Optical film, polarizing plate, image display device, and optical film selection method

An image display device comprising a display element, and a polarizer and an optical film disposed on a light emitting surface side of the display element, wherein

• disposition is such that an angle formed by a direction of the absorption axis of the polarizer and a horizontal or vertical direction of the display element falls within ±5 degrees, and
• that the angle formed by the absorption axis of the polarizer and a slow axis of a biaxially stretched plastic film of the optical film falls within 90 degrees±5 degrees, and
• the optical film comprises a low-refractive index layer on the biaxially stretched plastic film with an in-plane phase difference of less than 2500 nm, the low-refractive index layer is located on an outermost surface of the optical film, and the optical film comprises a region in which a difference between a maximum value and a minimum value of ΔEab is less than 17.0, wherein the ΔEab is calculated as follows:
• a laminate 1A is subjected to measurement 1A to calculate L* value, a* value, and b* value in a L*a*b* color system;
• a laminate 2A is subjected to measurement 2A to calculate L* value, a* value, and b* value in a L*a*b* color system;
• and based on a results of the measurement 1A and the measurement 2A, the ΔEab is calculated under condition 1A:
• <Measurement 1A>
• the laminate 1A is produced by laminating the polarizer and the optical film on the display element in this order;
• in the laminate 1A, the optical film is disposed so that the surface on the low-refractive index layer side faces a side opposite to the polarizer;
• and the polarizer is disposed so that the angle formed by the absorption axis of the polarizer and the horizontal or vertical direction of the display element falls within ±5 degrees, and the angle formed by the absorption axis of the polarizer and the slow axis of the biaxially stretched plastic film of the optical film falls within 90 degrees±5 degrees,
• the display element of the laminate 1A is displayed in white, a transmitted light emitted from the low-refractive index layer side of the laminate 1A is measured at 1 degree intervals in ranges of an elevation angle of 0 degrees or more and 80 degrees or less and an azimuth angle of 0 degrees or more and 359 degrees or less, to calculate L* value, a* value, and b* value in the L*a*b* color system based on the transmitted light at each angle;
• and the transmitted light measurement area is set to any in-plane area of 1 mm2 or more and 10 mm2 or less;
• <Measurement 2A>
• the laminate 2A is produced by laminating the polarizer on the display element that is the same as the display element in the measurement 1A,
• the display element of the laminate 2A is displayed in white, transmitted light emitted from the polarizer side of the laminate 2A is measured at 1 degree intervals in ranges of an elevation angle of 0 degrees or more and 80 degrees or less and an azimuth angle of 0 degrees or more and 359 degrees or less, to calculate L* value, a* value, and b* value in the L*a*b* color system based on the transmitted light at each angle;
• and transmitted light measurement area substantially coincides with that in the measurement 1A in the plane;
• and
• <Condition 1A>
• at each elevation angle and each azimuth angle, ΔL* is calculated by subtracting the L* value in the measurement 2A from the L* value in the measurement 1A;
• values from a maximum value to a minimum value of the ΔL* are converted to grayscale to a predetermined gradation and displayed in grayscale in two-dimensional coordinates with the elevation angle as concentric circles and the azimuth angle in the vertical and horizontal directions,
• it is confirmed that there are two regions in which the ΔL* is concentrically distributed in the two-dimensional coordinates, and that the two regions are in substantially symmetric positions in the two-dimensional coordinates,
• with respect to the elevation angle located at a center of the region where the ΔL* is concentrically distributed, one elevation angle is referred to as α degree(s) and the other elevation angle is referred to as β degree(s), and
• the ΔEab at each azimuth angle is calculated from the differences between the L* value, the a* value, and the b* value in the measurement 1A with an azimuth angle of 0 degrees or more and 359 degrees or less at an elevation angle of (α+β)/2 and the L* value, the a* value, and the b* value in the measurement 2A with an azimuth angle of 0 degrees or more and 359 degrees or less at an elevation angle of (α+β)/2, respectively.

A method for selecting an optical film of an image display device comprising a display element, and a polarizer and an optical film on a light emitting surface of the display element, wherein disposition is such that a direction of an absorption axis of the polarizer is parallel to a horizontal or vertical direction of the display element, the method comprising

• selecting, as the optical film, an optical film X satisfying determination conditions that the optical film X comprises a low-refractive index layer on a biaxially stretched plastic film with an in-plane phase difference of less than 2500 nm, the low-refractive index layer is located on an outermost surface of the optical film X, and the optical film X comprises a region in which a difference between a maximum value and a minimum value of ΔEab is less than 17.0, wherein the ΔEab is calculated as follows:
• a laminate 1B is subjected to measurement 1B to calculate L* value, a* value, and b* value in a L*a*b* color system;
• a laminate 2B is subjected to measurement 2B to calculate L* value, a* value, and b* value in a L*a*b* color system;
• and based on a results of the measurement 1B and the measurement 2B, the ΔEab is calculated under condition 1B:
• <Measurement 1B>
• the laminate 1B is produced by laminating the polarizer and the optical film X on the display element in this order;
• in the laminate 1B, the optical film is disposed so that the surface on the low-refractive index layer side faces a side opposite to the polarizer;
• and the polarizer is disposed so that n angle formed by the absorption axis of the polarizer and the horizontal or vertical direction of the display element falls within ±5 degrees, and the angle formed by the absorption axis of the polarizer and a slow axis of the biaxially stretched plastic film of the optical film X falls within 90 degrees±5 degrees,
• the display element of the laminate 1B is displayed in white, transmitted light emitted from the low-refractive index layer side of the laminate 1B is measured at 1 degree intervals in ranges of an elevation angle of 0 degrees or more and 80 degrees or less and an azimuth angle of 0 degrees or more and 359 degrees or less, to calculate L* value, a* value, and b* value in the L*a*b* color system based on the transmitted light at each angle;
• and the transmitted light measurement area is set to any in-plane area of 1 mm2 or more and 10 mm2 or less;
• <Measurement 2B>
• the laminate 2B is produced by laminating the polarizer on the display element that is the same as the display element in the measurement 1B,
• the display element of the laminate 2B is displayed in white, transmitted light emitted from the polarizer side of the laminate 2B is measured at 1 degree intervals in ranges of an elevation angle of 0 degrees or more and 80 degrees or less and an azimuth angle of 0 degrees or more and 359 degrees or less, to calculate L* value, a* value, and b* value in the L*a*b* color system based on the transmitted light at each angle;
• and a transmitted light measurement area substantially coincides with that in the measurement 1B in the plane;
• and
• <Condition 1B>
• at each elevation angle and each azimuth angle, ΔL* is calculated by subtracting the L* value in the measurement 2B from the L* value in the measurement 1B;
• values from a maximum value to a minimum value of the ΔL* are converted to grayscale to a predetermined gradation and displayed in grayscale in two-dimensional coordinates with the elevation angle as concentric circles and the azimuth angle in the vertical and horizontal directions,
• it is confirmed that there are two regions in which the ΔL* is concentrically distributed in the two-dimensional coordinates, and that the two regions are in substantially symmetric positions in the two-dimensional coordinates,
• with respect to the elevation angle located at the center of a region where the ΔL* is concentrically distributed, one elevation angle is referred to as α degree(s) and the other elevation angle is referred to as β degree(s), and
• the ΔEab at each azimuth angle is calculated from the differences between the L* value, the a* value, and the b* value in the measurement 1B with an azimuth angle of 0 degrees or more and 359 degrees or less at an elevation angle of (α+β)/2 and the L* value, the a* value, and the b* value in the measurement 2B with an azimuth angle of 0 degrees or more and 359 degrees or less at an elevation angle of (α+β)/2, respectively.