JPH028815A - Projection type color liquid crystal display element - Google Patents

Abstract

PURPOSE:To make a green display with high saturation by specifying the kinds and mixing amounts of nematic liquid crystal and cholestric liquid. CONSTITUTION:The kinds and mixing amounts of the nematic liquid crystal and cholestric liquid crystal are adjusted to hold the product of the refractive index anisotropy of a liquid crystal compound and panel thickness between 1,400-1,500nm and also spiral pitch below 1.3mum. The saturation increases at the time of scattering by diffraction as the diffraction efficiency increases. Namely, the spiral pitch is shortened to obtain effect, the mixing amount of the cholestric liquid crystal of phase shift type liquid crystal is adjusted, and the spiral pitch of the phase shift type liquid crystal is reduced. Consequently, clear green is displayed with good reproducibility.

Description

[Detailed Description of the Invention] [Summary] The purpose of this invention is to adjust the type and mixing amount of nematic liquid crystal and cholesteric liquid crystal in order to display a green color with high saturation in a projection type color display using a panel stacking method. Refractive Index Anisotropy of Liquid Crystal Composition [Field of Industrial Application] The present invention relates to a projection type color liquid crystal display element that displays green color with high saturation.

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 traditionally been used for such applications.

However, rather than projectors that periodically change information and display it on a screen, there is an increasing demand for display devices that input information in the form of signals to a liquid crystal display element and can rewrite the content at will and display it on a screen. ing.

[Conventional technology]

There is a demand for a projection-type LCD display that has the following performance features: ■ A bright and easy-to-read screen, ■ A large-capacity large-screen display, ■ The ability to rewrite information, and ■ A compact and inexpensive device. 5TN (Supe
Projection type liquid crystal displays using the Twist Nematic method and the laser writing method have been developed, but since the STN method requires a polarizing plate, there is a problem that the optical loss is large and the screen is dark.

Furthermore, the laser writing method has the problem that the device is large and expensive, and has not yet become widespread.

Next, the following method is known for displaying color in a projection type liquid crystal display.

(2) A method using RGB (Red-Green-Blue) filters.

■ A method in which the light that passes through multiple LCD panels is mixed and projected onto a screen using an optical system.

Here, method (2) uses an RGB filter, so the amount of projected light is less than 173, resulting in a display with low contrast, and the resolution is also reduced to 1/3.

Further, in the method (2), it is possible to mix and color by using a chromic mirror or the like, but there is a problem that the apparatus becomes large and expensive.

The inventors have been researching nematic-cholesteric phase transition type liquid crystal displays for some time, and have discovered that light transmittance becomes wavelength-dependent due to diffraction and scattering caused by the helical structure of the cholesteric phase in the cloudy state of liquid crystals. We have proposed a projection-type color display method using .

(Japanese Patent Application No. 1986-194617, filed on August 20, 1986) The outline of this is explained as follows.

Figure 5 shows the display principle of a nematic cholesteric phase change liquid crystal. transmittance).

In other words, when a liquid crystal composition consisting of a nematic liquid crystal and a cholesteric liquid crystal is mixed, when no voltage is applied or when the applied voltage is low, the liquid crystal assumes a cholesteric phase with a spiral molecular arrangement, and light is scattered and collected for projection. Since the light is transmitted outside the 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 is scattered and directly enters the condenser lens, 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.

This indicates that optically different bistable states can be achieved with the same driving voltage (Vd), and this phenomenon is utilized to perform large-capacity displays.

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 that refractive index modulation occurs in response to the helical pitch, and light is diffracted by this. We focused on

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 formed.

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,,X is approximated by the following equation under the condition of Bragg angle incidence.

ηIIIIX = Sin2(k7(Δ, d/λ”)
...(ll However, Δ7 ... refractive index anisotropy d ... cell thickness λ ... wavelength of light) (as seen from equation 11, light transmittance (non-scattering 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 performs color display by utilizing the fact that the image on the screen is colored due to the wavelength dependence of light transmittance.

[Problem to be solved by the invention]

The projection color display method proposed by the inventor uses the diffraction and scattering of liquid crystals, but in order to display color, it is necessary to clarify the conditions for obtaining a specific hue, such as green, and the conditions for high saturation. is necessary.

[Means to solve the problem]

The above problem is solved by adjusting the type and mixing amount of nematic liquid crystal and cholesteric liquid crystal in a projection type liquid crystal display that employs a panel stacking method using nematic liquid crystal and cholesteric phase change liquid crystal with positive dielectric constant anisotropy. The product of the refractive index anisotropy of the liquid crystal composition and the panel thickness is 1400 to 1500n.
By keeping the helical pitch at 1.3 μm or less and the helical pitch at 1.3 μm or less, a highly saturated green color can be displayed.

[Effect]

The inventors calculated the refractive index anisotropy (Δn) of the liquid crystal and the panel thickness (d
), but in order to achieve reliable color display, it is necessary to clarify the relationship between the product of Δn and d, that is, the optical path difference, and hue. be.

It is also known that in scattering due to diffraction, the higher the diffraction efficiency, the higher the saturation, that is, the larger the value of the Q parameter expressed by the following equation, the higher the diffraction efficiency.

Q=2πλd/(nΔ”) −f2) Here, λ
...Wavelength d ...Panel thickness n ...Average refractive index Δ ...Diffraction grating spacing From equation (2), in order to increase the value of Q, the diffraction grating spacing (Δ), that is, the helical pitch It turns out that shortening the interval is effective.

Therefore, the problem of low saturation of displayed colors can be solved by adjusting the amount of cholesteric liquid crystal mixed in the phase change liquid crystal and reducing the helical pitch of the phase change liquid crystal.

From the above, in order to perform color display with high reproducibility, it is necessary to clarify the relationship between helical pitch and saturation.

Therefore, the inventor decided to display green as the display color, and by measuring the three elements of color (lightness, two hues, and saturation) in accordance with JIS Z 8721 and using a liquid crystal luminance meter, it was possible to display a clear green color with good reproducibility. conditions have been clarified.

Nine types of phase change liquid crystals with different nd ) was sought.

Table 1 shows the results.

Table 1 [Example] Example 1 (Determination of green display conditions) Chiral nematic liquid crystal having two chiral centers as cholesteric liquid crystal was mixed with nematic liquid crystal mainly composed of ethane liquid crystal as shown in Figure 2. , the product (Δ) of the refractive index anisotropy and the panel thickness, then the driving voltage (V
The three elements of display color when d) was applied, namely brightness, two hues, and chroma, were measured using a liquid crystal luminance meter (LC-35, manufactured by Canon Inc.).

Table 2, that is, L*a according to JIS-Z-8721
The color coordinates (L"a"b") in the llbm color system were determined, and H'' and C'' were calculated.

Here, L9 corresponds to brightness, I("corresponds to hue, and C" corresponds to saturation.

Table 2 on the previous page shows the L” for each of the nine liquid crystals.
, H" and C1, and FIG. 1 shows the relationship between Δnd in Table 1 and H"" in Table 2.

From Table 2, for samples No. 1 to 9, Ll(
Although the values of brightness) and C0 (chroma) are almost the same, H” (
Hue) is 205 for NO65 than 113.54 for NO36
76°, and when plotted with Δnd on the horizontal axis and H” on the vertical axis, it shows that there is a linear relationship as shown in Figure 1, and the range where Hl is green (Munsell color system) 2.5G-10G in hue in the system)
It was found that Δnd was in the range of 1400 to 1500 nm.

From the above, if it is desired to obtain green as a projected display color in a liquid crystal display, Δnd may be set in the range of 1400 to 1500 nm.

The refractive index anisotropy (Δn) of the phase change type liquid crystal is determined by rubbing the surface of the sample stage of a refractometer (2T, manufactured by Atsube Corporation) that has a polarizer only at the eyepiece, and then dropping the liquid crystal material. The polarizer is rotated while the light is incident perpendicularly to the optical axis of the liquid crystal, and the refractive index (n) with respect to ordinary light is determined.

) and the refractive index (n, ) for the above light were measured and calculated from the difference.

Example 2: (Determination of a green color with good saturation) A chiral nematic liquid crystal having two chiral centers as a cholesteric liquid crystal was mixed with a nematic liquid crystal mainly composed of ethane liquid crystal as shown in Fig. 4, and a chiral nematic liquid crystal with two chiral centers was mixed as shown in Fig. Five different types of phase change liquid crystal samples were prepared.

Table 3 shows the relationship among the average refractive index, panel thickness, helical pitch, and driving voltage for each sample.

Next, for each sample, the driving voltage (Vd) was set at 25°C.
The three elements of displayed color when applying , lightness, hue, and saturation were measured.

Table 4 shows the results.

Table 3 Table 4: μμ path FIG. 3 shows the relationship between C and helical pitch.

From Table 4 and Figure 3, it can be seen that L”
(brightness) and I]*(hue) are almost the same, but C”
It is clear that the saturation (chroma) improves as the helical pitch becomes shorter, and from Figure 3, the saturation is 4 or more (C"22") in the Munsell display system.
It can be seen that in order to achieve the above), it is necessary to set the helical pitch to 1°3 μm or less.

FIG. 4 is a diagram showing the structural formula of the liquid crystal used in the measurement, and FIG. 5 is a diagram of the display principle of the phase change type liquid crystal.

(Effects of the invention) As described above, in a phase change type liquid crystal display panel, Δ
By setting the product of nd in the range of 1400 to L500 nm, the projected display color can be made green, and by shortening the helical pitch to 1.3 μm or less, a highly saturated display can be performed.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between Δrzf and H", Figure 2 is a diagram showing the structural formula of the liquid crystal used in the measurement, and Figure 3 is a diagram showing the relationship between helical pitch and C". Each envy) Between i machine △ sail d and H eel (Ice'*-I ■ +r for the remaining 40 food bullets! 5 nights of yarn play, : i'li +=, 4ttE [(Js shi 4 已) rh(7
) i−'k”n< a*T (u half 2 figure evening “5.’) 30'4 Shiteko loquat chicken 1 chicken purchase second flight Two power [su2 child d mouth]

Claims (1)

[Claims] In a projection type liquid crystal display that employs a panel stacking method using a nematic-cholesteric phase transition type liquid crystal with positive dielectric constant anisotropy, the refraction of the liquid crystal composition can be improved by adjusting the type and mixing amount of the nematic liquid crystal and cholesteric liquid crystal. A projection type color liquid crystal display device characterized by displaying a highly saturated green color by keeping the product of index anisotropy and panel thickness at 1400 to 1500 nm and by keeping the helical pitch at 1.3 μm or less.