Patent ID: 8742058

Claim:
A biaxially oriented polyester film having a ratio of Young's modulus in a transverse direction ETD to Young's modulus in a machine direction EMD, ETD/EMD, of 1.5 to 3, an n_bar, which represents an average of a refractive index in a machine direction nMD, a refractive index in a transverse direction nTD, and a refractive index in a thickness direction nZD ((nMD+nTD+nZD)/3), of 1.590 to 1.680, a minor melting peak temperature T-meta of 160 to 190° C., and a coefficient of hygroscopic expansion in the transverse direction of 0 to 6 ppm/% RH, and a heat of crystal melting ΔHm of 32 to 45 J/g, a degree of crystal orientation in the crystal backbone direction by wide-angle x-ray in the transverse direction of 0.75 to 0.90.