Patent ID: 8722842

Claim:
An optical film comprising: a polyimide-based polymer represented by the following general formula (I′): wherein m is 70% by mol or more and 100% by mol or less (m+n≦100% by mol); A, A′, B and B′ each denote a substituent, a and a′ denote substitution number of the corresponding A and A′ (an integer of 0 to 4), and b and b′ denote substitution number of the corresponding B and B′ (an integer of 0 to 3); A, A′, B and B′ each independently denote a halogen or an alkyl group having 1 to 4 carbon atoms, and are each identical or different in the case of being plural; R 3 and R 4 each independently denote an alkyl group having 1 to 10 carbon atoms or CR 6 R 7 (OH) (R 6 and R 7 are each an alkyl group having 1 to 4 carbon atoms); X and Y each independently denote a covalent bond, or an atom or a group selected from the group consisting of a CH 2 group, a C(CH 3 ) 2 group, a C(CZ 3 ) 2 group (Z is a halogen), a CO group, an O atom, an S atom, an SO 2 group, an Si(CH 2 CH 3 ) 2 group, and an N(CH 3 ) group; E is a substituent and e denotes substitution number thereof (an integer of 0 to 3); E denotes an atom or a group selected from the group consisting of a halogen, an alkyl group having 1 to 3 carbon atoms, an alkyl halide group having 1 to 3 carbon atoms, a phenyl group, and a substituted phenyl group, and are each identical or different in the case of being plural; G and H are substituents, g denotes substitution number of G (an integer of 0 to 4), and h denotes substitution number of H (an integer of 0 to 4); G and H each independently denote an atom or a group selected from the group consisting of a halogen, an alkyl group, a substituted alkyl group, a nitro group, a cyano group, a thioalkyl group, an alkoxy group, an aryl group, a substituted aryl group, an alkyl ester group, and a substituted alkyl ester group, and are each identical or different in the case of being plural; and q1 denotes an integer of 0 to 3 and q2 denotes an integer of 1 to 3; wherein an index ellipsoid satisfies a relationship of nx≈ny>nz, wherein nx denotes a refractive index in a direction for a maximum refractive index in a film plane (X-axis direction), ny denotes a refractive index in a direction orthogonal to the X-axis direction in the plane (Y-axis direction), and nz denotes a refractive index in the thickness direction; and wherein the optical film satisfies a relationship of Rth(450)/Rth(550)≦0.98, wherein Rth(450) and Rth(550) denote a retardation value in the thickness direction at a wavelength of 450 nm and a wavelength of 550 nm, respectively.