Patent ID: 8859648

Claim:
A retardation film comprising polyester having a photoelastic coefficient of 18×10 −12 Pa −1 to 27×10 −12 Pa −1 , wherein the polyester includes a structural unit expressed by the chemical formulas (I), (II), (III), or (IV) and wherein the mole fractions (%) of structural units expressed by the chemical formulas (I), (II), (III) and (IV) are denoted by l, m, n, and o, respectively, with l, m, n, and o satisfying the following equations (4) and (5): 80< l+m+n+o≦ 100 (4), and 10≦ l+m≦ 100 (5), wherein formulas (I), (II), (III) and (IV) are as follows: R 1 : a decalin structure, and R 2 : hydrogen, halogen or a hydrocarbon group having 1 to 3 carbon atoms; R 3 : an aromatic group, and R 4 : hydrogen, halogen or a hydrocarbon group having 1 to 3 carbon atoms; R 5 : a decalin structure, and R 6 : spiro glycol residues and optionally also ethylene structures; and R 7 : an aromatic group, and R 8 : spiro glycol residues and optionally also ethylene structures.