Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-01-20
2002-04-16
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S113000
Reexamination Certificate
active
06373538
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a reflection type liquid crystal display device, and to a method for driving a reflection type color liquid crystal device.
BACKGROUND TECHNOLOGY
A display equipped in a portable information terminal should be of the low power consumption type. Thus, a reflection type liquid crystal device, which does not need back light, is most suited for such an application. However, the dominating conventional reflection type liquid crystal device is of the monochromatic display type. Therefore, good reflection type color display has not been obtained yet.
Full-scale development of the reflection type color liquid crystal device was started about the middle of 1980's. There had been only a superficial understanding before then. For example, the reflection type color display could be obtained by replacing the backlight unit of the transmission type color liquid crystal device.
However, when the liquid crystal device is actually made of such a configuration, it has been found that only very dark display could be achieved and that the device of such a configuration was not practical. There have been three causes. First, the polarizing plate has discarded more than half of the quantity of light. Second, the color filter has discarded more than ⅔ of the quantity of light. Finally, there has been the problem of a parallax. Parallax has been the inevitable problem in the case of TN (twisted nematic) mode and STN (supertwisted nematic) mode. This is because of the fact that two polarizing plates are necessarily used in these modes and thus there is a distance created between the reflector plate and the liquid crystal layer, which cannot be disregarded, unless the polarizing plates are not built in a cell. Incidentally, the problem of the parallax referred to herein is not the problem of a double-image in the display which has occurred even in the conventional reflection type monochromatic liquid crystal device. Namely, the parallax is a problem that occurs in and is peculiar to the reflection type color liquid crystal device.
Problems of parallax will be described hereunder by referring to the accompanying drawings. FIGS.
7
(
a
) and
7
(
b
) are sectional views of a reflection type color liquid crystal device utilizing TN mode or STN mode. This liquid crystal device consists of an upper polarizing plate
1
. an upper glass substrate
2
, a liquid crystal layer
3
, a lower glass substrate
4
, a lower polarizing plate
5
, an optical reflector plate
6
and red-green-blue (RGB) three-color filter
7
. Additionally, there are transparent electrodes, an orientation film and insulating film between the upper and lower glass substrates. These composing elements are, however, unnecessary for describing the problem of a parallax. Thus, the drawing thereof is omitted.
Meanwhile, there are two problems with the parallax. First, one of the two problems is color cancellation. As shown in FIG.
7
(
a
), an observer
32
watches reflection light
31
having passed through a green filter. This reflection light has been mixed with incident light
30
which has been incident thereon through the red-green-blue color filter and has been diffused and reflected by the reflector plate. If the thickness of the lower glass substrate
4
is sufficiently large in comparison with the pitch of filter elements of the color filter
7
, a light ray propagated through any color filter element is mixed with the reflection light
31
with same probability.
However, the light of any wavelength, which has passed through the paths “red to green” and “blue to green”, is inevitably absorbed by one of the filter elements. Thus, only the light having passed through the path “green to green” remains. This is the same with reflection light rays having transmitted from the blue and red filter elements. Consequently, the brightness of displayed white is decreased to ⅓ that of the case in which there is no parallax.
A second problem is that the displayed colors become dark. FIG.
7
(
b
) illustrates a green displaying state. Further, the cross hatched portion of the liquid crystal layer
3
indicates that such a portion is in a non-illuminated state (namely, a dark state). Incident light
30
passes through each of red, green and blue dots with the same probability. However, ⅔ of the quantity of the incident light
30
are absorbed by the red and blue dots which are in an off-state. Furthermore, after diffused by the reflector plate
6
and mixed with the reflection light, ⅔ of the quantity of such light are absorbed by the red and blue dots which are put into the off-state again. Then, the light reaches the observer
32
. Therefore, the brightness of the display of green is obtained by subtracting the quantity of light, which is absorbed by the green filter element, from {fraction (1/9)} of the brightness of the displayed white (namely, {fraction (1/9)} of the brightness of the display of white—the quantity absorbed by the green filter element), and thus becomes very dark.
As is understood from the foregoing description, it is very difficult to apply TN mode and STN mode, in each of which the problem of the parallax occurs, to the reflection type color liquid crystal device.
Thus, hitherto, attempts have been made to obtain bright reflection type color displays by changing the liquid crystal mode.
For example, according to the article by Mr. Tatsuo Uchida et al. (IEEE Transactions on Electron Devices, Vol ED-33, No. 8, pp. 1207-1211 (1986)), as illustrated in
FIG. 2
therein, the comparisons of the brightness among various liquid crystal modes are made. As a consequence, PCGH (Phase-Change type Guest Host) mode, which does not need polarizing plates, is employed. Furthermore, in the case of the Japanese Unexamined Patent Publication No. 5-241143 Official Gazette, PDLC (Polymer-Dispersed type liquid crystal) mode, which does not require the polarizing plates, is employed so as to realize a reflection type color liquid crystal device.
In the case of using the liquid crystal mode which does not need polarizing plates, there are obtained the merits in that the absorption of light by the polarizing plate is eliminated and that the problem of the parallax can be settled completely by providing a reflector plate in such a manner so as to be adjacent to the liquid crystal layer.
However, on the other hand, in the case of the liquid crystal modes requiring no polarizing plates, the contrast is usually low. Further, especially, PCGH mode has encountered the problem in that halftone display cannot be performed owing to the presence of a hysteresis in the voltage-transmittance characteristic. Furthermore, these liquid crystal modes, by which foreign substance is added to the liquid crystal, have encountered the problem in degradation in the reliability.
Therefore, TN mode and STN mode, which have been widely used heretofore and achieved satisfactory results, are the best modes to be employed, if can be used even in the aforementioned conditions.
Additionally, hitherto, attempts have been made to obtain a bright reflection type color display by using the bright color filter. An example of this is disclosed in the Japanese Unexamined Patent Publication NO. 5-241143 Official Gazette. In this case, a reflection type color liquid crystal device is configured by using a color filter consisting of filter elements respectively corresponding to the subtractive primaries, namely, yellow, cyan and magenta.
This method has profound effects in obtaining bright displays but has faced the following problems.
Namely, ordinary color liquid crystal devices perform the additive mixing process by using a set of small color points, and thus use the color filter consisting of filter elements respectively corresponding to the additive primaries, namely, red, green and blue. However, according to the aforementioned Official Gazette, the additive mixing process is performed by using the color filter corresponding to the subtractive primaries. Thus, the degree of the color saturation o
Maeda Tsuyoshi
Matsushima Toshiharu
Okumura Osamu
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