JPS629303A - Polarizing plate - Google Patents

Abstract

PURPOSE:To increase the brightness of transmitted light and to prevent effectively a contrast ratio and visibility from decreasing by forming a layer of a birefringent medium on one surface of the polarizer of a polarizing film. CONSTITUTION:One surface of the polarizer 1 of the polarizing film is covered with the layer 2 of the birefringent medium to constitute a polarizing plate 3, whose thickness is so set that the value obtained by dividing the retardation value of the layer 2 by designed wavelength is large. The axis of absorption of the polarizer 1 is at an angle theta of 45 deg. to the horizontal direction, the axis of absorption of the polarizing plate 5 crosses it at an angle theta+90 deg., and the angle of the layer 2 is set to theta/2 which is 0.4-0.6 time the angle of the polarizer 1. Thus, the brightness of the transmitted light is increased to prevent effectively the contrast ratio and visibility from decreasing.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a novel polarizing plate that is suitable for automotive liquid crystal display devices and that improves visibility when viewed through a polarizing medium such as polarized sunglasses. It is something.

Prior Art and Problems New problems have arisen as liquid crystal display devices have become widespread in various fields. For example, in a vehicle-mounted liquid crystal display device, a problem arises in that the visibility of the display contents of the liquid crystal display device is reduced when the driver wears polarized sunglasses. In this way, the problem of decreased visibility due to a decrease in the contrast of displayed characters when viewing a conventional liquid crystal display device using a polarizing plate that has only a polarizing function through a polarizing medium can be solved by using polarized sunglasses, etc. Since the purpose is to improve the visibility of objects by blocking reflected light, etc., and especially when used in vehicles, it is important to overcome this problem as it can affect the lives of drivers etc. desired.

Means for Solving the Problems The present invention makes it possible to overcome the above problems by using a polarizing plate having a polarizing function and a birefringence function.

That is, the present invention provides a polarizing plate having a layer made of a birefringent medium on one side of a polarizer.

The polarizer used in the present invention may be any material as long as it can be molded into a plate or sheet shape and has a polarizing function. A typical example thereof is a polarizing film. Examples of polarizing films include hydrophilic polymer films such as polyvinyl alcohol films, partially formalized polyvinyl alcohol films, and saponified ethylene-vinyl acetate copolymer films in which iodine and/or dichroic dyes are added. Examples include those obtained by adsorption and orientation, those obtained by dehydrating a polyvinyl alcohol film to orient the polyene, and those obtained by dehydrochlorinating a polyvinyl chloride film and orienting the polyene. The appropriate thickness of the polarizer is usually 10 to 80 μm.

Note that in the present invention, the polarizer may have a surface protective layer.

The polarizing plate of the present invention has a layer 2 made of a birefringent medium on one side of a polarizer 1, as shown in the example shown in FIG. The layer 2 made of this birefringent medium may be made of a modified polarizer, or may be made of a birefringent plastic film or the like. That is, in the present invention, there is no limitation on the method of forming the layer made of the birefringent medium. A formation method in which a birefringent plastic film is used and bonded to one side of a polarizer is desirable in terms of production efficiency and controllability of the optical axis direction of the layer. Examples of plastic films with birefringence include films made of plastics such as polyvinyl alcohol, cellulose, polyvinyl chloride, polypropylene, polystyrene, acrylic copolymer, polycarbonate, polyester, and polyamide. Examples include stretched products. In addition, adhesives with good transparency such as polyvinyl adhesives, polyisocyanate adhesives, and methyl methacrylate adhesives can be used to bond birefringent plastic films to polarizers as needed. are preferably used.

The thickness d) of the layer made of birefringent medium is the value obtained by dividing the retardation value (4nd: product of refractive index difference (Δn) in birefringence (Δn) and layer thickness) by the design wavelength (λ) (And/λ> If the value is small, the occurrence of interference color becomes noticeable, so it is preferable to set the value to be large, preferably 15 or more, and also to set the value to be an integer value. The upper limit of the thickness of the layer is generally about several mm in practice, but there is no particular limitation.

On the other hand, in the present invention, the layer 2 made of a birefringent medium has an angle between its optical axis direction and the lower side direction of the polarizing plate that is 0 of the angle (θ) between the absorption axis direction of the polarizer 1 and the lower side direction of the polarizing plate. ．．
4 to 0.6 times, that is, 1/2 times the angle (θ) or a value close to this (see Figure 3), the intensity of the light when viewed through a polarizing medium, and therefore the visual recognition. Desirable in terms of sex.

Incidentally, the respective absorption axes of the polarizing medium and the polarizer in the polarizing plate intersect at an angle θ, and a layer made of a birefringent medium is provided between the polarizing medium and the polarizer, with the front axis of the layer intersecting the absorption axis of each of the above. When intervening in a state that occupies the middle (θ/2) of the axis, the intensity of light (1) for visibility is expressed by the following formula (a) (where A.

is the amplitude of light.

1=Ao (cos"θ+sin"θ・cos' (,
and ·K/λ) - (a) On the other hand, the intensity of light when there is no intervening layer made of a birefringent medium is expressed by the following formula (b).

1 = A: cosLθ (b) Since θ is generally not O, the intensity of light for visibility increases by interposing a layer made of a birefringent medium.

The polarizing plate of the present invention is preferably applied as a viewing-side polarizing plate in a device that can be viewed through a polarizing medium such as polarized sunglasses, such as a vehicle-mounted liquid crystal display device or a liquid crystal display wristwatch.

FIG. 2 shows an example of its application. That is, the polarizing plate 3 of the present invention is bonded to the viewing side of the liquid crystal cell 4, and the conventional polarizing plate 5 having only a polarization function is bonded to the other side. As shown in FIG. 3, in general, in a liquid crystal display device, upper and lower polarizing plates 3 and 5, which are arranged opposite to each other with a liquid crystal cell 4 interposed therebetween, are usually arranged so that their absorption axes (arrows) are perpendicular to each other. , are arranged opposite to each other so that the angle (θ) formed by the lower side direction of the device, that is, the lower side direction (generally horizontal direction) of the polarizing plate and the absorption axis direction are orthogonal to each other at 45 degrees and 135 degrees. In addition, in this facing arrangement, the polarizing plate of the present invention on the viewing side is arranged so that the layer 2 made of the birefringent medium is on the outside (viewing side). Note that 6 in the figure is a polarizing medium such as polarized sunglasses, and the arrow thereof indicates the absorption axis direction.

Effects of the Invention According to the present invention, since the polarizing plate is composed of a polarizer and a layer of a birefringent medium, the brightness of transmitted light when viewed through the polarizing medium is lower than that of t) consisting of only a polarizer. It is possible to increase the Therefore, it is possible to effectively prevent a decrease in contrast ratio when viewing the display contents of a liquid crystal display device or the like through polarized sunglasses or the like, and thus to prevent a decrease in visibility, which has great practical significance.

Application example A polarizing plate with a light transmittance of 39% made by laminating a stretched polyester film with a thickness of 85 um and a refractive index difference of 0.11 in birefringence and a polyvinyl alcohol/iodine polarizing film with a thickness of 50 um, and a polarizing plate with a light transmittance of 39% and a Light transmittance 41 obtained by laminating a cellulose film and the polarizing film
% polarizing plates were bonded to the front and back surfaces of a liquid crystal display cell so that the absorption axes of the polarizing films were perpendicular to each other, thereby obtaining a liquid crystal display device. In addition, the polarizing plate having a birefringent stretched polyester film is placed on the viewing side so that the stretched polyester film is on the outside, and
The polarizing film was attached so that its absorption axis was at an angle (θ) of 45 degrees with respect to the lower side of the device. In addition, the optical axis of the stretched polyester film is 22.5 mm with respect to the lower side of the device.
The polarizing film is attached to the polarizing film at an angle (θ/2) of degrees.

Comparative Example A liquid crystal display device was obtained in the same manner as in the application example, except that a polarizing plate having a cellulose film and equivalent to the facing side was used as the viewing side instead of a polarizing plate having a stretched polyester film.

For the devices of the evaluation test application example and comparative example, the brightness was measured when viewing the device as it was or in the off state and on state of the drive voltage of the device through polarized sunglasses, and the contrast ratio was determined. The results are shown in the table.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of an embodiment of the present invention, FIG. 2 is a sectional view of an application example thereof, and FIG. 3 is an exploded explanatory view thereof.

Claims (1)

[Claims] 1. A polarizing plate having a layer made of a birefringent medium on one side of the polarizer. 2. The polarizing plate according to claim 1, wherein the polarizer is a polarizing film, and the layer made of a birefringent medium is a plastic film having birefringence. 3. The angle between the optical axis direction of the layer made of birefringent medium and the lower side direction of the polarizing plate is 0.4 to 0.6 times the angle between the absorption axis direction of the polarizer and the lower side direction of the polarizing plate. A polarizing plate according to claim 1.