Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Patent
1991-04-29
1992-03-03
Miller, Stanley D.
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
359102, G02F 1133
Patent
active
050937391
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a liquid crystal display device and a retardation film, in which coloring in a birefringent liquid crystal display device is decolored by compensation using a retardation film.
PRIOR ART
As an improved twisted liquid crystal display device in which a nematic liquid crystal panel having a twisting angle of 90.degree. is arranged between a pair of polarizing plates, a device which realizes a sharp change in light transmittance with respect to a drive voltage by increasing the twisting angle to be about 270.degree. is known as an STN (Super example, U.S. Pat. No. 4,634,229).
In this STN liquid crystal display device, the polarizing direction of polarizing plates is arranged to cross the orientation direction of liquid crystal molecules, and high contrast is obtained by a birefringent effect derived from this arrangement.
Because the birefringent effect has wavelength dependency, when white light is transmitted through a birefringent liquid crystal display device, the transmitted light is undesirably colored.
In order to obtain full color display by using color filters of primaries R, G, and B in combination with this liquid crystal display device, therefore, coloring caused by the wavelength dependency of the birefringent effect must be decolored.
As a method of decoloring coloring in the STN liquid crystal display device, a method in which an optical anisotropic layer is arranged together with an STN liquid crystal panel between a pair of polarizing plates so that the effect of the former is compensated for by the birefringent effect of the latter to obtain a colorless state is proposed (see, for example, U.S. Pat. No. 4,844,569).
In this case, although a second twisted liquid crystal panel or a retardation film is used as the optical anisotropic layer, the retardation film consisting of a stretched polymeric film is more advantageous in terms of a manufacturing cost.
When a polymeric film is uniaxially or biaxially stretched, polymer molecules are mainly oriented in a direction along which a stretching ratio is highest, i.e., in a main stretching direction to obtain a retardation film having optical anisotropy. In this case, assuming that a refractive index in the stretching direction is n.sub.e and that in a direction perpendicular to the stretching direction in a film plane, i.e., in a widthwise direction is n.sub.o, a value of n.sub.e -n.sub.o =.DELTA.n is called a refractive index anisotropy or orientation birefringence. This value is determined by an intrinsic birefringent value, i.e., a specific optical anisotropy of a monomer which is a constituting unit of the polymeric film and a degree of orientation of high molecules. n.sub.e >n.sub.o is obtained in an orientation film formed by stretching a polymeric material in which a refractive index in a molecular axis direction is larger than that in a direction perpendicular to the molecular axis direction, i.e., a polymeric material having a positive intrinsic birefringence value.
n.sub.e <n.sub.o, on the other hand, is obtained in an orientation film formed by stretching a polymeric material having a negative intrinsic birefringence value.
A product .DELTA.n.multidot.d of the refractive index anisotropy .DELTA.n and a film thickness d is called a retardation. In addition, the main stretching direction is sometimes called an optical axis for convenience.
By optimally- balancing the retardation .DELTA.n.multidot.d of a retardation film with the product .DELTA.n.multidot.d of the refractive index anisotropy .DELTA.n of a liquid crystal and the thickness d of a liquid crystal layer and arranging the optical axis direction of the retardation film through an optimal angle with respect to the orientation direction of liquid crystal molecules, coloring caused by the birefringent effect of a liquid crystal panel can be compensated for to decolor a liquid crystal display device.
The above description is based on observation obtained when the liquid crystal display device is viewed from the front. When the
REFERENCES:
patent: 4844569 (1989-07-01), Wada et al.
patent: 4889412 (1989-12-01), Clerc et al.
patent: 4957349 (1990-09-01), Clerc et al.
Aida Kazuhiko
Arai Motonao
Koike Satofumi
Yoshimura Osamu
Citizen Watch Co.
Gross Anita Pellman
Miller Stanley D.
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