Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-08-27
2002-12-31
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S102000
Reexamination Certificate
active
06501523
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display element and a liquid crystal display device employing the same, e.g., a liquid crystal projector.
2. Description of the Related Art
The liquid crystal display modes for the reflection type liquid crystal light valve employing a nematic liquid crystal are roughly classified into the three kinds of systems. That is, there are a tilted homeotropic ECB (Electrically Controlled Birefringence) mode, a homogenious ECB mode and a reflection type twisted nematic mode.
By the tilted homeotropic ECB mode is meant the alignment system in which the liquid crystal molecules are arranged in such a way as to be slightly inclined with respect to the perpendicular direction of each of a pair of substrates by utilizing the oblique evaporation method or the like, and this mode is described in an article of Applied Physics Letters 20, 199 (1972) for example.
By the homogenious ECB mode is meant the alignment system in which the liquid crystal molecules are arranged in such a way as to be roughly parallel to each of a pair of substrates, and this mode is described in JP-A-1-7021.
Also, by the reflection type twisted nematic mode is meant the system in which the liquid crystal molecules are arranged in such a way as to be roughly parallel to each of a pair of substrates and also the alignment directions in the vicinity of the pair of substrates holding therebetween the liquid crystal molecules shows the twisted positional relationship. By the way, for the twisted nematic mode, the several systems have been further proposed. As an example, there are an HFE (Hybrid Field Effect) mode, a TN-ECB (Twisted Nematic-Electrically Controlled Birefringence) mode, an SCTN (Self-Compensated Twisted Nematic) mode, an MTN (Mixed mode Twisted Nematic) mode, and the like.
The TN-ECB mode is described in an article of Japan Display '89, p. 192 (1989), the SCTN mode is described in JP-A-10-090731, and the MTN mode is described in an article of Applied Physics Letters 68, p. 1455 (1996). In addition, the inclusive analysis of the overall reflection type twisted nematic mode is described in an article of Proceedings of SPIE 3635, p. 87 (1999) and an article of Proceedings of IDW '99, p. 985 (1999).
By the way, of these modes, the HFE mode is the so-called normally black type display mode which becomes the black (dark) display when the voltage applied thereto is 0 Vrms, while becomes the white (light) display when a suitable voltage is applied thereto. Each of other three modes is the so-called normally white display mode which becomes the white (light) display when the applied voltage is 0 Vrms, while becomes the black (dark) display when a suitable voltage is applied thereto.
The typical system as the optical system employing these reflection type liquid crystal light valves is the optical system employing the polarizing beam splitter and it is described in JP-A-61-13885. In addition, as for other example, the optical system in which the main optical axis is made oblique with respect to the vertical direction of the liquid crystal light valve is described in JP-A-4-319910.
By the way, as for the technique in which the reflection type liquid crystal light valve and the retardation plate are combined with each other, the technique in which the polarizing beam splitter, the ¼ wave plate and the reflection type liquid crystal light valve are combined with one another and these elements are arranged in such a way that the fast axis or the slow axis of the ¼ wave plate intersects perpendicularly the flat surface containing the incident light axis and the reflected light axis of the polarizing beam splitter is disclosed in JP-A-2-250026 and U.S. Pat. No. 5,327,270. In addition, the technique in which the retardation of the retardation plate combined with the polarizing beam splitter is made equal to or larger than 0.25 (i.e., ¼ wavelength) is disclosed in U.S. Pat. No. 5,576,854. Also, the description of the polarizing controller including the double refraction material layer is made in JP-A-1-7021.
SUMMARY OF THE INVENTION
The above-mentioned article of Proceedings of SPIE 3635 p. 87 (1999) reports that the conditions containing the MTN mode in the normally white reflection type twisted nematic mode is most excellent in the balance of the characteristics of the light utilization efficiency, the chromaticity characteristics, and the driving voltage (contrast ratio).
However, if the high speed response characteristics of the liquid crystal is regarded as important, the conditions containing the TN-ECB mode is more desirable. The importance of the high speed response characteristics of the liquid crystal molecules will hereinbelow be described by taking as an example the case where the liquid crystal display element is employed in the display device of the personal computer.
The image signals in the personal computer normally consist of the image signals corresponding to at least equal to or larger than 60 frames per second. That is, in the case where the image is displayed on the display device of the personal computer, in order that the display may surely follow the image signals, the liquid crystal needs to have the response time which is equal to or shorter than a time corresponding to one frame, i.e., equal to or shorter than {fraction (1/60)} seconds=16.6 milliseconds. If the response time of the liquid crystal is equal to or larger than a time corresponding to one frame, then the image which is different from that obtained on the basis of the image signals is displayed on the display device, and hence it is recognized on the residual image, which is expected not to essentially appear, at the back of the movement direction of the image. The phenomenon is particularly remarkable in the moving image, which reduces remarkably the quality of the image. Therefore, in order to realize the liquid crystal display device having the excellent image quality, it is necessary to select the display mode of the liquid crystal showing the high speed response.
Then, since it is in general known that the response time of the liquid crystal is in proportion to the square of the thickness of the liquid crystal, the high speed response can be expected as the thickness of the liquid crystal is thinner. In this respect, since the conditions containing the TN-ECB mode has the desired standardized retardation which is smaller than that in the conditions containing the MTN mode, even when the same liquid crystal material is employed, the thickness of the liquid crystal layer in the conditions containing the TN-ECB mode can be made narrower than that in the conditions containing the MTN mode. As a result, in the case of the conditions containing the TN-ECB mode, the shortening of the response time of the liquid crystal, i.e., the high speed operation of the liquid crystal response can be promoted as compared with the MTN mode.
Furthermore, the reflection type liquid crystal valve other than the transmission type liquid crystal light valves is employed, whereby the response time can be shortened. This will hereinbelow be described.
There is the retardation as the component contributing to the modulation of light in the light valve. The retardation is expressed by the product of the thickness d of the liquid crystal layer and the anisotropy &Dgr;n of refractive index of the liquid crystal.
While in the case of the transmission type liquid crystal light valve, the retardation is expressed by d&Dgr;n, in the case of the reflection type liquid crystal light valve, since the optical path is folded back by the reflection mirror, the incident light reciprocates through the liquid crystal layer. In other words, in the case of the reflection type liquid crystal light valve, the retardation contributing to the modulation of light becomes two times as large as that in the case of the transmission type liquid crystal light valve, i.e., becomes 2d&Dgr;n and hence though the liquid crystal layers have the same thic
Aoto Katsuhide
Hirota Shoichi
Takemoto Iwao
Tsumura Makoto
Dickstein , Shapiro, Morin & Oshinsky, LLP
Dudek James
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