Patent ID: RE39753

Claim:
A retardation film, characterized in that said retardation film is a single oriented film, the retardation at wavelengths of 450 nm and 550 nm of said retardation film satisfies the following formulae (1) and/or (2), and the water absorption of said retardation film is no greater than 1%: R(450)/R(550)<1 (1) K(450)/K(550)<1 (2) where R(450) and R(550) represent the in-plane retardation of the oriented polymer film at wavelengths of 450 nm and 550 nm, respectively, and 450 nm and 550 nm, respectively, and K(450) and K(550) are the values calculated by K=[ n z −( n x +n y )/2]×d (where n x , n y and n z represent the three-dimensional refractive indexes of the oriented polymer film as the refractive indexes in the direction of the x-axis, y-axis and z-axis, respectively, and d represents the thickness of the film) for the oriented polymer film at a wavelength of 450 nm an and 550 nm, respectively , wherein the absolute value of R( 550 ) is 20 nm or more and/or K ( 550 nm ) is 20 nm or more .