Polarizing plate, method for manufacturing polarizing plate, image display device, method for manufacturing image display device, and method for improving transmittance of polarizing plate

Abstract

The present invention provides a polarizer excellent in transmittance even if it includes a light-transmitting substrate having no in-plane phase difference. The present invention relates to a polarizer configured to be disposed on a backlight source side in an image display device, the polarizer including at least a light-transmitting substrate having in-plane birefringence and a polarizing element layered in said order from the backlight source side, the light-transmitting substrate receiving incidence of polarized light, the light-transmitting substrate and the polarizing element being layered such that a fast axis of the light-transmitting substrate along a direction in which the substrate shows a smaller refractive index and a transmission axis of the polarizing element form an angle of 030 or 9030.

Claims

1. An image display device comprising: an image display element; a backlight source emitting light to the image display element; and a polarizer disposed between the backlight source and the image display element and comprising at least a light-transmitting polyester substrate having in-plane birefringence and a polarizing element layered in said order from the backlight source side, the light-transmitting substrate having in-plane birefringence receiving incidence of polarized light, the light-transmitting substrate having in-plane birefringence and the polarizing element being layered such that a fast axis of the light-transmitting substrate having in-plane birefringence along a direction in which the substrate shows a smaller refractive index and a transmission axis of the polarizing element form an angle of 9030, the light-transmitting substrate having in-plane birefringence exhibiting a difference (nxny) between the refractive indexes of 0.05 or greater, where nx represents a refractive index in the direction of the slow axis in which the substrate shows a greater refractive index, and ny represents a refractive index in the direction of the fast axis that is orthogonal to the slow axis, and the thickness of the substrate being 10 to 300 m.

2. The image display device according to claim 1, wherein the light-transmitting substrate having in-plane birefringence and the polarizing element are layered such that the fast axis of the light-transmitting substrate having in-plane birefringence along a direction in which the substrate shows a smaller refractive index and the transmission axis of the polarizing element form an angle of 9015.

3. The image display device according to claim 1, wherein the light-transmitting substrate having in-plane birefringence satisfies the relation of nx>N>ny, where N represents an average refractive index of the light-transmitting substrate.

4. The image display device according to claim 1, wherein a polarization separation film is provided between the backlight source and the light-transmitting substrate having in-plane birefringence.

5. The image display device of claim 1, wherein the image display element comprises a liquid crystal cell.

6. The image display device of claim 1, further comprising a second polarizer on a viewer side of the image display element.

7. The image display device according to claim 6, wherein the second polarizer comprises an upper light-transmitting substrate having in-plane birefringence and an upper polarizing element, the upper light-transmitting substrate being provided on the upper polarizing element, wherein the upper light-transmitting substrate having in-plane birefringence and the upper polarizing element are arranged such that a fast axis of the upper light-transmitting substrate having in-plane birefringence along a direction in which the upper light-transmitting substrate shows a smaller refractive index and a transmission axis of the upper polarizing element do not form an angle of 90.

8. A method for producing an image display device that comprises a polarizer disposed between a backlight source and an image display element in an image display device, the polarizer including a light-transmitting polyester substrate having in-plane birefringence and a polarizing element layered in said order, the method comprising the step of layering the light-transmitting substrate having in-plane birefringence and the polarizing element such that a fast axis of the light-transmitting substrate having in-plane birefringence along a direction in which the substrate shows a smaller refractive index and a transmission axis of the polarizing element form an angle of 9030, the light-transmitting substrate having in-plane birefringence exhibiting a difference (nxny) between the refractive indexes of 0.05 or greater, where nx represents a refractive index in the direction of the slow axis in which the substrate shows a greater refractive index, and ny represents a refractive index in the direction of the fast axis that is orthogonal to the slow axis, and the thickness of the substrate being 10 to 300 m.

9. A method for improving the transmittance of a polarizer disposed between a backlight source and an image display element in an image display device, the polarizer comprising at least a light-transmitting polyester substrate having in-plane birefringence and a polarizing element layered in said order, the method comprising the step of layering the light-transmitting substrate having in-plane birefringence and the polarizing element such that a fast axis of the light-transmitting substrate having in-plane birefringence along a direction in which the substrate shows a smaller refractive index and a transmission axis of the polarizing element form an angle of 9030, the light-transmitting substrate having in-plane birefringence exhibiting a difference (nxny) between the refractive indexes of 0.05 or greater, where nx represents a refractive index in the direction of the slow axis in which the substrate shows a greater refractive index, and ny represents a refractive index in the direction of the fast axis that is orthogonal to the slow axis, and the thickness of the substrate being 10 to 300 m.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a graph showing a wavelength dispersion of the average refractive index of a cycloolefin polymer film having no in-plane birefringence.

(2) FIG. 2 is a graph showing a wavelength dispersion of the three-dimensional refractive index of a cycloolefin polymer film having in-plane birefringence.

(3) FIG. 3 is a graph showing five-degree reflectances of nx and ny measured with a spectrophotometer.

(4) FIG. 4 is a schematic view illustrating a layer structure of a polarizer according to each example and the like.

(5) FIG. 5 is a spectrum of a light source used in examples and the like.

(6) FIG. 6 is a graph showing a wavelength dispersion of the refractive index of a protective film used in examples and the like.

(7) FIG. 7 is a graph showing a refractive index and an extinction coefficient of a polarizing element used in examples and the like.

(8) FIG. 8 is a schematic view illustrating an image display device.

DESCRIPTION OF EMBODIMENTS

(9) The present invention is more specifically described with reference to, but not limited to, examples and comparative examples.

Production of Light-Transmitting Substrate

Production of Light-Transmitting Substrate A Having No In-Plane Birefringence

(10) Cellulose acetate propionate (CAP504-0.2, Eastman Chemical Company) was dissolved in methylene chloride as a solvent to a solid content concentration of 15%, casted on glass, and dried to give a light-transmitting substrate A. It had a n of 0.00002 at a wavelength of 550 nm and an average refractive index N of 1.4838.

Production of Light-Transmitting Substrate A Having In-Plane Birefringence

(11) The light-transmitting substrate A was uniaxially stretched from a free end by 1.5 times at 160 C. to give a light-transmitting substrate a having in-plane birefringence. Based on the calculation of the wavelength dispersion of the three-dimensional refractive index, the refractive indexes at a wavelength of 550 nm were nx=1.4845, ny=1.4835 (n=0.001), and nz=1.4834.

Production of Light-Transmitting Substrate B Having No In-Plane Birefringence

(12) As a light-transmitting substrate B, an unstretched ZEONOR (Zeon Corporation) made of cycloolefin polymers was prepared. It had a n of 0.00004 at a wavelength of 550 nm and an average refractive index N of 1.5177.

Production of Light-Transmitting Substrate B Having In-Plane Birefringence

(13) The light-transmitting substrate B was uniaxially stretched from a free end by 1.5 times at 150 C. to give a light-transmitting substrate b having in-plane birefringence. Based on the calculation of the wavelength dispersion of the three-dimensional refractive index, the refractive indexes at a wavelength of 550 nm were nx=1.5186, ny=1.5172, and nz=1.5173.

Production of Light-Transmitting Substrate C Having No In-Plane Birefringence

(14) A polyethylene terephthalate material was molten at 290 C. and slowly cooled on glass to give a light-transmitting substrate C. It had a n of 0.00035 at a wavelength of 550 nm and an average refractive index N of 1.6167.

Production of Light-Transmitting Substrate c1 Having In-Plane Birefringence

(15) The light-transmitting substrate C was uniaxially stretched from a fixed end by 4.0 times at 120 C. to give a light-transmitting substrate c1 having in-plane birefringence. The wavelength dispersion of the refractive indexes (nx, ny) was calculated using a spectrophotometer. The refractive indexes at a wavelength of 550 nm were nx=1.701, ny=1.6015, and nz=1.5476.

Production of Light-Transmitting Substrate c2 Having In-Plane Birefringence

(16) The light-transmitting substrate C was uniaxially stretched from a free end by 2.0 times at 120 C. to give a light-transmitting substrate c2 having in-plane birefringence. The wavelength dispersion of the refractive indexes (nx, ny) were calculated using a spectrophotometer. The refractive indexes at a wavelength of 550 nm were nx=1.6511, ny=1.5998, and nz=1.5992.

Production of Light-Transmitting Substrate c3 Having In-Plane Birefringence

(17) The light-transmitting substrate C was biaxially stretched at an appropriate stretching ratio at 120 C. to give a light-transmitting substrate c3 having in-plane birefringence. The wavelength dispersion of the refractive indexes (nx, ny) were calculated using a spectrophotometer. The refractive indexes at a wavelength of 550 nm were nx=1.6652, ny=1.6153, and nz=1.5696.

Production of Light-Transmitting Substrate c4 Having In-Plane Birefringence

(18) The light-transmitting substrate C was biaxially stretched at an appropriate stretching ratio at 120 C. to give a light-transmitting substrate c4 having in-plane birefringence. The wavelength dispersion of the refractive indexes (nx, ny) was calculated using a spectrophotometer. The refractive indexes at a wavelength of 550 nm were nx=1.6708, ny=1.6189, and nz=1.5604.

Production of Light-Transmitting Substrate D Having No In-Plane Birefringence

(19) A polyethylene naphthalate material was molten at 290 C. and slowly cooled on glass to give a light-transmitting substrate D. It had a n of 0.0004 at a wavelength of 550 nm and an average refractive index N of 1.6833.

Production of Light-Transmitting Substrate D Having In-Plane Birefringence

(20) The light-transmitting substrate D was uniaxially stretched from a fixed end by 4.0 times at 120 C. to give a light-transmitting substrate d having in-plane birefringence. The wavelength dispersion of the refractive indexes (nx, ny) was calculated using a spectrophotometer. The refractive indexes at a wavelength of 550 nm were nx=1.8472, ny=1.6466, and nz=1.5561.

Calculation of Transmittance of Polarizer

(21) The transmittance can be calculated by the 22 matrix method, 44 matrix method, or extended Jones matrix method. The transmittance of the polarizer was calculated using simulation software (LCD Master, Shintech, Inc.) in examples, comparative examples, and reference examples. FIG. 4 illustrates the layer structure of the polarizer. The wavelength dispersion of the three-dimensional refractive index of each light-transmitting substrate was applied to the portion Examples and Comparative Examples in FIG. 4 for the above calculation. In the case of the light-transmitting substrate determined not to have in-plane birefringence, the average refractive indexes were set to N=nx=ny=nz. In the case of the light-transmitting substrate having in-plane birefringence, actual measured values were used. The portion of Examples and Comparative Examples and the portion of Protective film each had a thickness of 80 m and the portion Polarizing element had a thickness of 20 m.

(22) FIG. 5 shows a spectrum of a light source. The incident light was linearly polarized light to have the same state of polarization as light after passing through the polarization separation film, and vibrated in a direction of the transmission axis of the polarizing element.

(23) FIG. 6 shows the wavelength dispersion of the refractive index of the protective film used, and the protective film was an isotropic material.

(24) FIG. 7 shows the refractive index and the extinction coefficient of the polarizing element used. In FIG. 7, the direction of the absorption axis and the direction of the transmission axis are almost completely overlapped with each other.

Example 1

(25) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate a, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 0, and the transmittance of the polarizer was calculated.

Example 2

(26) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate a, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 90, and the transmittance of the polarizer was calculated.

Comparative Example 1

(27) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate a, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 45, and the transmittance of the polarizer was calculated.

Example 3

(28) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate b, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the polarizing element form an angle of 0, and the transmittance of the polarizer was calculated.

Example 4

(29) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate b, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 90, and the transmittance of the polarizer was calculated.

Comparative Example 2

(30) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate b, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 45, and the transmittance of the polarizer was calculated.

Example 5

(31) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c1, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 0, and the transmittance of the polarizer was calculated.

Example 6

(32) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c1, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 2, and the transmittance of the polarizer was calculated.

Example 7

(33) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c1, the light-transmitting substrate and a polarizing element were disposed such that the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 30, and the transmittance of the polarizer was calculated.

Example 8

(34) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c1, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 60, and the transmittance of the polarizer was calculated.

Example 9

(35) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c1, the light-transmitting substrate and the polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 90, and the transmittance of the polarizer was calculated.

Comparative Example 3

(36) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c1, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 45, and the transmittance of the polarizer was calculated.

Example 10

(37) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c2, the light-transmitting substrate and the polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 0, and the transmittance of the polarizer was calculated.

Example 11

(38) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c2, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 90, and the transmittance of the polarizer was calculated.

Comparative Example 4

(39) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c2, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the polarization axis of the polarizing element form an angle of 45, and the transmittance of the polarizer was calculated.

Example 12

(40) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c3, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 0, and the transmittance of the polarizer was calculated.

Example 13

(41) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c3, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 90, and the transmittance of the polarizer was calculated.

Comparative Example 5

(42) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c3, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 45, and the transmittance of the polarizer was calculated.

Example 14

(43) Based on the Wavelength Dispersion of the Three-Dimensional refractive index of the light-transmitting substrate c4, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 0, and the transmittance of the polarizer was calculated.

Example 15

(44) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c4, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 90, and the transmittance of the polarizer was calculated.

Comparative Example 6

(45) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate c4, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 45, and the transmittance of the polarizer was calculated.

Example 16

(46) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate d, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 0, and the transmittance of the polarizer was calculated.

Example 17

(47) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate d, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 90, and the transmittance of the polarizer was calculated.

Comparative Example 7

(48) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate d, the light-transmitting substrate and a polarizing element were disposed such that the fast axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle of 45, and the transmittance of the polarizer was calculated.

Reference Example 1

(49) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate A, the transmittance of the polarizer was calculated.

Reference Example 2

(50) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate B, the transmittance of the polarizer was calculated.

Reference Example 3

(51) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate C, the transmittance of the polarizer was calculated.

Reference Example 4

(52) Based on the wavelength dispersion of the three-dimensional refractive index of the light-transmitting substrate D, the transmittance of the polarizer was calculated.

Reference Example 5

(53) The transmittance of the polarizer was calculated in the same manner as in Example 5, except that the incident light was randomly polarized light.

Reference Example 6

(54) The transmittance of the polarizer was calculated in the same manner as in Example 9, except that the incident light was randomly polarized light.

Reference Example 7

(55) The transmittance of the polarizer was calculated in the same manner as in Comparative Example 3, except that the incident light was randomly polarized light.

Reference Example 8

(56) The transmittance of the polarizer was calculated in the same manner as in Reference Example 3, except that the incident light had a state of polarization of random light.

(57) Table 1 shows the results of the evaluation related to examples, comparative examples, and reference examples.

(58) Regarding the transmittance in the case where the incident light was linearly polarized light, the transmittance of the polarizer having in-plane birefringence is shown relative to the transmittance of the polarizer having no in-plane birefringence taken as 100 for each material. Regarding the transmittance in the case where the incident light was randomly polarized light, similarly, the transmittance of the polarizer having in-plane birefringence is shown relative to the transmittance of the polarizer having no in-plane birefringence taken as 100.

(59) TABLE-US-00001 TABLE 1 Angle between fast axis of light- Material transmitting of light- substrate and transmitting transmission axis Transmit- substrate n of polarizing element tance (%) Example 1 CAP 0.001 0 100.004 Example 2 90 99.991 Comparative 45 80.827 Example 1 Example 3 COP 0.0014 0 100.006 Example 4 90 99.987 Comparative 45 67.009 Example 2 Example 5 PET 0.0995 0 100.279 Example 6 2 100.035 Example 7 30 62.562 Example 8 60 61.681 Example 9 90 98.517 Comparative 45 49.817 Example 3 Example 10 0.0513 0 100.279 Example 11 90 99.425 Comparative 45 49.565 Example 4 Example 12 0.0499 0 100.029 Example 13 90 99.158 Comparative 45 49.436 Example 5 Example 14 0.0519 0 99.944 Example 15 90 99.068 Comparative 45 49.393 Example 6 Example 16 PEN 0.2006 0 100.606 Example 17 90 96.681 Comparative 45 50.781 Example 7 Reference CAP 100.000 Example 1 Reference COP 100.000 Example 2 Reference PET 100.000 Example 3 Reference PEN 100.000 Example 4 Reference PET 0.0995 0 100.278 Example 5 Reference 90 98.518 Example 6 Reference 45 99.398 Example 7 Reference 100.000 Example 8

(60) As shown in Table 1, based on the comparison between Examples 1-2 and Comparative Example 1, between Examples 3-4 and Comparative Example 2, between Examples 5-9 and Comparative Example 3, between Examples 10-11 and Comparative Example 4, between Examples 12-13 and Comparative Example 5, between Examples 14-15 and Comparative Example 6, and between Examples 16-17 and Comparative Example 7, the polarizers according to examples in which the slow axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle within a predetermined range had better light transmittance than the polarizers according to comparative examples in which the angle is not within the predetermined range.

(61) Based on the comparison between Example 1 and Reference Example 1, between Example 3 and Reference Example 2, between Example 5, 10, and 12 and Reference Example 3, and between Example 16 and Reference Example 4, the polarizers including a light-transmitting substrate having in-plane birefringence according to the examples had better light transmittance than the polarizers including a light-transmitting substrate having no in-plane birefringence according to the reference examples.

(62) Based on the comparison between Reference Examples 5-8 and a group including Examples 5 and 9, Comparative Example 3, and Reference Example 3, the polarizers according to the examples in which the slow axis of the light-transmitting substrate and the transmission axis of the polarizing element form an angle within a predetermined range had better light transmittance than the polarizer according to the comparative example in which the angle formed is not within the predetermined range, when the polarized light was incident thereon.

INDUSTRIAL APPLICABILITY

(63) The polarizer of the present invention is, even in the case of including a light-transmitting substrate having in-plane birefringence, excellent in the light transmittance. Moreover, even if the polarizer of the present invention includes a conventional film of cellulose esters typified by triacetyl cellulose which have no in-plane phase difference, the polarizer imparted with birefringence has excellent transmittance and is suitably used as a polarizer disposed on a backlight source side of a liquid crystal display (LCD).