Liquid crystal display including filter means with 10-70%...

Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only

Reexamination Certificate

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C349S106000, C349S074000

Reexamination Certificate

active

06597419

ABSTRACT:

This application is based on Japanese Patent Application Nos. 11-189334 and 11-220557 respectively filed on Jul. 2, 1999 and Aug. 3, 1999 in Japan, the entire content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display in which a plurality of liquid crystal light control layers is stacked. In particular, the present invention relates to a reflective type liquid crystal color display having a plurality of liquid crystal light control layers, each of which includes a liquid crystal light control material exhibiting a cholesteric phase.
2. Description of the Related Art
In recent years, research and development of thin, low-power-consumption and bright reflective type color displays has been performed. Of such displays, a reflective type color display using a selective reflection characteristic of a cholesteric liquid crystal material receives attention. In the reflective type color display, the following layers are stacked: a liquid crystal light control layer having the peak of the selective reflection wavelength in the wavelength region of red liquid crystal light control layer); a liquid crystal light control layer having the peak of the selective reflection wavelength in the wavelength region of green (G liquid crystal light control layer); and a liquid crystal light control layer having the peak of the selective reflection wavelength in the wavelength region of blue (B liquid crystal light control layer). The reflective type color display achieves high brightness because it does not use a polarizing plate. In addition, since the cholesteric liquid crystal material exhibits a memory effect, the reflective type color display is low in power consumption, can be simple-matrix-driven, and can be manufactured inexpensively. For these reasons, the reflective type color display receives attention.
However, since the selective reflection wavelength of cholesteric liquid crystal has angle dependence in principle, the color tone varies according to the viewing position (viewing angle). With respect to this problem, Japanese Laid-open Patent Publication No. 10-31210A proposes a reflective type color display in which a B liquid crystal light control layer, a G liquid crystal light control layer and an R liquid crystal light control layer are stacked in this order from the extraneous light incident side. In this reflective type color display, a filter layer absorbing blue light and transmitting green light and red light is provided between the B liquid crystal light control layer and the G liquid crystal light control layer, and a filter layer absorbing blue light and green light and transmitting red light is provided between the G liquid crystal light control layer and the R liquid crystal light control layer.
However, the transmittance of the filter layers shown in that publication is substantially zero in the wavelength regions absorbed by the filter layers (see
FIG. 4
of the publication). In a display provided with such filter layers, the transmittance is as low as approximately 80% even in the wavelength regions to be transmitted by the filters, so that the display screen is extremely dark.
Moreover, the wavelength regions absorbed by the filter layers are extremely close to the selective reflection wavelengths of the liquid crystal light control layers. Therefore, only by changing the viewing angle several degrees from the front, the selective reflection wavelengths of the liquid crystal light control layers shift into the regions absorbed by the filter layers, so that the display on the screen disappears. As described above, a problem of reduction in viewing angle characteristic arises in the color display of the prior art taught by the publication.
Apart from the above-mentioned problem, a multilayer liquid crystal display in which a plurality of liquid crystal light control layers using a liquid crystal material exhibiting a cholesteric phase is stacked is subject to various limitations in order to provide display with excellent color balance. In particular, the reflection characteristic of each of the stacked liquid crystal light control layers and the transmission characteristics of members such as the substrates sandwiching the liquid crystal light control layers and the electrodes formed on the substrates are extremely important. As an example, an RGB multilayer liquid crystal display in which an R liquid crystal light control layer, a G liquid crystal light control layer and a B liquid crystal light control layer are stacked in this order from the viewing side will be examined.
In the multilayer liquid crystal display, light reflected from the B liquid crystal light control layer disposed farthest from the viewing side cannot be viewed unless it passes through the two R and G liquid crystal light control layers disposed on the viewing side of the B liquid crystal light control layer and the members such as the substrates sandwiching the liquid crystal light control layers and the electrodes formed on the substrates. Therefore, the use efficiency of the light incident on the B liquid crystal light control layer and the light reflected at the B liquid crystal light control layer is low. Consequently, when the liquid crystal light control layers have the same reflectance, red is comparatively intense and blue is comparatively weak in the display, so that the color balance is inferior. When indium tin oxide (ITO) electrodes, which have the property of absorbing blue light, are used as the electrodes, the display characteristic of blue further deteriorates.
Moreover, as mentioned above, the selective reflection of cholesteric phase liquid crystal has viewing angle dependence. Therefore, when light is obliquely incident on the display screen or when the display screen is viewed from a slanting direction, the selective reflection wavelength shifts toward the shorter wavelength side, so that the hue of the displayed color changes. The amount of the shift increases as the selective reflection wavelength increases. In the above-described RGB multilayer liquid crystal display, in a case where the half widths of the liquid crystal light control layers are substantially the same, when the display screen is viewed from a slanting direction, the selective reflection wavelength of the R liquid crystal light control layer shifts comparatively substantially, so that the hue of the displayed red significantly changes. As a result, it is difficult to provide excellent display.
SUMMARY OF THE INVENTION
Accordingly, a primary object of the present invention is to provide an improved multilayer liquid crystal color display.
Another object of the present invention is to provide a liquid crystal display in which reduction in contrast is suppressed so that the display screen does not become dark.
Yet another object of the present invention is to provide a liquid crystal display in which reduction in contrast is suppressed so that the viewing angle characteristic does not deteriorate
Still another object of the present invention is to provide a liquid crystal display in which deterioration in viewing angle characteristic and reduction in the brightness of the display screen are suppressed.
A still further object of the present invention is to provide a multilayer liquid crystal display in which a plurality of liquid crystal light control layers is stacked, wherein the light use efficiency is high, the change in hue caused when the viewing direction is changed is small and the color balance of the displayed color is excellent.
To achieve at least one of the above-mentioned objects, a liquid crystal display according to a first aspect of the present invention comprises: a plurality of liquid crystal light control layers each of which includes liquid crystal exhibiting a cholesteric phase and having a peak wavelength of a selective reflection wavelength range of a reflection spectrum in the visible wavelength range, the liquid crystal light control layers being stacked in such order that the peak

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