Patent ID: 7479312

Claim:
A retardation film, containing: a cellulose derivative; and a plasticizer in an amount of 1 to 20% by mass to the cellulose derivative; wherein the thickness of the film is 10 to 50 micrometer, wherein the radius of a free volume of the film, obtained by Positron Annihilation Lifetime Spectroscopy, is 0.250 to 0.310 nm, Ro in Equation (i) is in the range of 30 to 100 nm, Rt in Equation (ii) is in the range of 100 to 300 nm, and S in Equation (iii) is 0.002 or more but not exceeding 0.030, Ro =( nx−ny )× d Equation (i) Rt=S×d Equation (ii) S =(( nx+ny )/2 −nz ) Equation (iii) where “nx” is a maximum refraction index of the film surface, “ny” is a refraction index in the direction perpendicular to “nx,” “nz” is a refraction index along the thickness of the film, and “d” is the thickness of the film, wherein the plasticizer contains a polyvalent alcohol ester type plasticizer and an aromatic terminal ester type plasticizer, and the aromatic terminal ester type plasticizer is represented by the following general formula: B-(G-A)n-G-B where B represents a benzenemonocarboxylic acid residue, G represents an alkylene glycol residue having 2-12 carbon atoms, an aryl glycol residue having 6-12 carbon atoms, or an oxyalkylene glycol residue having 4-12 carbon atoms, A represents an alkylene dicarboxylic acid residue having 4-12 carbon atoms, or an aryl dicarboxylic acid residue having 6-12 carbon atoms, and n represents an integer of 1 or more.