Patent ID: 7128952

Claim:
An optical film comprising: a transparent polymer film (b); and a biaxially birefringent layer (a) formed of a polymer and laminated above the transparent polymer film (b), wherein the birefringent layer (a) and the transparent polymer film (b) satisfy a formula (1) below, the birefringent layer (a) satisfies formulae (2) and (3) below, and the polymer forming the birefringent layer (a) has a weight-average molecular weight in a range between 10,000 and 400,000 inclusive, Δ n ( a )>Δ n ( b )×10 (1) 1<( nx−nz )/( nx−ny )<100 (2) 0.0005 ≦Δn ( a )≦0.5 (3) where Δn(a) is a birefringence of the birefringent layer (a) and is represented by [(nx+ny)/2]−nz, Δn(b) is a birefringence of the transparent polymer film (b) and is represented by [(nx′+ny′)/2]−nz′, nx, ny, and nz represent refractive indices in an X-axis direction, a Y-axis direction, and a Z-axis direction in the birefringent layer (a), respectively, with the X-axis direction being an axial direction exhibiting a maximum refractive index within a plane of the birefringent layer (a), the Y-axis direction being an axial direction perpendicular to the X-axis within the plane, and the Z-axis direction being a thickness direction perpendicular to the X-axis and the Y-axis, and nx′, ny′, and nz′ represent refractive indices in an X-axis direction, a Y-axis direction, and a Z-axis direction in the transparent polymer film (b), respectively, with the X-axis direction being an axial direction exhibiting a maximum refractive index within a plane of the transparent polymer film (b), the Y-axis direction being an axial direction perpendicular to the X-axis within the plane, the Z-axis direction being a thickness direction perpendicular to the X-axis and the Y-axis.