Patent ID: 7790249

Claim:
A stretched cellulose ester film produced by stretching after film forming employing cellulose acetate propionate or cellulose triacetate including a polyester polyol obtained from (a) a glycol having an average carbon atom number of 2 to 3.5 and (b) a dibasic or an anhydrous dibasic acid having an average carbon number of 4 to 5.5, wherein the stretched cellulose ester film exhibits retardation value Ro in the range of 30 to 200 nm, and a retardation value Rt in the range of 70 to 400 nm, Ro and Rt are each defined by Formulas (1) and (2); and the polyester polyol has a number average molecular weight of from 1,200 to 4,000: Ro =( nx−ny )× d , and Formula (1) Rt ={( nx+ny )/2 −nz}×d, Formula (2) wherein nx is a refractive index in an x direction which gives a maximum refractive index in a plane of the film, ny is a refractive index in a y direction which is orthogonal to the x direction in the plane of the film, nz is a refractive index in a thickness direction of the film, and d is a thickness (nm) of the film.