements containing dichroic dyes and display elements without additives, and the chroma equivalent Cdl and hue equivalent H゜ of the L"a"b" color system according to JIS-Z 872l. This is the result of measuring 9, which corresponds to brightness.

Table 2 From the results, it can be seen that the display color changed from green to black.

〔Effect of the invention〕

As mentioned above, in a projection display using a phase change type liquid crystal, by adding a dichroic dye to the liquid crystal that absorbs light at the wavelength projected by the diffraction phenomenon of the liquid crystal, it is possible to improve the wavelength dependence of transmitted light. This makes it possible to eliminate black and white display with good contrast.

[Brief explanation of drawings]

Figure 1 shows the spectrum of display light in a scattered state. Figure 2 is a diagram of the display principle of phase change type liquid crystal. It is. wings 1 Leap t 2 figure

Claims (1)

[Claims] Utilizing the hysteresis loop between applied voltage and light transmittance of a nematic-cholesteric phase transition type liquid crystal, by changing the driving voltage application conditions, a transparent state and a scattering state can be selected and displayed at the same voltage. What is claimed is: 1. A projection type liquid crystal display element, characterized in that a dichroic dye is added to the liquid crystal, which absorbs light of a wavelength that is transmitted when the liquid crystal is in a scattering state.