Patent ID: 7410680

Claim:
A retardation film comprising any one of a stretched film having an achromatic property formed from cellulose n-pentanate having a degree of substitution for hydroxyl group per one cellulose monomer unit of 2.0 to 2.8, cellulose n-hexanate having a degree of substitution for hydroxyl group per one cellulose monomer unit of 2.0 to 2.6 or cellulose n-heptanate having a degree of substitution for hydroxyl group per one cellulose monomer unit of 1.0 to 2.99, or a stretched film having a biaxial property prepared by uniaxially stretching cellulose n-heptanate having a degree of substitution for hydroxyl group per one cellulose monomer unit of 2.5 to 2.99 or a cellulose derivative substituted by a straight chained (C 8 -C 20 ) acyl group in a degree of substitution for hydroxyl group per one cellulose monomer unit of 1.00 to 2.99, wherein said retardation film satisfies the relations represented by the following formulae (1) and 2): ( Re 750/ Re 550)= A 1 /Δn+ 1 (1) ( Re 450/ Re 550)= A 2 /Δn+ 1 (2) wherein, (Re750/Re550) is a ratio of a retardation value at a wavelength of 750 nm to a retardation value at a wavelength of 550 nm, showing a value of 0.05 to 1.95; (Re450/Re550) is a ratio of a retardation value at a wavelength of 450 nm to a retardation value at a wavelength of 550 nm, showing a value of 0.05 to 1.95; Δn is a birefringence of the retardation film at a wavelength of 550 nm, showing a value of 0.0001 to 0.06; A1 and A2 are each a constant showing a value of −0.06 to 0.06, and when A1 is a positive value, A2 becomes a negative value, and when A1 is a negative value, A2 becomes a positive value.