Optical biaxially stretched plastic film, polarizing plate, image display device, and method of selecting optical biaxially stretched plastic film
Provided are an optical biaxially stretched plastic film, a polarizing plate, and an image display device that can suppress blackouts when viewed with polarized sunglasses, polarized goggles or the like without increasing the in-plane phase difference. In addition, a method for selecting an optical biaxially stretched plastic film is provided. The optical biaxially stretched plastic film has a region satisfying condition 1 and condition 2 below: <Condition 1> the difference between a luminance obtained in a specific measurement 1 and a luminance obtained in a specific measurement 2 (L1.n−L2.n) is calculated at each of 100 measurement points, and the “luminance difference variation 3σ” calculated from the luminance differences at the 100 measurement points is 100 or more; and <Condition 2> the in-plane phase difference (Re) is 2500 nm or less.
1 . An optical biaxially stretched plastic film comprising a region satisfying <Condition 1>, <Condition 2>, and <Condition 4> below:
<Condition 1>
Measurement 1 below and Measurement 2 below are conducted;
<<Measurement 1>>
a measurement sample 1 includes a first polarizer, the optical biaxially stretched plastic film, and a second polarizer disposed in this order on a surface light source with a slow axis direction of the optical biaxially stretched plastic film disposed substantially perpendicular to an absorption axis direction of the first polarizer, and an absorption axis of the second polarizer is disposed substantially perpendicular to the absorption axis direction of the first polarizer,
the surface light source of the measurement sample 1 is displayed in white having a color temperature in a range of 5000K to 13000K, and luminance of transmitted light emitted from the second polarizer side is measured for a first region defined by 100 vertical columns of measurement sections×100 horizontal rows of measurement sections set at equal intervals;
a size of the first region is 100 mm in vertical and ×100 mm in horizonal; and
luminance measurement results for the first region at 100 measurement sections in any one of the horizontal rows of the first region are extracted from the luminance measurement results and are sequentially identified as a first measurement point of Measurement 1 to a 100th measurement point of Measurement 1, with the luminance at the first measurement point of Measurement 1 defined as L1.1, the luminance at the 100th measurement point of Measurement 1 defined as L1.100, and the luminance at a n-th measurement point of Measurement 1 defined as L1.n;
<<Measurement 2>>
a measurement sample 2 includes the first polarizer and the second polarizer disposed in this order on the surface light source used in the Measurement 1 with the absorption axis of the second polarizer disposed substantially perpendicular to the absorption axis direction of the first polarizer,
the surface light source of the measurement sample 2 is displayed in white having the same color temperature as that for Measurement 1, and luminance of transmitted light emitted from the second polarizer side is measured for a second region defined by 100 vertical columns of measurement sections×100 horizontal rows of measurement sections set at equal intervals with a location of the second region substantially coinciding with a location of the first region; and
luminance measurement results for the second region in Measurement 2 at 100 measurement sections in a horizontal row of the second region corresponding in location to the any one of the horizontal rows selected in Measurement 1 are extracted from the luminance measurement results for the second region in Measurement 2 and are sequentially identified as a first measurement point of Measurement 2 to a 100th measurement point of Measurement 2, and the luminance at the first measurement point of Measurement 2 is defined as L2.1, the luminance at the 100th measurement point of Measurement 2 is defined as L2.100, and the luminance at a n-th measurement point of Measurement 2 is defined as L2.n; and
when a luminance difference between the luminances obtained for the 100 measurement points of Measurement 1 and the 100 measurement points of Measurement 2 is determined as L1.n−L2.n for each n in a range from 1 to 100, a luminance difference variation 3σ calculated from the luminance differences between the 100 measurement points of Measurement 1 and the 100 measurement points of Measurement 2 is 100 or more and 800 or less;
<Condition 2>
wherein when a sample of 100 mm in vertical×100 mm in horizonal is taken from the biaxially stretched plastic film, and an in-plane phase difference (Re) are measured at a total of five points including a center of the sample and four points spaced 10 mm inward from respective ones of four corners of the sample toward the center of the sample, an average value of an in-plane phase difference (Re) at the five points of the sample is 2500 nm or less; and
<Condition 4>
wherein when slow axis directions are measured at the five points of the sample of 100 mm in vertical×100 mm in horizonal, and the slow axis directions of the five points respectively are defined as D 1 , D 2 , D 3 , D 4 , and D 5 , a difference between a maximum value and a minimum value of D 1 to D 5 is 5.0 degrees or more.
2 . The optical biaxially stretched plastic film according to claim 1 , wherein the average value of the in-plane phase difference is 520 nm or more and 2500 nm or less.
3 . The optical biaxially stretched plastic film according to claim 1 , wherein the difference between the maximum value and the minimum value of D 1 to D 5 is 5.0 degrees or more and 20.0 degrees or less.
4 . The optical biaxially stretched plastic film according to claim 1 , wherein
the in-plane phase difference with respect to a phase difference in a thickness direction is 0.10 or less.
5 . The optical biaxially stretched plastic film according to claim 1 , having a film thickness of 20 μm or more and 200 μm or less.
6 . A functional film comprising a functional layer on one side of the optical biaxially stretched plastic film according to claim 1 .
7 . A polarizing plate comprising: a polarizer; a first transparent protective plate disposed on one side of the polarizer; and a second transparent protective plate disposed on the other side of the polarizer, wherein
at least one selecting from the group consisting of the first transparent protective plate and the second transparent protective plate is the optical biaxially stretched plastic film according to claim 1 .
8 . An image display device comprising: a display element; and a plastic film disposed on a light emitting surface side of the display element, wherein
the plastic film is the optical biaxially stretched plastic film according to claim 1 .
9 . The image display device according to claim 8 , further comprising a polarizer between the display element and the plastic film.
10 . The image display device according to claim 8 , further comprising a functional layer on the side opposite to the display element of the optical biaxially stretched plastic film.