US 12,216,302 B2
Optical film, polarizing plate, image display device, and optical film selection method
Shosei Kubota, Kashiwa (JP); Yoshiko Tanaka, Tokorozawa (JP); Norio Ishii, Nagareyama (JP); Takashi Kuroda, Moriya (JP); Hiroaki Segawa, Nagareyama (JP); and Akinobu Ushiyama, Moriya (JP)
Assigned to DAI NIPPON PRINTING CO., LTD., Tokyo (JP)
Appl. No. 17/764,456
Filed by DAI NIPPON PRINTING CO., LTD., Tokyo (JP)
PCT Filed Sep. 30, 2020, PCT No. PCT/JP2020/037067
§ 371(c)(1), (2) Date Mar. 28, 2022,
PCT Pub. No. WO2021/065978, PCT Pub. Date Apr. 8, 2021.
Claims priority of application No. 2019-181043 (JP), filed on Sep. 30, 2019.
Prior Publication US 2022/0365265 A1, Nov. 17, 2022
Int. Cl. G02B 5/30 (2006.01); G02F 1/1335 (2006.01); G02F 1/13363 (2006.01)
CPC G02B 5/3083 (2013.01) [G02B 5/3025 (2013.01); G02F 1/133528 (2013.01); G02F 1/13363 (2013.01)] 8 Claims
OG exemplary drawing
 
1. An optical film comprising a low-refractive index layer on a plastic film, wherein
the plastic film is a biaxially stretched plastic film with an in-plane phase difference of 2500 nm or less,
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 1 is subjected to measurement 1 to calculate L* value, a* value, and b* value in a L*a*b* color system; a laminate 2 is subjected to measurement 2 to calculate L* value, a* value, and b* value in a L*a*b* color system; and based on a results of the measurement 1 and the measurement 2, the ΔEab is calculated under condition 1:
<Measurement 1>
the laminate 1 is produced by laminating a polarizer and the optical film on a surface light source in this order; in the laminate 1, the optical film is disposed so that the surface on the low-refractive index layer side faces the side opposite to the polarizer; and the polarizer is disposed so that an angle formed by an absorption axis of the polarizer and a horizontal or vertical direction of the surface light source 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 falls within 90 degrees±5 degrees,
the surface light source of the laminate 1 is displayed in white, a transmitted light emitted from the low-refractive index layer side of the laminate 1 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 2>
the laminate 2 is produced by laminating a polarizer on a surface light source that is the same as the surface light source in the measurement 1; the polarizer is disposed so that a direction of the absorption axis of the polarizer with respect to the surface light source is the same direction as in the measurement 1,
the surface light source of the laminate 2 is displayed in white, transmitted light emitted from the polarizer side of the laminate 2 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 1 in the plane; and
<Condition 1>
at each elevation angle and each azimuth angle, ΔL* is calculated by subtracting the L* value in the measurement 2 from the L* value in the measurement 1; 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 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 1 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 2 with an azimuth angle of 0 degrees or more and 359 degrees or less at an elevation angle of (α+β)/2, respectively.