JPH0380227A - Projection type color liquid crystal display element - Google Patents

Abstract

PURPOSE:To suppress the change in display colors and to make stable display by constituting the element so as to allow the selective transmission of light of a green wavelength and mixing a yellow dichromatic dyestuff with a liquid crystal. CONSTITUTION:Colors change from green to blue in a clockwise direction with an increase in driving voltage in an L*a*b* color specification system. Color is determined by three elements; lightness L*, hue H* and saturation C*; the hue H<0> is expressed by a central angle and the saturation C* by the length of segments. The value of the hue H<0> increases with an increase in the driving voltage and enters a 3rd quadrant from a 2nd quadrant in the case of making of green display. The yellow dichromatic dyestuff is, thereupon, previously mixed in the liquid crystal, by which the increase of the value of the hue H<0> to enter the 3rd quadrant is prevented. The stable display is made in this way.

Description

[Detailed Description of the Invention] [Summary] Regarding a liquid crystal display element that performs projection type color display, the aim is to stably display a green color by utilizing the optical wavelength selectivity of a nematic-cholesteric phase transition type liquid crystal. In a display element that displays a green image on a screen by irradiating a liquid crystal cell with light from a light source, a projection type color liquid crystal display element is constructed by mixing a yellow dichroic dye with the liquid crystal.

[Industrial application field]

The present invention relates to a projection type color liquid crystal display element that can stably display green color.

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]

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 Unexamined Patent Publication No. 63-49736. Published on March 21, 1986) The outline is as follows.

Figure 4 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 consisting of a mixture of 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 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.

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 defined as the difference between the applied voltage value (VO2°) at which the transmittance is 90% in the falling curve 2 and the applied voltage value (Vuz.) at which the transmittance is 20% in the rising curve 1. This intermediate voltage value is the driving voltage (
V, ).

This indicates that optically different bistable states can be achieved with the same driving voltage (vo), 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,,1X is approximated by the following equation under the condition of Bragg angle incidence.

7i' raax = sin" (k KΔnd/λ)
-(1) However, Δ7...Refractive index anisotropy d...Cell thickness λ...Light wavelength As can be seen from equation (1), 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 type color display method proposed by the inventors utilizes the diffraction and scattering of liquid crystals, and has been successful in displaying green color.

However, when color display is performed using this method, there is a problem in that the hue of the displayed color changes as the applied voltage increases.

In other words, the helical pitch of the liquid crystal forming the cholesteric phase increases as the applied voltage increases, and eventually the helix unravels to form the nematic phase, but in this process, the refractive index anisotropy Δa of the liquid crystal decreases. Since the value of is also decreased, there is a problem that the hue changes and becomes bluish.

[Means to solve the problem]

The above problem is solved by using the light wavelength selectivity of nematic cholesteric phase transition type liquid crystal to irradiate a liquid crystal cell with light from a white light source to display a green image on the screen. This problem can be solved by configuring a projection type color liquid crystal display element by mixing dichroic dyes.

[As mentioned above, in the color display using the phase change type liquid crystal according to the present invention, there is a problem that the hue changes depending on the driving voltage. To explain using the b* color system, the color changes counterclockwise (from green to blue) as the drive voltage increases.

Figure 3 shows the chromaticity diagram of the Lma*b* color system. Color is determined by three elements: lightness (L"), hue (H"), and saturation (C"). ”) is the central angle, and the saturation (C1) is expressed by the length of the line segment, but when displaying green, the value of the hue (Ho) increases as the drive voltage increases, and the color saturation (C1) is expressed by the length of the line segment. Enter the third quadrant.

Therefore, the present invention prevents the hue (Ho) value from increasing and entering the third quadrant by mixing a yellow dichroic dye into the liquid crystal.

〔Example〕

As a phase change type liquid crystal, 10 chiral nematic liquid crystals having two chiral centers are added to the nematic liquid crystals shown in Table 1.
It was formed by mixing the amount of %.

In Table 1, Nos. 1 to 4 are all ethane-based liquid crystals, with a mixing ratio of 20:20:10:10 by weight, and the remaining 40% is other-based liquid crystals.

Ab (A) is used as a yellow dichroic dye in phase change liquid crystals.
zo) derivative (product name 5I-209, Mitsui Toatsu Chemical ■) and perylene derivative (product name M-36)
1. Two types of liquid crystals were prepared by mixing 2% by weight of each of Mitsui Toatsu Chemical ■).

The pixel size of this liquid crystal is 350μm x 330μm.
μm, and was encapsulated in a 12-inch liquid crystal display device having 640×400 pixels.

In addition, for comparison, a display element was formed using a liquid crystal containing no dichroic dye.

Then, this was attached to a projection display device, and the transmission spectrum of the displayed color in each display element was measured with a spectrometer (product name MCPD-100, Otsuka Electronics Co., Ltd.) while changing the applied voltage at room temperature, and L" a " b” color system (JIS
Z 8721) (7) Saturation equivalent c”, hue equivalent HO
The lightness equivalent L* was determined.

Figure 1 shows an azo derivative (product name 5l-20) as a dichroic dye.
9), and Figure 2 shows the measurement results for a display element mixed with a perylene derivative (Product name M-361), compared with a display element mixed with no dichroic dye. It is.

From this figure, it can be seen that by mixing the yellow dichroic dye, the dependence of applied voltage on saturation and hue is clearly reduced.

〔Effect of the invention〕

As described above, in a phase change type liquid crystal display element configured to selectively transmit green wavelength light, changes in display color can be suppressed by mixing a yellow dichroic dye into the liquid crystal. and stable display can be done.

[Brief explanation of drawings]

Figure 1 shows the voltage dependence of the displayed color of an element mixed with a dichroic dye (Sr-209). Figure 2 shows the voltage dependence of the displayed color of an element mixed with a dichroic dye (M-361). Figure 3 is a chromaticity diagram of the L"a9 bl color system, and Figure 4 is a diagram of the display principle of phase change liquid crystal. Voltage dependence of voltage of 1,000 display colors fID@pressure M Applied voltage (V) Two colors I, Ikihe (rl-361) ε7 Voltage dependence with display of combined 粂÷ Figure 2 Front Direction Label Diagram Diagram

Claims (1)

[Claims] Utilizing the light wavelength selectivity of nematic-cholesteric phase transition type liquid crystal, the liquid crystal cell is irradiated with light from a white light source,
A projection type color liquid crystal display element for displaying a green image on a screen, characterized in that the liquid crystal is mixed with a yellow dichroic dye.