Optical film, polarizing plate, image display device, and method for selecting optical film

Abstract

To provide an optical film capable of suppressing rainbow unevenness when viewed with the naked eye without increasing an in-plane phase difference.SOLUTION: An optical film having a low-refractive-index layer on a plastic film, wherein the plastic film is a biaxially stretched plastic film having an in-plane retardation of 2,500 nm or less, and the low-refractive-index layer is located on the outermost surface, wherein Δ Eab calculated from a difference between L *, a * and b * values measured under specific conditions of a laminate 1 including the optical film and a polarizer and a surface light source and L *, a * and b * values measured under specific conditions of a laminate 2 including the polarizer and the surface light source satisfies a specific condition.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to an optical film, a polarizing plate, an image display device, and a method for selecting an optical film. [Background technology]

[0002] Various optical plastic films are often used in optical components of image display devices, etc. For example, in image display devices having a polarizing plate on a display element, a plastic film is used to protect the polarizer that constitutes the polarizing plate.

[0003] Plastic films for image display devices used as polarizer protective films etc. preferably have excellent mechanical strength, and therefore stretched plastic films are preferably used as plastic films for image display devices.

[0004] When a stretched plastic film is placed on a polarizer, the stretched plastic film disrupts the polarization state of linearly polarized light that has passed through the polarizer, resulting in the problem of rainbow pattern unevenness being observed. To solve this problem, techniques such as those in Patent Documents 1 to 3 have been proposed. Hereinafter, in this specification, "rainbow pattern unevenness" may be referred to as "rainbow unevenness."

[0005] Patent Document 1 discloses a liquid crystal display device that can eliminate rainbow unevenness when viewing an image through polarized sunglasses by using a specific white light source as the light source of the image display device, increasing the in-plane phase difference (retardation) of the stretched plastic film to 3000 nm or more and 30,000 nm or less, and arranging the absorption axis of the polarizer and the slow axis of the stretched plastic film at approximately 45 degrees. However, the method of Patent Document 1 requires the use of a stretched plastic film with a large in-plane retardation. Stretched plastic films with a large in-plane retardation are usually uniaxially stretched, which poses problems such as the tendency to tear in the stretching direction.

[0006] Patent Document 2 discloses a polarizing plate protective film having a specific range of reflectance at Brewster's angle. Patent Document 3 discloses a polarizing plate protective film having a difference of 20% or less between the reflectance of P waves and the reflectance of S waves at an incident angle of 50 degrees. The polarizing plate protective films of Patent Documents 2 and 3 aim to eliminate the visible rainbow unevenness by reducing the reflectance difference between P waves and S waves, which are the polarized components of light traveling from inside the image display device toward the viewer, without increasing the in-plane retardation of the film as in Patent Document 1.

[0007] Biaxially stretched plastic films are generally obtained by melt-extruding a plastic material to form a cast film, and then stretching the cast film in the machine direction and width direction. During this process, it is known that the orientation angle varies depending on the position in the width direction due to the so-called bowing phenomenon. It is generally said that the optical performance of a biaxially stretched plastic film is stable near the center in the width direction. However, in the polarizing plate protective films of Patent Documents 2 and 3, even when biaxially stretched plastic films obtained near the center were used, rainbow unevenness could not be eliminated in some cases. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-107198 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-14886 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-204630 Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present disclosure is to provide an optical film that can suppress rainbow unevenness when viewed with the naked eye without increasing in-plane retardation, and a polarizing plate and an image display device using the optical film. Another object of the present disclosure is to provide a method for selecting an optical film that can suppress rainbow unevenness when viewed with the naked eye without increasing in-plane retardation. [Means for solving the problem]

[0010] The present disclosure provides the following [1] to [5]. [1] An optical film having a low refractive index layer on a plastic film, the plastic film is a biaxially stretched plastic film having an in-plane retardation of 2500 nm or less, the low refractive index layer is located on the outermost surface of the optical film, An optical film having a region in which the difference between the maximum and minimum values ​​of ΔEab is less than 17.0. Here, Measurement 1 is performed on Laminate 1 to calculate the L* value, a* value, and b* value in the L*a*b* color system. Measurement 2 is performed on Laminate 2 to calculate the L* value, a* value, and b* value in the L*a*b* color system. Based on the results of Measurement 1 and Measurement 2, ΔEab is calculated under Condition 1. <Measurement 1> A laminate 1 is prepared by laminating a polarizer and the optical film in this order on a surface light source. In the laminate 1, the optical film is arranged so that the surface on the low refractive index layer side faces away from the polarizer. The polarizer is also arranged so that the angle between the absorption axis of the polarizer and the left-right or up-down direction of the surface light source is within ±5 degrees. Furthermore, the polarizer is also arranged so that the angle between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film is within 90 degrees ±5 degrees. The surface light source of the laminate 1 is displayed in white, and the transmitted light emitted from the low refractive index layer side of the laminate 1 is measured at 1-degree intervals at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees. Based on the transmitted light at each angle, the L* value, a* value, and b* value of the L*a*b* color system are calculated. The measurement area of ​​the transmitted light is an arbitrary 1 mm 2 More than 10mm 2 The areas are as follows: <Measurement 2> Laminate 2 is prepared by laminating a polarizer on the same surface light source as in Measurement 1. The polarizer is arranged so that the direction of the absorption axis relative to the surface light source is the same as in Measurement 1. The surface light source of the laminate 2 is displayed as white, and the transmitted light emitted from the polarizer side of the laminate 2 is measured at 1-degree intervals at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees, and the L*, a*, and b* values ​​of the L*a*b* color system are calculated based on the transmitted light at each angle. The measurement area of ​​the transmitted light is set to approximately the same as Measurement 1 within the plane. <Condition 1> For all elevation angles and all azimuth angles, ΔL* is calculated by subtracting the L* value of measurement 2 from the L* value of measurement 1. ΔL* is converted into a grayscale from the maximum to minimum value in a specified gradation and displayed in grayscale on a two-dimensional coordinate system where the elevation angle is represented by concentric circles and the azimuth angle is represented by vertical and horizontal lines. It is confirmed that there are two regions in the two-dimensional coordinate system where ΔL* is distributed concentrically, and that the two regions are located at approximately symmetrical positions in the two-dimensional coordinate system. Regarding the elevation angles at the center of the region where ΔL* is distributed concentrically, one elevation angle is set to α degrees and the other elevation angle is set to β degrees. ΔEab at each azimuth angle is calculated from the difference between the L*, a*, and b* values ​​of measurement 1 at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α + β) / 2, and the L*, a*, and b* values ​​of measurement 2 at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α + β) / 2.

[0011] [2] A polarizing plate having a polarizer, a first transparent protective plate arranged on one side of the polarizer, and a second transparent protective plate arranged on the other side of the polarizer, wherein at least one of the first transparent protective plate and the second transparent protective plate is the optical film described in [1]. [3] An image display device having a display element, and a polarizer and an optical film arranged on the light-emitting surface side of the display element, wherein the optical film is the optical film described in [1], and is arranged so that the angle between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film is within 90 degrees ± 5 degrees, and the surface of the optical film on the low refractive index layer side faces away from the display element.

[0012] [4] An image display device having a polarizer and an optical film on a light exit surface of a display element, The polarizer is arranged so that the angle formed between the direction of the absorption axis of the polarizer and the left-right direction or the up-down direction of the display element is within ±5 degrees, the polarizer and the biaxially stretched plastic film of the optical film are disposed so that the angle formed between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film is within 90 degrees ± 5 degrees, The optical film has a low refractive index layer on a biaxially stretched plastic film having an in-plane retardation of less than 2500 nm, the low refractive index layer being located on the outermost surface of the optical film, and the image display device has a region in which the difference between the maximum and minimum values ​​of ΔEab is less than 17.0. Here, Measurement 1A is performed on Laminate 1A to calculate the L*, a*, and b* values ​​in the L*a*b* color system. Measurement 2A is performed on Laminate 2A to calculate the L*, a*, and b* values ​​in the L*a*b* color system. ΔEab is calculated under Condition 1A based on the results of Measurement 1A and Measurement 2A. <Measurement 1A> A laminate 1A is prepared by laminating a polarizer and the optical film in this order on a display element. In the laminate 1A, the optical film is arranged so that the surface on the low refractive index layer side faces away from the polarizer. The polarizer is also arranged so that the angle between the absorption axis of the polarizer and the left-right or up-down direction of the display element is within ±5 degrees. Furthermore, the polarizer is also arranged so that the angle between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film is within ±5 degrees. The display element of the laminate 1A is set to white, and the transmitted light emitted from the low refractive index layer side of the laminate 1A is measured at 1-degree intervals at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees. Based on the transmitted light at each angle, the L* value, a* value, and b* value of the L*a*b* color system are calculated. The measurement area of ​​the transmitted light is an arbitrary 1 mm 2 More than 10mm 2 The areas are as follows: <Measurement 2A> A laminate 2A is prepared by laminating a polarizer on the same display element as in the measurement 1A. The display element of the laminate 2A is set to white, and the transmitted light exiting the polarizer side of the laminate 2A is measured at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees, one degree at a time, and the L*, a*, and b* values ​​of the L*a*b* color system are calculated based on the transmitted light at each angle. The measurement area of ​​the transmitted light is set to approximately the same as Measurement 1A in the plane. <Condition 1A> At all elevation angles and all azimuth angles, ΔL* is calculated by subtracting the L* value of measurement 2A from the L* value of measurement 1A. ΔL* is converted from the maximum to minimum values ​​into a grayscale at a specified gradation and displayed in grayscale on a two-dimensional coordinate system where the elevation angle is represented by concentric circles and the azimuth angle is represented by vertical and horizontal lines. It is confirmed that there are two regions in the two-dimensional coordinate system where ΔL* is distributed concentrically, and that the two regions are located at approximately symmetrical positions in the two-dimensional coordinate system. Regarding the elevation angles at the center of the region where ΔL* is distributed concentrically, one elevation angle is set to α degrees and the other elevation angle is set to β degrees. ΔEab at each azimuth angle is calculated from the difference between the L*, a*, and b* values ​​of Measurement 1A at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α+β) / 2, and the L*, a*, and b* values ​​of Measurement 2A at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α+β) / 2.

[0013] [5] A method for selecting an optical film for an image display device, which comprises a polarizer and an optical film on a light-emitting surface of a display element, and which is arranged so that the direction of the absorption axis of the polarizer is parallel to the left-right direction or the up-down direction of the display element, comprising: A method for selecting an optical film for an image display device, comprising the steps of: selecting an optical film X that satisfies a determination condition that the optical film X comprises a biaxially stretched plastic film having an in-plane retardation of less than 2500 nm and a low refractive index layer on the biaxially stretched plastic film, the low refractive index layer being located on the outermost surface of the optical film X, and having a region in which the difference between the maximum and minimum values ​​of ΔEab is less than 17.0; Here, Measurement 1B is performed on Laminate 1B to calculate the L*, a*, and b* values ​​in the L*a*b* color system. Measurement 2B is performed on Laminate 2B to calculate the L*, a*, and b* values ​​in the L*a*b* color system. Based on the results of Measurement 1B and Measurement 2B, ΔEab is calculated under Condition 1B. <Measurement 1B> A laminate 1B is prepared by laminating a polarizer and the optical film X in this order on a display element. In the laminate 1B, the optical film is arranged so that the surface on the low refractive index layer side faces away from the polarizer. The polarizer is also arranged so that the angle between the absorption axis of the polarizer and the left-right or up-down direction of the display element is within ±5 degrees. Furthermore, the angle between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film X is within 90 degrees ±5 degrees. The display element of the laminate 1B is set to white, and the transmitted light emitted from the low refractive index layer side of the laminate 1B is measured at 1-degree intervals at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees. Based on the transmitted light at each angle, the L* value, a* value, and b* value of the L*a*b* color system are calculated. The measurement area of ​​the transmitted light is an arbitrary 1 mm 2 More than 10mm 2 The areas are as follows: <Measurement 2B> A laminate 2B is prepared by laminating a polarizer on the same display element as in the measurement 1B. The display element of the laminate 2B is set to white, and the transmitted light exiting the polarizer side of the laminate 2B is measured at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees, one degree at a time, and the L*, a*, and b* values ​​of the L*a*b* color system are calculated based on the transmitted light at each angle. The measurement area of ​​the transmitted light is set to approximately the same as that of Measurement 1B within the plane. <Condition 1B> At all elevation angles and all azimuth angles, ΔL* is calculated by subtracting the L* value of measurement 2B from the L* value of measurement 1B. ΔL* is converted from the maximum to minimum values ​​into a grayscale at a specified gradation and displayed in grayscale on a two-dimensional coordinate system where the elevation angle is represented by concentric circles and the azimuth angle is represented by vertical and horizontal lines. It is confirmed that there are two regions in the two-dimensional coordinate system where ΔL* is distributed concentrically, and that the two regions are located at approximately symmetrical positions in the two-dimensional coordinate system. Regarding the elevation angles at the center of the region where ΔL* is distributed concentrically, one elevation angle is set to α degrees and the other elevation angle is set to β degrees. When ΔEab at each azimuth angle is calculated from the difference between the L*, a*, and b* values ​​of measurement 1B at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α + β) / 2, and the L*, a*, and b* values ​​of measurement 2B at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α + β) / 2, the difference between the maximum and minimum ΔEab values ​​is less than 17.0. [Effects of the Invention]

[0014] The optical film of the present disclosure, and the polarizing plate and image display device using the optical film, can suppress rainbow unevenness when viewed with the naked eye without increasing the in-plane retardation to 3000 nm or more. Furthermore, the method for selecting an optical film of the present disclosure can efficiently select an optical film that can suppress rainbow unevenness when viewed with the naked eye without increasing the in-plane retardation. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional view illustrating an embodiment of an optical film of the present disclosure. [Figure 2] FIG. 1 is a cross-sectional view for explaining the arrangement in the thickness direction of the laminate 1 used in measurement 1. [Figure 3] FIG. 10 is a cross-sectional view illustrating the arrangement in the thickness direction of the laminate 2 used in measurement 2. [Figure 4] The value (ΔL*) obtained by subtracting the L* value of measurement 2 from the L* value of measurement 1 is grayscaled to a specified tone and displayed in grayscale on a two-dimensional coordinate system in which the elevation angle is represented by concentric circles and the azimuth angle is represented by vertical and horizontal axes. [Figure 5] FIG. 1 is a cross-sectional view showing an embodiment of a polarizing plate according to the present disclosure. [Figure 6] 1 is a cross-sectional view illustrating an embodiment of an image display device according to the present disclosure. [Figure 7] FIG. 2 is a plan view for explaining five measurement positions in a sample when calculating an in-plane retardation or the like from the sample. [Figure 8] 1 is a cross-sectional photograph of an example of a low refractive index layer in which hollow particles and non-hollow particles are uniformly dispersed. [Figure 9] 1 is a cross-sectional photograph of an example of a low refractive index layer in which hollow particles and non-hollow particles are not uniformly dispersed. [Figure 10] FIG. 1 is a diagram showing a schematic diagram of a continuous folding test. [Figure 11] FIG. 10 is a diagram for explaining [+αB-(-αB)], [+αG-(-αG)], and [+αR-(-αR)] under condition A. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the present disclosure will be described. [Optical film] The optical film of the present disclosure comprises a low refractive index layer on a plastic film, the plastic film being a biaxially stretched plastic film having an in-plane retardation of 2500 nm or less, the low refractive index layer being located on the outermost surface of the optical film, and having a region in which the difference between the maximum and minimum values ​​of ΔEab is less than 17.0. Here, Measurement 1 is performed on Laminate 1 to calculate the L* value, a* value, and b* value in the L*a*b* color system. Measurement 2 is performed on Laminate 2 to calculate the L* value, a* value, and b* value in the L*a*b* color system. Based on the results of Measurement 1 and Measurement 2, ΔEab is calculated under Condition 1.

[0017] <Measurement 1> A laminate 1 is prepared by laminating a polarizer and the optical film in this order on a surface light source. In the laminate 1, the optical film is arranged so that the surface on the low refractive index layer side faces away from the polarizer. The polarizer is also arranged so that the angle between the absorption axis of the polarizer and the left-right or up-down direction of the surface light source is within ±5 degrees. Furthermore, the polarizer is arranged so that the angle between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film is within 90 degrees ±5 degrees. The surface light source of the laminate 1 is displayed in white, and the transmitted light emitted from the low refractive index layer side of the laminate 1 is measured at 1-degree intervals at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees. Based on the transmitted light at each angle, the L* value, a* value, and b* value of the L*a*b* color system are calculated. The measurement area of ​​the transmitted light is an arbitrary 1 mm 2 More than 10mm 2 The areas are as follows:

[0018] <Measurement 2> A laminate 2 was prepared by laminating a polarizer on the same surface light source as in the measurement 1. The surface light source of the laminate 2 is displayed as white, and the transmitted light emitted from the polarizer side of the laminate 2 is measured at 1-degree intervals at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees, and the L*, a*, and b* values ​​of the L*a*b* color system are calculated based on the transmitted light at each angle. The measurement area of ​​the transmitted light is set to approximately the same as Measurement 1 within the plane. From the measurement results, the L* value, a* value, and b* value at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α+β) / 2 are confirmed.

[0019] <Condition 1> For all elevation angles and all azimuth angles, ΔL* is calculated by subtracting the L* value of measurement 2 from the L* value of measurement 1. ΔL* is converted into a grayscale from the maximum to minimum value in a specified gradation and displayed in grayscale on a two-dimensional coordinate system where the elevation angle is represented by concentric circles and the azimuth angle is represented by vertical and horizontal lines. It is confirmed that there are two regions in the two-dimensional coordinate system where ΔL* is distributed concentrically, and that the two regions are located at approximately symmetrical positions in the two-dimensional coordinate system. Regarding the elevation angles at the center of the region where ΔL* is distributed concentrically, one elevation angle is set to α degrees and the other elevation angle is set to β degrees. ΔEab at each azimuth angle is calculated from the difference between the L*, a*, and b* values ​​of measurement 1 at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α + β) / 2, and the L*, a*, and b* values ​​of measurement 2 at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α + β) / 2.

[0020] In this specification, Measurement 1, Measurement 2, and the measurements described below (in-plane retardation, thickness direction retardation, slow axis direction, surface roughness of the low refractive index layer, etc.) are performed in an atmosphere at a temperature of 23°C ± 5°C and a relative humidity of 40% RH to 65% RH, unless otherwise specified. Furthermore, before each measurement, the measurement sample is exposed to the atmosphere for 30 minutes or more.

[0021] 1 is a cross-sectional view showing an embodiment of an optical film 100 of the present disclosure. As shown in FIG. 1, the optical film 100 of the present disclosure has a low refractive index layer 30 on a plastic film 10. The optical film 100 of the present disclosure may have layers other than the plastic film 10 and the low refractive index layer 30. Examples of layers other than the plastic film 10 and the low refractive index layer 30 include a hard coat layer, an antiglare layer, and a high refractive index layer. The optical film 100 of FIG. 1 has a hard coat layer 20 between the plastic film 10 and the low refractive index layer 30.

[0022] Regarding measurement 1: Measurement 1 can be carried out as follows (1-1) to (1-2). (1-1) A laminate 1 is prepared by laminating a polarizer and an optical film in this order on a surface light source. In the laminate 1, the optical film is arranged so that the surface on the low refractive index layer side faces away from the polarizer. The polarizer is also arranged so that the angle between the direction of the absorption axis of the polarizer and the left-right or up-down direction of the surface light source is within ±5 degrees. Furthermore, the angle between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film is within 90 degrees ±5 degrees.

[0023] FIG. 2 is a cross-sectional view for explaining the arrangement in the thickness direction of the laminate 1 used in (1-1) above. The laminate 1(X) in FIG. 2 includes a surface light source 200, a polarizer 300, and an optical film 100 laminated together with an adhesive layer 400 interposed therebetween. As shown in FIG. 2, the surface light source, the polarizer, and the optical film are preferably bonded together via an adhesive layer or the like. Note that the above-mentioned bonding only needs to be an apparent bond, and may be temporary bonding via a liquid such as water or a solvent. The laminate 1 may also include an optically isotropic film. For example, an optically isotropic film may be provided as a polarizer protective film on one or both sides of the polarizer.

[0024] The adhesive layer used to prepare the laminate 1 and the laminate 2 described later preferably has a refractive index of 1.42 or more and 1.53 or less, and a thickness of 15 μm or more and 40 μm or less. The adhesive layer more preferably has a refractive index of 1.45 or more and 1.50 or less, and a thickness of 20 μm or more and 30 μm or less. If the refractive index and thickness of the adhesive layer are within the above ranges, it can be said that there is no substantial effect on ΔEab described later. In this specification, the refractive index is a dimensionless parameter. The adhesive layer used to prepare the laminate 1 and the laminate 2 described later preferably has substantially no internal haze. The adhesive layer may be a general-purpose adhesive layer such as a curing adhesive layer, a pressure-sensitive adhesive layer (a so-called pressure-sensitive adhesive layer), or a heat-sensitive adhesive layer (a heat-sealing layer).

[0025] In the above (1-1), the polarizer is arranged so that the absorption axis of the polarizer is approximately parallel to the left-right direction or the up-down direction of the surface light source. In this specification, approximately parallel means that the difference between the absorption axis of the polarizer and the left-right direction or the up-down direction of the surface light source is within ±5 degrees, preferably within ±3 degrees, and more preferably within ±1 degree. If the planar shape of the surface light source is rectangular or square, it is easy to identify the left-right or up-down direction. However, it is not necessary to distinguish between left-right and up-down. If the planar shape of the surface light source is a shape other than a rectangle or square (circle, triangle, etc.), draw a rectangle or square that has the largest area that does not extend beyond the outer frame shape of the surface light source, and determine the left-right or up-down directions based on the drawn rectangle or square. In the above (1-1), the polarizer is arranged so that the angle formed between the direction of the absorption axis and the left-right or up-down direction of the surface light source is within ±5 degrees, taking into consideration that the polarizer on the light exit surface side of a general-purpose image display device is arranged in this manner.

[0026] In the above (1-1), the polarizer and the biaxially stretched plastic film of the optical film are arranged so that their absorption axes are substantially perpendicular to each other. In this specification, "substantially perpendicular" means that the angle between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film is within 90°±5°, preferably within 90°±3°, and more preferably within 90°±1°. The slow axis refers to the direction in the plane of the biaxially stretched plastic film in which the refractive index is greatest. When the direction of the slow axis is not uniform in the plane of the biaxially stretched plastic film, the direction of the slow axis of the biaxially stretched plastic film refers to the average direction of the slow axis in the plane of the biaxially stretched plastic film.

[0027] In the above (1-1), the polarizer preferably has a polarization degree of 99.00% or more and an average transmittance of 35% or more, more preferably a polarization degree of 99.90% or more and an average transmittance of 37% or more, and even more preferably a polarization degree of 99.95% or more and an average transmittance of 40% or more. In this specification, the average transmittance means the average of the spectral transmittance at a wavelength of 400 nm or more and 700 nm or less. The wavelength interval for measuring the average transmittance is 5 nm. The polarizer may have an optically isotropic film on one or both sides thereof, and the polarizer and the optically isotropic film may be bonded to each other via an adhesive layer.

[0028] The polarizer used in Measurement 1 and Measurement 2 may be a polarizer that is disposed on the display element in advance, or may be a polarizer that is separately prepared.

[0029] In the above (1-1), the surface light source is not particularly limited as long as it can display white. Light emitted from a surface light source passes through a polarizer to become linearly polarized light, and the linearly polarized light is incident on the optical film. The linearly polarized light incident on the optical film can be assumed to be light emitted from a display element of a general-purpose image display device and further passed through a viewer-side polarizer (linearly polarized light). The surface light source can be, for example, a general-purpose image display device such as a liquid crystal display device or an organic EL display device. However, if the image display device has a viewer-side polarizer on the display element, the surface light source is considered to be the device excluding the viewer-side polarizer. This is because the viewer-side polarizer can be the polarizer of laminate 1 and laminate 2. Furthermore, if the surface light source is a liquid crystal display device, examples of the backlight of the liquid crystal display device include a backlight using quantum dots and a backlight using white light-emitting diodes. A laminate in which an optical film is disposed on an image display device having a viewer-side polarizer on a display element can be regarded as Laminate 1 used in Measurement 1 as long as it satisfies the other conditions of Measurement 1. Furthermore, an image display device having a viewer-side polarizer on a display element can be considered as laminate 2 used in measurement 2, as long as it satisfies the other conditions of measurement 2.

[0030] (1-2) The surface light source of the laminate 1 is displayed in white, and the transmitted light emitted from the low refractive index layer side of the laminate 1 is measured at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees, one degree at a time, and the L* value, a* value, and b* value of the L*a*b* color system are calculated based on the transmitted light at each angle. The measurement area of ​​the transmitted light is an arbitrary 1 mm 2 More than 10mm 2 The areas are as follows:

[0031] In (1-2) above, the measurement area is 1 mm 2 The reason for limiting the measurement area to 10 mm is that if the area is too small, it is difficult for the human eye to recognize it. 2 The reason for using the values ​​below is to suppress the influence of the emission angle distribution of the surface light source, and also to take into consideration that if the area is too large, ΔEab will be averaged, making it difficult to correlate with the visual result. The maximum elevation angle is set to 80 degrees because generally, light emitted from an elevation angle greater than 80 degrees is nearly parallel to the plane, making it difficult to detect. In the above (1-2), the measurement area is 1 mm 2 More than 5mm 2 It is preferable to do the following:

[0032] In the above (1-2), the distance between the laminate 1 and the transmitted light measuring device is preferably more than 0 mm and not more than 1.5 mm, more preferably 0.5 mm to 1.5 mm, and even more preferably 1.0 mm. By setting the distance in this range, deformation of the laminate 1 due to the weight of the measuring device is suppressed, and the spread of the emitted light is suppressed, making it easier to reduce measurement errors. Similarly, in the later-described (2-2), the distance between the laminate 2 and the transmitted light measuring device is preferably within the above range.

[0033] The measurements and analyses in (1-2) above, (2-2) described below, and (3-1) to (3-4) described below can be carried out using, for example, ELDIM's product name "EzContrast."

[0034] In this specification, the L* value, a* value, and b* value are based on the L*a*b* color system standardized by the International Commission on Illumination (CIE) in 1976. The L*a*b* color system is adopted in JIS Z8781-4:2013.

[0035] Regarding measurement 2: Measurement 2 can be carried out as follows (2-1) to (2-2).

[0036] (2-1) A laminate 2 is prepared by laminating a polarizer on the same surface light source as in Measurement 1. The polarizer is arranged so that the direction of the absorption axis relative to the surface light source is the same as in Measurement 1. The polarizer used in Measurement 2 is the same as the polarizer used in Measurement 1.

[0037] FIG. 3 is a cross-sectional view for explaining the arrangement in the thickness direction of the laminate 2 used in the above (2-1). In the laminate 2(Y) of Fig. 3, a surface light source 200 and a polarizer 300 are laminated via an adhesive layer 400. As shown in Fig. 3, the surface light source and the polarizer are preferably bonded together via an adhesive layer or the like. Note that the above-mentioned bonding only needs to be an apparent bond, and may be temporary bonding via a liquid such as water or a solvent.

[0038] In the above (2-1), the polarizer is arranged so that the direction of the absorption axis relative to the surface light source is the same as in the above measurement 1. For example, if the polarizer is arranged so that the absorption axis direction is parallel to the left-right direction of the surface light source in measurement 1, the same arrangement is used in the above (2-1).

[0039] (2-2) The surface light source of the laminate 2 is displayed as white, and the transmitted light emitted from the polarizer side of the laminate 2 is measured at one-degree intervals at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees, and the L*, a*, and b* values ​​of the L*a*b* color system are calculated based on the transmitted light at each angle. The measurement area of ​​the transmitted light is set to approximately the same as Measurement 1 within the plane.

[0040] In the above (2-2), the measurement area of ​​the transmitted light being approximately the same in the plane as that of Measurement 1 means that when laminate 1 and laminate 2 are superimposed and viewed in a plane, the distance between the center of the measurement area of ​​the transmitted light in Measurement 1 and the center of the measurement area of ​​the transmitted light in Measurement 2 is within 0.5 mm, preferably within 0.3 mm, and more preferably within 0.1 mm.

[0041] As described in (1-1) above, the surface light source used in Measurement 1 and Measurement 2 is not particularly limited as long as it can display white, but in order to perform stable measurements, it is preferable to use a surface light source whose average L* value, a* value, and b* value satisfy the following ranges (a1) to (a3) ​​in the state of laminate 2. In other words, in order to perform stable measurements, it is preferable that the average L* value, a* value, and b* value in Measurement 2 satisfy the following ranges. (a1) The average L* value for all angles obtained by the measurement in (2-2) above is 95 or more and 105 or less. The average L* value for all angles is more preferably 95 or more and 100 or less. (a2) The average a* value for all angles obtained in the measurement in (2-2) above is −10 or more and 10 or less. The average a* value for all angles is more preferably −5 or more and 5 or less. (a3) The average b* value for all angles obtained in the measurement in (2-2) above is −10 or more and 10 or less. The average b* value for all angles is more preferably −5 or more and 5 or less.

[0042] Furthermore, in order to perform more stable measurements, it is more preferable for the surface light source to have the variations (3σ) in the L* value, a* value, and b* value satisfy the following ranges (b1) to (b3) in the state of the laminate 2. In other words, in order to perform stable measurements, it is preferable that the variations (3σ) in the L* value, a* value, and b* value in measurement 2 satisfy the following ranges. (b1) The variation (3σ) of the L* values ​​across all angles obtained by the measurement in (2-2) above is 120 or less. The variation (3σ) of the L* values ​​across all angles is more preferably 115 or less. (b2) The variation (3σ) of the absolute values ​​of a* values ​​for all angles obtained by the measurement in (2-2) above is 15 or less. (b3) The variation (3σ) of the absolute values ​​of the b* values ​​across all angles obtained by the measurement in (2-2) above is 15 or less.

[0043] Furthermore, in order to more easily suppress rainbow unevenness, the surface light source preferably satisfies the following condition A. Satisfying condition A means that at least one of the full widths at half maximum of the intensity peaks present in the blue wavelength range, the green wavelength range, and the red wavelength range is equal to or greater than a predetermined value (10 nm or greater). Figure 11 shows the [+α B -(-α B )], [+α G -(-α G )] and [+α R -(-α R 11 is a diagram for explaining the spectrum of a surface light source of a general-purpose organic EL element.

[0044] <Condition A> A first polarizer is placed on a surface light source, and the intensity of light L1 emitted vertically from the first polarizer side is measured at wavelength intervals of 1 nm. The blue wavelength range is set to 400 nm or more and less than 500 nm, the green wavelength range is set to 500 nm or more and less than 570 nm, and the red wavelength range is set to 570 nm or more and less than 780 nm. The maximum intensity of the blue wavelength range of L1 is defined as B. max , the maximum intensity of the green wavelength region of L1 is G max , the maximum intensity of the red wavelength region of L1 is R max Let's say. B max The wavelength at which B , the G max The wavelength at which G , the R max The wavelength at which R Let's say. B max The wavelength at which the intensity is 1 / 2 or less of L1λ B The minimum wavelength located on the negative side of B , B max The wavelength at which the intensity is 1 / 2 or less of L1λ B The minimum wavelength located on the positive side of B , the G max The wavelength at which the intensity is 1 / 2 or less of L1λ G The maximum wavelength located on the negative side of G , the G max The wavelength at which the intensity is 1 / 2 or less of L1λ G The minimum wavelength located on the positive side of G , the R max The wavelength at which the intensity is 1 / 2 or less of L1λ R The maximum wavelength located on the negative side of R , the R max The wavelength at which the intensity is 1 / 2 or less of L1λ R The maximum wavelength located on the positive side of R Let's say. [+α B -(-α B )], [+α G -(-α G )] and [+α R -(-α R)] is 10 nm or more.

[0045] Condition A is more preferably such that two or more of [+αB-(-αB)], [+αG-(-αG)] and [+αR-(-αR)] are 10 nm or more, and even more preferably such that all three are 10 nm or more.

[0046] Regarding Condition 1: In Condition 1, the difference between the maximum and minimum values ​​of ΔEab calculated according to the following procedure is calculated based on the L* value, a* value, and b* value obtained in Measurement 1 and Measurement 2. The optical film of the present disclosure is required to have a region where the difference is less than 17.0. <Condition 1> For all elevation angles and all azimuth angles, ΔL* is calculated by subtracting the L* value of measurement 2 from the L* value of measurement 1. ΔL* is converted into a grayscale from the maximum to minimum value in a specified gradation and displayed in grayscale on a two-dimensional coordinate system where the elevation angle is represented by concentric circles and the azimuth angle is represented by vertical and horizontal lines. It is confirmed that there are two regions in the two-dimensional coordinate system where ΔL* is distributed concentrically, and that the two regions are located at approximately symmetrical positions in the two-dimensional coordinate system. Regarding the elevation angles at the center of the region where ΔL* is distributed concentrically, one elevation angle is set to α degrees and the other elevation angle is set to β degrees. ΔEab at each azimuth angle is calculated from the difference between the L*, a*, and b* values ​​of measurement 1 at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α + β) / 2, and the L*, a*, and b* values ​​of measurement 2 at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α + β) / 2.

[0047] ΔEab is the so-called color difference. When the "L* value, a* value, and b* value" of Measurement 1 and Measurement 2 at a specific azimuth angle are defined as "L1*, a1*, and b1*" and "L2*, a2*, and b2*," respectively, the color difference (ΔEab) at the specific azimuth angle can be expressed by the following formula. <Formula for color difference (ΔEab)> ΔEab={(L1*-L2*) 2+(a1*-a2*) 2 +(b1*-b2*) 2} 1 / 2

[0048] ΔEab under Condition 1 can be calculated according to the following procedures (3-1) to (3-4) based on the L*, a*, and b* values ​​obtained in Measurements 1 and 2. That is, ΔEab under Condition 1 defines the color difference between a state with an optical film and a state without an optical film. This is to cancel the influence of the surface light source and provide a correlation with rainbow unevenness. (L1*) can be calculated based only on the results of Measurement 1 without using the results of Measurement 2. 2 +a1* 2 +b1* 2 ) 1 / 2 " has no correlation with the visibility of rainbow unevenness. This is thought to be because the L* value of the surface light source has a large influence.

[0049] (3-1) For all elevation angles and all azimuth angles, ΔL* is calculated by subtracting the L* value of measurement 2 from the L* value of measurement 1. ΔL* is converted into a grayscale from the maximum to minimum value in a specified gradation, and displayed in grayscale on a two-dimensional coordinate system in which the elevation angle is represented by concentric circles and the azimuth angle is represented by vertical and horizontal lines.

[0050] Figure 4 shows the range of ΔL* from maximum to minimum values ​​in a grayscale representation at a predetermined gradation, and displays it in grayscale on a two-dimensional coordinate system where the elevation angle is represented by concentric circles and the azimuth angle is represented by vertical and horizontal lines. In other words, Figure 4 is the two-dimensional coordinate system of (3-1) above. In Figure 4, the closer to white the value, the larger the ΔL*. In Figure 4, the center of the circular two-dimensional coordinate system indicates an elevation angle of 0 degrees (perpendicular to the surface light source), and the edges indicate the maximum elevation angle. Also, in Figure 4, the direction to the right from the center indicates an azimuth angle of 0 degrees, the direction above from the center indicates an azimuth angle of 90 degrees, the direction to the left from the center indicates an azimuth angle of 180 degrees, and the direction below from the center indicates an azimuth angle of 270 degrees.

[0051] The gradation in (3-1) above is usually the nth power of 2, and examples include 16 gradations, 32 gradations, 64 gradations, 128 gradations, and 256 gradations. The two-dimensional coordinates of the grayscale display in (3-1) above can be created using, for example, ELDIM's product name "EzContrast."

[0052] (3-2) It is confirmed that there are two regions in which ΔL* is distributed concentrically within the two-dimensional coordinate system, and that the two regions are located at approximately symmetrical positions within the two-dimensional coordinate system. Here, "substantially symmetric" means that when the elevation angle and azimuth angle at the center of one region where ΔL* is distributed concentrically are defined as X1 and Y1, and the elevation angle and azimuth angle at the center of the other region where ΔL* is distributed concentrically are defined as X2 and Y2, the difference between X1 and X2 is ±3 degrees or less, and the absolute value of the difference between Y1 and Y2 is 180 degrees ±5 degrees or less. The difference between X1 and X2 is preferably ±1 degree or less, and the absolute value of the difference between Y1 and Y2 is preferably 180 degrees ±3 degrees or less.

[0053] In the two-dimensional coordinates shown in FIG. 4, there are two regions where ΔL* is distributed concentrically, and the two regions are located at approximately symmetrical positions in the two-dimensional coordinates. These symmetrically positioned concentric circles are thought to be formed by fluctuations in the reflectance of P waves and S waves due to changes in the refractive index distribution within the measurement region (biaxially stretched plastic film has a high refractive index in the slow axis direction, so concentric circles are formed along the slow axis. In the case of Figure 4, the direction of the slow axis of the biaxially stretched plastic film is the left-right direction in Figure 4) and the optical distance of the low refractive index layer within the measurement region (the optical distance (the distance of transmitted light passing through the low refractive index layer) increases with distance from the measurement center). In other words, such symmetrically positioned concentric circles are normally formed in an optical film having a low refractive index layer on a biaxially stretched plastic film (Measurement 1), and in the biaxially stretched plastic film alone (and further, the higher the in-plane retardation, the more likely it is that concentric circles will be formed in symmetric positions). Furthermore, since measurement 2 only contains either P waves or S waves and does not form the concentric circles described above, the difference in the L* values ​​(ΔL*) between measurements 1 and 2 maintains the concentric circles formed by the L* values ​​in measurement 1 (since the influence of the light source is canceled out in ΔL*, the fluctuations in the reflectance of P waves and S waves become clearer, and the concentric circles are also formed more clearly).

[0054] (3-3) Regarding the elevation angles at the center of the region where ΔL* is distributed concentrically, one elevation angle is set to α degrees and the other elevation angle is set to β degrees.

[0055] In the above (3-3), the "elevation angle at the center of the area where ΔL* is distributed concentrically" can be determined, for example, by displaying two-dimensional coordinates on the software of the measuring device and selecting the center of the area where ΔL* is distributed concentrically (for example, by clicking the center of the area where ΔL* is distributed concentrically with the mouse).

[0056] (3-4) From the measurement results, ΔEab at each azimuth angle is calculated from the difference between the L*, a*, and b* values ​​of Measurement 1 at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α+β) / 2 and the L*, a*, and b* values ​​of Measurement 2 at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α+β) / 2. Then, the difference between the maximum and minimum values ​​of ΔEab is calculated.

[0057] In the above (3-4), if a fraction occurs in (α+β) / 2, the value shall be rounded up. For example, if α is 30 degrees and β is 31 degrees, ΔEab shall be calculated from the L*, a*, and b* values ​​at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is 31 degrees.

[0058] (α+β) / 2 is preferably 50 degrees or less, more preferably 40 degrees or less, and even more preferably 30 degrees or less. By setting (α+β) / 2 to 50 degrees or less, the value of (α+β) / 2 moves away from the vicinity of the Brewster angle, making it easier to suppress fluctuations in the reflectance of P waves and S waves at angles before and after (α+β) / 2. There is no particular lower limit for (α+β) / 2, but it is preferably 5 degrees or more, more preferably 10 degrees or more, and even more preferably 15 degrees or more. By reducing the refractive index nz in the thickness direction of the biaxially stretched plastic film, it becomes easier to bring (α+β) / 2 into the above range.

[0059] The optical film of the present disclosure requires that the difference between the maximum and minimum values ​​of ΔEab calculated at each azimuth angle (0 degrees or more and 359 degrees or less) be less than 17.0. A large difference means that a color is perceived as being strong at a specific azimuth angle. Therefore, if the difference is 17.0 or more, a color is perceived as being strong at the azimuth angle at which ΔEab is at its maximum value, and rainbow unevenness visible to the naked eye cannot be suppressed. The difference is preferably 16.0 or less, more preferably 15.0 or less, and even more preferably 14.0 or less. The lower limit of the difference is not particularly limited, but is usually about 3.0. The difference can be easily made less than 17.0 by using, for example, a biaxially stretched plastic film that satisfies condition 2 described below.

[0060] In condition 1, the reason for using the measured values ​​at an azimuth angle of 0 degrees or more and 359 degrees or less passing through (α + β) / 2 is that in the direction passing through (α + β) / 2, the bright and dark areas of the concentric circles are measured alternately (see Figure 4), and the difference between the maximum and minimum values ​​of ΔEab is theoretically the largest. Another feature of the present disclosure is that it uses the azimuth angle passing through the center of the concentric circles ((α + β) / 2) instead of the Brewster angle. The concentric circles of ΔL* described above are affected by the in-plane retardation (the larger the in-plane retardation, the smaller the concentric circles). On the other hand, the Brewster angle is governed by the refractive index and is therefore not affected by the in-plane retardation. Therefore, even if the "difference between the maximum and minimum values ​​of ΔEab" calculated from the measured values ​​of the azimuth angle passing through the Brewster angle is small, it is difficult to reduce the "difference between the maximum and minimum values ​​of ΔEab" calculated from the measured values ​​of the azimuth angle passing through (α + β) / 2. In other words, the present disclosure has technical significance superior to Patent Documents 2 and 3 in that it makes it possible to suppress the visibility of rainbow unevenness by making the "difference between the maximum and minimum values ​​of ΔEab" calculated from the measured values ​​of the azimuth angle passing through (α + β) / 2, rather than the Brewster angle, satisfy a predetermined condition. Also, as will be described later, n x >n y ≧n z In a biaxially stretched plastic film that satisfies this relationship, the retardation in the oblique direction gradually decreases as the angle is tilted along the slow axis direction, reaching 0 nm at around (α + β) / 2. The inventors have found that when the elevation angle is fixed at (α + β) / 2 and the color is observed at an azimuth angle of 0 to 359 degrees, a color unevenness similar to rainbow unevenness is strongly observed. Usually, rainbow unevenness is observed in a direction where the retardation has a predetermined value. However, when the elevation angle is fixed at (α + β) / 2 and the observation is performed at an azimuth angle of 0 to 359 degrees, the oblique retardation in the slow axis direction is 0 nm, which is completely different from normal observation. For this reason, in this specification, the color unevenness that is visually observed at an elevation angle of (α + β) / 2 and the azimuth angle of 0 to 359 degrees is sometimes referred to as "color distortion" to distinguish it from "rainbow unevenness."

[0061] Within the optical film, the area where the difference between the maximum and minimum values ​​of ΔEab is less than 17.0 is preferably 50% or more, more preferably 70% or more, even more preferably 90% or more, and even more preferably 100%. Similarly, the area that satisfies various parameters other than the above difference (in-plane retardation, retardation in the thickness direction, condition 2, surface roughness of the low refractive index layer, etc.) is preferably 50% or more, more preferably 70% or more, even more preferably 90% or more, and even more preferably 100% within the optical film.

[0062] The maximum value of ΔEab calculated at each azimuth angle (0 degrees or more and 359 degrees or less) is preferably 16.0 or less, more preferably 15.0 or less, and even more preferably 14.0 or less. By setting the maximum value of ΔEab within the above range, it is possible to make iridescence less visible to the naked eye.

[0063] In the optical film of the present disclosure, when the Brewster angle for light in the entire visible light range incident on the optical film from the biaxially stretched plastic film side is defined as X degrees, it is preferable that (α+β) / 2 and X satisfy the following formula (A): X-(α+β) / 2≦20° (A) The Brewster angle (X) can be calculated from the refractive indices of the two materials using the following formula (B), where n1 is the refractive index on the incident side and n2 is the refractive index on the transmitting side. X=Arctan(n2 / n1) (B)

[0064] The optical film of the present disclosure can exhibit more significant effects when the Brewster angle and the center of the concentric circle ((α+β) / 2) are misaligned. In other words, an optical film that satisfies the above formula (A) is preferred in that it can easily exhibit the effects of the present disclosure. X-(α+β) / 2 is more preferably 30 degrees or more, and even more preferably 35 degrees or more. There are no particular restrictions on the upper limit of X-(α+β) / 2, but it is about 40 degrees.

[0065] <Plastic film> The plastic film is a biaxially stretched plastic film having an in-plane retardation of 2500 nm or less. By using a biaxially stretched plastic film, it is possible to improve the mechanical strength. Furthermore, by setting the in-plane retardation of the biaxially stretched plastic film to 2500 nm or less, the longitudinal and transverse stretching ratio can be kept within an appropriate range, thereby improving the mechanical strength and improving the tear resistance. Furthermore, by setting the in-plane retardation of the biaxially stretched plastic film to 2500 nm or less, it is possible to contribute to reducing the thickness of the biaxially stretched plastic film.

[0066] In order to easily suppress rainbow unevenness, the in-plane retardation of the biaxially stretched plastic film is preferably 2000 nm or less, more preferably 1500 nm or less, more preferably 1400 nm or less, more preferably 1150 nm or less, more preferably 1000 nm or less, and more preferably 600 nm or less. When the thickness of the biaxially stretched plastic film is reduced to 10 μm or more and 50 μm or less, the in-plane retardation is preferably 1400 nm or less.

[0067] If the in-plane retardation of the biaxially stretched plastic film is too small, the biaxially stretched film may not have sufficient mechanical strength. Therefore, the in-plane retardation of the biaxially stretched plastic film is preferably 20 nm or more, more preferably 100 nm or more, even more preferably 300 nm or more, and even more preferably 520 nm or more.

[0068] Preferred ranges of the in-plane retardation of the biaxially stretched plastic film are 20 nm or more and 2000 nm or less, 20 nm or more and 1500 nm or less, 20 nm or more and 1400 nm or less, 20 nm or more and 1150 nm or less, 20 nm or more and 1000 nm or less, 20 nm or more and 600 nm or less, 100 nm or more and 2000 nm or less, 100 nm or more and 1500 nm or less, 100 nm or more and 1400 nm or less, 100 nm or more and 1150 nm or less, 100 nm or more and 1000 nm or less, Examples include 300nm to 2000nm, 300nm to 1500nm, 300nm to 1400nm, 300nm to 1150nm, 300nm to 1000nm, 300nm to 600nm, 520nm to 2000nm, 520nm to 1500nm, 520nm to 1400nm, 520nm to 1150nm, 520nm to 1000nm, and 520nm to 600nm.

[0069] The thickness direction retardation (Rth) of the biaxially stretched plastic film is preferably 2000 nm or more, more preferably 3000 nm or more, and even more preferably 4000 nm or more. The upper limit of Rth is about 10000 nm, preferably 8000 nm or less, more preferably 7000 nm or less. By setting Rth within the above range, it is possible to more easily suppress rainbow unevenness. Preferred ranges for Rth of the biaxially stretched plastic film include 2000 nm or more and 10000 nm or less, 2000 nm or more and 8000 nm or less, 2000 nm or more and 7000 nm or less, 3000 nm or more and 10000 nm or less, 3000 nm or more and 8000 nm or less, 3000 nm or more and 7000 nm or less, 4000 nm or more and 10000 nm or less, 4000 nm or more and 8000 nm or less, and 4000 nm or more and 7000 nm or less. In order to set the Rth of the biaxially stretched plastic film within the above range, it is preferable to increase the stretching ratios in the machine direction and the cross direction. By increasing the stretching ratios in the machine direction and the cross direction, the refractive index nz in the thickness direction of the biaxially stretched plastic film decreases, making it easier to increase Rth. Furthermore, by reducing the nz of the biaxially stretched plastic film, it is easier to reduce the value of "(α + β) / 2". When the value of "(α + β) / 2" is reduced, the value of (α + β) / 2 moves away from the vicinity of the Brewster angle, making it easier to suppress fluctuations in the reflectance of P waves and S waves at angles before and after (α + β) / 2.

[0070] By setting the in-plane retardation and thickness direction retardation within the above ranges, the degree of stretching of the biaxially stretched plastic film can be made closer to uniform biaxiality, and the mechanical strength of the biaxially stretched plastic film can be improved.

[0071] The in-plane retardation (Re) and thickness direction retardation (Rth) are expressed by the following formulas (1) and (2), where nx is the refractive index in the slow axis direction, which is the direction in which the refractive index is greatest, ny is the refractive index in the fast axis direction, which is the direction perpendicular to the slow axis direction, nz is the refractive index in the thickness direction of the plastic film, and T [nm] is the thickness of the plastic film. In this specification, the in-plane retardation and thickness direction retardation refer to values ​​at a wavelength of 550 nm. Re=(nx-ny)×T[nm] (1) Rth=((nx+ny) / 2-nz)×T[nm] (2)

[0072] The direction of the slow axis, the in-plane retardation, and the retardation in the thickness direction can be measured, for example, by using a product name "RETS-100" manufactured by Otsuka Electronics Co., Ltd. When measuring the in-plane retardation and the like using the product name "RETS-100" manufactured by Otsuka Electronics Co., Ltd., it is preferable to prepare for the measurement according to the following steps (A1) to (A4).

[0073] (A1) First, turn on the RETS-100 light source and leave it for at least 60 minutes to stabilize it. Then, select the rotating analyzer method and the θ mode (a mode for measuring angular phase difference and calculating Rth). By selecting this θ mode, the stage becomes a tilting rotation stage. (A2) Next, enter the following measurement conditions into the RETS-100. (Measurement conditions) Retardation measurement range: Rotating analyzer method Measurement spot diameter: φ5mm Tilt angle range: 0° Measurement wavelength range: 400nm to 800nm The average refractive index of the plastic film. For example, in the case of PET film, N = 1.617. The average refractive index N of a plastic film can be calculated using the formula (N = (nx + ny + nz) / 3) based on nx, ny, and nz. Thickness: Thickness measured separately using an SEM or optical microscope (A3) Next, background data is obtained without placing a sample in the device, using the device as a closed system, and this is performed every time the light source is turned on. (A4) Then, the sample is placed on the stage inside the device and measured.

[0074] The in-plane retardation, thickness direction retardation, and the direction of the slow axis described below are preferably measured at five points on a sample 50 mm long x 50 mm wide cut out from a biaxially stretched plastic film, and the average values ​​of the measured values ​​are determined by the five measurement points, one in the center and four points 10 mm from each corner of the sample toward the center (the five points indicated by black circles in Figure 7).

[0075] The biaxially stretched plastic film preferably has an in-plane retardation relative to the thickness direction retardation (in-plane retardation / thickness direction retardation) of 0.10 or less. In this specification, the in-plane retardation relative to the thickness direction retardation may be expressed as "Re / Rth". Re / Rth can be measured, for example, as follows.

[0076] The in-plane retardation values ​​measured at five points on the sample are defined as Re1, Re2, Re3, Re4, and Re5, respectively, and the thickness direction retardation values ​​measured at five points on the sample are defined as Rth1, Rth2, Rth3, Rth4, and Rth5, respectively. The biaxially stretched plastic film preferably has an average of Re1 / Rth1, Re2 / Rth2, Re3 / Rth3, Re4 / Rth4 and Re5 / Rth5 of 0.10 or less.

[0077] A small ratio (Re / Rth) of the in-plane retardation to the thickness direction retardation means that the biaxial stretching of the biaxially stretched plastic film approaches uniform biaxiality. Therefore, by setting Re / Rth to 0.10 or less, the mechanical strength of the biaxially stretched plastic film can be improved. Re / Rth is more preferably 0.07 or less, and even more preferably 0.05 or less. The lower limit of Re / Rth is about 0.01. The Re / Rth of a completely uniaxially stretched plastic film is 2.0. General-purpose uniaxially stretched plastic films are also stretched slightly in the machine direction. For this reason, the Re / Rth of general-purpose uniaxially stretched plastic films is about 1.0.

[0078] Each of Re1 / Rth1, Re2 / Rth2, Re3 / Rth3, Re4 / Rth4, and Re5 / Rth5 is preferably 0.10 or less, more preferably 0.07 or less, and even more preferably 0.05 or less. The lower limit of these ratios is about 0.01.

[0079] The biaxially stretched plastic film preferably satisfies the following condition 2. <Condition 2> The direction of the slow axis is measured at five points on the sample. When the angles formed between any one side of the sample and the direction of the slow axis at each measurement point are defined as D1, D2, D3, D4, and D5, respectively, the difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is 5.0 degrees or more.

[0080] When the slow axis of a biaxially stretched plastic film is neatly aligned, rainbow unevenness tends to be easily visible. On the other hand, when the slow axis of a biaxially stretched plastic film is uneven, the rainbow unevenness becomes blurred and difficult to see. Therefore, by satisfying condition 2, it is possible to easily prevent rainbow unevenness from being visible to the naked eye. In other words, by satisfying condition 2, it is possible to easily satisfy the condition that the difference between the maximum and minimum values ​​of ΔEab is less than 17.0. General-purpose stretched plastic films are designed so that the direction of the slow axis does not shift. However, as mentioned above, by deliberately shifting the direction of the slow axis of the plastic film, it is possible to more easily suppress rainbow unevenness. Furthermore, although the effect of suppressing rainbow unevenness is small even if the slow axis varies over a large area, it is possible to more easily suppress rainbow unevenness by varying the slow axis over a relatively small area of ​​50 mm length x 50 mm width.

[0081] In condition 2, any one side of the sample that serves as the reference for the angle with the direction of the slow axis may be either a vertical or horizontal side of the sample, as long as the same side is used as the reference for all of D1 to D5.

[0082] Furthermore, satisfying condition 2 is preferable in that the biaxially stretched plastic film can have good bending resistance. On the other hand, general-purpose oriented films in which the slow axes are aligned without satisfying condition 2 break after bending tests or retain a strong bending tendency. Specifically, uniaxially stretched films such as those described in Patent Document 1 break when bent along the slow axis, and retain a strong bending tendency when bent in a direction perpendicular to the slow axis. Furthermore, general-purpose biaxially stretched films retain a strong bending tendency when bent in a direction perpendicular to the slow axis. A biaxially stretched plastic film that satisfies condition 2 is preferable in that it can prevent the film from retaining a bending habit or breaking after a bending test, regardless of the bending direction.

[0083] The difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is preferably 6.0 degrees or more, more preferably 8.0 degrees or more, and even more preferably 10.0 degrees or more. If the difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is too large, the orientation of the plastic film tends to decrease, and the mechanical strength tends to decrease. Therefore, the difference is preferably 20.0 degrees or less, more preferably 17.0 degrees or less, and even more preferably 15.0 degrees or less.

[0084] In condition 2, preferred ranges for the difference between the maximum and minimum values ​​of D1 to D5 are, for example, 5.0 degrees or more and 20.0 degrees or less, 6.0 degrees or more and 20.0 degrees or less, 8.0 degrees or more and 20.0 degrees or less, 10.0 degrees or more and 20.0 degrees or less, 5.0 degrees or more and 17.0 degrees or less, 6.0 degrees or more and 17.0 degrees or less, 8.0 degrees or more and 17.0 degrees or less, 10.0 degrees or more and 17.0 degrees or less, 5.0 degrees or more and 15.0 degrees or less, 6.0 degrees or more and 15.0 degrees or less, 8.0 degrees or more and 15.0 degrees or less, and 10.0 degrees or more and 15.0 degrees or less.

[0085] In the biaxially stretched plastic film, D1 to D5 are preferably 5 degrees or more and 30 degrees or less, or 60 degrees or more and 85 degrees or less, more preferably 7 degrees or more and 25 degrees or less, or 65 degrees or more and 83 degrees or less, and even more preferably 10 degrees or more and 23 degrees or less, or 67 degrees or more and 80 degrees or less. By setting D1 to D5 to 5 degrees or more or 85 degrees or less, respectively, it is possible to easily prevent blackout when viewed through polarized sunglasses. Furthermore, by setting D1 to D5 to 30 degrees or less or 60 degrees or more, respectively, it is possible to easily prevent a decrease in mechanical strength due to a decrease in the orientation of the plastic film.

[0086] Biaxially oriented plastic films may be in the form of a sheet or a roll. In either case, a 50 mm x 50 mm sample may be cut from any location on the plastic film. However, if the longitudinal and transverse directions of the sheet or roll can be confirmed, the sample should be cut along the confirmed longitudinal and transverse directions. For example, in the case of a roll, the machine direction (MD) of the roll can be considered the machine direction, and the width direction (TD) of the roll can be considered the transverse direction. Furthermore, if the machine and transverse directions of the sheet can be confirmed, the machine direction can be considered the machine direction and the transverse direction can be considered the transverse direction. If it is difficult to confirm the machine and transverse directions of the sheet, and the sheet is rectangular or square, the machine and transverse directions can be confirmed by the four sides of the rectangle or square. If it is difficult to confirm the machine and transverse directions of the sheet, and the sheet is a shape other than a rectangle or square (e.g., a circle, a triangle), the machine and transverse directions can be confirmed by drawing a rectangle or square with the largest area that does not extend beyond the outer frame of the sheet, and confirming the machine and transverse directions by the sides of the drawn rectangle or square. When multiple samples each measuring 50 mm long x 50 mm wide can be taken from a sheet-like plastic film, the proportion of the samples that satisfy condition 2 among the multiple samples is preferably 50% or more, more preferably 70% or more, even more preferably 90% or more, and even more preferably 100%. The same applies to the in-plane retardation, thickness direction retardation, and Re / Rth.

[0087] It is preferable that the biaxially stretched plastic film does not crack or break after 100,000 cycles (more preferably 300,000 cycles) of the folding test shown in the examples. Furthermore, after 100,000 cycles (more preferably 300,000 cycles) of the biaxially stretched plastic film, when a measurement sample is placed on a horizontal table, the angle at which the edge of the sample rises from the table is preferably 20 degrees or less, more preferably 15 degrees or less. A sample edge rise angle of 15 degrees or less means that the film is less likely to develop a crease due to folding. Furthermore, it is preferable that the biaxially stretched plastic film exhibits the above-mentioned results (no cracks, breaks, or creases due to folding) in both the average direction of the slow axis and the average direction of the fast axis of the film. When a uniaxially stretched plastic film is subjected to a folding test, it breaks in the stretching direction and retains a strong bending tendency in the direction perpendicular to the stretching direction.

[0088] <<Specific Configuration of Biaxially Stretched Plastic Film>> The biaxially stretched plastic film may have a single-layer structure or a multi-layer structure, with the single-layer structure being preferred. In order to suppress rainbow unevenness while improving mechanical strength, it is preferable that the in-plane retardation of a biaxially stretched plastic film be small. To reduce the in-plane retardation of a stretched plastic film, it is important to precisely control the stretching, such as by uniformly stretching the film in the longitudinal and transverse directions. While precise control of the stretching is difficult in a multilayer structure due to differences in the physical properties of each layer, a single-layer structure is preferable because it is easy to precisely control the stretching.

[0089] Examples of resin components constituting the biaxially stretched plastic film include polyester, polyimide, polyethersulfone, polysulfone, polypropylene, polymethylpentene, polyvinyl chloride, polyvinyl acetal, polyether ketone, polymethyl methacrylate, polycarbonate, polyurethane, and amorphous olefin (cycloolefin polymer: COP). Among these, polyester is preferred because it is easy to improve mechanical strength. That is, the biaxially stretched plastic film is preferably a polyester film, and more preferably a biaxially stretched polyethylene terephthalate film.

[0090] In addition, biaxially oriented plastic films are made of a material that has good mechanical strength. x >n y ≧n z It is preferable that the relationship between x >n y ≧n z In order to satisfy the above relationship, the resin component constituting the biaxially stretched plastic film is preferably a positive birefringent material. n x >n y ≧n z In a biaxially stretched plastic film that satisfies the above relationship, the retardation in the oblique direction gradually decreases as the angle of inclination along the slow axis increases, and becomes 0 nm at around "(α+β) / 2". As mentioned above, "color distortion" is easily felt when the oblique retardation is 0 nm when the elevation angle along the slow axis is "(α+β) / 2". In the present disclosure, x >n y ≧n z Even when a biaxially stretched plastic film that satisfies the above relationship is used, color distortion can be easily suppressed by making the difference between the maximum and minimum values ​​of ΔEab less than 17.0.

[0091] Positively birefringent materials include polyesters, polycarbonates, cycloolefin polymers, polyimide films, and polyamides.

[0092] Examples of polyesters that can be used to form biaxially stretched polyester films include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polybutylene terephthalate (PBT). Among these, PET is preferred because it has low intrinsic birefringence and can easily reduce in-plane retardation.

[0093] The biaxially stretched plastic film may contain additives such as ultraviolet absorbers, light stabilizers, antioxidants, antistatic agents, flame retardants, antigelling agents, and surfactants.

[0094] The lower limit of the thickness of the biaxially stretched plastic film is preferably 10 μm or more, more preferably 15 μm or more, more preferably 20 μm or more, more preferably 25 μm or more, and more preferably 30 μm or more, and the upper limit is preferably 200 μm or less, more preferably 180 μm or less, more preferably 150 μm or less, more preferably 100 μm or less, more preferably 80 μm or less, more preferably 60 μm or less, and more preferably 50 μm or less. To reduce the thickness, the thickness of the biaxially stretched plastic film is preferably 50 μm or less. By making the thickness 10 μm or more, it is possible to easily obtain good mechanical strength, and by making the thickness 200 μm or less, it is possible to easily obtain an in-plane retardation of 2500 nm or less.

[0095] Preferred ranges of the thickness of the biaxially stretched plastic film are, for example, 10 μm or more and 200 μm or less, 15 μm or more and 200 μm or less, 20 μm or more and 200 μm or less, 25 μm or more and 200 μm or less, 30 μm or more and 200 μm or less, 10 μm or more and 180 μm or less, 15 μm or more and 180 μm or less, 20 μm or more and 180 μm or less, 25 μm or more and 180 μm or less, 30 μm or more and 180 μm or less, 10 μm or more and 150 μm or less, 15 μm or more and 150 μm or less, 20 μm or more and 150 μm or less, 25 μm or more and 150 μm or less, 30 μm or more and 150 μm or less, 10 μm or more and 100 μm or less, 15 μm or more and 100 μm or less, 20 μm or more and 100 μm or less, 25 μm or more and 100 μm or less, 30 μm or more and 100 μm or less, 10 μm or more and 80 μm or less, 15 μm or more and 80 μm or less, 20 μm or more and 80 μm or less, 25 μm or more and 80 μm or less, 30 μm or more and 80 μm or less, 10 μm or more and 60 μm or less, 15 μm or more and 60 μm or less, 20 μm or more and 60 μm or less, 25 μm or more and 60 μm or less, 30 μm or more and 60 μm or less, 10 μm or more and 50 μm or less, 15 μm or more and 50 μm or less, 20 μm or more and 50 μm or less, 25 μm or more and 50 μm or less, 30 μm or more and 50 μm or less.

[0096] The biaxially stretched plastic film preferably has a haze according to JIS K7136:2000 of 3.0% or less, more preferably 2.0% or less, even more preferably 1.5% or less, and even more preferably 1.0% or less. Furthermore, the biaxially stretched plastic film preferably has a total light transmittance according to JIS K7361-1:1997 of 80% or more, more preferably 85% or more, and even more preferably 90% or more.

[0097] A biaxially stretched plastic film can be obtained by stretching a resin layer containing components constituting the plastic film. Stretching techniques include sequential biaxial stretching and simultaneous biaxial stretching.

[0098] -Sequential biaxial stretching- In the sequential biaxial stretching, the cast film is stretched in the machine direction, and then stretched in the width direction of the film. Stretching in the machine direction is usually performed by the difference in peripheral speed between a pair of stretching rolls. Stretching in the machine direction may be performed in one stage, or may be performed in multiple stages using multiple pairs of stretching rolls. In order to suppress excessive variations in optical properties such as in-plane retardation, it is preferable to place multiple nip rolls close to the stretching roll. The stretching ratio in the machine direction is usually 2 to 15 times, and in order to suppress excessive variations in optical properties such as in-plane retardation, it is preferably 2 to 7 times, more preferably 3 to 5 times, and even more preferably 3 to 4 times. To suppress excessive variations in optical properties such as in-plane retardation, the stretching temperature is preferably from the glass transition temperature of the resin to the glass transition temperature + 100°C. In the case of PET, the stretching temperature is preferably from 70°C to 120°C, more preferably from 80°C to 110°C, and even more preferably from 95°C to 110°C. Regarding the stretching temperature, the average in-plane retardation tends to decrease by shortening the low-temperature stretching section, for example by increasing the temperature of the film quickly, whereas the average in-plane retardation tends to increase and the variation in the slow axis tends to decrease by increasing the low-temperature stretching section, for example by increasing the temperature of the film slowly. It is preferable to use a heater that generates turbulent airflow when heating during stretching. Heating with turbulent airflow generates temperature differences in minute regions within the film plane, which in turn causes minute deviations in the orientation axis, making it easier to satisfy condition 2. Furthermore, when a plastic film satisfies condition 2, it becomes easier to make the difference between the maximum and minimum values ​​of ΔEab less than 17.0.

[0099] The film stretched in the machine direction may be imparted with properties such as easy slippage, easy adhesion, antistatic properties, etc. Furthermore, before in-line coating, the film may be subjected to a surface treatment such as corona treatment, flame treatment, or plasma treatment, as necessary. The coating film formed by in-line coating is very thin, with a thickness of approximately 10 nm to 2000 nm (and the coating film is stretched even thinner by stretching treatment). In this specification, such a thin layer is not counted as part of the number of layers constituting the plastic film.

[0100] The width direction stretching is usually performed using a tenter method, in which the film is conveyed while being held at both ends with clips, and stretched in the width direction. The width direction stretching ratio is usually 2 to 15 times, and in order to suppress excessive variation in optical properties such as in-plane retardation, it is preferably 2 to 5 times, more preferably 3 to 5 times, and even more preferably 3 to 4.5 times. It is also preferable that the width direction stretching ratio is higher than the longitudinal direction stretching ratio. The stretching temperature is preferably from the glass transition temperature of the resin to the glass transition temperature + 120°C, and preferably increases from upstream to downstream. Specifically, when the transverse stretching section is divided into two, the difference between the upstream and downstream temperatures is preferably 20°C or more, more preferably 30°C or more, even more preferably 35°C or more, and still more preferably 40°C or more. In the case of PET, the first-stage stretching temperature is preferably 80°C or more and 120°C or less, more preferably 90°C or more and 110°C or less, and even more preferably 95°C or more and 105°C or less.

[0101] The plastic film sequentially biaxially stretched as described above is preferably heat-treated in a tenter at a temperature equal to or higher than the stretching temperature but lower than the melting point to impart flatness and dimensional stability. Specifically, in the case of PET, heat setting is preferably performed at a temperature in the range of 150°C to 255°C, more preferably 200°C to 250°C. Furthermore, in order to suppress excessive variation in optical properties such as in-plane retardation, it is preferable to perform additional stretching of 1% to 10% in the first half of the heat treatment. After the plastic film is heat-treated, it is slowly cooled to room temperature and then wound up. If necessary, a relaxation treatment or the like may be performed during the heat treatment and slow cooling. The relaxation rate during the heat treatment is preferably 0.5% to 5%, more preferably 0.5% to 3%, even more preferably 0.8% to 2.5%, and even more preferably 1% to 2% in order to suppress excessive variation in optical properties such as in-plane retardation. The relaxation rate during slow cooling is preferably 0.5% to 3%, more preferably 0.5% to 2%, even more preferably 0.5% to 1.5%, and even more preferably 0.5% to 1.0% in order to suppress excessive variation in optical properties such as in-plane retardation. To obtain good flatness, the temperature during slow cooling is preferably 80°C or higher and 150°C or lower, more preferably 90°C or higher and 130°C or lower, even more preferably 100°C or higher and 130°C or lower, and even more preferably 100°C or higher and 120°C or lower.

[0102] -Simultaneous biaxial stretching- In simultaneous biaxial stretching, the cast film is introduced into a simultaneous biaxial tenter, and while both ends of the film are held by clips, the film is conveyed and stretched simultaneously and / or stepwise in the machine direction and width direction. Simultaneous biaxial stretching machines include pantograph, screw, drive motor, and linear motor types, but drive motor or linear motor types are preferred, as they allow the stretching ratio to be changed as desired and relaxation treatment to be performed at any location.

[0103] The area ratio of the simultaneous biaxial stretching is usually 6 to 50. In order to suppress excessive variation in optical properties such as in-plane retardation, the area ratio is preferably 8 to 30, more preferably 9 to 25, even more preferably 9 to 20, and still more preferably 10 to 15. In simultaneous biaxial stretching, the stretch ratio in the machine direction and the stretch ratio in the width direction are preferably adjusted to the above area ratio within the range of 2 to 15. In the case of simultaneous biaxial stretching, it is preferable to make the stretching ratios in the machine direction and the width direction almost the same and to make the stretching speeds in the machine direction and the width direction almost the same in order to suppress in-plane orientation differences.

[0104] The stretching temperature for simultaneous biaxial stretching is preferably from the glass transition temperature of the resin to the glass transition temperature + 120°C in order to suppress excessive variation in optical properties such as in-plane retardation. In the case of PET, the temperature is preferably from 80°C to 160°C, more preferably from 90°C to 150°C, and even more preferably from 100°C to 140°C.

[0105] The simultaneously biaxially stretched film is preferably subsequently heat-treated in a heat-setting chamber in a tenter at a temperature equal to or higher than the stretching temperature but lower than the melting point in order to impart flatness and dimensional stability. The heat-treatment conditions are the same as those after the successive biaxial stretching.

[0106] <Low refractive index layer> The low refractive index layer has the role of enhancing the anti-reflection properties of the optical film and also of making it easier to suppress rainbow irregularities when viewed with the naked eye. Light traveling from the inside of an image display device toward the viewer is linearly polarized light when it passes through a polarizer. However, after passing through a biaxially stretched plastic film, the polarization state of the linearly polarized light is disrupted, resulting in a mixture of P-wave and S-wave light. Because there is a difference between the reflectance of P-waves and the reflectance of S-waves, and the reflectance difference is wavelength-dependent, it is believed that rainbow unevenness is visible to the naked eye. Here, if a low-refractive index layer is provided on a biaxially stretched plastic film, the reflectance difference can be reduced, which is thought to facilitate the suppression of rainbow unevenness. However, simply forming a low-refractive index layer on a general-purpose biaxially stretched plastic film makes it difficult to achieve the difference between the maximum and minimum values ​​of ΔEab within the above range, and therefore rainbow unevenness cannot be suppressed to a high level. To facilitate achieving the difference between the maximum and minimum values ​​of ΔEab within the above range, it is preferable to impart variation to the direction of the slow axis of the biaxially stretched plastic film, as described above. The low refractive index layer is preferably formed on the side of the optical film farthest from the biaxially stretched plastic film. By forming a high refractive index layer (described later) adjacent to the low refractive index layer on the biaxially stretched plastic film side of the low refractive index layer, it is possible to further improve antireflection properties and more easily suppress rainbow unevenness.

[0107] The refractive index of the low refractive index layer is preferably 1.10 or more and 1.48 or less, more preferably 1.20 or more and 1.45 or less, more preferably 1.26 or more and 1.40 or less, more preferably 1.28 or more and 1.38 or less, and more preferably 1.30 or more and 1.32 or less. The thickness of the low refractive index layer is preferably 80 nm to 120 nm, more preferably 85 nm to 110 nm, and even more preferably 90 nm to 105 nm, and is preferably larger than the average particle size of the low refractive index particles such as hollow particles.

[0108] The method for forming a low refractive index layer can be roughly divided into a wet method and a dry method. The wet method includes a method of forming a low refractive index layer by a sol-gel method using a metal alkoxide or the like, a method of forming a low refractive index layer by coating a resin having a low refractive index such as a fluororesin, and a method of forming a low refractive index layer by coating a coating liquid for forming a low refractive index layer in which low refractive index particles are contained in a resin composition. The dry method includes a method of selecting particles having a desired refractive index from the low refractive index particles described below and forming the layer by a physical vapor deposition method or a chemical vapor deposition method. The wet method is superior to the dry method in terms of production efficiency, suppression of oblique reflection hue, and chemical resistance. Among the wet methods, it is preferable to form the low refractive index layer using a coating liquid for forming the low refractive index layer, in which low refractive index particles are contained in a binder resin composition, in order to improve adhesion, water resistance, scratch resistance, and to reduce the refractive index.

[0109] The low refractive index layer is usually located on the outermost surface of the optical film, and therefore is required to have good scratch resistance, and general-purpose low refractive index layers are also designed to have a predetermined scratch resistance. In recent years, hollow particles with large particle diameters have been used as low-refractive-index particles to lower the refractive index of low-refractive-index layers. The inventors have discovered a problem: even if the surface of a low-refractive-index layer containing such hollow particles with large particle diameters is rubbed with an object with only fine solid matter (e.g., sand) or only oil, no visible scratches are visible, the layer is still scratched when rubbed with an object with both solid matter and oil (hereinafter, this problem may be referred to as "oil dust resistance"). The action of rubbing with an object with solid matter and oil corresponds to, for example, a user operating a touch-panel image display device with a finger that has oil contained in cosmetics, food, etc., and sand contained in the air attached to it. Improving the oil dust resistance of the low refractive index layer is preferable because it leads to maintaining the rainbow unevenness suppression effect for a long period of time.

[0110] As a result of our investigations, we found that the above-mentioned scratches are primarily caused by partial chipping of hollow particles contained in the low-refractive index layer or the detachment of hollow particles. We believe that this is due to the large unevenness caused by hollow particles formed on the surface of the low-refractive index layer. Specifically, when the surface of the low-refractive index layer is rubbed with a finger having solid and oily particles attached, the oil acts as a binder, causing the solid particles to adhere to the finger as the finger moves across the surface of the low-refractive index layer. This tends to cause a portion of the solid (e.g., sharp points of sand) to enter a depression on the surface of the low-refractive index layer, or for the solid that has entered the depression to pass through the depression with the finger and climb over the protrusion (hollow particle). This exerts a large force on the protrusion (hollow particle), which is thought to damage or cause the hollow particle to fall off. Furthermore, we believe that the resin itself located in the depression is also scratched by friction with the solid, making the hollow particle more susceptible to detachment due to the damage to the resin. The inventors conducted extensive research and found that, in order to impart oil dust resistance, it is effective to use a combination of hollow and solid particles as low-refractive index particles and to uniformly disperse the hollow and solid particles. Fig. 8 shows a cross-sectional photograph of a low-refractive index layer in which hollow and solid particles are uniformly dispersed, and Fig. 9 shows a cross-sectional photograph of a low-refractive index layer in which hollow and solid particles are not uniformly dispersed. The cross-sectional photographs of Figs. 8 and 9 were obtained by observation using an electron microscope (product number: H-7650) manufactured by Hitachi High-Tech Corporation under conditions of an emission current of 10 μA, an acceleration voltage of 100 keV, and a filament voltage of 20 V.

[0111] In order to improve oil dust resistance, the low refractive index particles preferably include hollow particles and non-hollow particles. The material for the hollow particles and non-hollow particles may be either an inorganic compound such as silica or magnesium fluoride, or an organic compound, but silica is preferred for its low refractive index and strength. The following description will focus on hollow silica particles and non-hollow silica particles.

[0112] Hollow silica particles refer to particles that have an outer shell layer made of silica, and the particles surrounded by the outer shell layer are hollow, with air contained inside the hollow. Hollow silica particles are particles whose refractive index decreases in proportion to the occupancy rate of the gas compared to the refractive index of silica itself due to the inclusion of air. Non-hollow silica particles are particles that do not have a hollow interior like hollow silica particles. Non-hollow silica particles are, for example, solid silica particles. The shape of the hollow silica particles and non-hollow silica particles is not particularly limited, and may be a perfect sphere, a spheroid, a nearly spherical shape such as a polyhedron that can approximate a sphere, etc. Among these, in consideration of scratch resistance, a perfect sphere, a spheroid, or a nearly spherical shape is preferred.

[0113] Since hollow silica particles contain air inside, they serve to lower the refractive index of the entire low refractive index layer. By using hollow silica particles with a large particle size and a high air content, the refractive index of the low refractive index layer can be further lowered. However, hollow silica particles tend to have poor mechanical strength. In particular, when hollow silica particles with a large particle size and a high air content are used, the scratch resistance of the low refractive index layer tends to be reduced. The non-hollow silica particles, when dispersed in the binder resin, play a role in improving the scratch resistance of the low refractive index layer.

[0114] To incorporate hollow silica particles and non-hollow silica particles into a binder resin at a high concentration and to uniformly disperse the particles in the resin in the film thickness direction, it is preferable to set the average particle size of the hollow silica particles and the average particle size of the non-hollow silica particles so that the hollow silica particles are close to each other and the non-hollow particles can be inserted between the hollow silica particles. Specifically, the ratio of the average particle size of the non-hollow silica particles to the average particle size of the hollow silica particles (average particle size of non-hollow silica particles / average particle size of hollow silica particles) is preferably 0.29 or less, more preferably 0.27 or less. Furthermore, the ratio of the average particle sizes is preferably 0.05 or more, more preferably 0.10 or more. Considering optical properties and mechanical strength, the average particle size of the hollow silica particles is preferably 20 nm or more and 100 nm or less. Since it is easy to lower the refractive index of the entire low refractive index layer, the average particle size of the hollow silica particles is more preferably 50 nm or more and 100 nm or less, and even more preferably 60 nm or more and 80 nm or less. Furthermore, considering dispersibility while preventing aggregation of the non-hollow silica particles, the average particle size of the non-hollow silica particles is preferably 5 nm or more and 20 nm or less, and more preferably 10 nm or more and 15 nm or less.

[0115] The surfaces of the hollow silica particles and non-hollow silica particles are preferably coated with a silane coupling agent, and it is more preferable to use a silane coupling agent having a (meth)acryloyl group or an epoxy group. By treating the silica particles with a silane coupling agent, the affinity between the silica particles and the binder resin is improved, making the silica particles less likely to aggregate, and therefore making it easier for the silica particles to be dispersed uniformly.

[0116] Examples of silane coupling agents include 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, and 3-triethoxysilyl-N-(1,3-dimethoxysilane). methyl-butylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, tris-(trimethoxysilylpropyl)isocyanurate, 3-mercaptopropylmethyldimethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-isocyanatopropyltriethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, phenyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, hexyltrimethoxysilane, hexyltriethoxysilane, octyltriethoxysilane, decyltrimethoxysilane, 1,6-bis(trimethoxysilyl)hexane, trifluoropropyltrimethoxysilane, vinyltrimethoxysilane, and vinyltriethoxysilane. In particular, it is preferable to use one or more selected from 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, and 3-methacryloxypropyltriethoxysilane.

[0117] The higher the content of hollow silica particles, the higher the filling rate of hollow silica particles in the binder resin, and the lower the refractive index of the low refractive index layer. Therefore, the content of hollow silica particles is preferably 100 parts by mass or more, more preferably 125 parts by mass or more, per 100 parts by mass of binder resin. On the other hand, if the content of hollow silica particles relative to the binder resin is too high, the number of hollow silica particles exposed from the binder resin increases, and the amount of binder resin bonding between the particles decreases. As a result, the hollow silica particles tend to be easily damaged or fall off, and the mechanical strength of the low refractive index layer, such as scratch resistance, tends to decrease. For this reason, the content of hollow silica particles is preferably 400 parts by mass or less, more preferably 300 parts by mass or less, and even more preferably 200 parts by mass or less, relative to 100 parts by mass of the binder resin.

[0118] If the content of non-hollow silica particles is small, even if the non-hollow silica particles are present on the surface of the low refractive index layer, it may not affect the increase in hardness.In addition, if the content of non-hollow silica particles is large, it can reduce the influence of shrinkage unevenness caused by polymerization of the binder resin, and reduce the unevenness that occurs on the surface of the low refractive index layer after the resin is cured.Therefore, the content of non-hollow silica particles is preferably 20 parts by mass or more, more preferably 40 parts by mass or more, even more preferably 90 parts by mass or more, and even more preferably 100 parts by mass or more, relative to 100 parts by mass of the binder resin. On the other hand, if the content of the non-hollow silica particles is too high, the non-hollow silica particles tend to aggregate, causing uneven shrinkage of the binder resin and increasing surface irregularities. Therefore, the content of the non-hollow silica particles is preferably 200 parts by mass or less, more preferably 150 parts by mass or less, and even more preferably 120 parts by mass or less, relative to 100 parts by mass of the binder resin.

[0119] By incorporating hollow silica particles and non-hollow silica particles in the binder resin in the above ratio, the barrier properties of the low refractive index layer can be improved, presumably because the silica particles are uniformly dispersed at a high filling rate, thereby inhibiting the permeation of gases and the like. Furthermore, various cosmetics such as sunscreens and hand creams may contain low-volatility low-molecular-weight polymers. By improving the barrier properties of the low-refractive index layer, it is possible to prevent the low-molecular-weight polymer from penetrating into the coating film of the low-refractive index layer, and to prevent problems (such as abnormal appearance) caused by the low-molecular-weight polymer remaining in the coating film for a long period of time.

[0120] The binder resin of the low refractive index layer preferably contains a cured product of an ionizing radiation curable resin composition. The ionizing radiation curable compound contained in the ionizing radiation curable resin composition is preferably a compound having an ethylenically unsaturated bond group. Among these, a (meth)acrylate compound having a (meth)acryloyl group is more preferred. Hereinafter, a (meth)acrylate compound having four or more ethylenically unsaturated bond groups will be referred to as a "polyfunctional (meth)acrylate compound," and a (meth)acrylate compound having two to three ethylenically unsaturated bond groups will be referred to as a "low-functional (meth)acrylate compound."

[0121] The (meth)acrylate compound may be either a monomer or an oligomer. In particular, the ionizing radiation-curable compound preferably contains a low-functional (meth)acrylate compound in order to suppress uneven shrinkage during curing and to facilitate smoothing of the irregularities on the surface of the low refractive index layer. The proportion of the low-functional (meth)acrylate compound in the ionizing radiation-curable compound is preferably 60% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 100% by mass. In addition, in order to suppress the uneven shrinkage during curing and to easily smooth the uneven shape of the surface of the low refractive index layer, the low-functional (meth)acrylate compound is preferably a (meth)acrylate compound having two ethylenically unsaturated bond groups.

[0122] Among the (meth)acrylate compounds, examples of bifunctional (meth)acrylate compounds include isocyanuric acid di(meth)acrylate, polyalkylene glycol di(meth)acrylates such as ethylene glycol di(meth)acrylate, polyethylene glycol diacrylate, and polybutylene glycol di(meth)acrylate, bisphenol A tetraethoxydiacrylate, bisphenol A tetrapropoxydiacrylate, and 1,6-hexanediol diacrylate. Examples of trifunctional (meth)acrylate compounds include trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and isocyanuric acid-modified tri(meth)acrylate. Examples of the tetrafunctional or higher polyfunctional (meth)acrylate compounds include pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and dipentaerythritol tetra(meth)acrylate. These (meth)acrylate compounds may be modified as described below.

[0123] Examples of the (meth)acrylate oligomer include acrylate polymers such as urethane (meth)acrylate, epoxy (meth)acrylate, polyester (meth)acrylate, and polyether (meth)acrylate. Urethane (meth)acrylates can be obtained, for example, by reacting a polyhydric alcohol and an organic diisocyanate with a hydroxy (meth)acrylate. In addition, preferred epoxy (meth)acrylates are (meth)acrylates obtained by reacting a tri- or higher functional aromatic epoxy resin, alicyclic epoxy resin, aliphatic epoxy resin, or the like with (meth)acrylic acid; (meth)acrylates obtained by reacting a di- or higher functional aromatic epoxy resin, alicyclic epoxy resin, aliphatic epoxy resin, or the like with a polybasic acid and (meth)acrylic acid; and (meth)acrylates obtained by reacting a di- or higher functional aromatic epoxy resin, alicyclic epoxy resin, aliphatic epoxy resin, or the like with a phenol and (meth)acrylic acid.

[0124] Furthermore, the (meth)acrylate compound may have a portion of its molecular skeleton modified to suppress uneven shrinkage due to crosslinking and improve surface smoothness. For example, the (meth)acrylate compound may be modified with ethylene oxide, propylene oxide, caprolactone, isocyanuric acid, alkyl, cyclic alkyl, aromatic, bisphenol, or the like. In particular, in order to increase affinity with low refractive index particles (including silica particles) and suppress aggregation of the low refractive index particles, the (meth)acrylate compound is preferably modified with an alkylene oxide such as ethylene oxide or propylene oxide. The proportion of the alkylene oxide-modified (meth)acrylate compound in the ionizing radiation-curable compound is preferably 60% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 100% by mass. The alkylene oxide-modified (meth)acrylate compound is preferably a low-functionality (meth)acrylate compound, and more preferably a (meth)acrylate compound having two ethylenically unsaturated bond groups.

[0125] Examples of alkylene oxide-modified (meth)acrylate compounds having two ethylenically unsaturated bond groups include bisphenol F alkylene oxide-modified di(meth)acrylate, bisphenol A alkylene oxide-modified di(meth)acrylate, isocyanuric acid alkylene oxide-modified di(meth)acrylate, and polyalkylene glycol di(meth)acrylate, of which polyalkylene glycol di(meth)acrylate is preferred. The average repeating unit of the alkylene glycol contained in the polyalkylene glycol di(meth)acrylate is preferably 3 to 5. Furthermore, the alkylene glycol contained in the polyalkylene glycol di(meth)acrylate is preferably ethylene glycol and / or polyethylene glycol. Examples of alkylene oxide-modified (meth)acrylate compounds having three ethylenically unsaturated bond groups include trimethylolpropane alkylene oxide-modified tri(meth)acrylate and isocyanuric acid alkylene oxide-modified tri(meth)acrylate. The above ionizing radiation curable resins can be used alone or in combination of two or more.

[0126] The low refractive index layer preferably contains a leveling agent for stain resistance and surface smoothness. The leveling agent may be a fluorine-based or silicone-based agent, with a silicone-based agent being preferred. The inclusion of a silicone-based leveling agent can further smoothen the surface of the low-reflectivity layer. Furthermore, the low-reflectivity layer surface can be improved in terms of slipperiness and antifouling properties (ease of wiping off fingerprints, large contact angles with pure water and hexadecane).

[0127] The content of the leveling agent is preferably 1 part by mass or more and 25 parts by mass or less, more preferably 2 parts by mass or more and 20 parts by mass or less, and even more preferably 5 parts by mass or more and 18 parts by mass or less, relative to 100 parts by mass of the binder resin. By making the content of the leveling agent 1 part by mass or more, it is possible to easily impart various properties such as antifouling properties. Furthermore, by making the content of the leveling agent 25 parts by mass or less, it is possible to suppress a decrease in scratch resistance.

[0128] The low refractive index layer preferably has a maximum height roughness Rz of 110 nm or less, more preferably 90 nm or less, even more preferably 70 nm or less, and even more preferably 60 nm or less. Furthermore, the Rz / Ra (Ra is the arithmetic mean roughness) is preferably 12.0 or less, more preferably 10.0 or less. Setting the Rz / Ra in the above range is particularly effective when the Rz is large, approximately 90 nm or more and 110 nm or less. In this specification, Ra and Rz are two-dimensional roughness parameters that are expanded to three dimensions as described in the Shimadzu Corporation Scanning Probe Microscope SPM-9600 Upgrade Kit Instruction Manual (SPM-9600, February 2016, pp. 194-195). Ra and Rz are defined as follows:

[0129] (arithmetic mean roughness Ra) When a reference length (L) is cut out from the roughness curve in the direction of the average line, and the X axis is taken in the direction of the average line of this cut-out portion and the Y axis is taken in the direction of the vertical magnification, and the roughness curve is expressed as y = f(x), the following formula can be used.

[0130]

number

[0131] (Maximum height roughness Rz) A reference length is cut out from the roughness curve in the direction of the average line, and the distance between the peak line and the valley line of this cutout portion is measured in the direction of the vertical magnification of the roughness curve.

[0132] When using a scanning probe microscope SPM-9600 manufactured by SHIMADZU CORPORATION, it is preferable to measure and analyze Ra and Rz under the following conditions, for example. <Measurement conditions> Measurement mode: Phase Scanning range: 5 μm x 5 μm Scanning speed: 0.8Hz to 1Hz Number of pixels: 512 x 512 Cantilever used: NanoWorld Holding AG, product number "NCHR", resonance frequency: 320 kHz, spring constant: 42 N / m <Analysis conditions> Tilt Correction: Line Fit

[0133] A small Rz means that the convex portions caused by the hollow silica particles in the micro-region are small. Also, a small Rz / Ra means that the irregularities caused by the silica particles in the micro-region are uniform and do not have protruding irregularities relative to the average elevation difference of the irregularities. Note that, although the value of Ra is not particularly limited in the present disclosure, Ra is preferably 15 nm or less, more preferably 12 nm or less, even more preferably 10 nm or less, and even more preferably 6.5 nm or less. By uniformly dispersing the low refractive index particles in the low refractive index layer and suppressing uneven shrinkage of the low refractive index layer, it becomes easier to satisfy the above ranges of Rz and Rz / Ra.

[0134] By ensuring that the Rz and Rz / Ra of the low refractive index layer surface are within the above ranges, the resistance when solid matter moves over the convex portions (caused by hollow silica particles present near the surface) on the low refractive index layer surface can be reduced. Therefore, even when rubbed with oily sand under a load, the solid matter is believed to move smoothly over the low refractive index layer surface. It is also believed that the hardness of the concave portions themselves is increased. As a result, it can be assumed that breakage or detachment of the hollow silica particles is prevented, and damage to the binder resin itself is also prevented.

[0135] Unless otherwise specified, surface roughness such as Rz and Ra means the average value of measurements taken at 14 locations excluding the minimum and maximum values ​​of the measurements taken at 16 locations. In this specification, the 16 measurement points are preferably determined by dividing the area inside the 0.5 cm margin from the outer edge of the measurement sample into five equal parts vertically and horizontally, and the measurement centers are preferably the 16 intersections. The measurement sample is preferably 5 cm x 5 cm.

[0136] The low refractive index layer can be formed by applying and drying a low refractive index layer-forming coating liquid in which each component constituting the low refractive index layer is dissolved or dispersed. Usually, a solvent is used in the low refractive index layer-forming coating liquid to adjust the viscosity or to make each component soluble or dispersible. Examples of the solvent include ketones (acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, etc.), ethers (dioxane, tetrahydrofuran, etc.), aliphatic hydrocarbons (hexane, etc.), alicyclic hydrocarbons (cyclohexane, etc.), aromatic hydrocarbons (toluene, xylene, etc.), halogenated carbons (dichloromethane, dichloroethane, etc.), esters (methyl acetate, ethyl acetate, butyl acetate, etc.), alcohols (butanol, cyclohexanol, etc.), cellosolves (methyl cellosolve, ethyl cellosolve, etc.), cellosolve acetates, sulfoxides (dimethyl sulfoxide, etc.), glycol ethers (1-methoxy-2-propyl acetate, etc.), amides (dimethylformamide, dimethylacetamide, etc.), and mixtures thereof may also be used.

[0137] If the solvent evaporates too quickly, the solvent will convect violently when the coating liquid for forming a low refractive index layer dries. Therefore, even if the silica particles in the coating liquid are uniformly dispersed, the state of uniform dispersion is likely to be disrupted by the violent convection of the solvent during drying. For this reason, it is preferable to include a solvent with a slow evaporation rate. Specifically, it is preferable to include a solvent with a relative evaporation rate (relative evaporation rate when the evaporation rate of n-butyl acetate is set to 100) of 70 or less, more preferably 30 to 60. Furthermore, the solvent with a relative evaporation rate of 70 or less preferably accounts for 10 to 50% by mass of the total solvent, and more preferably 20 to 40% by mass. Examples of relative evaporation rates for slow-evaporating solvents are isobutyl alcohol (64), 1-butanol (47), 1-methoxy-2-propyl acetate (44), ethyl cellosolve (38), and cyclohexanone (32). The remaining solvent (solvent other than the solvent with a slow evaporation rate) is preferably one that has excellent resin solubility. The remaining solvent preferably has a relative evaporation rate of 100 or more.

[0138] In addition, in order to suppress convection of the solvent during drying and improve the dispersibility of the silica particles, it is preferable that the drying temperature during the formation of the low refractive index layer is as low as possible. The drying temperature can be appropriately set in consideration of the type of solvent, the dispersibility of the silica particles, the production rate, etc.

[0139] <Reflectance> The optical film of the present disclosure has a reflectance measured from the low refractive index layer side of 2.00% or less, preferably 1.70% or less, more preferably 1.20% or less, and even more preferably 1.00% or less.

[0140] In this specification, reflectance refers to the luminous reflectance Y value of the CIE 1931 standard color system. The reflectance is preferably calculated as the average value of 10 arbitrary points. In this specification, the reflectance of an optical film is measured by preparing a sample by attaching a black plate to the side of the optical film opposite to the side where the reflectance is to be measured via a transparent adhesive layer, and irradiating light from the low refractive index layer side of the sample at an incident angle of 5°. The light source used for measuring the reflectance is preferably light source C. The difference in refractive index between the transparent adhesive layer and a member (e.g., a biaxially stretched plastic film) in contact with the sample's transparent adhesive layer is preferably within 0.15, more preferably within 0.10, more preferably within 0.05, and even more preferably within 0.01. Furthermore, the black plate preferably has a total light transmittance according to JIS K7361-1:1997 of 1% or less, more preferably 0%. Furthermore, the difference in refractive index between the resin constituting the black plate and the transparent adhesive layer is preferably within 0.15, more preferably within 0.10, more preferably within 0.05, and even more preferably within 0.01.

[0141] <Haze, total light transmittance> The optical film preferably has a haze of 5% or less, more preferably 4% or less, and even more preferably 3% or less according to JIS K7136:2000.The optical film preferably has a haze of 0.5% or more, more preferably 1.0% or more, and even more preferably 1.5% or more according to JIS K7136:2000. Furthermore, the optical film preferably has a total light transmittance according to JIS K7361-1:1997 of 90% or more, more preferably 91% or more, and even more preferably 92% or more.

[0142] <Other layers> The optical film of the present disclosure may have layers other than the biaxially stretched plastic film and the low refractive index layer. The low refractive index layer and the layers other than the low refractive index layer are preferably optically isotropic. An optically isotropic layer refers to a layer having an in-plane retardation of less than 20 nm, preferably 10 nm or less, more preferably 5 nm or less. For example, the optical film of the present disclosure preferably has one or more layers selected from a hard coat layer, an antiglare layer, and a high refractive index layer between the biaxially stretched plastic film and the low refractive index layer.

[0143] <High refractive index layer> The high refractive index layer is formed on the biaxially stretched plastic film side of the low refractive index layer as needed. When a hard coat layer described later is provided, the high refractive index layer is preferably formed between the hard coat layer and the low refractive index layer.

[0144] The high refractive index layer preferably has a refractive index of 1.53 or more and 1.85 or less, more preferably 1.54 or more and 1.80 or less, more preferably 1.55 or more and 1.75 or less, and even more preferably 1.56 or more and 1.70 or less. The thickness of the high refractive index layer is preferably 200 nm or less, more preferably 50 nm or more and 180 nm or less, and even more preferably 70 nm or more and 150 nm or less. When a high refractive index hard coat layer is used, the thickness is preferably similar to that of the hard coat layer.

[0145] The high refractive index layer can be formed from a coating liquid for forming a high refractive index layer, which contains, for example, a binder resin composition and high refractive index particles. Examples of the binder resin composition include the curable resin compositions exemplified for the hard coat layer described below.

[0146] Examples of high refractive index particles include antimony pentoxide, zinc oxide, titanium oxide, cerium oxide, tin-doped indium oxide, antimony-doped tin oxide, yttrium oxide, and zirconium oxide. Note that the refractive index of antimony pentoxide is about 1.79, that of zinc oxide is about 1.90, that of titanium oxide is about 2.3 to 2.7, that of cerium oxide is about 1.95, that of tin-doped indium oxide is about 1.95 to 2.00, that of antimony-doped tin oxide is about 1.75 to 1.85, that of yttrium oxide is about 1.87, and that of zirconium oxide is 2.10.

[0147] The average particle size of the high refractive index particles is preferably 2 nm or more, more preferably 5 nm or more, and even more preferably 10 nm or more. In order to suppress whitening and ensure transparency, the average particle size of the high refractive index particles is preferably 200 nm or less, more preferably 100 nm or less, more preferably 80 nm or less, more preferably 60 nm or less, and even more preferably 30 nm or less. The smaller the average particle size of the high refractive index particles, the better the transparency, and by making it 60 nm or less, the transparency can be made extremely good.

[0148] The average particle size of the high refractive index particles or the low refractive index particles can be calculated by the following steps (y1) to (y3). (y1) A cross section of the high refractive index layer or low refractive index layer is imaged by TEM or STEM. The acceleration voltage of the TEM or STEM is preferably 10 kV to 30 kV, and the magnification is preferably 50,000 to 300,000. (y2) Randomly extract 10 particles from the observed image and calculate the particle diameter of each particle. The particle diameter is measured as the distance between two parallel lines that maximize the distance between the two lines when the cross section of the particle is sandwiched between the two lines. If the particles are aggregated, the aggregated particles are considered to be a single particle and are measured. (y3) The same procedure is carried out five times on a separate observation image of the same sample, and the value obtained from the number average of the particle diameters of a total of 50 particles is taken as the average particle diameter of the high refractive index particles or low refractive index particles.

[0149] <Hard coat layer> The hard coat layer is formed as needed to improve the scratch resistance of the optical film. The hard coat layer is preferably formed between the biaxially stretched plastic film and the low refractive index layer. When the optical film further has a high refractive index layer, it is preferable to arrange the hard coat layer, the high refractive index layer, and the low refractive index layer in this order on the biaxially stretched plastic film.

[0150] In order to improve scratch resistance, the hard coat layer preferably contains a cured product of a curable resin composition such as a thermosetting resin composition or an ionizing radiation-curable resin composition, and more preferably contains a cured product of an ionizing radiation-curable resin composition.

[0151] A thermosetting resin composition is a composition containing at least a thermosetting resin, and is a resin composition that cures when heated. Examples of thermosetting resins include acrylic resins, urethane resins, phenolic resins, urea-melamine resins, epoxy resins, unsaturated polyester resins, and silicone resins. In a thermosetting resin composition, a curing agent is added to the curable resin as needed.

[0152] The ionizing radiation-curable resin composition is a composition containing a compound having an ionizing radiation-curable functional group (hereinafter also referred to as "ionizing radiation-curable compound"). Examples of the ionizing radiation-curable functional group include ethylenically unsaturated bond groups such as (meth)acryloyl groups, vinyl groups, and allyl groups, as well as epoxy groups and oxetanyl groups. As the ionizing radiation-curable compound, a compound having an ethylenically unsaturated bond group is preferred, and a compound having two or more ethylenically unsaturated bond groups is more preferred, and among these, a (meth)acrylate-based compound having two or more ethylenically unsaturated bond groups is even more preferred. As the (meth)acrylate-based compound having two or more ethylenically unsaturated bond groups, either a monomer or an oligomer can be used. Ionizing radiation refers to electromagnetic waves or charged particle beams that have an energy quantum capable of polymerizing or crosslinking molecules. Usually, ultraviolet (UV) rays or electron beams (EB) are used, but other types of radiation such as electromagnetic waves (X-rays and gamma rays), alpha rays, and charged particle beams (ion beams) can also be used. In this specification, (meth)acrylate means acrylate or methacrylate, (meth)acrylic acid means acrylic acid or methacrylic acid, and (meth)acryloyl group means acryloyl group or methacryloyl group.

[0153] The thickness of the hard coat layer is preferably 0.1 μm or more, more preferably 0.5 μm or more, even more preferably 1.0 μm or more, and even more preferably 2.0 μm or more to improve scratch resistance. Furthermore, the thickness of the hard coat layer is preferably 100 μm or less, more preferably 50 μm or less, more preferably 30 μm or less, more preferably 20 μm or less, more preferably 15 μm or less, and even more preferably 10 μm or less to suppress curling. The thickness of the hard coat layer is preferably 10 μm or less, more preferably 8 μm or less to improve flex resistance.

[0154] 《Anti-glare layer》 The antiglare layer can be formed from a coating liquid for forming an antiglare layer, which contains, for example, a binder resin composition and particles. As the binder resin composition, for example, the curable resin composition exemplified for the hard coat layer can be used.

[0155] The particles may be either organic or inorganic. Examples of organic particles include particles made of polymethyl methacrylate, polyacrylic-styrene copolymer, melamine resin, polycarbonate, polystyrene, polyvinyl chloride, benzoguanamine-melamine-formaldehyde condensate, silicone, fluorine-based resin, and polyester-based resin. Examples of inorganic particles include particles made of silica, alumina, antimony, zirconia, and titania.

[0156] The average particle size of the particles in the antiglare layer cannot be generalized because it varies depending on the thickness of the antiglare layer, but is preferably from 1.0 μm to 10.0 μm, more preferably from 2.0 μm to 8.0 μm, and even more preferably from 3.0 μm to 6.0 μm.

[0157] The average particle size of the particles in the antiglare layer can be calculated by the following steps (z1) to (z3). (z1) A transmission observation image of the cross section of the antiglare layer is taken using an optical microscope. The magnification is preferably 500 times or more and 2000 times or less. (z2) Randomly extract 10 particles from the observed image and calculate the particle diameter of each particle. The particle diameter is measured as the distance between two parallel lines that maximizes the distance between the two lines when the cross section of the particle is sandwiched between the two lines. (z3) The same procedure is repeated five times on a separate observation image of the same sample, and the value obtained from the number average of the particle diameters of a total of 50 particles is regarded as the average particle diameter of the particles in the antiglare layer.

[0158] The content of particles in the antiglare layer cannot be generalized because it varies depending on the desired level of antiglare properties, but it is preferably 1 part by mass or more and 100 parts by mass or less, more preferably 5 parts by mass or more and 50 parts by mass or less, and even more preferably 10 parts by mass or more and 30 parts by mass or less, per 100 parts by mass of the resin component. The antiglare layer may contain fine particles having an average particle size of less than 500 nm in order to impart antistatic properties, control the refractive index, and adjust the shrinkage of the antiglare layer due to curing of the curable resin composition.

[0159] The thickness of the antiglare layer is preferably 0.5 μm or more, more preferably 1.0 μm or more, and even more preferably 2.0 μm or more. The thickness of the antiglare layer is preferably 50 μm or less, more preferably 30 μm or more, more preferably 20 μm or less, more preferably 15 μm or less, and more preferably 10 μm or less. To improve bending resistance, the thickness of the antiglare layer is preferably 10 μm or less, and more preferably 8 μm or less.

[0160] <Example of layer structure> The following (1) to (5) are examples of layer configurations of the optical film of the present disclosure. (1) A structure having a low refractive index layer on a biaxially stretched plastic film. (2) A structure having a hard coat layer and a low refractive index layer in this order on a biaxially stretched plastic film. (3) A structure having a high refractive index layer and a low refractive index layer in this order on a biaxially stretched plastic film. (4) A structure having an antiglare layer and a low refractive index layer in this order on a biaxially stretched plastic film. (5) A structure having a hard coat layer, a high refractive index layer, and a low refractive index layer in this order on a biaxially stretched plastic film.

[0161] The total thickness of the optical film is preferably 100 μm or less, more preferably 60 μm or less, in order to maintain mechanical properties, suppress excessive variations in optical properties such as in-plane retardation, and effectively suppress blackout. In addition, in the optical film, the balance between the thickness of the biaxially stretched plastic film and the thickness of the layer other than the biaxially stretched plastic film is preferably 10:4 to 10:0.5.

[0162] <Shape, size> The optical film may be in the form of a sheet cut to a predetermined size, or in the form of a roll obtained by winding a long sheet into a roll. The size of the sheet is not particularly limited, but the maximum diameter is approximately 2 inches to 500 inches, and in the present disclosure, 30 inches to 80 inches is preferred. The "maximum diameter" refers to the maximum length when any two points on the optical film are connected. For example, if the optical film is rectangular, the diagonal line of the rectangular area is the maximum diameter. If the optical film is circular, the diameter is the maximum diameter. The width and length of the roll are not particularly limited, but generally, the width is about 500 mm to 3000 mm, and the length is about 100 m to 5000 m. The optical film in roll form can be cut into sheets according to the size of an image display device or the like. When cutting, it is preferable to remove the end of the roll, which has unstable physical properties. The shape of the sheet is also not particularly limited, and may be, for example, a polygon (triangle, square, pentagon, etc.), a circle, or a random, indeterminate shape. More specifically, when the optical film is square, the aspect ratio is not particularly limited as long as it does not cause any problems as a display screen. Examples include width:height = 1:1, 4:3, 16:10, 16:9, 2:1, etc.

[0163] <Application> The optical film of the present disclosure can be suitably used as an optical film for an image display device. The optical film of the present disclosure can be suitably used as an optical film to be disposed on the light-emitting surface side of a display element of an image display device, and in this case, it is preferable to have a polarizer between the display element and the optical film of the present disclosure. When the biaxially stretched plastic film satisfies condition 2, it is possible to prevent the film from retaining a bending habit or breaking after a bending test, regardless of the bending direction. Therefore, when the biaxially stretched plastic film satisfies condition 2, it can be more suitably used as a plastic film for a curved image display device or a foldable image display device.

[0164] [Polarizing plate] The polarizing plate of the present disclosure has a polarizer, a first transparent protective plate arranged on one side of the polarizer, and a second transparent protective plate arranged on the other side of the polarizer, and at least one of the first transparent protective plate and the second transparent protective plate is the optical film of the present disclosure described above. In the polarizing plate, the optical film is preferably arranged so that the surface on the biaxially stretched plastic film side faces the polarizer side.

[0165] Fig. 5 is a cross-sectional view showing an embodiment of a polarizing plate 700 of the present disclosure. The polarizing plate 700 of Fig. 5 has a polarizer 300, a first transparent protective plate (500) arranged on one side of the polarizer, and a second transparent protective plate (600) arranged on the other side of the polarizer. The polarizing plate 700 of Fig. 5 uses an optical film 100 as the first transparent protective plate (500). The polarizing plate 700 of Fig. 5 includes the polarizer 300, the first transparent protective plate (500), and the second transparent protective plate (600) laminated together with an adhesive layer 400 interposed therebetween.

[0166] The polarizing plate is used to impart anti-reflection properties, for example, in combination with a λ / 4 retardation plate. In this case, the λ / 4 retardation plate is disposed on the display element of the image display device, and the polarizing plate is disposed closer to the viewer than the λ / 4 retardation plate. Furthermore, when the polarizing plate is used for a liquid crystal display device, it is used to provide a liquid crystal shutter function. In this case, the liquid crystal display device is arranged in the order of a lower polarizing plate, a liquid crystal display element, and an upper polarizing plate, and the absorption axis of the polarizer of the lower polarizing plate and the absorption axis of the polarizer of the upper polarizing plate are arranged perpendicular to each other. In the configuration of the liquid crystal display device, it is preferable to use the polarizing plate of the present disclosure as the upper polarizing plate.

[0167] <Transparent protection plate> The polarizing plate of the present disclosure uses the optical film of the present disclosure as at least one of the first transparent protective plate and the second transparent protective plate, and it is preferable that both the first transparent protective plate and the second transparent protective plate are the optical film of the present disclosure.

[0168] When one of the first and second transparent protective plates is the optical film of the present disclosure, the other transparent protective plate is not particularly limited, but is preferably an optically isotropic transparent protective plate. In this specification, an optically isotropic transparent protective plate refers to one having an in-plane retardation of less than 20 nm, preferably 10 nm or less, and more preferably 5 nm or less. Examples of optically isotropic transparent protective plates include acrylic films, cyclic polyolefin films, and triacetyl cellulose (TAC) films. Acrylic films and cyclic polyolefin films are preferred because they have moisture permeability similar to that of biaxially stretched plastic films, making them less likely to distort when the polarizing plate absorbs water and providing excellent protection for the polarizer. Furthermore, when only one of the first transparent protective plate and the second transparent protective plate is the optical film of the present disclosure described above, it is preferable to use the optical film of the present disclosure described above as the transparent protective plate on the light exit side.

[0169] <Polarizer> Examples of polarizers include sheet-type polarizers (such as polyvinyl alcohol films, polyvinyl formal films, polyvinyl acetal films, and saponified ethylene-vinyl acetate copolymer films) made by stretching a film dyed with iodine or the like, wire-grid polarizers made of a large number of parallelly arranged metal wires, coated polarizers coated with a lyotropic liquid crystal and a dichroic guest-host material, and multilayer thin-film polarizers. These polarizers may also be reflective polarizers that have the function of reflecting polarized light components that are not transmitted.

[0170] The polarizer is preferably arranged so that the angle between its absorption axis and the slow axis of the biaxially stretched plastic film is within 90°±5°, more preferably within 90°±3°, and even more preferably within 90°±1°.

[0171] [Image display device (1)] The image display device (1) of the present disclosure is an image display device having a display element, and a polarizer and an optical film arranged on the light-emitting side of the display element, wherein the optical film is the optical film of the present disclosure described above, and the optical film is arranged so that the angle between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film is within 90°±5°, and the surface of the optical film facing the low refractive index layer faces away from the display element. The angle is preferably within 90°±3°, more preferably within 90°±1°.

[0172] FIG. 6 is a cross-sectional view showing an embodiment of an image display device (1) of the present disclosure and an embodiment of an image display device (2) described later. 6 includes an optical film 100 on the light-emitting surface side (upper side in FIG. 6) of a display element 800. Each of the image display devices 100 in FIG. 6 includes a polarizer 300 between the display element 800 and the optical film 100.

[0173] It should be noted that the image display device 1000 is not limited to the configuration shown in Fig. 6. For example, in Fig. 6, the components constituting the image display device 1000 are arranged at predetermined intervals, but it is preferable that the components are laminated and integrated together via an adhesive layer or the like. The image display device may also include components (other optical films, etc.) not shown.

[0174] <Display element> Examples of display elements include liquid crystal display elements, EL display elements (organic EL display elements, inorganic EL display elements), plasma display elements, and the like. Examples also include LED display elements such as mini LEDs and micro LED display elements, and liquid crystal display elements and LED display elements using QDs. When the display element of the display device is a liquid crystal display element, a backlight is required on the surface of the liquid crystal display element opposite to the resin sheet.

[0175] The image display device may also be an image display device equipped with a touch panel function. Examples of touch panels include resistive, capacitive, electromagnetic induction, infrared, and ultrasonic types. The touch panel function may be added to the display element, such as an in-cell touch panel liquid crystal display element, or may be a display element on which a touch panel is placed.

[0176] Furthermore, if the biaxially stretched plastic film satisfies condition 2, the optical film can be prevented from retaining a bending habit or from breaking after a bending test. Therefore, if the biaxially stretched plastic film satisfies condition 2, the image display device is preferably a curved image display device or a foldable image display device. When the image display device is a curved image display device or a foldable image display device, the display element is preferably an organic EL display element.

[0177] <Other plastic films> The image display device of the present disclosure may include other plastic films as long as the effects of the present disclosure are not impaired. As the other plastic film, one having optical isotropy is preferred.

[0178] [Image display device (2)] The image display device of the present disclosure is an image display device including a polarizer and an optical film on a light exit surface of a display element, The polarizer is arranged so that the angle formed between the direction of the absorption axis of the polarizer and the left-right direction or the up-down direction of the display element is within ±5 degrees, the polarizer and the biaxially stretched plastic film of the optical film are disposed so that the angle formed between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film is within 90 degrees ± 5 degrees, The optical film has a low refractive index layer on a biaxially stretched plastic film having an in-plane retardation of less than 2500 nm, the low refractive index layer being located on the outermost surface of the optical film, and has a region in which the difference between the maximum and minimum values ​​of ΔEab is less than 17.0. Here, Measurement 1A is performed on Laminate 1A to calculate the L*, a*, and b* values ​​in the L*a*b* color system. Measurement 2A is performed on Laminate 2A to calculate the L*, a*, and b* values ​​in the L*a*b* color system. ΔEab is calculated under Condition 1A based on the results of Measurement 1A and Measurement 2A.

[0179] <Measurement 1A> A laminate 1A is prepared by laminating a polarizer and the optical film in this order on a display element. In the laminate 1A, the optical film is arranged so that the surface on the low refractive index layer side faces away from the polarizer. The polarizer is also arranged so that the angle between the absorption axis of the polarizer and the left-right or up-down direction of the display element is within ±5 degrees. Furthermore, the polarizer is also arranged so that the angle between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film is within ±5 degrees. The display element of the laminate 1A is set to white, and the transmitted light emitted from the low refractive index layer side of the laminate 1A is measured at 1-degree intervals at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees. Based on the transmitted light at each angle, the L* value, a* value, and b* value of the L*a*b* color system are calculated. The measurement area of ​​the transmitted light is an arbitrary 1 mm 2 More than 10mm 2 The areas are as follows:

[0180] <Measurement 2A> A laminate 2A is prepared by laminating a polarizer on the same display element as in the measurement 1A. The display element of the laminate 2A is set to white, and the transmitted light exiting the polarizer side of the laminate 2A is measured at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees, one degree at a time, and the L*, a*, and b* values ​​of the L*a*b* color system are calculated based on the transmitted light at each angle. The measurement area of ​​the transmitted light is set to approximately the same as Measurement 1A in the plane.

[0181] <Condition 1A> At all elevation angles and all azimuth angles, ΔL* is calculated by subtracting the L* value of measurement 2A from the L* value of measurement 1A. ΔL* is converted from the maximum to minimum values ​​into a grayscale at a specified gradation and displayed in grayscale on a two-dimensional coordinate system where the elevation angle is represented by concentric circles and the azimuth angle is represented by vertical and horizontal lines. It is confirmed that there are two regions in the two-dimensional coordinate system where ΔL* is distributed concentrically, and that the two regions are located at approximately symmetrical positions in the two-dimensional coordinate system. Regarding the elevation angles at the center of the region where ΔL* is distributed concentrically, one elevation angle is set to α degrees and the other elevation angle is set to β degrees. ΔEab at each azimuth angle is calculated from the difference between the L*, a*, and b* values ​​of Measurement 1A at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α+β) / 2, and the L*, a*, and b* values ​​of Measurement 2A at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α+β) / 2.

[0182] "Laminate 1A" in Measurement 1A of the image display device (2) refers to the image display device (2). Also, "Laminate 2" in Measurement 1B of the image display device (2) refers to the image display device (2) excluding the optical film of the present disclosure.

[0183] Measurement 1A and Measurement 2A for the image display device (2) of the present disclosure are the same as Measurement 1 and Measurement 2 for the optical film of the present disclosure described above, except that the surface light source and the display element are different. In addition, the preferred embodiments of Measurement 1A and Measurement 2A are the same as the preferred embodiments of Measurement 1 and Measurement 2 (for example, the preferred ranges of the L* value, a* value, and b* value in the state of laminate 2A are the same as the preferred ranges of the L* value, a* value, and b* value in the state of laminate 2). In addition, the preferred embodiment of Condition 1A is the same as the preferred embodiment of Condition 1 described above.

[0184] [Selection method for optical films for image display devices] The method for selecting an optical film for an image display device according to the present disclosure is a method for selecting an optical film for an image display device, which comprises a polarizer and an optical film on a light exit surface of a display element, and which is arranged so that the direction of the absorption axis of the polarizer is parallel to the left-right direction or the up-down direction of the display element, and the method comprises the steps of: selecting an optical film X that satisfies a determination condition that the optical film X is an optical film X having a low refractive index layer on a biaxially stretched plastic film having an in-plane retardation of less than 2500 nm, the low refractive index layer being located on the outermost surface of the optical film X, and having a region in which the difference between the maximum and minimum values ​​of ΔEab is less than 17.0; Here, Measurement 1B is performed on Laminate 1B to calculate the L*, a*, and b* values ​​in the L*a*b* color system. Measurement 2B is performed on Laminate 2B to calculate the L*, a*, and b* values ​​in the L*a*b* color system. Based on the results of Measurement 1B and Measurement 2B, ΔEab is calculated under Condition 1B.

[0185] <Measurement 1B> A laminate 1B is prepared by laminating a polarizer and the optical film X in this order on a display element. In the laminate 1B, the optical film is arranged so that the surface on the low refractive index layer side faces away from the polarizer. The polarizer is also arranged so that the angle between the absorption axis of the polarizer and the left-right or up-down direction of the display element is within ±5 degrees. Furthermore, the angle between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film X is within 90 degrees ±5 degrees. The display element of the laminate 1B is set to white, and the transmitted light emitted from the low refractive index layer side of the laminate 1B is measured at 1-degree intervals at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees. Based on the transmitted light at each angle, the L* value, a* value, and b* value of the L*a*b* color system are calculated. The measurement area of ​​the transmitted light is an arbitrary 1 mm 2 More than 10mm 2 The areas are as follows:

[0186] <Measurement 2B> A laminate 2B is prepared by laminating a polarizer on the same display element as in the measurement 1B. The display element of the laminate 2B is set to white, and the transmitted light exiting the polarizer side of the laminate 2B is measured at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees, one degree at a time, and the L*, a*, and b* values ​​of the L*a*b* color system are calculated based on the transmitted light at each angle. The measurement area of ​​the transmitted light is set to approximately the same as that of Measurement 1B within the plane.

[0187] <Condition 1B> At all elevation angles and all azimuth angles, ΔL* is calculated by subtracting the L* value of measurement 2B from the L* value of measurement 1B. ΔL* is converted from the maximum to minimum values ​​into a grayscale at a specified gradation and displayed in grayscale on a two-dimensional coordinate system where the elevation angle is represented by concentric circles and the azimuth angle is represented by vertical and horizontal lines. It is confirmed that there are two regions in the two-dimensional coordinate system where ΔL* is distributed concentrically, and that the two regions are located at approximately symmetrical positions in the two-dimensional coordinate system. Regarding the elevation angles at the center of the region where ΔL* is distributed concentrically, one elevation angle is set to α degrees and the other elevation angle is set to β degrees. ΔEab at each azimuth angle is calculated from the difference between the L*, a*, and b* values ​​of Measurement 1B at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α+β) / 2, and the L*, a*, and b* values ​​of Measurement 2B at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α+β) / 2.

[0188] In the method for selecting an optical film for an image display device according to the present disclosure, "an image display device in which the absorption axis direction of the polarizer is arranged parallel to the left-right direction or the up-down direction of the display element" means an image display device in which the angle formed between the absorption axis direction of the polarizer and the left-right direction or the up-down direction of the display element is within ±5 degrees. The angle is preferably within ±3 degrees, more preferably within ±5 degrees.

[0189] Measurement 1B and Measurement 2B in the method for selecting an optical film for an image display device of the present disclosure are the same as Measurement 1 and Measurement 2 of the optical film of the present disclosure described above, except that the surface light source and the display element are different. In addition, preferred embodiments of Measurement 1B and Measurement 2B are the same as preferred embodiments of Measurement 1 and Measurement 2 (for example, preferred ranges of the L* value, a* value, and b* value in the state of laminate 2B are the same as preferred ranges of the L* value, a* value, and b* value in the state of laminate 2). In addition, preferred embodiments of Condition 1B are the same as preferred embodiments of Condition 1 described above.

[0190] The method for selecting an optical film for an image display device according to the present disclosure preferably further includes additional criteria, such as preferred embodiments of the optical film according to the present disclosure (e.g., retardation in the thickness direction, surface roughness, etc.). The method for selecting an optical film for an image display device according to the present disclosure is useful as a method for selecting an optical film for an image display device having a polarizer on the light-emitting surface side of a display element, and is particularly useful as a method for selecting an optical film for an image display device in which the direction of the absorption axis of the polarizer is arranged parallel to the left-right direction or the up-down direction of the display element. [Example]

[0191] Next, the present disclosure will be described in more detail by way of examples, but the present disclosure is not limited in any way by these examples.

[0192] 1. Measurement, Evaluation The atmosphere for the following measurement and evaluation shall be at a temperature of 23°C ± 5°C and a relative humidity of 40% RH or more and 65% RH or less. Also, before the measurement and evaluation, the sample shall be exposed to the said atmosphere for 30 minutes or more.

[0193] 1-1. In-plane retardation (Re), retardation in the thickness direction (Rth), and direction of the slow axis Samples of 50 mm in length × 50 mm in width were cut out from the plastic films used in the examples, comparative examples, and reference examples prepared or prepared in "2" described below. At that time, the flow direction (MD direction) of the plastic film was regarded as the longitudinal direction, and the width direction (TD direction) of the plastic film was regarded as the transverse direction. For a total of five locations, namely, four locations 10 mm from the four corners of the sample toward the central part and the central part of the sample, the in-plane retardation, retardation in the thickness direction, and direction of the slow axis were measured. The averages of Re1 to Re5 calculated from the measurement results are shown in Table 1. The measuring device used was the product named "RETS-100 (measurement spot: diameter 5 mm)" manufactured by Otsuka Electronics Co., Ltd. The direction of the slow axis was measured within the range of 0 degrees or more and 90 degrees or less with the flow direction (MD direction) of the plastic film as the reference 0 degrees.

[0194] 1-2. Flexural resistance Samples in the form of strips with a short side (TD direction) of 30 mm and a long side (MD direction) of 100 mm were cut from the plastic films used in the examples, comparative examples, and reference examples prepared or provided in "2" described below. Both ends of the short side (30 mm) of the sample were fixed to a durability tester (product name "DLDMLH-FS", Yuasa System Co., Ltd.) (the area 10 mm from the tip was fixed), and a continuous folding test of folding 180 degrees was conducted 100,000 times. The folding speed was 120 times per minute. A more detailed method of the folding test is shown below. After the folding test, the strip-shaped sample was placed on a horizontal table, and the angle at which the end of the sample lifted from the table was measured. If the angle is 15 degrees or less, it is the passing level. The results are shown in Table 1. Samples that broke during the process were regarded as "broken". <MD direction> Samples in the form of strips with a short side (MD direction) of 30 mm and a long side (TD direction) of 100 mm were cut from the plastic films used in the examples, comparative examples, and reference examples prepared or provided in "2" described below, and the same evaluation as above was conducted.

[0195] <Details of the folding test> As shown in FIG. 10(A), in the continuous folding test, first, the side portion 10C of the plastic film 10 and the side portion 10D facing the side portion 10C are respectively fixed by the fixing portions 60 arranged in parallel. The fixing portion 60 is slidable in the horizontal direction. Next, as shown in FIG. 10(B), by moving the fixing portions 60 closer to each other, the plastic film 10 is deformed to be folded. Further, as shown in FIG. 10(C), after moving the fixing portions 60 to a position where the distance between the two opposite side portions fixed by the fixing portions 60 of the plastic film 10 becomes 2 mm, the fixing portions 60 are moved in the reverse direction to eliminate the deformation of the plastic film 10. 10(A) to 10(C), the plastic film 10 can be folded 180 degrees by moving the fixing part 60. Furthermore, by conducting a continuous folding test so that the bent part 10E of the plastic film 10 does not protrude from the lower end of the fixing part 60 and controlling the distance between the fixing parts 60 to 2 mm when they are closest to each other, the distance between the two opposing sides of the optical film 10 can be set to 2 mm.

[0196] 1-3. Calculating the difference between the maximum and minimum values ​​of ΔEab A liquid crystal display device (EIZO Corporation's trade name "EV2450", width: 527.0 mm, height: 596.4 mm, absorption axis of the polarizer parallel to the vertical direction of the screen, backlight: backlight using white light-emitting diodes) was prepared, which had a polarizer on a liquid crystal display element. This liquid crystal display device was regarded as laminate 2. The liquid crystal display device (laminate 2) was set to display white in a darkroom environment, and measurement 2 described in this specification was carried out at each angle using ELDIM Corporation's trade name "EzContrast". The measurement area was a circle with a diameter of 2 mm (area: approximately 3.14 mm 2 is. Next, a laminate 1 was prepared by placing the optical films of the Examples and Comparative Examples on the liquid crystal display device via an adhesive layer. At this time, the optical film was placed so that the absorption axis of the polarizer and the slow axis of the plastic film of the optical film were at 90 degrees. Then, the laminate 1 was displayed in white in a darkroom environment, and measurement 1 of this specification was carried out at each angle using ELDIM's product name "EzContrast." The measurement area was a circle with a diameter of 2 mm (area of ​​approximately 3.14 mm). 2 and matched the area of ​​measurement 1. Measurements 1 and 2 were carried out at the center positions in the plane of laminate 1 and laminate 2. Next, ΔEab was calculated for each azimuth angle based on steps (3-1) to (3-4) in the main text of the specification, and the difference between the maximum and minimum values ​​of ΔEab was calculated. Steps (3-1) to (3-4) were performed using ELDIM's product name "EzContrast" and its accompanying software "EzCom" (the gradation level in (3-1) above is 16 levels). The results are shown in Table 1. In the state of laminate 2, the averages of the L* value, a* value, and b* value, and the variations (3σ) of the L* value, a* value, and b* value were as follows: In other words, in measurement 2, the averages of the L* value, a* value, and b* value, and the variations (3σ) of the L* value, a* value, and b* value were as follows: Average L* value across all angles: 95.9 Average a* value for all angles: 4.2 Average b* value for all angles: -4.6 - Variation of L* value across all angles (3σ): 113.3 Variation of a* value across all angles (3σ): 10.7 Variation of b* value across all angles (3σ): 14.1

[0197] 1-4. Evaluation of rainbow unevenness Laminate 1 was prepared in the same manner as in 1-3 above. The slow axis of the biaxially stretched plastic film of the optical film was arranged so that it was perpendicular to the absorption axis of the polarizer of the polarizing plate. A surface light source (liquid crystal display element) was set to display white in a darkroom environment, and the subject was asked to move their face up and down and left and right from a distance of 30 cm to 100 cm from Laminate 1, and to view the laminate from ±90 degrees up and down and ±90 degrees left and right. The evaluators were healthy people in their 20s with visual acuity of 0.7 or higher, and evaluated the presence or absence of rainbow unevenness with the naked eye according to the following criteria. The visual acuity mentioned above includes corrected visual acuity. A: No rainbow irregularities are visible when viewed from any position or direction. B: Rainbow unevenness is visible in a small number of positions and / or directions in a very small area. C: Rainbow unevenness is visible in a small area in many positions and / or directions. D: Rainbow unevenness is visible in most areas at many positions and / or directions.

[0198] 1-5. Evaluation of color distortion Laminate 1 was prepared in the same manner as in 1-3 above. The slow axis of the biaxially stretched plastic film of the optical film was arranged so that it was perpendicular to the absorption axis of the polarizer of the polarizing plate. The surface light source (liquid crystal display element) was set to display white in a darkroom environment, and the laminate 1 was observed around the entire circumference with the elevation angle fixed at approximately (α + β) / 2. The observation can be said to be observation at an azimuth angle of 0 to 359 degrees with the elevation angle fixed at approximately (α + β) / 2. The observation was carried out from a distance of 30 cm to 100 cm from the laminate 1. The evaluators were healthy people in their twenties with visual acuity of 0.7 or higher, and evaluated the presence or absence of color distortion with the naked eye according to the following criteria. The visual acuity mentioned above includes corrected visual acuity. A: Colors appear the same at all azimuth angles. B: There are a few azimuth angles where the colors appear different. C: There are many azimuth angles at which the colors appear different. D: Colors appear different at most azimuth angles.

[0199] 1-6. Reflectance measurement A black plate (KURARAY CO., LTD., trade name "COMOGLAS DFA2CG 502K (Black) type", total light transmittance 0%, thickness 2 mm, refractive index 1.49) was attached to the biaxially stretched plastic film side of the optical film of each of the examples and comparative examples via a 25 μm-thick transparent pressure-sensitive adhesive layer (PANAC CO., LTD., trade name "Panaclean PD-S1", refractive index 1.49) to prepare a sample (5 cm × 5 cm). The normal direction to the surface of the low refractive index layer of the sample was defined as 0°, and light was incident on the sample from a 5° angle. The reflectance (luminous reflectance Y value) was measured based on the specular reflection of the incident light. The reflectance was measured using a spectral reflectance meter (Shimadzu Corporation, product name: MPC3100) to measure the 5° specular reflectance over a wavelength range of 380 nm to 780 nm. The measured value was then converted into the brightness perceived by the human eye using software (built into the MPC3100; reflectance calculation conditions: Illuminant C, viewing angle 2°). The reflectance was measured at 10 locations for each sample, and the average value was used as the reflectance for each sample.

[0200] 2. Fabrication and Preparation of Biaxially Stretched Polyester Films [Biaxially oriented polyester film 1] 1 kg of PET (melting point 258°C, absorption center wavelength: 320 nm) and 0.1 kg of UV absorber (2,2'-(1,4-phenylene)bis(4H-3,1-benzoxazinon-4-one) were melt-mixed in a kneader at 280°C to produce pellets containing the UV absorber. The pellets and PET with a melting point of 258°C were placed in a single-screw extruder and melt-mixed at 280°C. The mixture was extruded through a T-die and cast onto a casting drum with a surface temperature controlled at 25°C to obtain a cast film. The amount of UV absorber in the cast film was 1 part by mass per 100 parts by mass of PET. The resulting cast film was heated with a group of rolls set at 95°C, and then stretched 3.3 times in the machine direction while heating both the front and back sides of the film with a radiation heater so that the film temperature at 250 mm into the 400 mm stretching section (starting point is stretching roll A and ending point is stretching roll B; stretching rolls A and B each have two nip rolls) would be 103°C, and then cooled once. During heating with the radiation heater, air at 92°C and 4 m / s was blown toward the film from the opposite side of the film from the radiation heater, creating turbulence on both sides of the film and disrupting the temperature uniformity of the film. Next, both sides of this uniaxially stretched film were subjected to a corona discharge treatment in air to set the wetting tension of the base film to 55 mN / m, and the corona discharge-treated surfaces of both sides of the film were inline coated with a "slip layer coating solution containing a polyester resin with a glass transition temperature of 18°C, a polyester resin with a glass transition temperature of 82°C, and silica particles with an average particle size of 100 nm" to form a slip layer. The uniaxially stretched film was then introduced into a tenter, preheated with hot air at 95°C, and stretched 4.5 times in the width direction at a temperature of 105°C in the first stage and 140°C in the second stage. When the transverse stretching section was divided into two, the stretching amount of the film at the midpoint of the transverse stretching section (film width at the measurement point - film width before stretching) was 80% of the stretching amount at the end of the transverse stretching section. The transversely stretched film was then heat-treated in the tenter with hot air at temperatures ranging from 180°C to 245°C, followed by 1% relaxation in the width direction at the same temperature conditions, and then rapidly cooled to 100°C and then 1% relaxation in the width direction. The film was then taken up and taken up to obtain a 40 μm-thick biaxially stretched polyester film 1 (the biaxially stretched polyester film used in Example 1).

[0201] [Biaxially oriented polyester film 2] As the biaxially stretched polyester film used in Comparative Example 1, a commercially available biaxially stretched polyester film (TOYOBO CO., LTD., trade name: Cosmoshine A4100, thickness: 50 μm) was prepared.

[0202] [Biaxially oriented polyester film 3] A biaxially oriented polyester film 3 (biaxially oriented polyester film used in Example 2) having a thickness of 40 μm was obtained in the same manner as in the biaxially oriented polyester film 1, except that the stretching ratio in the width direction was changed from 4.5 times to 5.1 times.

[0203] [Biaxially oriented polyester film 4] Biaxially oriented polyester film 4 (biaxially oriented polyester film used in Example 3) was obtained in the same manner as biaxially oriented polyester film 3, except that the thickness of the casting film of biaxially oriented polyester film 1 was increased and the final thickness was changed to 80 μm.

[0204] [Biaxially oriented polyester film 5] As the biaxially stretched polyester film used in Comparative Example 2, a commercially available biaxially stretched polyester film (Toray Industries, Inc., product name: 75U403, thickness: 75 μm) was prepared.

[0205] [Biaxially oriented polyester film 6] As the biaxially stretched polyester film used in Comparative Example 3, a commercially available biaxially stretched polyester film (TOYOBO CO., LTD., trade name: Cosmoshine A4300, thickness: 23 μm) was prepared.

[0206] 3. Fabrication of Optical Film [Example 1] A coating solution for forming a hard coat layer having the following formulation was applied onto the biaxially stretched polyester film 1 prepared in the above 2, and then dried at 70°C for 1 minute to volatilize the solvent. Subsequently, the film was irradiated with ultraviolet light (100 mJ / cm 2 ) to form a hard coat layer (dry thickness: 10 μm). The coating solution 1 for forming a low refractive index layer having the following formulation was applied onto the hard coat layer, and then dried at 60°C for 1 minute to volatilize the solvent. 2 ) to form a low refractive index layer (dry thickness: 100 nm), and an optical film of Example 1 was obtained.

[0207] <Coating liquid for forming hard coat layer> UV-curable acrylate-containing composition: 22 parts by weight (Nippon Kayaku Co., Ltd., product name "KAYARAD PET-30", solid content 100%) UV-curable acrylate-containing composition: 17 parts by mass (Dai-ichi Kogyo Seiyaku Co., Ltd. (DKS Co. Ltd.), product name "New Frontier R-1403MB (NewFrontier R-1403M)", solid content 80%) Fluorine-based leveling agent: 1 part by weight (DIC Corporation, product name "MEGAFACE F-568") Photopolymerization initiator: 1 part by mass (IGM Resins BV, product name "Omnirad 184") Methyl isobutyl ketone: 15 parts by weight Methyl ethyl ketone: 44 parts by weight

[0208] <Coating liquid for forming low refractive index layer> UV-curable acrylate-containing composition: 1 part by mass (Nippon Kayaku Co., Ltd., product name "KAYARAD PET-30", solid content 100%) Photopolymerization initiator: 0.2 parts by mass (IGM Resins BV, product name "Omnirad 127") Hollow silica particles: 1.3 parts by mass (Average primary particle diameter 60nm) Solid silica particles: 0.7 parts by weight (Average primary particle diameter 15nm) Leveling agent: 0.1 parts by weight (Dainichiseika Color & Chemicals Mfg. Co., Ltd., product name "SEIKABEAM 10-28(MB)") ·Dilution solvent: 90 parts by mass (MIBK / AN=7 / 3)

[0209] [Examples 2 to 3] Optical films of Examples 2 and 3 were obtained in the same manner as in Example 1, except that the biaxially stretched polyester film 1 was changed to the biaxially stretched polyester films 3 and 4 prepared in the above "2".

[0210] [Comparative Example 1] An optical film of Comparative Example 1 was obtained in the same manner as in Example 1, except that the biaxially stretched polyester film 1 was changed to the biaxially stretched polyester film 2 prepared in "2" above.

[0211] [Comparative Examples 2 to 3] Optical films of Comparative Examples 2 and 3 were obtained in the same manner as in Example 1, except that the biaxially stretched polyester film 1 was changed to the biaxially stretched polyester films 5 and 6 prepared in "2" above.

[0212] [Reference example 1] The biaxially stretched polyester film 3 alone (the biaxially stretched polyester film 3 on which neither a hard coat layer nor a low refractive index layer was formed) was used as the optical film of Reference Example 1.

[0213] [Reference example 2] The biaxially stretched polyester film 2 alone (the biaxially stretched polyester film 2 on which neither a hard coat layer nor a low refractive index layer was formed) was used as the optical film of Reference Example 2.

[0214] [Reference example 3] A commercially available uniaxially stretched polyester film (TOYOBO CO., LTD., trade name "Cosmoshine TA048", thickness: 80 μm) was used as the optical film of Reference Example 3.

[0215] [Table 1]

[0216] The results in Table 1 confirm that the optical films of the examples, in which the difference between the maximum and minimum values ​​of ΔEab is less than 17.0, can suppress rainbow unevenness when viewed with the naked eye without increasing the in-plane retardation. Furthermore, the results in Table 1 confirm that the optical films of the examples, in which the difference between the maximum and minimum values ​​of ΔEab is less than 17.0, can suppress color distortion. In contrast, the optical films of the comparative examples, although having a low refractive index layer and having the same reflectance as the optical films of the examples, cannot suppress rainbow unevenness and color distortion. It can also be confirmed that the optical films of the examples can be prevented from retaining a bending habit or breaking after a bending test, regardless of the bending direction (the optical films of the examples are less likely to retain a bending habit and are less likely to break than uniaxially stretched polyester films (Reference Example 3) and general biaxially stretched films (Comparative Examples 1 to 3)). It should be noted that ΔEab in Table 1 is based on the measurement results at the central position within the plane of Laminate 1 and Laminate 2, but similar results were obtained even when the measurement location was shifted (for example, ΔEab at a location 130 mm to the left of the central position (other measurement location 1), a location 130 mm to the right of the central position (other measurement location 2), a location 75 mm above the central position (other measurement location 3), and a location 75 mm below the central position (other measurement location 4) were roughly equivalent to the values ​​in Table 1). [Explanation of symbols]

[0217] 10: Plastic film 20: Hard coat layer 30: Low refractive index layer 100: Optical film 200: Surface light source 300: Polarizer 400: Adhesive layer 500: First transparent protective plate 600: Second transparent protective plate 700: Polarizing plate 800: Display element 1000: Image display device X: Laminate 1 Y: Laminate 2

Claims

1. An optical film having a low refractive index layer on a plastic film, the plastic film is a biaxially stretched plastic film having an in-plane retardation of 2500 nm or less, the low refractive index layer is located on the outermost surface of the optical film, An optical film having a lower region in which the difference between the maximum and minimum values ​​of ΔEab is less than 17.

0. Here, measurement 1 is carried out for laminate 1 to calculate the L* value, a* value, and b* value in the L*a*b* color system. Measurement 2 is carried out for laminate 2 to calculate the L* value, a* value, and b* value in the L*a*b* color system. Based on the results of measurements 1 and 2, ΔEab is calculated under condition 1. <Measurement 1> A laminate 1 is prepared by laminating a polarizer and the optical film in this order on a surface light source. In the laminate 1, the optical film is arranged so that the surface on the low refractive index layer side faces the opposite side to the polarizer. The polarizer is also arranged so that the angle between the absorption axis of the polarizer and the left-right or up-down direction of the surface light source is within ±5 degrees. Furthermore, the angle between the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film is within ±5 degrees. The surface light source of the laminate 1 is displayed in white, and the transmitted light emitted from the low refractive index layer side of the laminate 1 is measured at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees, one degree at a time, and the L* value, a* value, and b* value of the L*a*b* color system are calculated based on the transmitted light at each angle. ２ More than 10 mm ２ The areas are as follows: <Measurement 2> A laminate 2 is prepared by laminating a polarizer on the same surface light source as in Measurement 1. The polarizer is arranged so that the direction of the absorption axis relative to the surface light source is the same as in Measurement 1. The surface light source of the laminate 2 is displayed as white, and the transmitted light emitted from the polarizer side of the laminate 2 is measured at one-degree intervals at an elevation angle of 0 to 80 degrees and an azimuth angle of 0 to 359 degrees, and the L* value, a* value, and b* value of the L*a*b* color system are calculated based on the transmitted light at each angle. The measurement area of ​​the transmitted light is set to approximately the same as Measurement 1 within the plane. <Condition 1> At all elevation angles and all azimuth angles, ΔL* is calculated by subtracting the L* value of measurement 2 from the L* value of measurement 1. ΔL* is converted into a grayscale from the maximum value to the minimum value in a predetermined gradation, and displayed in grayscale on a two-dimensional coordinate system in which the elevation angle is represented by concentric circles and the azimuth angle is represented by vertical and horizontal lines. It is confirmed that there are two regions in the two-dimensional coordinate system where ΔL* is distributed concentrically, and that the two regions are located at approximately symmetrical positions in the two-dimensional coordinate system. With respect to the elevation angles at the center of the region where ΔL* is distributed concentrically, one elevation angle is set to α degrees and the other elevation angle is set to β degrees. ΔEab at each azimuth angle is calculated from the difference between the L* value, a* value, and b* value of measurement 1 at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α + β) / 2, and the L* value, a* value, and b* value of measurement 2 at an azimuth angle of 0 degrees or more and 359 degrees or less when the elevation angle is (α + β) / 2.

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