Optical: systems and elements – Extended spacing structure for optical elements – Extension of tubular element adjustable
Reexamination Certificate
2002-07-16
2004-09-21
Chang, Andrey (Department: 2872)
Optical: systems and elements
Extended spacing structure for optical elements
Extension of tubular element adjustable
C359S483010, C359S494010, C359S490020, C349S118000
Reexamination Certificate
active
06795246
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical film in which axial displacement of a laminate hardly occurs even when a viewing point is changed, and which is adapted for forming a liquid-crystal display device good in display quality, a circularly or elliptically polarizing plate or an anti-reflection plate.
2. Description of the Related Art
If a plate, such as a phase retarder disposed between a polarizer and a liquid-crystal cell for improving display quality of a liquid-crystal display device, or a quarter-wave plate used for forming a circularly polarizing plate or an anti-reflection plate, is formed from one birefringent film, birefringence is dispersed in accordance with wavelengths on the basis of dispersion peculiar to the material of the birefringent film. As a result, variation in retardation occurs in accordance with wavelengths so that birefringence generally becomes higher as the wavelength becomes shorter. As a result, the state of polarization does not change evenly. Under such circumstances, there has been heretofore proposed an optical film having two birefringent films which are different in dispersion characteristic of birefringence in accordance with wavelengths, and which are laminated on each other so that slow axes of the two birefringent films intersect each other perpendicularly (Unexamined Japanese Patent Publication No. Hei. 5-27118 and Unexamined Japanese Patent Publication No. Hei. 10-239518).
The proposed optical film is provided so that dispersion characteristic of birefringence in accordance with wavelengths is controlled on the basis of lamination of birefringent films so that birefringence becomes lower as the wavelength becomes shorter. As a result, a uniform compensation effect can be obtained so that a uniform change in state of polarization can be achieved in a wide wavelength range. The orthogonal relation is retained on an optical axis so that an expected effect can be fulfilled. The orthogonal relation is, however, corrupted when observation is made in an oblique direction at an azimuth displaced from the optical axis due to the change of apparent axial angles, so that the expected effect cannot be fulfilled. There is therefore a problem that the state of polarization changes. Even in the case where Nz values of the birefringent films are controlled to compensate for axial displacement relative to the polarizer as described in Unexamined Japanese Patent Publication No. Hei. 5-27118, this control is not effective in compensating for axial displacement of the birefringent film laminate itself.
SUMMARY OF THE INVENTION
An object of the invention is to develop an optical film in which the crossing relation between optical axes (slow axes) is kept good even when a viewing point is changed, and which can be used for forming a liquid-crystal display device good in display quality, a quarter-wave plate or the like.
According to the invention, there is provided an optical film including: a birefringent film A having Re of from 200 to 350 nm and Nz of from 0.6 to 0.9; and a birefringent film B having Re of from 100 to 175 nm and Nz of from 0.3 to 0.7, and laminated on the birefringent film A so that optical axes of the birefringent films A and B intersect each other, in which Re and Nz are given by the relations (nx−ny)d=Re and (nx−nz)/(nx−ny)=Nz, nz is a refractive index of corresponding one of the birefringent films A and B in a direction of a Z axis indicating a direction of the thickness of the birefringent film, nx is a refractive index of the birefringent film in a direction of an X axis indicating a direction of the highest refractive index in a plane perpendicular to the Z axis, ny is a refractive index of the birefringent film in a direction of a Y axis perpendicular both to the X axis and to the Z axis, and d is the thickness of the birefringent film.
Further, according to the invention, there is provided a polarizer including: an optical film defined above; and a film having a polarizing function and laminated on the birefringent film A side of the optical film. There is also provided a liquid-crystal display device including: a liquid-crystal cell; and a polarizer defined above, and disposed on at least one of opposite sides of the liquid-crystal cell so that the film included in the polarizer and having the polarizing function is located on the outer side. Alternatively, there is also provided a display device including: a polarizer defined above, and disposed at an anterior surface of the display device so that the film included in the polarizer and having the polarizing function is located on the outer side.
According to the invention, an optical film in which axial angles hardly change at respective azimuths on optical axes and in accordance with the change of a viewing point and in which the retardation caused by birefringence hardly changes in a wide viewing angle range can be obtained on the basis of the combination of the birefringent films A and B and the relation in arrangement between the birefringent films A and B. As a result, the optical film can be used for obtaining a circularly polarizing plate or the like having polarizing characteristic hardly changing in accordance with the viewing angle or the wavelength, or an anti-reflection plate little in viewing angle dependence. Further, the circularly or elliptically polarizing plate having polarizing characteristic hardly changing in a wide azimuth range and in a wide wavelength range can be used for forming a liquid-crystal display device exhibiting good display quality at a wide viewing angle. Further, the anti-reflection plate having circularly polarizing characteristic or the like hardly changing in a wide azimuth range and in a wide wavelength range can be used for obtaining various kinds of display devices good in display quality.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in
FIG. 1
, an optical film
1
according to the invention includes: a birefringent film A (first birefringent film) having Re of from 200 to 350 nm and Nz of from 0.6 to 0.9; and a birefringent film B (second birefringent film) having Re of from 100 to 175 nm and Nz of from 0.3 to 0.7, and laminated on the birefringent film A so that optical axes of the birefringent films A and B intersect each other, in which Re and Nz are given by the relations (nx−ny)d=Re and (nx−nz)/(nx−ny)=Nz, nz is a refractive index of corresponding one of the birefringent films A and B in a direction of a Z axis indicating a direction of the thickness of the birefringent film, nx is a refractive index of the birefringent film in a direction of an X axis indicating a direction of the highest refractive index in a plane perpendicular to the Z axis, ny is a refractive index of the birefringent film in a direction of a Y axis perpendicular both to the X axis and to the Z axis, and d is the thickness of the birefringent film.
The optical film can be formed from the birefringent films A and B which are laminated on each other so that optical axes of the birefingent films A and B intersect each other. In this case, a film having Re of from 200 to 350 nm and Nz of from 0.6 to 0.9, preferably Nz of from 0.7 to 0.8 is used as the birefringent film A. A film having Re of from 100 to 175 nm and Nz of from 0.3 to 0.7, preferably Nz of from 0.4 to 0.6 is used as the birefringent film B.
Incidentally, in the description, Re and Nz are defined by Re=(nx−ny)d and Nz=(nx−nz)/(nx−ny) in which nz is a refractive index of corresponding one of the birefringent films A and B in a direction of a Z axis indicating a direction of the thickness of the birefringent film, nx is a refractive index of the birefringent film in a direction of an X axis indicating a direction of the highest refractive index in a plane
Umemoto Seiji
Yano Shuuji
Chang Andrey
Curtis Craig
Nitto Denko Corporation
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