Optical biaxially stretched plastic film, polarizing plate, image display device, and method of selecting optical biaxially stretched plastic film

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 D1, D2, D3, D4, and D5, a difference between a maximum value and a minimum value of D1 to D5 is 5.0 degrees or more.